durability and performance specifications · pdf fileaci 318 (2008) with regard to the...

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Durability and PerformanceSpecifications

Introduction: Building a Case for Performance Specifications

Mixture design of concrete involves a step by step procedure for the selection of types and proportions of ingredients in order to achieve the correct workability and compressive strength. The conventional approach assumes that if the strength criteria are satisfactory, the concrete would be sufficiently durable. Once the concrete has been mixed and placed, only the compressive strength is measured on specially prepared (and standard-cured) samples to ensure compliance with the design requirements and specifications. There is an inherent flaw here, as this assumption does not take into account the variability in concrete resulting from the actual concreting practices, such as placement, consolidation, finishing, and curing. In other words, concrete in the specimens cast for compression testing bear little resemblance to the concrete in the actual structure.

Concrete quality in the cover zone is often dictated by factors other than mixture proportioning. These factors arise as a result of the quality of the materials and, on a particular site, from a wide range of construction practices such as handling, placement, consolidation, and curing of the concrete. Thus, a reliable measure of the quality of the cover zone can only be obtained by assessing the concrete after hardening in the structure, rather than on the companion specimens that are cast to determine the strength development. This is primarily the basis for the call to develop suitable strategies to implement what are commonly known as ‘performance-based specifications’. In particular, performance specifications are intended to assess and ensure the required level of concrete quality in relation to long-term durability, in a given service environment of the structure. Lobo et al. (2005) describe performance specifications as ‘a set of clear, measurable, and enforceable instructions that outline the application-

specific functional requirements for hardened concrete’.

According to Taylor (2004), while the primary risk in a prescriptive specification is placed on the owner and designer, performance specifications separate and allocate risk and responsibility more clearly. By separately specifying and testing the concrete as supplied and the concrete as placed into the structure, the risk and responsibility appropriate to the supplier of the concrete is distinguished from that appropriate to the constructor who places the concrete. Furthermore, prescriptive specifications are mainly concerned with providing complete details of inputs (i.e. materials) and processes in an attempt to ensure that quality will be adequate and that the appropriate degree of supervision and inspection will be provided. On the other hand, in a performance specification, it would be necessary to wait for the allotted time before the durability (or other performance) parameters are checked and payment for construction could be made. Taylor (2004) argues that the two methodologies, in their purest forms, are impractical and that an intermediate approach of adopting hybrid specifications (with greater emphasis on the performance criteria) should be used. In this system, the owner and designer decide on the desired performance level in the specific service environment and propose appropriate ‘index’ or indicator tests, which are used to prepare specifications. The supplier and contractor then provide a concrete system (which is prequalified using tests conducted before actual construction) that satisfies the index parameters (or limits) set forth by the owner/designer. The ‘concrete system’ not only describes the mixture proportions, but it also encompasses the details of the concreting procedures adopted.

Day (2005) suggests that prescriptive specifications offer little advantage to the concrete producers, as they limit the extent of application of the latest developments to mixture proportioning techniques. The view is therefore

Manu SanthanamAssociate Professor, Department of Civil Engineering, IIT Madras

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Location Pertaining to Details Remarks

Section 25.3.1

Unconventional aggregate (such as slag, crushed overburnt brick/tile)

Limits for aggregates:< 0.5% SO3< 10% absorption

Section 25.5.2

Admixtures General statement that admixtures should not impair durability of concrete

Section 25.5.5

Admixtures Chloride content to be checked

Section 28.1

Durability Section defines durability, and identifies factors affecting it Mentions regarding accuracy of current testing regimes for control and compliance

8.2.1 Size and shape effect on durability

(i) Good drainage (ii) Adequate curing

8.2.2.1, Table 3

Exposure conditions Mild, moderate, severe, very severe, and extreme conditions identified

Too general – need to be revised in tune with international developments

8.2.2.3 Freezing and thawing Entrained air % w.r.t. max. aggregate size specified

8.2.2.4 Sulphate attack Table 4, giving recommendations for type of cement, max. free w/c, and min. cement contentFor Class 5 exposure in Table 4, liners and surface coatings also recommended

