40_recycled aggregate from c&d waste & its use in concrete

8/17/2019 40_Recycled Aggregate From C&D Waste & Its Use in Concrete

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Review

Recycled aggregate from C&D waste & its use in concrete – A

breakthrough towards sustainability in construction sector: A review

Monalisa Behera ⇑, S.K. Bhattacharyya, A.K. Minocha, R. Deoliya, S. Maiti

CSIR-Central Building Research Institute, Roorkee 247667, India

h i g h l i g h t s

Usage of RAC will possibly help theglobal community towards

sustainable development.

The strength and durability

performance of RAC arecomprehensively documented.

ITZ of RAC is porous, presence of cracks and fissures are commonlyfound due to the residual mortar.

The deficiencies observed in RAC canbe significantly overcome by the

incorporation of mineral additives tothe concrete.

g r a p h i c a l a b s t r a c t

Recycled aggregate concrete -

Review

C&D WasteCrushers Durability performance

Crushing process Microstructure

Recycled aggregate Hardened properties

Recycled aggregate concrete Workability

a r t i c l e i n f o

Article history:

Received 24 March 2014

Received in revised form 27 June 2014

Accepted 2 July 2014

Available online 26 July 2014

Keywords:

Construction and demolition waste

Recycled aggregate

Recycled aggregate concrete

Strength

Durability

Interfacial transition zone

Microstructure

a b s t r a c t

The issues of sustainability are of prime concerns these days as we use large amount of natural resources

for producing materials such as concrete. Depletion of natural resources is one of such sustainabilityissues which we need to address in an efficient manner. The recent trend in construction industry is to

use the alternative source of construction materials which can substitute the use of virgin materials inorder to reduce environmental impact in terms of energy consumption, pollution, waste disposal andglobal warming. On the other hand, the waste generated from the demolition of old structure and con-

struction activity is a matter of concern all over the world. Thus, recycling and reuse of these wastes

may reduce the usage of natural resources and it can also serve towards the demand of environment.The present paper gives a brief status of recycled aggregate concrete made out of recycled aggregate,summarizes and critically analyses some of the most important research findings over the past few years

regarding the material aspects. It also attempts to elucidate the approaches for the better performances,identifies the gaps in the existing knowledge and underlines the reasons why this promising technology

has not become widely accepted by the construction industry. The practical problems with application of recycled aggregate in concrete are also discussed.

2014 Elsevier Ltd. All rights reserved.

http://dx.doi.org/10.1016/j.conbuildmat.2014.07.003

0950-0618/ 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Tel.: +91 1332283317, +91 9045379496.

E-mail address: [email protected] (M. Behera).

Construction and Building Materials 68 (2014) 501–516

Contents lists available at ScienceDirect

Construction and Building Materials

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c o n b u i l d m a t

http://dx.doi.org/10.1016/j.conbuildmat.2014.07.003mailto:[email protected]://dx.doi.org/10.1016/j.conbuildmat.2014.07.003http://www.sciencedirect.com/science/journal/09500618http://www.elsevier.com/locate/conbuildmathttp://www.elsevier.com/locate/conbuildmathttp://www.sciencedirect.com/science/journal/09500618http://dx.doi.org/10.1016/j.conbuildmat.2014.07.003mailto:[email protected]://dx.doi.org/10.1016/j.conbuildmat.2014.07.003http://-/?-http://-/?-http://-/?-http://crossmark.crossref.org/dialog/?doi=10.1016/j.conbuildmat.2014.07.003&domain=pdfhttp://-/?-http://-/?-


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Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 502

1.1. Essence of recycling of Construction & Demolition (C&D) wastes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5022. Recycled aggregate (RA). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 503

2.1. Recycling process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 503

2.2. Physical and mechanical properties of recycled aggregate. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5033. Recycled aggregate concrete (RAC). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 504

3.1. Properties of recycled aggregate concrete . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5043.1.1. Green state . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 504

3.1.2. Hardened properties. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5053.1.2.1. Compressive strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 506

3.1.2.2. Split tensile strength and flexural strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5073.1.2.3. Drying shrinkage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 507

3.1.2.4. Creep and modulus of elasticity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5083.1.2.5. Bond strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 509

3.2. Microstructure of recycled aggregate concrete . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 509

3.3. Techniques for improving properties of recycled aggregate concrete . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5103.3.1. Incorporation of mineral admixture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 510

3.3.2. Impregnation of RA in cement slurry or other mineral admixture solution. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5113.3.3. Modifying mixing process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 511

3.4. Durability properties. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5123.4.1. Techniques for improving durability properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 513

4. Summary and conclusion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5135. Future aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 514References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 514

1. Introduction

Concrete is a composite material, basically consisting of different constituents such as binding materials, water, aggregatesand admixtures. Among these ingredients, aggregate plays a very

crucial role in concrete which occupy the largest volume of about60–75% of total concrete volume [1]. It is indispensable for anyconstruction work. The versatility of concrete as a construction

material for large construction work lies in its high strength, lowmaintenance cost, resistant to weathering effect, economical over

other construction materials and its excellent structuralperformance.

Moreover, the extensive increase in the rate of industrializationand urbanization due to the parallel growth in economy and pop-

ulation has made the use of concrete as the most non-sustainablematerial as it is consuming the maximum amount of naturalresources. Concrete has a very important role in the economydevelopment of a country due to its large volume utilization. Ituses approximately 20 billion tons of raw materials (coarse aggre-

gate) each year. According to Mehta and Meryman [2], the currentusage of concrete is approximately 20 billion metric tons perannum. However, from the forecast of the research group of Fredo-

nia, it was mentioned that the global consumption of aggregateused in construction may exceed 26 billion tons by 2012 [3]. Withthis increase in rate of consumption, it is expected that the demandof aggregates will be doubled in the next two to three decades [4].

Amongst different countries, India has occupied a place in the topten users of the leading countries to use natural resources.

Thus, the concrete industry consumes a large amount of naturalresources that cause substantial environmental, energy and eco-

nomic losses as it exploits 50% raw material, 40% of total energy,as well as generates 50% of total waste [5]. So, minimizing the envi-ronmental impact, energy consumption and the increase in CO2intensity for the concrete to be used for construction have become

more evident for construction industry which can lead towardssustainable development.

The present review paper accounts for the state-of the art reporton the usage of recycled aggregate (RA) as construction material in

developing new concrete. This paper includes a brief information

regarding the properties of RA and its effects on different proper-ties of fresh and hardened concrete (mechanical, durability etc.).It also emphasizes on different processing techniques of RA.

1.1. Essence of recycling of Construction & Demolition (C&D) wastes

In recent years, the large investment in construction sector and

the increasing requirement of habitats in urban areas due to thegrowth in economy and the high growth rate in population havecreated a large demand of conventional building materials. Again,the depletion of good quality aggregates along with the increasein aggregate requirement makes the availability of raw materials

scarcer. In addition to this, the materials tend to become moreexpensive due to the increase in transportation costs accompaniedwith the increasing haulage in some regions. As a result, there is anincrease in the cost of construction materials. Further, rapid rate of

modernization and industrialization have also led to the genera-tion of sheer amounts of debris from construction and demolition(C&D) wastes. Major volume of these wastes emerges from demo-

lition of old construction work. New construction works also gen-erates waste almost to a smaller volume from the left over concrete

of ready mix concrete plants, precast concrete plants, shot creteoperations and the tested samples in compliance to laboratory

applications. These C&D wastes are the largest waste streams of solid waste in many countries all over the world. In addition tothese, large amounts of industrial and mining by-products suchas fly ash, slag, and limestone powder are being generated annu-

ally. These large quanta of debris or by-product materials fromindustry are simply used as back filling material for low layingareas or illegally dumping material for vacant lands and theirquantity has been increasing with time. All these have led to an

increasing dearth of landfill areas; useful lands becoming dumpingyard, increase in the price of land in recent years and highlyincreased dumping costs at landfill sites. So, handling of such deb-ris has become one of the important issues in developed countries

and it has become a global concern that requires sustainable solu-tion. Moreover, the global concern about the reduction of carbon

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footprint is also playing a crucial role during the extraction process

of RA than virgin aggregate. Because, extracting and crushing of virgin aggregate cause use of substantial amount of energy andmore emission of CO2 [6]. Apart from these major problemsmentioned above, there are some other minor issues, promoting

the recycling of C&D waste [7–15].

