strength and durability studies of self compacting rubberised concreteicjonline.com/views/2012.09....

15SEPTEMBER 2012 The IndIan ConCreTe Journal

Point of View

Strength and durability studies of self compacting rubberised concrete

N. Ganesan, Bharati Raj. J and A.P. Shashikala

This paper describes the development of Self Compacting Rubberised Concrete (SCRC) using the advantages of both Self Compacting Concrete (SCC) and Rubberised Concrete. The development presents an opportunity for the utilisation of waste tyres. It deals with SCRC with and without steel fibres and provides a correlation between compressive strength and the various durability parameters.

IntroductionDiscarded vehicle tyres are one of the important solid waste challenges needing more useful applications than just becoming a material for landfilling. Due to the rapid depletion of available sites for waste disposal, many countries discourage the disposal of waste tyre rubber in landfills and encourage the construction sector to use these waste materials in concrete in place of fine or coarse aggregate.1

The production of shredded or ground tyre rubber is now well developed, making the reuse of this material in concrete practicable.2 Developing such construction materials could have both environmental and economic advantages. However, concrete with scrap tyre aggregates must satisfy the minimum requirements of strength and durability. The idea of developing Self Compacting Concrete (SCC) incorporating rubber

aggregates is a novel approach to combine the advantages of both SCC and rubberised concrete. To attain the required self-compacting properties, the new material Self Compacting Rubberised Concrete (SCRC), requires a slightly higher super plasticiser than conventional SCC at the same water/powder ratios.3 Even though this technology has the potential for obtaining an interesting mechanical behaviour, few studies have been carried out on Self Compacting Rubberised Concrete.4-7 Past investigations suggest that the partial replacement of coarse or fine aggregate of concrete with waste tyre can improve properties such as abrasion resistance, shock absorption, vibration absorption and ductility.8-10 The use of steel fibres in SCC improves the engineering properties such as ductility, post crack resistance and energy absorption capacity.11, 12 However, no attempts have been made so far to evaluate the effect of addition of steel fibres to SCRC.

This paper reports the strength and durability characteristics of self compacting rubberised concretes with and without steel fibres for specimens of 30 to 50 MPa. The durability properties were also investigated. Tests were performed for permeability, water absorption, sorptivity and chloride diffusion and resistance to marine and acid attacks. Regression equations correlating the various durability indices and compressive strength are proposed.

The IndIan ConCreTe Journal SEPTEMBER 201216

Point of View

experimental programmeMaterialsThe materials used in this study were:

Ordinary Portland cement conforming to IS 12269:198713

Flyash with a consistency of 45% obtained from Neyveli Lignite Power Plant and conforming to Type F as per ASTM C618 14

River sand pass ing through 4 .75 mm IS sieve conforming to grading zone II of IS 383:1970 and having specific gravity of 2.54.15

Coarse aggregate with a maximum size of 12 mm.

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The mix design was based on the method proposed by Nan et. al. It gives an indication of the target strength at 28 day. Figure 1 shows the flowchart used for obtaining 30, 40 and 50 MPa concrete.16 The water powder ratio (w/p) was varied to obtain the strengths. The mixes were checked for self compactability following EFNARC acceptance criteria.17 A naphthalene based superplasticiser and calcium sulphate dihydrate viscosity modifying admixture (VMA) were added to obtain the required workability.

Development of SCRC and SFRSCRCFine rubber was obtained by shredding the worn out tyres and sieving the product gave particles of 4.75 mm (Figure 2(a)). The specific gravity of fine rubber was 1.14. Figure 3 shows the gradation curves of both fine aggregates and fine rubber. In Self Compacting Rubberised Concrete (SCRC), the addition of fine rubber was 15% by volume replacing the fine aggregates. The addition of crimped steel fibres 0.5% by volume (diameter 0.45 mm, length 30 mm and aspect ratio 66) gave the Steel Fibre Reinforced Self Compacting Rubberised Concrete (SFRSCRC) (Figure 2(b)).

When the fine rubber replaced the fine aggregate, the mix became less workable. So the superplasticiser dosage was increased to meet the acceptance criteria of SCC. The viscosity modifying admixture was also added (0.01% of water content) to avoid segregation. Poly Vinyl Alcohol (PVA) was added to compensate for the strength loss due to the addition of rubber. PVA undergoes polymerisation in the presence of water and roughens the surface of the rubber aggregate bringing about a better interfacial bond between the matrix and


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rubber particles.18 Table 1 gives the details of the mix. The self compactability of the mixes was checked by the Flow test, V-funnel test and L-Box test. The compressive strength of the various mixes including SCC, SCRC and SFRSCRC was tested using 150 mm cube specimens. Table 2 gives the fresh state properties and compressive strength.