Prescriptive; does not allow for innovations from concrete producers

8.2.3.2 Cover to reinforcement Refers to 26.4, for nominal cover for concrete durability and fire resistance considerations (Tables 16 and 16A)

Limiting cases: Columns – min. 40 mm or diameter of bar (greater of the two) Footings – 50 mmShould be revised based on new environmen-tal classification

8.2.4.1 Mix proportioning for durability Tables 5 and 6 for max. free w/c, min. cement content, and min. grade of concrete for different exposure conditions

Prescriptive; does not allow for innovations from concrete producers

8.2.4.2 Max. cement content Limited to 450 kg/m3 To avoid shrinkage and thermal cracks;Maximum value with additives should also be proposed (probably, 550 kg/m3)

8.2.5.2 Chloride content Table 7 listing the limits of chloride contents for different types of concrete

8.2.5.3 Sulphate content Total should not exceed 4% (expressed as SO3) by mass of cement in the mix

Except for supersulphated cement

8.2.5.4 Alkali aggregate reaction Protection suggested includes:(i) Low alkali cement (ii) Use of fly ash or slag(iii) Use of impermeable membranes(iv) Lowering cement content

8.2.6.1 Concrete in aggressive soils and water

Recognizes the more serious nature of one-sided chemical attackAlso refers to the importance of proper drainage

Appropriate environmental classification will address this issue

8.2.7 Compaction, finishing and curing Stresses the importance of good concreting practices Direct link of concreting practices to durability must be mentioned

8.2.8 Concreting in sea water Minimum quality: M20 plain concrete / M30 reinforced concreteNo construction joints within 600 mm below low water level, or within 600 mm of upper and lower planes of wave action

Use of slag or pozzolanic cement advanta-geousBituminous or silico-fluoride coatings recom-mended for abrasionProtection of reinforcement on site recom-mended

35.3.2 Limit states of serviceability – cracking

Not > 0.3 mm for general casesNot > 0.2 mm for members exposed to weather / in contact with soil or groundwaterNot > 0.1 mm for severe environment and worse

Should be revised based on new environmen-tal classification

Table 1. Clauses in IS456 (2000) pertaining to concrete durability

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that prescriptive specifications stifle innovation in the manufacture and use of concrete.

Current Practice in India

IS 456 (2000), or the Indian Standard for Plain and Reinforced Concrete – Code of Practice, was revised in 2000 for the fourth time. This new revision, although still lacking in many respects with regard to concrete, does reflect to some extent the advances in concrete technology since the previous revision in 1978. Particularly, the clause on durability has been elaborated, and sampling and acceptance criteria for concrete have been revised. Specific clauses from the code that address concrete durability are covered in Table 1. A glance through the table indicates that there are several shortcomings (see the remarks column) and sufficient modifications can be taken up to bring the code up to date with international developments. Especially in terms of the environmental classifications, some thoughts have already been put forward by Kulkarni (2009), who compared the developments in EN206-1 (2000), AS3600 (1994), and ACI 318 (2008) with regard to the concrete exposure conditions. These need to be developed further. Specific comments pertaining to the new work and understanding that is necessary with respect to the development of the IS 456 (2000), are presented in the ‘Remarks’ column of the table.

International Developments

While prescriptive specifications are still the benchmark around the world, there are several examples from where performance criteria for strength and durability have been adopted successfully, resulting in high quality structures. The current EN206-1 (2000) allows the use of performance criteria for concrete design – the specific performance parameters need to be worked out between the specifier and the producer. Additionally, according to Day (2005) and Bickley et al. (2006), the Australian standards (AS 1379) allow for performance requirements in the special class concretes. The concrete producer has a choice of a performance specification or a prescription.