Therefore, the use of RA in construction work as structuralgrade concrete may yield as a means of economic viability andenvironmental consciousness along with bulk reduction of waste

materials. The benefits of recycling of C&D waste have been shownschematically in Figs. 1 and 2. In the recent past, extensive researchworks have been carried out to evaluate different properties of

concrete by using RA. Previous studies have indicated that RAcould successfully be used as a substitute of natural aggregatesto produce concrete, meeting required performance of normalstructural concrete [16,17]. Now a days recycled aggregate

concrete (RAC) is being used for both structural and non-structuralapplications. It has been established that its use is viable both com-mercially and technically.

2. Recycled aggregate (RA)

RAs are extracted through the processing of the debrisgenerated from the demolition of concrete structures and otherconstruction debris such as waste concrete, rejected precast con-crete members, broken masonry, concrete road beds and asphalt

pavement, leftover concrete from ready mix concrete plant andthe waste generated from different laboratories [18]. RAs may beof different types such as brick aggregate, glass aggregates, asphaltand bitumen aggregate, concrete aggregates, tiles and marbles

recycled from flooring, finishes and ceramic products. Aggregatetypically processed by the crushing of parent or old concrete suchas demolished waste concrete is regarded as recycled concreteaggregate (RCA). Generally RCAs are mixed with bricks, tiles, met-

als and other miscellaneous such as glass, wood, paper, plastic andother debris [18,19]. The concept of use of RA from demolished

concrete structures was introduced into practice dates back to

the time of world war II in Europe [18,21]. Earlier it had been usedas unbound sub base materials for pavement. Now-a-days it isbeing used for construction purposes also.

2.1. Recycling process

Recycling is the act of processing the used material for furtheruse in developing new value added products. The integraltechnique behind recycling process includes the breaking of

demolished concrete to produce smaller size fragments by subject-ing to a series of performances such as removal of contaminants(reinforcement, wood, plastic etc.), different stages of screening,and sorting. Higher quality aggregates can also be processed in

steps with time and effort involved in stock piling, crushing, pre-sizing, sorting (pre-crushing and post crushing), screening andcontaminant elimination depending upon the level of contamina-

tion and the application for which the recycled materials will beused [18].

Demolition debris can be crushed by several crushers such as jaw crusher, hammer mill, impact crusher, and cone crusher or

manually by hammer [22]. Different crushers have differentconsequences on the physical and mechanical properties of RAsdepending upon the effectiveness of crushing processes [23] andconsequently it affects the concrete performance also. Jaw crushers

are mainly used for primary crushing as it can crush oversized con-crete pieces into comparable size for secondary crushing. Impactcrushers are preferred for secondary crushing as they produce abetter quality of aggregate with less adhered mortar content

[24]. Desirable grading for RAs can be achieved by crushingthrough primary crushers successively through secondary crushers[15]. The selection of crushers at various stages depends on severalfactors such as maximum feed size, quality of output, desirable

particle size and shape of the various fractions, and amount of finesproduced. These days with the help of mobile crushing plants andsome portable equipment, recycling facility can be established on

site for immediate use of product and also the freight distancecan be reduced [2]. Along with the above mentioned dry processes,wet processing technique for RA provides better quality aggregatewith less organic and inorganic impurity. However, in some devel-oped countries like Japan, China, USA and Netherland, the

researchers have developed some advanced processing techniquesto minimize the adverse effect of RA. By adopting these methods,high quality aggregates can be produced by removing the adheredmortar without losing the integrity of original coarse aggregate.

Some of these techniques are nitric acid dissolution method [25],presoaking treatment [26], freeze–thaw method [27], thermalexpansion method [28], microwave heating method [29], heating

and rubbing method [30–32], mechanical grinding method [33]and ultrasonic treatment method [34] etc.

2.2. Physical and mechanical properties of recycled aggregate

RA, derived from C&D waste generally consists of natural coarseaggregate and adhered mortar. The old clinging mortar mostlycontains fine aggregate, hydrated and unhydrated cement parti-

cles. The quality of RA mostly depends on the methods of recyclingprocess to be adopted but the properties of RA mainly depend onthe water/cement (w/c) ratio of the original concrete from whichit is obtained [35,36]. The most distinguished feature of RA is its

old adhered mortar which makes it porous due to high mortar con-tent, inhomogeneous and less dense [8,34,37–40]. The volume of the residual mortar in RA varies from 25% to 60% according tothe size of aggregate [35]. Some researchers have reported in their

studies that around 20% of cement paste is found attached to thesurface of RA for particle size range from 20 to 30 mm [41,42].

Fig. 1. Benefits of recycling of C&D waste.

Fig. 2. Schematic representation of recycling technique.

M. Behera et al. / Construction and Building Materials 68 (2014) 501–516 503

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Whereas Poon et al. [38] reported that RA extracted from crushing

of waste concrete consists of 65–70% natural coarse and fine aggre-gate and 35–30% of cement paste by volume.

The amount of adhered mortar signifies strength characteristicsof original concrete, effectiveness of crushing procedure, method of crushing and the particle size of RA. There is always a reciprocal

relationship between this adhered mortar and the quality of RA.

The presence of attached mortar is greatly responsible for itsanomalously high water absorption capacity [39,43–47]. The por-ous nature of RA is responsible for the higher water absorption

capacity of RA as these pores allow to absorb more water [48].The water absorption capacity of RA is 2–3 times higher than nat-ural aggregate and it may range up to 12% for coarse and fine RA[34,37,38,49]. Poon et al. [38] in their study mentioned that it

may vary up to 15%. The water absorption capacity of RA is morefor smaller particle size; since greater is the specific surface areaso greater is the mortar content [35]. Water absorption capacity

of RA varies depending on the amount of cement paste attachedto the surface of the aggregate particles [50]. According to the rec-ommendation by some international committee, coarse RA havingwater absorption capacity more than 7% and fine aggregate more

than 13% is not desirable to be used in concrete [51]. The waterabsorption capacity of RA also reflects the water absorption of RAC. The density and specific gravity of the attached mortar isquite less and accounts for the low specific gravity, bulk density

and SSD density of RA [19,41,44–46,52,53,47]. Limbachiya et al.[54], from the experimental investigation concluded that the rela-tive density of RA or surface saturated density (SSD) is approxi-mately 7–9% lower than that of natural aggregate. The porosity

of RA also determines its specific gravity consequently the density[15].

RA is generally poorly graded due to its poor particle sizedistribution [47]. It may be too coarse or too fine as a result of

the processing and crushing through various types of crushers.The quantity of finer fractions in RA is more. It has an old interfa-cial transition zone (ITZ) due to the presence of the old adhered

mortar or cement paste surrounding it. This is weak in naturebecause of the presence of minute pores in the clinging mortar,continuous cracks and fissures developed inside the aggregate inconsequence to the crushing process [55,56]. These basic featuresof RA are presented schematically in a pictorial format in Fig. 3.

It has rough surface texture and irregular shape, mostly roundedin nature due to the wrapped mortar. RA is having inferiormechanical properties such as low crushing strength, low impactresistance and low abrasion resistance than natural aggregate

[19,53,54,57]. It may be contaminated with organic impurity suchas textiles, fabrics, polymeric materials [15] and inorganic impuri-ties due to the internal chemical reaction such as alkali aggregate

reaction, high alumina cement, silt, clay, sulphate, chloride and

increased quantity of dust particles [19,58,59]. Although the poten-tial for the use of RA has now been acknowledged, there are somefactors which hinder the large use of RA in concrete as it affects theperformance of concrete in terms of workability, strength anddurability. Thus, some salient properties of RA such as particle size

distribution, shape and size of aggregate, porosity, absorption,

toughness, hardness, strength and the impurity level, are necessar-ily to be assessed before its use in concrete. So, the above men-tioned inferior qualities and the weaknesses present in RA shown

in Fig. 3 hinders the use of RA in concrete and the lack of properspecification also discourages the recycling technique.

3. Recycled aggregate concrete (RAC)

The use of RA in concrete has generated interest in civil

engineering construction regarding sustainable development as itis the means of achieving more environment friendly concrete.Concrete made up of RA in terms of fine or coarse or both, pro-cessed from C&D waste either as a partial or 100% replacement

of conventional natural aggregates is known as RAC. RAC mainly

consists of three phases such as the aggregate phase, mortar phaseand the interfacial transition zones between the coarse aggregateand the matrix and the adhered mortar as another matrix. Fig. 4a

and b shows the schematic diagrams of natural aggregate concreteand RAC respectively, showing the basic difference of matrix inbetween two concrete. These three phases are responsible for lim-iting the properties of RAC. So it needs more attention regarding

the performance of concrete when RA is to be used in concrete.In practice, RA is obtained from different types of constructionand demolished structure as a result the properties of these RAobtained from various sources also vary from structure to struc-

ture. Since the early 1980s, European countries have endeavoredto use RAs for new concrete structures. A diversified study onRAC demonstrates the feasibility of the use of RA in concrete and

it can be an alternative source to natural aggregate that has beenexplained later.