Durability tests The durability properties such as water permeability, chloride ion permeability, water absorption, abrasion resistance, sorptivity, and resistance to seawater attack and acid attack were investigated. Six specimens were cast for each mix and the average value was used for the analysis. The durability test details are discussed below.

Table 1. Mix designDesignation Target

strength, MPa

Cement, kg/m3

Fly ash, kg/m3

Fine aggregate,

kg/m3

Coarse aggregate,

kg/m3

Rubber, kg/m3

Steel fibres, kg/m3

Super plasticizer,

% of powder content

VMA, kg/m3

PVA, kg/m3

w/p Water, kg/m3

SCC 130

267 161 887.00 710 - - 1.00 - - 0.49 209.00SCRC 1 267 161 753.95 710 133.05 - 1.26 0.134 5.029 0.50 214.00SFRSCRC 1 267 161 753.95 710 133.05 39.250 1.31 0.134 5.029 0.50 214.00SCC 2

40339 130 887.00 710 - - 1.30 - - 0.44 205.00

SCRC 2 339 130 753.95 710 133.05 - 1.39 0.542 4.849 0.44 206.36SFRSCRC 2 339 130 753.95 710 133.05 39.250 1.43 0.542 4.849 0.44 206.36SCC 3

50410 112 887.00 710 - - 1.60 - - 0.37 193.00

SCRC 3 410 112 753.95 710 133.05 - 1.66 0.533 4.661 0.38 198.36SFRSCRC 3 410 112 753.95 710 133.05 39.250 1.74 0.533 4.661 0.38 198.36

Table 2. Properties of fresh and hardened concreteDesignation Slump flow, mm V-Funnel

time(s)L-Box value, mm Compressive strength, MPa

7-days 28-days 90-days

SCC 1 700 9 0.8 18.36 30.10 39.04

SCRC 1 700 9 0.82 15.54 25.48 33.48

SFRSCRC 1 700 10 0.86 18.22 29.88 35.26

SCC 2 700 10 0.84 28.87 47.33 54.52

SCRC 2 700 11 0.85 24.76 40.59 44.44

SFRSCRC 2 700 11 0.87 25.94 42.52 48.74

SCC 3 700 11 0.86 32.26 52.89 61.63

SCRC 3 700 11 0.89 25.04 41.05 50.82

SFRSCRC 3 700 12 0.9 29.42 48.22 56.81


Point of View

Water permeabilityThe water permeability test was performed following IS 3085:1965.19 Figure 4 shows the test setup including the cylindrical specimens (150 mm diameter and height) centred in the cell, with the lower end resting on the support. The annular space between the specimen and the cell was carefully filled with a molten mixture of bee wax and rosin. The cell assembly was connected to a reservoir and a pressure of 12 kg/cm2 was applied for a period of 100 hours. The total quantity of water permeation was noted. The coefficient of permeability was calculated as follows:

, ......(1)

where K is the coefficient of permeability in cm/sec, Q is ml of water percolating over the entire period, T is the time in seconds over which Q is measured, A is the area of specimen in cm2 and H/L is the ratio of pressure head to the thickness of specimen.

Figure 5 shows the coefficient of permeability within the range of 1 x 10-10 to 7 x 10-10 cm/sec, far less than the limit for normal strength concrete suggested by other researchers.20 The specimens became more impermeable with increasing compressive strength. The coefficient of permeability for SCRC was found to be 57% lower than that of SCC apparently because the rubber particles act as a barrier for the passage of water. When steel fibres were included, the permeability was only 34% of the

SCC’s permeability. This may be due to the crimped fibres entrapping air during mixing. These results show that SCRC was two times more impermeable than the conventional SCC.

Chloride ion penetrationThe vulnerability of rubber composites to chloride ion penetration from seawater and other chloride environment was tested following ASTM C1202.21 A water saturated 100 mm diameter, 50 mm thick concrete specimen was subjected to a 60 V DC for 6 hours. One end of the specimen was maintained in 3% NaCl solution and the other in 0.3 M NaOH solution. Figure 6 shows the stainless steel electrodes used in the experiment. The current flow through the specimen was


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noted at regular intervals for 6 hours to determine the total charge passed.

The chloride ion permeability ranged between 300 to 400 Coulombs indicating a dense microstructure.21 From Figure 7, it can be seen that the charge passed reduced as the concrete strength increased. The reduction was by 10% in the case of SCRC specimens due to the insulating property of rubber. SFRSCRC specimens showed a 7% higher affinity to current compared to SCRC specimens. This increase was attributed to the conducting nature of steel fibres.