Day (2005) speaks of the use of a control system by the concrete producers at the Petronas twin towers, which enable the production of 80 MPa concrete with a coefficient of variation of only 3%. He also cites several other examples from Australia where the use of performance criteria have helped in producing good quality concrete. According to him, performance specifications provide a win-win situation for all concerned in the concreting process, as (i) the owner gets a cost effective and reliably uniform product (concrete), and can also get additional information on the concrete in terms of the heat generation, shrinkage and other characteristics (which is possible because

of the producer having standardized the mixes), and (ii) the producer can be innovative with regard to materials selection, as the specification does not place any curbs on that aspect. Furthermore, with increasing experience, the specifier can also be able to arrive at optimal values of strength and durability parameters for the specifications.

Bickley et al. (2006) indicate that the Canadian standards (CSA A23.1) use performance requirements such as total charge passed (Coulombs) for special categories of chloride exposures, in addition to the routine prescriptive requirements. They also state that currently, a number of standardized testing methods are available to use in performance specifications, namely, the rapid chloride permeability test (ASTM C1202), air void system (ASTM C457), sorptivity (ASTM C1585), rapid migration test (AASHTO TP64), and chloride bulk diffusion (ASTM C1556). These methods have been used extensively in Canada and Australia; further, these tests can be conducted either on samples cast during concreting or from cores drilled through the actual structure.

Three durability index (DI) tests have been developed in South Africa to characterize the potential durability of concrete according to the transport mechanisms of oxygen permeability for permeation, water sorptivity for absorption, and chloride conductivity for diffusion (Alexander et al. 1999, 2001). The tests are simple and practical to perform, and can be applied either on lab specimens or on as-built structures. Test samples are generally discs of 70 mm diameter and 30 mm thick, extracted from the surface or cover zone of concrete.

An example of the application of DI based design approach is in concrete marina construction in the Gulf region. This region is characterized by high temperatures, low humidity and highly saline conditions. In this case, the contractor was initially required to provide reasonable assurance of a minimum 100-year maintenance-free design life. This was done by using available service life models. The assurance expectation was later reduced to a 50 year design life, with no evidence of major deterioration and no major maintenance within 25 years. A series of different service life models were used to obtain an idea of the required design parameters for concrete materials selection and mix design. A trial mix was made up in University of Cape Town lab to test key durability and other parameters. Simultaneously, mixes were made up in contractor’s lab on site, for evaluation. Decision was taken to use the Rapid Chloride Permeability Test (RCPT) as the ‘DI’ test – since it is well-known in Gulf region, and also because it is an ASTM standard test (ASTM C1202-2010). ‘The Concepts of ‘Material Potential’ and ‘As-built values’ were used, with actual measured coefficient of variations. Limiting

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RCPT values were decided on for lab and in-situ samples. Sampling was more frequent at start of construction, to assist contractors to achieve required performance. Using the continuous on-site testing regime, deviations from acceptable limits could be easily detected.

Of all international developments, the use of performance specifications in South Africa needs to be followed with great interest. The climatic exposure conditions in South Africa, for the most part, resemble those in India, and the concrete construction industry there is undergoing similar changes and upheavals as in India. Further, the extensive experience gained using the durability index approach can be utilized beneficially. Moreover, the concrete codes in South Africa are currently being revamped to be more in tune with worldwide developments, which will also give them an opportunity to incorporate useful lessons learnt from their durability index approach experience. Thus, Indian concrete industry has a clear cut model to follow, towards the development of performance specifications for concrete.

Framework for Development of Performance Specifications in India

The development should be taken up in 4 stages, which are discussed below. It is expected that the final stage, after the initial implementation, will continue long into the future, as the guidelines would have to be revised based on inputs from the field implementation.

Stage 1: Background Work

(a) Critical evaluation of durability provisions in Indian standards

(b) Re-look at the environmental classification system, with inputs from recent changes worldwide.

Stage 2: Preparation of distress map for different geographic locations across India

(a) Collection of information from different geographical locations in India about types of distress observed in concrete

(b) Obtaining cores from concrete from these locations,

and laboratory evaluation to determine (i) mix design (unless original available), (ii) depth of carbonation, and (iii) chloride profile

(c) Redefining exposure conditions in India based on the studies in Stages 1 and 2.