3.1. Properties of recycled aggregate concrete

3.1.1. Green state

Fresh properties of concrete such as workability and wetdensity are greatly affected by a number of factors such as w/cratio; the characteristics of the constituent materials of concrete,

especially the aggregate i.e. type of aggregate, maximum size of aggregate, water absorption of aggregate etc. Workability of con-crete also gets affected by other physical parameters of aggregate

Fig. 3. Pictorial representation of physical characteristics of recycled aggregate.



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such as surface texture, aggregate size, and shape of aggregate. Incase of RAC, the slump loss is more than natural aggregate concrete

and it is difficult to meet the required workability [60–63]. Beingporous in nature, it requires more water than conventional con-

crete to obtain the same workability [64]. Tabsh and Abdelfatah[65] in their study concluded that RAC demands 10% extra waterto achieve the same slump when RA is used instead of natural

aggregate. Even if keeping w/c ratio constant; aggregate type andthe quantity of RCA can also affect the slump of RAC mixes [66].Moreover, concrete made from RA, generally has a harsher andgranular texture due to the adhered mortar which causes a higher

water demand and more energy for compaction due to interparticle friction [24,45,67,68]. Similarly Sagoe-Crentsil et al. [47]identified that commercially produced RA showed better workabil-ity performance than laboratory produced RA as it produces com-

paratively smoother aggregate. The angular shaped aggregate also

demands more water than rounded shape [69]. As the size of RAdecreases, water demand increases due to more adhered mortarquantity. This higher water demand leads to difficulties in control-

ling the properties of fresh concrete and consequently influencesthe strength and durability properties of hardened concrete [64].If coarse RA is used in dry condition, then the workability of con-crete gets hampered to a great extent depending upon the quantity

of RA used. The loss in workability or slump in RAC is quite prom-inent at higher percentage replacement especially when itexceeded 50% [49].

RA absorbs free water from the mixture during mixing process

due to its reduced water content in adhered mortar phase of aggre-gate, which causes high water demand of mix to maintain the sameworkability. In order to compensate the low workability problem

with RA, some researchers have adopted some novel proceduresby using presoaked aggregate instead of using dried one [24].The presoaked aggregates are used in surface saturated dry (SSD)condition. When RA was used in SSD condition, an increase in ini-tial slump has been observed with the increasing amount of RA.

This is associated with the high absorption capacity of RA leadingto larger amount of initial free water [62,47]. Sometimes thismay lead to bleeding of concrete [62], as a result the w/c ratio of the matrix can be slightly increased. But later on, the workabilitydeclines. The initial moisture state of RA also affects the fresh prop-

erties of RAC. The initial moisture state of RA mainly depends onthe type of dried condition of aggregate such as air dried, ovendried or SSD condition. Again, the water absorption capacity of RA increases with the increase in strength of parent concrete they

are derived from. Higher the strength of parent concrete, more isthe quantity adhered mortar due to strong bond between

aggregate and matrix. So, it results in the higher water absorptioncapacity that will affect the workability of the mix. Even with tech-

nical and environmental advantages, the wide spread use of RAmay be limited due to its influence on the workability of the

concrete, which turns out to be smaller when compared with theconventional concrete [61,63,70–73]. From literature, it has alsobeen found that the slump loss of RAC can be overcome by the

incorporation of some mineral admixtures, chemical admixtureor super plasticizers or by adding extra water corresponding tothe absorption of aggregates [49]. By the use of super plasticizers,water demand of aggregate can be compensated to maintain the

desired workability [23]. According to Saravana Kumar and Dhi-nakaran [74], the water demand could be reduced by 12.5% bythe use of RAs admixed with fly ash (20%) and superplasticizerthan that of without fly ash and super plasticizers.

Similarly the bulk density of green RAC is significantly lower

than natural aggregate concrete due to low density RA[34,75,76]. The lower density is also a consequence of the lowerspecific gravity of the RA.

3.1.2. Hardened properties

The hardened properties of RAC include the mechanical proper-ties such as compressive strength, split tensile strength, flexuralstrength, modulus of elasticity, creep, and bond strength. These

properties of concrete depend upon many parameters such as w/c ratio of mix, engineering and physical properties of RA to be usedin concrete and the microstructure. From the literature, it has been

observed that, RA are less resistant to mechanical action due to itspoor bond between old mortar and the RA, presence of transverse

cracks and fissures in RA during recycling processes and the pres-ence of weak porous mortar around RA. The potentially inferior

mechanical properties of RAC raise concerns regarding the properassessment of the properties of RA before its use. In addition tothis, the response of RAC towards mechanical action will alsodepend upon the level of replacement by RA, w/c ratio and the

moisture condition of the RA [13,47]. In RAC, w/c ratio also playsa very important role as it depends on many factors such as waterabsorption of RA, free moisture content of aggregate, and quantityof adhered mortar. Despite that the wide variation in results of RAC

may be due to the variation in w/c ratio and the variability in thequality of RA also. The most important conclusion drawn fromnumerous studies is that the cement mortar adhered to the aggre-gate surface influences the performance of RAC, basically the

strength characteristics. In the recent past, researchers haveworked on different hardened properties of RAC which revealed

(a) (b)

Old mortar

Matrix

Cracks

Fig. 4. Difference between matrixes of (a) natural aggregate concrete and (b) recycle aggregate concrete.


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that concrete properties get affected by the increase of RA content.

These properties have been discussed briefly in the following.

3.1.2.1. Compressive strength. Compressive strength of RAC depends

on many parameters like replacement level of RA, w/c ratio [13,47],moisture condition of RA etc. [38,62]. It is also influenced by thephysical and the mechanical properties of RA basically the crushing

strength of aggregate as crushing and impact strength of aggregategoverns the resistance towards compressive load. It has beenobserved from a number of experimental investigations that thecompressive strength of RAC is greatly influenced by the increment

in the replacement percentage (%) of RA using the same w/c ratio[77,78]. As widely reported, the reduction in compressive strengthis up to 30% as compared to natural aggregate concrete at 100%replacement [35,39,59,62,79–83]. Similarly some researchers con-

cluded that the reduction in strength is in between 12% and 25%with 100% replacement of RA [19,24,43,44,46]. In some other stud-ies, it has been reported that the compressive strength of RAC with100% RCA varies like 60% [70], 76% [34] of conventional concrete.

Katz [34] reported that age of parent concrete from which RA hasbeen derived is an influential factor on the strength development

of RAC. In compliance with this, the hydration stage of old matrixi.e. the residual cementing capacity of unhydrated cement present

in old matrix governs the strength properties of RAC. Numerousexperimental investigations showed that the reduction in com-pressive strength is not much prominent, when the RA replace-ment is up to 30% [12,21,78,83,84]. The possible reasons for this

may be due to the lower strength of RA, increased concrete poros-ity, weak interfacial bond between the aggregate and matrix [85]and the presence of micro cracks and fissures which may reducethe bonding with the surrounding matrix. Tam et al. [86] modeled

on the optimum RA substitution level and reported that the opti-mum replacement level is to be 25–40% for two stage mixingapproach. Thus, the influence of replacement of RA on 28 dayscompressive strength of RAC as reported by several authors has

been presented in Fig. 5.From earlier studies it has been observed that the same trend is

followed by RAC as there is a decrease in strength with the increasein w/c ratio as that of conventional concrete. The study by Hansen

reported that the compressive strength of RAC depends on thecombination of w/c ratio of original concrete and w/c ratio of RAC, when other factors are kept almost identical [8]. The strengthof RAC made out of 100% RA can be more than or equivalent to

source concrete if the w/c ratio of parent concrete is less than or

equal to the strength of RAC intended to be prepared [35].However, Li et al. [52] from their investigation reported that the

compressive strength decreased with the increase in w/c ratio forall replacement level except in case of concrete with 50% RA whichis shown in Fig. 6. The reason for this non-linearity betweenstrength and w/c ratio at 50% replacement is not clear. In contrary

to this, some authors have observed that compressive strength of RAC is sometimes higher than natural aggregate concrete [69,87].It may be due to the lowering of effective w/c ratio in RAC withhigh rate of water absorption of RA from concrete, which in turn

leads to increase the strength. Some other experimental investiga-tions showed that the compressive strength of RAC is equal to thestrength of natural aggregate concrete at higher w/c ratio such as0.40, 0.55 and 0.70. However, the strength is less at lower water

cement ratio of 0.25 [88]. It is attributed to the new ITZ which gov-erns the strength performance of RAC at higher w/c ratio, whereasthe old ITZ governs the strength performance of RAC at lower w/cratio. Yet in some other cases, it has been reported that when the