Water absorptionThe water absorption test was carried out following IS 1237:1959 on 100 mm cube specimens to determine the porosity of specimens containing rubber aggregates.22 Figure 8 shows the results on SCC, SCRC and SFRSCRC. The water absorption of all the mixes was well below the permissible value of 10%. As the compressive strength increased, the mixes showed a decreasing capacity for water absorption. The water absorption of SCRC was 50% of that of conventional SCC. The presence of

rubber particles which do not absorb water could be responsible for this result. The 5% higher absorption in SFRSCRC over SCRC may be attributed to the steel fibres entrapping air during mixing.

Abrasion resistancePrism specimens ( 70.7 x 70.7 x 25 mm ) weighed to the accuracy of 0.1 gm were tested following IS 1237:1959 to find the resistance to abrasion.22 The grinding path of the abrasion testing machine was evenly strewn with 20 gms of an abrasive powder. The specimen was fixed with the test surface facing down. A 30 kg weight at the centre loaded the specimen. The grinding disc rotated at a rate of 30 rpm. After every 22 revolutions, the disc was stopped, the abraded powder was removed and fresh 20-gms abrasive powder was applied each time. After 110 revolutions, the specimen was turned about its vertical axis by 90o and test was continued until 220 revolutions were completed. After testing, the specimens


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were weighed again and the average loss in thickness was calculated.

Figure 10 shows the specimens’ loss in thickness. The values are within the code limit of 2 mm. As expected, the abrasion resistance increased with increasing compressive strength. Abrasion of SCRC was less by 20% compared to that of SCC. Steel fibre addition reduced it by 6%.

SorptivitySorptivity is a measure of the capillary force exerted by the pore structure causing fluids to be drawn into the body of the material. It is calculated as the rate of capillary rise in a concrete prism placed in 2 to 5 mm deep water.23,24 For one-dimensional flow, the relation between absorption and sorptivity is given by, i = S t0.5

where, i is the cumulative water absorption per unit area of inflow surface, S is the sorptivity and t is the elapsed time. The test was conducted in the laboratory

on 100 mm diameter and 50 mm thick specimens preconditioned to a certain moisture level by drying in an oven at 50oC for 7 days. After cooling, the sides of the concrete samples were sealed and the initial weight was taken. The samples were then kept in a tray so that 2 to 5 mm depth was immersed in water as shown in Figure 11. At selected intervals of 1, 2, 3, 4, 5, 9, 12, 16, 20 and 25 minutes; the sample was removed and was weighed after blotting off excess water. The gain in mass per unit area over the density of water (gain in mass/unit area/density of water) versus the square root of time was plotted (not shown). The slope of the best fitting line was reported as the sorptivity.

Figure 12 shows that the sorptivity of SCRC and SFRSCRC were higher than that of SCC, which indicates that the rubberised composites have a higher initial water absorption. This may be due to the following reason. In the case of SCRC, the rubber particles finer than fine aggregate act as micro fillers and fill most of the pores in the core portion of concrete. However, it may be noted that the interfacial shear between the rubber particles and the rest of the matrix is less. This


Point of View

Table 3. Composition of marine waterComposition Concentration, g/lit

Sodium chloride 24.53

Magnesium chloride 5.2

Sodium sulphate 4.09

Calcium chloride 1.16

Potassium chloride 0.695

leads to a relatively porous concrete in the outer shell of the specimen when compared to the core portion as mentioned earlier. Since sorptivity measures the capillary flow over a very small depth of 2 to 5mm which invariably lies in the outer shell, rubberised concrete shows higher values of sorptivity in comparison to SCC. The sorptivity values of all the specimens were in the permissible range of 0.09 mm/min0.5and 0.17mm/min0.5

meant for normal concrete.25 The sorptivity decreased with increasing compressive strength.

Resistance to seawater and acidic solution The effect of seawater and acidic solutions on the durability of SCC with rubber aggregates was investigated by testing 100 mm cube specimens for loss in mass and reduction in compressive strength. Table 3 gives the composition of seawater prepared in the laboratory as per ASTM D1141.26 For determining the resistance to acid, the cubes were immersed in a 3% sulphuric acid (H2SO4) solution for 90-days.

Figure 13 indicates that the loss in mass was lesser than the loss in compressive strength. The reduction in mass was 2% for 50 MPa SCC while SCRC and SFRSCRC specimens having the same strength showed negligible reduction in mass (less than 1%). This may be due to

the replacement of fine aggregates by rubber which is less reactive in chloride environment. When fibres were added to SCRC, the mass loss increased by 12% compared to SCRC without fibres. This could be the consequence of the chemical affinity of steel fibres to chloride environment. The loss in mass and the reduction in compressive strength in acid solution were 8% and 25% respectively for SCRC. The corresponding losses in SFRSCRC were 10% and 30% owing to the corrosive nature of steel fibres in acidic environment. This may be the reason why the fibres in contact with aggressive solution showed signs of corrosion by turning brown.