Stage 3: Testing methods for performance specifications

(a) Selection of appropriate laboratory based performance indicators

(b) Selection of appropriate field based performance indicators

(c) Conduct of round robin tests at different laboratories

(d) Analysis of test data from different labs and evaluation of critical factors affecting results. Finalization of tests and test procedures.

Note: The scope of the project in Stage 3 is very extensive. However, instead of re-inventing the wheel, it would be advisable to directly implement valuable lessons learnt from studies of a similar nature in different parts of the world. Considerable information already exists in this regard from the work of several RILEM committees. Suitable inputs can be taken from RILEM technical committee (TC230) on Performance Specifications for Concrete (PSC), as well as from (TC189) Non-destructive Evaluation of the Penetrability and Thickness of the Concrete Cover (2007). Further, guidelines for conducting round robin tests can be based on the recent experience in South Africa, and analyzing the vast volume of information generated through the work done there in the past five years.

Stage 4: Final analysis and white paper on durability specifications and Beginning of Field implementation

(a) Prepare white paper on durability specifications, and set guidelines for implementation

(b) Identify suitable sites for carrying out field trials(c) Use durability specifications in concrete contracts

– select participating concrete producers

In the long term, one would need to follow up on testing of concrete from the sites using the selected testing methods, and refine the methodology.

Mix w/c Cement (kg/m3) Water (kg/m3) Coarse Aggregate (kg/m3) Sand (kg/m3) SP (% By weight of cement)

M1 0.35 350 122.5 1195 797 1%

M2 0.65 350 227.5 1011 706 Nil

M3 0.50 350 175 1120 740 Nil

M4 0.65 300 195 1120 730 Nil

M5 0.65 400 260 980 610 Nil

Table 2. Mixture proportions

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Description of Work at IIT Madras

Work related to performance specifications has been carried out at IIT Madras for the past 5 years. Much of the work is related to the study of different types of durability parameters, and their range of values for different types of concrete. Further, a critical analysis of the environmental exposure classifications has also been performed and a new exposure classification system has been proposed. The details from these studies are presented in this section.

Durability indices for performance specifications

The mixture proportions for concrete are presented in Table 2. A polycarboxylic ether based superplasticizer (SP) was used for the mix M1 with a water cement ratio of 0.35. All other mixtures were cast without SP.

Coarse aggregates were blended in the ratio 60: 40 of 20 and 10 mm sized aggregates respectively. The moisture content of each batch of aggregates was determined before mixing, by using the microwave method described in ASTM D 4643 (1987). The concrete was mixed in a pan type mixer having 60 litres capacity and the mixing time for all the mixes was kept as 5 minutes. The mixer drum was made just wet condition by wiping with a wet piece of sponge before the material is charged so that no water will draw from the mixing water for wetting the surface of the drum. Coarse aggregate was charged to the mixer followed by cement and sand. Dry mixing was done for one minute and 70% of the mixing water was added in the next 1.5 minutes. Then superplasticizer was added (for w/c ratio 0.35 mix only) followed by the remaining water and mixed for 2 minutes. The specimens were demolded after 24 hours and kept in curing tank containing water saturated with calcium hydroxide.

Cubes of 150 mm size were tested for compressive strength with a loading rate of 0.25 MPa/second as per IS 516 (1959). Apart from cubes, 100 x 200 mm cylinders were also cast for some durability tests.

Oxygen permeability

Oxygen permeability test was carried out using a permeability cell designed by Ballim (1991). The concrete specimens were cylindrical discs of 68 mm diameter and 25 mm thickness which were obtained by coring and slicing of concrete cube specimens. These were then oven dried at 50°C and 15% RH prior to testing, until the change in mass was less than 0.1% over a 24 hrs period. Preconditioned specimens were then placed in a falling head permeameter to measure the decay of oxygen pressure over a period of time. The oxygen permeability test up is as shown in Figure 1. The coefficient of permeability is determined from the slope of the line produced when the natural log of the ratio

of initial pressure to pressure at any time is plotted against the time elapsed. The coefficient of permeability is given by:

where

w = molecular mass of oxygen (O2) =32 kg/mol

V= Volume of oxygen under pressure in the permeameter (m3)

g = acceleration due to gravity (m/s2)

R = Universal gas constant = 8.313 Nm/K mol

A = superficial cross sectional area of the sample (m2)

d = sample thickness (m)

Ø = absolute temperature (K)

P0 = pressure at start of test (kPa)

P = pressure at time t (kPa)

The oxygen permeability index (OPI) is then determined from the negative log of the coefficient of permeability, i.e.