RA are used in dry condition the compressive strength of RACincreases with the increase in substitution percentage [89]. It isdue to the higher water absorption capacity of dry RA that causes

the reduction in effective w/c ratio. Furthermore, the use of superplasticizers also contributes towards the increase in compressivestrength as it compensates the total water demand. Rather thanthis in some literature it is reported that the early strength gainrate (up to 7 days) in case of RAC is more than natural aggregate

concrete [24,90]. Basically it is accomplished with the high waterabsorption capacity of the adhered mortar and also the rough tex-ture of RA which leads better bonding and interlocking propertiesbetween the mortar and RA surface [81]. The early strength gain

rate also depends upon the percentage replacement of RA. How-ever, later age strength gain of RAC from 21 days to 28 days is quiteless than that of natural aggregate concrete [12]. The strength

reduction in RAC is also associated with the reduction in the den-sity of RA. In addition to this, Poon et al. [38] from their investiga-

tion stated that RAC made up of RA derived from high strengthconcrete has higher rate of strength gain than conventional con-

crete. Ultimately, it is observed that the strength properties of RAC are associated with the quality of RA, strength of RA and themicrostructural properties of ITZ. Furthermore, Poon et al. [62] intheir studies reported that the moisture level of RA has also influ-

ence on the strength of RAC. Oven dried samples showed higherstrength than air dried and surface saturated samples. Few studieshave also been carried out to achieve similar compressive strengthand slump equivalent to natural aggregate concrete by using 100%

RA [91]. Many researchers have reported that the desired compres-sive strength can be achieved by using RAs in concrete by adjustingthe w/c ratio (4–10% lower) [24], by using more cement content(5–10% extra) than normal concrete [24] or by incorporating some

alternative cementitious materials. It is also possible to achievemore strength than conventional concrete by doing some

0

10

20

30

40

50

60

0% 20% 40% 60% 80% 100%

S t r e n g t h ( M P a )

RA content

Fig. 5. Variation in 28 days strength compressive strength w.r.t. RA replacement

percentage by various researchers (1) Rao et al. [12], (2) (3) Elhakam et al. [21], (4)

Kwan et al. [96], (5) (6) (7) Kou et al. [92], (8) (9) (10) Poon et al. [62], (11)

Limbachiya et al. [17], (12) (13) (14) Limbachiya et al. [84], (15) Etxeberria et al.[24], (16) Kou et al. [56].

12

17

22

27

32

37

42

0.3 0.35 0.4 0.45 0.5 0.55 0.6 C o m p r e s s i v e S t r e n g t h ( M P a )

100% RA 70% RA 50% RA 30% RA

Fig. 6. Variation in compressive strength of RAC w.r.t different w/c ratio [52,83].



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adjustments to the paste content and to the w/c ratio with the aid

of super plasticizers [20]. However, the drawbacks of RAC fromstrength point of view may be improved by adopting some noveltechniques such as modified mixing approach, by the incorpora-tion of alternative cementitious materials such as silica fumes, flyash, and GGBS. Apart from this, Saravan and Dhinakaran [74]

reported that a compressive strength of 5% lower than conven-

tional concrete could be achieved by the combined use of superplasticizer and fly ash (20%). Thus their influences on differentproperties have been discussed in details in the latter sections.

Hence, the results obtained from literature are not comparable toeach other due to a large non homogeneity in the quality of RA,the w/c ratio deployed and the type of cement used.

3.1.2.2. Split tensile strength and flexural strength. The other poten-tially inferior mechanical property of RAC is the split tensilestrength which exhibits similar behavior as compressive strength

with the increase in RA quantity. Several past investigations onRAC showed that the effect of RA content on split tensile strengthis less than that on compressive strength. Some authors in theirstudy have mentioned that the decrease in split tensile strength

is up to 10% w.r.t different RA replacement percentage.[13,19,44,61]. However, Rao et al. [12] from their experimentalstudy reported that the reduction in split tensile strength is up to24% at 100% replacement by RA. Generally, it has been found that

the split tensile strength and flexural strength of RAC mainlydepend on the quality and surface characteristics of RA regardlessthe replacement level of RA [3,61]. Contrary to this, in some casesRAC showed similar [47] or better performance [24,91] than

conventional concrete with respect to split tensile strength [24].Etxeberria et al. [24] mentioned that the higher split tensilestrength of RAC is due to the water absorption capacity of clingingmortar which developes a proper bonding between aggregate and

matrix. Matias et al. [23] concluded that the increase in split tensilestrength is due to the rough surface of RA which provides betteradherence to matrix. Concrete with RA which is derived from high

strength concrete shows better split tensile performance than thatwith RA derived from normal strength concrete. However, in someother cases significant differences up to certain limit have beennoticed in the split tensile strength of RAC and conventional con-crete [12,70,76]. McNeil and Kang [48] mentioned in their review

paper that the adhered mortar acts as a weak point to fail undercompressive load which results in lower split tensile strength. Inorder to improve the performances, it needs to create a smootherzone near ITZ. However, Kou et al. [92] in their study mentioned

that the split tensile strength of RAC increases on long term basisthan natural aggregate concrete. It can be explained by the modi-fication of pore structure of RAC [93]. So it can be summarized that

bond strength between aggregate surface and matrix has a verygreater influence on split tensile strength of RAC which increases

age due to more hydration. Though it is possible to achieve theequivalent target strength by modifying the w/c ratio and paste

content as those in case of natural aggregate concrete, but a notice-able difference has been observed in the split tensile strength of RAC by using 100% RA [91]. The influence of replacement of RAon 28-days split tensile strength of RAC as reported by different

researchers has been presented in Fig. 7. From the figure, it can eas-ily be visualized that the influence of RA quantity on split tensilestrength is quite less. Rather it is related to the RA quality andthe w/c ratio deployed.

Similarly, several attempts have been made on the modulus of rupture of RAC and the results have shown that the replacementlevel of RA has only marginal influence on the flexural strengthof RAC [83]. Some authors have reported that there was no signif-

icant difference found in flexural strength of RAC even if containing100% RA in comparison to that of conventional concrete [12,94].

Topçu and Sengel [63] found that flexural strength of RACdecreased with the increase in percentage of RA. In different

literature, it has been found that the flexural strength of RACdecreased up to 10% [13,19,61,44]. Bairagi et al. [70] also observeda significant difference in the flexural strength of RAC at differentw/c ratio than conventional concrete. Some other researchers

observed that flexural strength of RAC varied in the range of 16–23% with different percentage replacement ratio of RA [3,49]. Thetest results for flexural strength of RAC obtained by some research-ers are presented in Fig. 8. The figure reveals that, as the RA content

increases, the flexural strength decreases.

3.1.2.3. Drying shrinkage. Drying shrinkage is a very prominent

feature of RAC which is related with decrease in volume or contrac-

tion of hardened concrete due to loss of capillary moisture thatresults in the development of capillary tension developed insidethe meso-pore structure of cement matrix. The total amount of

water in fresh concrete and drying shrinkage is directly relatedto each other. Incorporation of RA into concrete exhibits moredrying shrinkage and it is significantly higher than that of conven-tional concrete [44]. It may be due to, RA having lower modulus of

elasticity, offers less resistance to the potential shrinkage of cement paste. Yang et al. [44] reported that the stiffness of aggre-gate significantly contributes to the amount of concrete shrinkage.It increases with the increase in RA quantity and with w/c ratio of

S p l i t T e n s i l e S t r e n g t h ( M P a )

RA content

Fig. 7. Variation in 28-days split tensile strength w.r.t RA replacement percentage

by various researchers (1) Rao et al. 2011: w/c-0.43 [12], (2) Xiao et al. [97], (3)

Etxeberria et al. [24], (4) (5) (6) Kou et al.: TKOS, TKOL, KTS respectively [92], (7)

Elhakam et al.: w/c-0.45 [21], (8) Kou et al.: w/c-0.45 [77], (9) Kou et al.: w/c-0.55

[77], (10) Elhakam et al.: w/c-0.60 [21].

F e x u r a l s t r e n g t h ( M P a )

RA replacement (%)

Fig. 8. Variation in 28-days flexural strength w.r.t RA replacement percentage by

various researchers (1) Rao et al.:w/c-0.43 [12], (2) Limbachiya et al. [84], (3)

Limbachiya et al.:GEN 3 [17], (4) Limbachiya et al.:RC 30 MPa [17], (5) Limbachiya

et al.:RC 35 MPa [17], (6) Limbachiya et al.:RC 40 MPa [17], (7) Sonawane et al.:M30[3], (8) Sonawane et al.:M40 [3].