The loss in mass and the reduction in compressive strength under acid medium shown in Figure 14 were also observed under chloride medium. However, the specimens were more vulnerable in acidic medium.

Figure 15 shows the physical appearance of the cubes after 90 days immersion in marine and sulphuric acid solution. The acid medium fully eroded the cover exposing the aggregates (Figure 15 b). In contrast, the specimen was more or less intact in chloride medium. (Figure 15 a).


Point of View

Statistical evaluation of the resultsAttempts made to correlate the water permeability, absorption and sorptivity characteristics of the composites with the chloride ion penetration values show that the permeability, absorption and sorptivity characteristics increase linearly with increase in chloride ion penetration (Figures 16 – 18).

A general equation relating the durability indices with the compressive strength for SCC, SCRC and SFRSCRC was obtained, equation (2). 27

......(2)

where, DI is the durability index, fck is the compressive strength and a and b are the regression coefficients.

Figure 19 shows a typical correlation curve for chloride ion permeability. Similar curves were obtained for all the other durability indices. The corresponding regression coefficients are given in Table 4. The authors hope that these correlation equations would be useful in designing self compacting rubberised concrete mixes.

ConclusionsThe strength and durability characteristics of self compacting rubberised concrete with and without the addition of fibres was investigated. The reduction in compressive strength due to the incorporation of scrap rubber in SCC could be compensated to some extent by the addition of steel fibres. All the evaluated durability characteristics were found to be within the limits prescribed by the codes for normal concrete. However, when compared to SCC, SCRC satisfies all the durability requirements better than SCC, except for the sorptivity index. The rubberised concrete with fibres


Point of View

Table 4. Regression coefficients for durability indices

Durability Index Regression coefficients SCC SCRC SFRSCRC

Coefficient of water permeability,

cm/sec

a 2.00E-06 1.00E-06 0.0006

b 2.292 2.513 4.024

R2 0.82 0.83 0.85

Chloride ion permeability

a 739.88 666.77 859.2

b 0.018 0.192 0.233

R2 0.84 0.72 0.93

Water absorption, %

a 3527.8 50.95 516.16

b 2.131 1.188 1.765

R2 0.80 0.74 0.88

Abrasion resistance

a 232.33 28.467 265.19

b 1.751 1.279 1.895

R2 0.92 0.75 0.97

Sorptivity mm/min0.5

a 0.1559 0.3452 0.1428

b 0.132 0.267 0.089

R2 0.84 0.92 0.92

Marine attack

Loss in mass,

%

a 3334.6 12869 174.63

b 1.771 2.643 1.817

R2 0.78 0.84 0.97

Loss in Compressive strength, %

a 288.09 273.01 204.4

b 0.628 0.803 0.596

R2 0.98 0.98 0.99

Acid attack

Loss in mass, %

a 20.268 28.279 15.774

b 0.138 0.366 0.099

R2 0.94 0.62 0.99

Loss in Compressive strength, %

a 62.119 115.53 302.8

b 0.209 0.387 0.744

R2 0.64 0.91 0.78

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was seen to have the best resistance against abrasion. The effect of fibres on the other durability indices was not significant in the rubberised concrete specimens. These results suggest that Self Compacting Rubberised Concrete may be a useful cementitious composite with better durability characteristics than conventional Self Compacting Concrete.


Point of View

Dr. N. Ganesan holds an M.E and Ph.D degree from I.I.Sc, Bangalore. He is currently the Dean (Planning and Development) and Professor of Civil Engineering at the National Institute of Technology, Calicut, India. His research interests include reinforced concrete, ferrocement, fibre reinforced concrete, self compacting concrete,

sustainable concrete, forensic engineering and rehabilitation of RCC structures. He is a fellow of The Institution of Engineers, India and International Ferrocement Information Centre consultant. Apart from being a visiting professor at the Asian Institute of Technology, Bangkok and King Khalid University, Kingdom of Saudi Arabia, his overseas visits include University of Dundee, Scotland, Queens University, Belfast, National University of Singapore, University of Stuttgart, Germany, & University of Michigan, USA.

Ms. Bharati Raj. J holds a B.Tech from Kerala University and an M.Tech in Structural Engineering from NIT Calicut. She is a Research Scholar at National Institute of Technology, Calicut, India. Her research interests include sustainable concrete using waste materials, rubberized concrete, fibre reinforced concrete and self compacting concrete.

Dr. A.P. Shashikala holds a B.Tech from Calicut University, an M.Sc (Engg) from REC Calicut and a Ph.D from IIT Madras. She is a Professor of Civil Engineering at National Institute of Technology, Calicut, India. She has around 29 years of teaching and research experience and 2 years of industrial experience. Her research

interests include seismic resistant structures and offshore structures, development of sustainable concrete using waste materials and rubberized concrete.

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