OPI = -log10k

Fig. 1 Oxygen permeability test set up showing the specimen holder (left) and the permeability cell and oxygen cylinder (right)

Three specimens were tested for each concrete, and the average result is reported in this paper.

Water sorptivity

Water sorptivity is an indicator of the potential for uptake of water by the concrete through capillary absorption. The test method consists of measuring the mass of water absorbed with time from the bottom of a concrete specimen, of which the sides are sealed with epoxy to ensure only uniaxial absorption. Sample preparation for this test was similar to that for oxygen permeability. 68 mm diameter and 25 mm thick concrete specimens, after drying in the oven for 7 days at 50°C, were placed on lime water saturated layers of an absorbent material (cotton cloth) in a plastic tray. The mass of the specimen was measured at prescribed time intervals (0, 3, 5, 7, 9, 12, 16, 20 and 25 minutes), until a period of 25 minutes. After the completion of the test, the

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specimen was immersed in a water bath inside a chamber connected to a vacuum pump. Vacuum was then applied inside the chamber to cause the penetration of water into the accessible pores of the concrete. After a period of 24hrs, the vacuum saturated mass of the specimen was measured. The test setup is as shown in Figure 2.

The mass of water absorbed at each weighing period is given by:

Mwt = Mst- Mso

whereMst is the mass at time tMso is the initial mass

The slope of the best fit straight line obtained by plotting the values of Mwt against the square root of time (√t), gives the coefficient of sorptivity (F). The water sorptivity is then calculated as:

Surface water permeability

To find Surface Water Permeability, Germann Water Permeability Test (GWT) apparatus was used. The GWT is also used for testing of microcracking and porosity of the skin concrete. 150 mm concrete cubes were used to test the surface permeability of the concrete. With the GWT a sealed pressure chamber is attached to the concrete surface, boiled water is filled into the chamber and a required water pressure is applied to the surface. The pressure is kept constant using a micrometer gauge with attached pin that substitutes the water leaving the chamber, to measure the amount of water penetrating the substrate.

The difference in the gauge position over a given time (e.g. 5 minutes) is taken as a measure of the water penetrability for given water pressure. The recommended pressure applied is 0.5 BAR or 1.0 BAR. The GWT allows testing up to a maximum of 6 BAR pressure, e.g. on dense high performance concrete or on coatings. The attachment of the GWT unit is either by means of two clamping pliers anchored to the surface, or by using a suction plate eliminating the need of anchoring. The pressure selected may be kept constant turning the micrometer gauge clockwise. The gauge reading is recorded for a set period, e.g. 5 minutes. The difference in gauge reading from the start of the test and after 5 minutes is an index for the water penetration. The test setup is as shown in Figure 3.

The flux q may be calculated for a given water pressure as:

where,

B is the area of the micrometer pin being pressed into the chamber, 78.6 mm2 for the 10 mm pin diameterg1 and g2 the micrometer gauge readings in mmA is the surface area, 3018 mm2 (gasket inner diameter 62

Fig. 2 Water sorptivity test (left) and vacuum saturation set up (right)

where

F is the coefficient of sorptivityd is the thickness of the sample (mm)Msv is the vacuum saturated mass of the sample (g)

Three specimens were tested for each concrete and the average is reported in this paper. Fig. 3 Surface water permeability test set up

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mm) t the time the test is performed in seconds

The surface permeability Coefficient (ccp) is then calculated using Darcy’s law:

where

ccp is the coefficient of permeability of the surface concreteq is the flux (mm/s)p is the pressure selected (BAR) andL is the length the pressure is applied over, 15 mm (the width of pressure gasket)