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RAC [77]. Drying shrinkage characteristics of RAC can be explained

with the higher water absorption characteristics and highly porousnature of RA. Additionally the old mortar adhered to RA surfacecontributes to increase in volume of total paste content whichcontributes more to the shrinkage of cement paste than naturalaggregate concrete [6,17,22,56,77]. A number of experimental

investigations revealed that the shrinkage and creep of RAC is quite

higher than conventional concrete due to the higher water absorp-tion of RA. Sanchez de Juan et al. [46] from their experimentalinvestigation reported that the shrinkage value lied in between

15% and 60%. Domingo-Cabo et al. [89] reported that shrinkagein RAC was 20% higher with a substitution level of 50% and thisreached up to 70% with 100% RA after a period of 180 days. Theyalso identified that RAC showed a shrinkage value equivalent to

conventional concrete at lower substitution level (20%) at earlyage. Some researchers stated that the increase in drying shrinkagecan be up to 50% [37,61,89]. However, Sagoe-Crentsil et al. [47]

revealed that the drying shrinkage of RAC was about 25% higherthan that of natural aggregate concrete. Whereas Limbachiyaet al. [17] stated that RA has no major effect on the drying shrink-age up to 30% replacement and after that it increases significantly

with the increase in RA content. The variation of drying shrinkageof RAC (100% RA) with respect to natural aggregate concretereported by some researchers is shown in Fig. 9. From the figureit is prominent that drying shrinkage of RAC with 100% RA is prom-

inently higher than conventional concrete. The short comings of RAC due to the use of RA such as drying shrinkage, creep can beminimized by incorporating fly ash whether as addition or replace-ment [56,77,114]. Zhu and Wu [78,95] reported that the drying

shrinkage of RAC can be minimized with the use of fly ash andsuper plasticizer in concrete. The reduction in creep of RAC dueto the addition of fly ash attributes to the reduction in w/c of themix and the dilution effect of fly ash particles [56].

3.1.2.4. Creep and modulus of elasticity. A series of investigations ondeformation characteristics of the RAC specimens revealed that the

creep increases with the increase in content of RA. This is due tothe increased volume of total mortar content in case of RAC ascompared to the conventional concrete. Creep of concrete is alsodirectly proportional to the content of the paste or mortar present

in concrete as drying shrinkage. In RAC, the residual mortar of RAcontributes towards more mortar content which results in consid-erable higher creep. Some researchers from their investigationsreported that the creep of RAC can be increased up to 50% than that

of conventional concrete [37,89]. Furthermore, Domingo-Caboet al. [89] concluded that the creep deformation was more than50% at 100% use of RA. On the other hand, Ajdukiewicz et al. [13]

suggested that the general tendency of the development of creep

in RAC was found to be reversed by the use of RA derived from high

strength concrete. It can be attributed to the higher water absorp-tion rate of RA, obtained from high strength concrete, whichreduces the effective w/c ratio of the matrix. However, it was alsofound from their study that the creep development in RAC wasonly 20% lower than conventional concrete after a period of 1 year

[13]. The potential drawbacks associated with the creep strain

development in RAC can be reduced with the use of fly ash as a par-tial replacement of cement or as addition to cement [55,77].Modulus of elasticity is another important mechanical property

which signifies the stiffness of concrete. Modulus of elasticity of concrete gets affected by so many parameters such as porosity of aggregate and matrix, dense nature of aggregate and the transitionzone characteristics. This is because the aggregate’s porosity and

density determines the stiffness of bulk matrix. Substitution of nat-ural aggregate by RA also affects the modulus of elasticity. How-ever, RA content has more has pronounced effect on the modulus

of elasticity than that of compressive strength due to its porousnature, low density and weak bond between old ITZ and new ITZdue to presence more capillary voids and cracks. Like compressivestrength, similar trend has also been observed for modulus of elas-

ticity with degree of substitution of RA. Modulus of elasticity of RAC decreases considerably than normal concrete and it reduceswith the increase in degree of substitution of RA [12,13,64]. Varia-tion of results of modulus of elasticity as reported by various

researchers with natural aggregate and RA (100%) are presentedin Fig. 10. It is found that the modulus of elasticity of RAC with100% RA can be lowered up to 45% than that of natural aggregateconcrete [12,13,43,44,52,61,78,96,97]. However, Kheder and Al-

Windawi [98] reported that it was lowered by 20–25% and Bairagiet al. [70] from their study mentioned that it was reduced by 39%.Despite that Tpocu et al. [75] reported that the reduction was up to80% at complete replacement by RA. From the failure pattern of

RAC, it is found that RAC behaves in a more brittle manner thanthe conventional concrete. Limbachiya et al. [6] reported that thelower values of elastic modulus of RAC may attribute to strength

characteristics of RA and its inferior quality compared to the natu-ral aggregate. Despite that Xiao et al. [78,83] reported that thedecrease in elastic modulus is due to the old adhered mortar tothe surface of RA which is comparatively of low elastic moduli thanthe aggregate. RA has adverse influence both on longitudinal and

transverse modulus of elasticity [99]. The reduced elastic moduliof RA is particularly responsible for the increase in the peak strainand ultimate strain of RAC [99] which leads to large deformation.Limbachiya et al. [6] in their empirical studies also concluded that

with the use of 100% RA, approximately 35% reduction in modulusof elasticity was observed. Similarly, Corinaldesi [100] reportedthat 15% less elastic modulus could be achieved by using only

D r y i n g s h r i n k a g e ( m i c r o s t r a i n

)

Fig. 9. Influence of RA on Drying Shrinkage (1) Kou et al. [56], (2) Limbachiya et al.

[84], (3) Limbachiya et al. C20 [6], (4) Limbachiya et al. C30 [6], (5) Limbachiya et al.

C35 [6], (6) Limbachiya et al. GEN3 [17], (7) Limbachiya et al. RC30 [17], (8)Limbachiya et al. RC35 [17].

0

5

10

15

20

25

30

35

1 2 3 4 5

M o d u l u s o f E l a s t i c i t y ( G P a )

0% RA 100% RA

Fig. 10. Variation in modulus of elasticity (28-days) w.r.t RA replacement by

different researchers (1) Etxeberria et al. [24], (2) Rao et al. [12], (3) Xiao et al. [97],(4) Kou et al. [92], (5) Li et al. [52].



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30% RA. The incorporation of RA into concrete reduces the stiffness

of concrete and increases the brittleness as a result modulus of elasticity decreases [40].

So, from the results obtained from various research works, itcan be well summarized that the influence of content of RA onshrinkage, creep and especially on the modulus of elasticity is sig-

nificantly higher than other strength properties [40,101]. Basically

the strength properties of RAC have an inverse relationship withRA content. Despite that all these properties depend more or lesson the w/c ratio of RAC and the variability in properties of

aggregate.

3.1.2.5. Bond strength. While considerable results are available onthe other strength properties of RAC, relatively few informationis reported on the bond strength between matrix and steelreinforcement. Some of the experimental investigations revealed

that the influence of RA and its quantity on the bond strength isquite smaller than other mechanical properties. RA content doesnot have much influence on the bonding between concrete andreinforcing bar. However, the factors which contribute to the bond

strength are type of rebar and rebar surface profile in case of RAC

[102]. From literature, it has been observed that the bond strengthmay reduce up to 10% at a replacement level up to 100% [13,102].Xiao et al. [103,104] studied the bond behavior of RAC with differ-

ent types rebar by pull out test and reported that the bond strengthdecreased by 12% and 6% at 50% and 100% replacement level of RAthan natural aggregate concrete respectively for plain rebar. Andfor deformed bar it was quite similar to each other irrespectiveof different level of replacement. The possible reason might be that

in case of deformed bar, the bond is due to mechanical anchorageand friction, whereas in case of plain bar, it is due to the bondingbetween the rebar and concrete [22,104]. The influence of thebar profile on bond strength development in between rebar and

concrete is shown in Fig. 11. However, Zuhud [105] in his studyreported that the bond strength between deformed bar and con-

crete increased with the increase in RA content. It was up to32.4% and 46.1% more than conventional concrete for a replace-

ment percentage of 30.0% and 60.0% respectively. The increase inbond strength attributed to the increase in friction due to roughsurface of RA and deformed bar. Similarly, Xiao et al. [104] fromtheir study also reported that the bond strength of RAC with

100% RA found to be more than conventional concrete. The higherbond strength of RAC is due to the internal curing effect of RAC dueto more hydration of cement paste by the use of SSD aggregateswhich could be attributed to the improved contact zone between

RA and matrix by the use of SSD aggregates [106]. Valeria et al.[20] studied the influence of fly ash on the bond strength of RACand concluded that the bond strength increased with theincorporation of fly ash. The improvement in strength can be

explained through the pore structure improvement due to

secondary C–S–H gel formation. In their study they reported that

the bond strength was more for RAC for both plain and deformedbar than conventional concrete and it was 6% and 15% higher thanconventional concrete in the absence and presence of fly ashrespectively. So it can be summarized that bar profile has moreinfluence on bond strength than the RA replacement.

3.2. Microstructure of recycled aggregate concrete

The microstructure of RAC is a very different and complicatedstructure than that of conventional concrete especially in the inter-facial transition zone (ITZ). Unlike normal concrete, it has two ITZs,

one is new ITZ lies between RA and new matrix and another is oldITZ lies between RA and old adhered mortar [59] which is shown inFig. 12c. So the mechanical performance of RAC is a consequence of dual performance of both the ITZs as ITZ significantly influence the

strength property. Many studies have revealed that the ITZ of RACis very weak in nature [38,77,107]. This ITZ forms the weakest linkand strength limiting phase in RAC as it acts like a wall betweenthe matrix phase and the coarse aggregate phase in concrete

[59,88,108] and acts as a barrier to transfer the load as the cracks

form first near the ITZ. It acts like a gradual transition zone whosethickness is associated with degree of hydration and the adheredmortar content of RA. In many studies, it is reported that for nor-

mal cement concrete the thickness of ITZ may range up to 50 lmaround the aggregate. It consists of less unhydrated particles,highly porous and greater concentration of Ca(OH)2 and ettringite.Moreover, from SEM analysis it is found that the ITZ of RAC is com-

posed of many minute intrinsic pores, cracks and fissures [59].Poon et al. [38] investigated the microstructure of ITZ throughSEM analysis and reported that the ITZ of RAC consists of looseand porous hydrates which are granular in nature. Thomas et al.[76] studied the surface morphology of RAC through SEM which

is shown in Fig. 12a, and observed that it is composed of looselyadhered mortar. The porous nature of ITZ causes the reduced elas-

tic modulus and lower strength in that particular area relative tothe surrounded mortar matrix [109–111]. Due to its poor micro-

structure, the stiffness of the concrete may be low, which doesnot withstand the stress transfer across this.

Khalaf and DeVenny [15] concluded that the pores and crackspresent on the surface and inside the RA respectively may vary

in size. Hence the cement paste cannot penetrate into these poresdue to more viscous whereas water can easily penetrate. It mayresult a poor bond between old ITZ and new ITZ. In addition to this,some amount of fine flake-like and whisker-like crystals were also

found in the voids of ITZ. Some studies on the ITZ of natural aggre-gate concrete have revealed that, the formation of large flatCa(OH)2 crystal perpendicular to the surface of aggregate grainresults in the formation of a highly porous structure in the ITZ

due to accumulation of water film in vicinity to aggregate surface.

Fig. 11. Influence of bar profile on bond strength.


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The presence of this weak ITZ makes the strength properties of RAC

inferior to those of natural aggregate concrete [59,62]. In order tocharacterize the strength of ITZ, micro hardness test showed that

the strength characteristic of both the ITZs depends on the w/cratio of both the ITZs [88]. The micro hardness of the interface

between the RA and new mortar matrix is very low in comparisonto other areas [83,112]. Rao et al. [113] from their study stated thatVickers micro hardness increases with the increase in distancefrom the surface of aggregate. Numerous studies on microstructure

of RAC revealed that the durability performance of RAC is directlyrelated to its porous microstructure (Figs. 12b and 12d). It is asso-ciated with its porous nature, high absorption capacity and the

loosely attached old mortar i.e. old ITZ. This porous nature of ITZis due to the incomplete hydration product Ca(OH)2, whichremains perpendicular to the surface of aggregate. In spite of this,the high water absorption property of RA causes the accumulation

of water film close to the aggregate surface, as a result weaker ITZforms between the cement paste and RA [107]. Similarly, Thomaset al. [76] from their study on porosity of RAC revealed that acces-

sible porosity of RAC is more than conventional concrete. The openporosity of RAC increases with the increase in w/c ratio of concreteand the replacement level of RA [76]. Therefore, modification of theporous nature has been one of the great concerns to improve thestrength and durability properties of RAC.

3.3. Techniques for improving properties of recycled aggregate

concrete

The potential benefits and drawbacks of using RA in concrete

have been investigated extensively. Taking all these into consider-ations, several approaches have been proposed in order to enhancethe performance of RAC. These approaches can be grouped in tothree broad aspects which have beenexplained in details as follows:

3.3.1. Incorporation of mineral admixture

In order to improve the properties of RAC, it is necessary toenhance the quality of concrete by modifying the weak ITZ of RAC and the bulk matrix of concrete. The microstructure of RACcan be improved by incorporating mineral admixtures such as fly

ash, meta kaolin, silica fume, ground granulated blast furnace slag(GGBS), and Nano silica. These mineral admixtures act as micro fil-ler, filling the ITZ between the aggregate surface and the matrix.The filler effect by pozzolanic reaction of these minerals has shown

in Figs. 13c–13e. Incorporation of these mineral admixtures intoconcrete led to the enhancement of its compactness through theformation of secondary C–S–H gel that fills up the open poresand empty capillary spaces within the hardened concrete and

consequently decreased the porosity of the concrete and helpedin enhancing the strength and durability [6,56,114,116,120].

Fig. 12a. Micrograph of RAC structure [76].

Fig. 12b. Microstructure of RA mortar matrix [76].

Fig. 12c. Sectional view of RAC [115].

Fig. 12d. Microstructure of RAC [38].



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3.3.2. Impregnation of RA in cement slurry or other mineral admixture

solution

Some other approaches like surface coating of RA with low w/cratio paste or by impregnating RA in silica fume solution or in other

mineral admixture solution also helped in healing the pores orcracks present in RA [107,117]. Impregnation of the RA with a solu-

tion of silica fume or any other mineral admixtures helps in pene-trating the silica fume particles into the cracked and loose layer of this aggregate [118]. Due to the filling effect of silica fume, it helpsin improving the ITZ during the hardening process of concrete. Fur-

thermore, the pozzolanic reaction of silica fume with Ca(OH)2 pro-duces secondary C–S–H gel which in turn strengthened the weakstructure of the RA to form an improved zone, penetrates from

the RA through the residues of the old cement paste into thenew cement matrix. Silica fume treatment at early age has a stron-ger effect on filling than the pozzolanic reaction, which is known todevelop more slowly. The similar effect is also shown by other poz-zolanic substances like GGBS, fly ash etc. This ultimately helps in

improving the performance of RAC regarding strength and durabil-ity. The improvement in the surface of the RA by surface treatmentis shown in Figs. 13c and 13d).

3.3.3. Modifying mixing process

Investigations have been carried out by some other researchersby doing trials with several new approaches in order to limit theshort comings of using RA in RAC. It has been noticed that the neg-

ative effects can be mitigated up to certain extent by adding differ-ent ingredients of the concrete in a modified manner. So, the new

Fig. 13a. Filled cracks in RA by two stage mixing approach [59].

Fig. 13b. Unfilled cracks in RA by normal mixing approach [59].

Fig. 13c. Without surface treatment (in pores) [107].

Fig. 13d. Without surface treatment (on surface) [107].

Fig. 13e. Surface microstructure of RA with fly ash [107].


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approaches tried by different researchers along with the abovementioned approaches and their beneficial effects have been sum-

marized in Table 1.These can be easily understood from the following SEM images

obtained from different sources as given below. Figs. 13a and 13bshow the influence of two stage mixing approach on filling the

cracks and fissures present on the surface of RA. SimilarlyFigs. 13c–13f represents the surface microstructure of RAC whichhas been densified by the pozzolanic reaction of fly ash. Fig. 13f represents surface improvement of RA filled with silica fume.