Rapid Chloride Permeability

Disc shaped specimens of size 100 mm diameter and 50 mm thickness, cut from 100 x 200 mm size cylinders were used for Rapid Chloride Permeability tests as per ASTM C 1202 -1997. The circumferential section of the specimen was coated using an epoxy paint, to ensure that the any penetration of the fluid was through the cut faces only. The diffusion cell consisted of two chambers, containing 3% NaCl on one side and 0.3N NaOH on the other side of the concrete specimen. A potential difference of 60 V was applied across the specimen, and the current passing the specimen was measured at 30 minute intervals for a total duration of 6 hours. The total charge passed through the specimen was then calculated and reported as the chloride permeability. The classification of concretes in terms of

Water absorption

The water absorption of selected high early strength concrete mixtures was determined on 100 mm cube specimens as per ASTM C 642-1997. The samples were weighed before drying. The drying was carried out in an oven at a temperature of 105°C. The drying process was continued until the difference in mass between successive measurements at 24 hours interval was less than 0.1%. The specimens were immersed in water and taken out at regular intervals of time, surface dried using a clean cloth and weighed. The process was continued till successive measurements were not different by more than 0.1%. The difference between the water saturated mass and oven dry mass expressed as a percentage of oven dry mass gives the water absorption.

Fig. 4 Rapid chloride permeability test set up

the total charge passed as per ASTM C1202-1997. Two specimens were tested for each concrete, and the results presented later are the average of the two readings. The test set up is as shown in Figure 4.

Fig. 5 Compressive strength plotted against the water to cement ratio (cement content fixed at 350 kg/m3)

Fig. 6 Compressive strength plotted against cement content (w/c fixed at 0.65)

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The water absorption was calculated as:

% Water absorption = (Ws-Wd)/Wd

Where

Ws is the weight of the specimen in fully saturated conditionWd is the weight of the oven dried specimen

The water absorption results presented in the discussions are the average of three readings.

Results

Compressive strength

The variation in compressive strength with water to cement ratio and cement content is plotted in Figures 5 and 6 respectively. As expected, the trends are good, with high regression coefficients in both the relationships.

Durability parameters

The results of the tests on durability parameters are presented in this section. The results are compared against the suggested values in the standards. The durability index parameters (water sorptivity and oxygen permeability) are compared with the values suggested by Ronne and Alexander (2002), while the chloride permeability values are compared against the criteria given in ASTM C1202 (1997).

Fig. 7 Oxygen permeability index values for the different concretes (classifica-tion by Ronne and Alexander also shown)

Fig. 8 Water sorptivity values for the different concretes (classification by Ronne and Alexander also shown)

Fig. 9 Total charge passed in 6 hours for the different concretes (classification by ASTM C1202 also shown)

The oxygen permeability index (OPI) values for the different concretes are shown in Figure 7. The OPI values only marginally increase when the w/c is decreased from 0.65 to 0.50, but there is a significant increase in the OPI when w/c is decreased from 0.50 to 0.35. Similarly when the cement content is increased from 300 to 350 kg/m3 at a water to cement ratio of 0.65, there is a significant enhancement in the OPI, but for the increase from 350 to 400 kg/m3 in cement content, there is only marginal increase in the OPI. As per the criteria suggested by Ronne and Alexander (2002), only the 0.35 w/c concrete is classified as ‘Good’, while all the other concretes fall in the regime of ‘Very Poor’. With the exception of the concrete with 0.65 w/c and 300 kg/m3 cement content, there is little enhancement in OPI values of the other concretes from 7 to 28 days.

The results of water sorptivity are presented in Figure 8. Unlike the oxygen permeability results, for all concretes, there is significant reduction in the water sorptivity between 7 and 28 days. Additionally, there is a good degree of reduction in the sorptivity when w/c is decreased, as well as when the cement content is increased at a constant w/c. Apart from the concrete with w/c of 0.65 and cement content of 300 kg/m3, which is in the ‘Poor’ category, all other concretes are in the ‘Good’ range as per the classification

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by Ronne and Alexander (2002).