3.4. Durability properties

Generally durability performance of concrete is a measure of permeation characteristics of concrete, as well as integrity of con-crete against aggressive agents in the environment like sulphate,

chlorides, acids, carbon dioxide, oxygen, etc. Investigations of theRAC have revealed that, the durability performance of RAC ispoorer than conventional concrete. The poor durability perfor-mance of RAC is associated with the inferior quality of RA due to

the presence of numerous cracks and fissures and pores presentinside the aggregate, thus making it more susceptible towards

permeation [55,56]. In addition to this, microstructure of ITZ alsoplays a significant role as it strongly depends on the nature of

aggregate, its porosity and its water absorption characteristics.However, microstructure of RAC is porous in nature due to thepresence of old adhesive mortar around the RA, high water absorp-tion characteristics of RA, which also contributes towards the per-

meability characteristics of RAC. This porous ITZ has a largeinfluence on permeation characteristics of concrete, especially if the individual ITZ regions are connected with each other acrossthe three dimensional microstructure. Apart from this, permeabil-

ity of RAC also depends upon the w/c ratio [77] and curing period[55]. So, more is the curing duration, denser is the microstructuredue to more hydration; hence better is the durability performance[55]. Thus, porosity of RA is the controlling factor for better dura-bility performance of RAC. So it needs special treatment to block

the open path for foreign agents. On the other hand, one of themajor difficulties with RA is the variability in their propertiesdue to processing, composition, contents and proportions largelylinked to the original source of debris, which consequently results

in the variability of concrete properties.

From literature, it has been observed that the permeability of concrete can be measured through different methods such as

Table 1

Different approaches adopted for the improvement of properties of RAC.

Authors Proposed methodology Significance

Otsuki et al.[88]

Double mixing method Compressive strength increased up to 12.6% than normal mixing Chloride penetration depth reduced to 22.7%

Carbonation depth was up to 12.3%

Tam et al.

[59]

Two stage mixing approach 28-Days Compressive strength increased up to 21.19% at different percentage

replacement

Developed a stronger ITZ by filling the cracks and pores in RACorinaldesi

et al.

[20]

Additions of fly ash or silica fume into concrete to replace

fine aggregate

Improvement of pore structure by reducing the volume of pores

As a result mechanical performance such as compressive strength, tensile and bond

strength could be improved

Limbachiya

et al.[17]

10% silica fume was used as a partial replacement of

Portland cement

Enhanced compressive strength and compactness

Target strength could be achieved with 100% RA Showed less resistance towards carbonation. Causes pore refinement, as a result

lower chloride ion ingression

Kou et al.

[56]

Incorporation of 25-35% class F fly ash as well as partial

replacement of cement

Strength gain was more in between 28–90 days

Increase in strength was up to 19.4%, 36.1% and 47.6% from 28 to 90 days for concrete

containing 0, 25, 35% fly ash respectively for 100% RA

Drying shrinkage, creep and chloride ion penetration reduced to a certain extent

Replacement of cement caused reduction in strength

Elhakam

et al.

[21]

Self-healing of RA,Modified two stage mixing method

Addition of 10% silica fume as cement

Unhydrated cement particles on RA got hydrated by self healing method thus

enhanced its properties

Two stage mixing approach showed better split tensile strength, bond strength andenhanced porosity of RAC

Addition of silica fume improved the porosity of RACKong et al.[107]

Surface coating of RA by pozzolanic substances. A noveltriple mixing method

A thin layer of pozzolanic particles formed around the aggregate which helped inimproving the ITZ through filler effect and pozzolanic reactive effect

Improved compressive strength and chloride ion penetration resistance

Katz [118] Pre-treating of RA with silica fume solution (10 wt%) Compressive strength increased up to 30% and 15% at ages of 7 days and 28 days

respectively

ITZ between RA and matrix could be improved

Kou et al.

[115]

Incorporated different mineral admixtures such as fly ash

(FA) (35%), silica fume (10%), meta kaolin (15%), GGBS (55%)

Silica fume and GGBS contributes to both short term and long term properties

FA and GGBS showed their beneficial effect on long term properties

Contributions of mineral admixtures to the performance improvement of RAC are

higher than that to natural aggregate concrete

Zhihui et al.[119]

Pre coating of RA surface with thin cement paste 28 days compressive strength of concrete increased up to 16% With the use of pre coated recycled fine aggregate with sulpho aluminate cement, the

compressive strength of mortar increased by 34.8% and with sodium silicate

increased by 32.4%

Ann et al.

[114]

Use of 35% pulverized fuel ash (PFA) and 65% ground

granulated blast furnace slag (GGBS)

Showed equivalent performance with conventional concrete for long term compres-

sive strength (180 days), ion penetrability in terms of chloride ion and corrosion

resistance (corrosion free life)

Somna et al.[120]

Ground fly ash was used by 20, 35 and 50% by weight of cement to replace cement and to improve properties of RAC

Result showed that it slightly improved the compressive strength only at 20% use of fly ash in RAC than concrete without fly ash

For all replacement% of fly ash compressive strength was less than conventional

concrete Did not show any effect on modulus of elasticity at all% replacement Ground fly ash had significant influence on reducing the water permeability

coefficient



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water permeability, capillary water absorption, air permeability,and oxygen permeability. On the basis of results obtained, it has

been observed that permeability of RAC is more than that of con-ventional concrete [76] and it increases with the increase in RAcontent and w/c ratio. The movement of various foreign agents(permeation) through the concrete occurs not only by the flow

through porous system but also by diffusion and sorption. There-fore, evaluation of water absorption of concrete is the first essentialstep to study the durability of concrete made with RA. In case of RAC, water absorption of concrete is directly related to the water

absorption of RA. Thus, higher water absorption value of RAimposes a high risk towards the durability of RAC. From variousresearch works, it is found that the water absorption of RAC is sig-nificantly higher than natural aggregate concrete [12,56,121,122].

Water absorption of RAC also depends on the quantity of adheredmortar and the substitution level of RA.

Some of the most known aspects of RAC related to its durability

are carbonation and chloride ion resistance. In general, it has beenobserved that the carbonation and chloride ion penetration resis-tance of RAC is poorer than the conventional concrete due to thepresence of intrinsic porosity. In particular, regarding carbonationprocess it has been observed that the carbonation depth of RAC

was 1.3–2.5 times of the conventional concrete [47,102,123]. CuiZL et al. [124] reported that the carbonation process of RAC is 3times that of the conventional concrete. Otsuki et al. [80] in theirstudy observed that carbonation depth of RAC was slightly higher

than that of natural aggregate concrete. Similarly, Crentsil et al.[47] reported that carbonation rate was found to be 10% more thanthe conventional concrete. The carbonation depth increases with

the increase in RA content [56] and also increases with the increasein w/c ratio [47]. Abbas et al. [125] reported that rate of carbon-

ation depends on the alkalinity of binding medium. Similarly fromprevious investigations, it has been reported that RA has a negative

effect on chloride ion penetration resistance of RAC and resistancedeclines with the increase in RA quantity [12,22,56]. Olorunsogoand Padayachee [55] reported that the RAC containing 100% RAshowed 73.2% increase in chloride conductivity at 28 days.

Similarly Kou et al. [126] in their study using different grades of RA (100%) reported that the chloride conductivity in terms of coulombs of charge increased up to 55%, 40%, 32% than that of con-ventional concrete.

In contrary to these, from a number of studies, it has beennoticed that concrete made up of RA shows better freeze and thawresistance than the conventional concrete though RA is moreporous [127,128]. Gokce et al. [129] reported that the adhered

mortar does not have much influence on freeze and thaw resis-tance of RAC if the quality of rubble in RA is good. Some other

authors also reported that RAC made up of RA derived from air

entrained concrete showed very good frost resistance[90,102,130]. Abbas et al. [125] studied the durability aspects of RAC by a novel approach known as equivalent mortar volume(EMV) method and they reported that the freeze and thaw perfor-mance of RAC was good for both conventional mix design method

and EMV method. The concrete produced through EMV method

showed better resistance than conventional mix design methodas the mortar volume in EMV method was less.RAs may be contaminated with several types of contaminants

such as sulphate, chloride and carbonates. Generally sulphateattack causes severe deterioration in concrete by spalling, soften-ing and expansion of concrete. Expansion of concrete occurs dueto the formation of calcium sulphate formed due to chemical reac-

tion of calcium hydroxide and sulphate and due to ettringite whichleads to the formation of crack leading to disruption [15,120].Hence, some of the researchers have studied the resistance of

RAC to sulphate exposed environment. Limbachiya et al. [6,17] intheir study reported that concrete containing higher amount of RA content showed lower resistance towards sulphate attack. Theyalso reported that incorporation of fly ash into RAC provides a bet-

ter resistance towards sulphate attack than ordinary Portlandcement containing RAC. Similarly, Hua and Song [22,130] fromtheir empirical studies reported that RAC has poor resistancetowards sulphate attack as the loss in concrete mass increased

with the increase of the RA in sulphate resistance test.

3.4.1. Techniques for improving durability properties

Modification of the porous nature has been one of the great con-

cerns to improve the durability properties of RAC. Some of thestudies have revealed that the durability properties can beimproved by modifying the microstructure of RAC by incorporatingmineral admixtures like silica fume, fly ash, GGBS, Nano silica etc.

into concrete [56,77,92,93,114] which has already been mentionedin Section 4.4. In contrary to this, the addition of fly ash to RAC

shows a negative response towards carbonation resistance[56,125]. Incorporating these admixtures or modifying the aggre-

gates with these pozzalanic materials leads to the enhancementof its compactness and the formation of secondary C–S–H that fillsup the empty capillary spaces within the hardened cement pasteand consequently refines the pore size distribution and pore shape

of concrete [17]. Hence, it increases the strength of concrete as wellas the durability performance. As the durability properties likechloride penetration resistance, carbonation resistance depend onthe strength of concrete, improving the strength can improve these

resistances. Basically, the properties like drying shrinkage andmodulus of elasticity have been improved to a great extent in com-parison to other properties with the incorporation of alternativecementitious materials like fly ash [6]. Furthermore, the reduction

in w/c ratio also makes the concrete more impermeable towardsthese agents. Moreover, Abbas et al. [125] in their empirical studiesreported that by adopting equivalent mortar volume (EMV)method, a novel mix design procedure, the different durability

properties such as freeze and thaw resistance, chloride ion pene-tration resistance, carbonation resistance could be improved.

4. Summary and conclusion

This review paper presents a comprehensive summary regard-

ing the production and the use of RA in concrete and an overviewon its effect on different properties of RAC. It will help for thefuture research progress in this field. Though it has been found thatthe mechanical and durability performance of RAC are generally

inferior to conventional concrete, in the recent years many studieshave revealed that RA is gaining wide spread attention day by day

Fig. 13f. surface treated RA in silica fume solution [118].


http://-/?-


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to be used as a construction material both from material and

structural point of view. Thus, the use of RA is from C&D waste iscontributing towards a sustainable development in constructionindustry. However, to use RA as a structural material, its qualityand properties need to be characterized very precisely as thequality of RA has significant influence on the performance of

RAC. Thus, instead of saying C&D debris as waste, it can be termed

as ‘‘raw material to produce economic resources for future’’. Thesematerials have been extensively used in some developed countries.However, in India, its use is limited to non-structural application

and to research purpose due to limited knowledge and experimen-tation is on to use for structural application. Thus, more focusedresearch needs to be conducted in this area for the efficient useof RA as a structural material in India. The use of RA, in particular

aggregates from C&D waste or precast concrete residues, seems tobe a promising contribution towards the sustainability of the con-struction industry. Some of the future research aspects are listed as

follows:

5. Future aspects

Long term behavior (mechanical and durability performance) of RAC is not well known. Thus more research needs to be con-ducted in this area.

Long term behavior and modification of microstructure needs tobe studied. Nano scale study of ITZ should be done.

Durability of RAC needs to be assessed both from material andstructural aspect.

Limited knowledge on the use of RA in other concrete such ashigh performance concrete, geopolymer concrete, and precastconcrete.

Proper modeling relationship should be established between

compressive strength, split tensile strength and flexuralstrength.

Optimal mix proportioning is to be formulated and proper mix

design procedure has to be established. Numerical modeling on the behavior of RAC is to be is to be

investigated for better understanding.

Even though researchers investigated some properties of RAC,more detailed studies on structural behavior of RAC is necessary

for its mass use in building. Moreover, a lack of reliable information on corrosion behavior

of reinforced RAC has to be established.

References

[1] Kosmatka SH, Kerkhoff B, Panarese WC, MacLeod NF, McGrath RJ. Design andcontrol of concrete mixtures. 7th ed. Ottawa, Ontario, Canada: CementAssociation of Canada; 2002.

[2] Mehta PK, Meryman H. Tools for reducing carbon emissions due to cementconsumption. Struct Mag 2009(Jan):11–5.[3] Sonawane TR, Pimplikar SS. Use of recycled aggregate in concrete. Int J Eng

Res Technol 2013;2(1):1–9.[4] Oikonomou ND. Recycled concrete aggregates. Cem Concr Compos

2005;27:315–8.[5] Anik D, Boonstra C, Mak J. Handbook of sustainable building. James & James;

1996.[6] Limbachiya M, Meddah MS, Ouchagour Y. Use of recycled concrete aggregate

in fly-ash concrete. Constr Build Mater 2012;27:439–49.[7] Kasai Y. Reuse of demolition waste. In: Proceedings of the second

international RILEM symposium on demolition and reuse of concrete andmasonry. Chapman and Hall, Tokyo, 7–11 November; 1988.

[8] Hansen TC. Recycled aggregate and recycled aggregate concrete, second stateof- the-art report, developments from 1945–1985. Mater Struct1986;19(3):201–46.

[9] Gilpin R, Menzie DW, Hyun H. Recycling of construction debris as aggregate inthe Mid-Atlantic Region, USA. Resour Conserv Recycle 2004;42(3):275–94.

[10] Schimmoller VE, Holtz K, Eighmy TT, et al. Recycled materials in European

highway environments: uses, technologies, and policies. FHA report,FHWAPL-00-025. Washington, DC: US Department of Transportation; 2000.

[11] Poon CS, Chan D. The use of recycled aggregate in concrete in Hong Kong.Resour Conser Recycle 2007;50:293–305.

[12] Rao MC, Bhattacharyya SK, Barai SV. Influence of field recycled coarseaggregate on properties of concrete. Mater Struct 2011;44:205–20.

[13] Ajdukiewicz A, Kliszczewicz A. Influence of recycled aggregates onmechanical properties of HS/HPC. Cem Concr Compos 2002;2:269–79.

[14] Mehta PK. Advancements in concrete technology. Concr Int1999;21(6):69–76.

[15] Khalaf FM, DeVenny AS. Recycling of demolished masonry rubble as coarseaggregate in concrete: review. J Mater Civ Eng 2004;16(4):331–40 .

[16] Abbas A, Fathifazl G, Isgor OB, Razaqpur AG, Fouriner B, Foo S. Environmentalbenefits of green concrete. EIC Clim Change Technol 2006 IEEE; 10–12 May2006:1–8.

[17] Limbachiya M, Meddah MS, Ouchagour Y. Performance of Portland/silicafume cement concrete produced with recycled concrete aggregate. ACI Mater J 2012;109(1):91–100.

[18] Best practice guide for the use of ‘‘recycled aggregates in new concrete’’.Technical report TR 14, Cement & Concrete Association of New Zealand(CCANZ); October 2011.

[19] Hansen TC, editor. Recycling of Demolished Concrete andMasonry. Oxfordshire, UK: Taylor and Francis; 1992.

[20] Corinaldesi V, Moriconi G. Influence of mineral additions on the performanceof 100% recycled aggregate concrete. Constr Build Mater 2009;23:2869–76.

[21] Elhakam AA, Mohamed AE, Awad E. Influence of self-healing, mixing methodand adding silica fume on mechanical properties of recycled aggregatesconcrete. Constr Build Mater 2012;35:421–7.

[22] Li X. Recycling and reuse of waste concrete in China Part I. Material behaviourof recycled aggregate concrete. Resour Conserv Recycle 2008;53:36–44.

[23] Matias D, de Brito J, Rosa A, Pedro D. Mechanical properties of concreteproduced with recycled coarse aggregates – influence of the use of superplasticizers. Constr Build Mater 2013;44:101–9.

[24] Etxeberria M, Vazquez E, Mari A, Barra M. Influence of amount of recycledcoarse aggregates and production process on properties of recycled aggregateconcrete. Cem Concr Res 2007;37(5):735–42.

[25] Movassaghi R. Durability of reinforced concrete incorporating recycledconcrete as aggregate, MASc Thesis. Waterloo, Ontario, Canada; Universityof Waterloo: 2006.

[26] Tam VWY, Tam CM, Le KN. Removal of cement mortar remains from recycledaggregate using pre-soaking approaches. Resour Conser Recycle2007;50:82–101.

[27] Abbas A, Fathifazl G, Isgor OB, Razaqpur AG, Fournier B, Foo S. Proposedmethod for determining the residual mortar content of recycled concreteaggregates. J ASTM Int 2008;5(1):1–12.

[28] Juan MS, Gutierrez PA. Study on the influence of attached mortar content onthe properties of recycled concrete aggregate. Constr Build Mater2009;23:872–7.

[29] Akbarnezhad A, Ong KCG, Zhang MH, Tam CT, Foo TWJ. Microwave-assisted

beneficiation of recycled concrete aggregates. Constr Build Mater2011;25(8):3469–79.

[30] Kuroda Y, Hashida H. A closed – loop concrete system on a construction site.In: Proc CANMET/ACI/JCI, three-day international symposium on sustai

40_recycled aggregate from c&d waste & its use in concrete

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