The total charge passed (Coulombs) for the different concretes in the rapid chloride permeability test is plotted in Figure 9. With the exception of the 0.35 w/c concrete that falls in the ‘Moderate’ permeability category, all other concretes are in the ‘High’ permeability category. Increase in w/c corresponds to a significant increase of the chloride permeability. On the other hand, the chloride permeability is only marginally reduced when cement content is increase from 300 to 350 kg/m3, while there is significant reduction from 350 to 400 kg/m3.

The results for surface water permeability, shown in Figure 10, reveal expected trends. Both w/c and cement content have significant impact on the surface water permeability. However, as standards are not available for comparison,

distinct judgements regarding the quality of the concretes cannot be made.

The results for water absorption test are presented in Figure 11. There is only marginal reduction in the absorption when the w/c is changed from 0.50 to 0.35, but there is a significant impact when the w/c is changed between 0.50 and 0.65. With increasing cement content, there is a decrease in the water absorption, in line with expectations.

The interrelationships between compressive strength and durability parameters, as well as amongst the durability parameters, were explored with the use of linear regression. The results of the regression are summarized in Table 3. Only those plots that generated a regression coefficient (R2) of greater than 0.8 are chosen for discussion.Fig. 10 Surface water permeability using Germann apparatus for the different

concretes

Fig. 11 Water absorption for the different concretes

Independent variable Response variable Proportionality R2

Compressive strength OPI Direct 0.546

Compressive strength Sorptivity Inverse 0.528

Compressive strength Charge Passed Inverse 0.898

Compressive strength Surface permeability Inverse 0.660

Compressive strength Water absorption Inverse 0.628

OPI Charge Passed Inverse 0.688

OPI Surface permeability Inverse 0.920

OPI Sorptivity Inverse 0.813

OPI Water absorption Inverse 0.752

Sorptivity Charge passed Direct 0.670

Sorptivity Surface permeability Direct 0.818

Sorptivity Water absorption Direct 0.892

Surface permeability Water absorption Direct 0.873

Table 3. (Linear) Regression coefficients for different combinations of parameters

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The summary provided in Table 3 clearly indicates that only the charge passed in the rapid chloride test is well correlated with the compressive strength. Thus, the compressive strength itself cannot be used as a sole parameter to indirectly express the durability of the concrete. If performance specifications are to become a reality, there should be a better use of one or more of the other durability parameters. It is evident from the results that the OPI, sorptivity and surface permeability of concrete are well correlated. Furthermore, the sorptivity seems to be well correlated to the water absorption too. Since the sorptivity, surface permeability, and water absorption are all related to the penetration of water into concrete, a suitable durability specification, on the lines of the durability index methodology from South Africa, should include the resistance of concrete to penetration against gases, water and chlorides. Thus, from the range of tests, given the good correlations between the methods, the following tests are proposed in addition to compressive strength to fulfill the requirements of performance specifications for concrete:

(a) Oxygen permeability(b) Water sorptivity, and (c) Rapid chloride permeability

New exposure classifications for concrete construction

As presented earlier, the Indian Code IS 456-2000 for plain and reinforced cement concrete specifies five exposure classifications namely, mild, moderate, severe, very severe and extreme, which seem to be arbitrary and prescriptive in nature regarding durability requirements. Further, they do not necessarily address the relevant mechanisms of concrete deterioration adequately. Given the importance of environmental effects on concrete service life and

performance, it is necessary to have detailed classifications catering to all exposure conditions, which address the appropriate mechanisms of deterioration.

A thorough investigation was undertaken by IIT Madras to evaluate the advances in systems of environmental classification from across the world. These included the EN, BS, AS, and ACI classifications, along with the new proposals from Kulkarni (2009). Different construction projects executed by CPWD across India were then analysed in the light of these new classification systems. The analysis showed several shortcomings with the current IS456, which need to be rectified. These have been analysed in detail the paper by Saravanan and Santhanam (2012). The excerpts from the new classification system are presented here.

1. Exposure classification for concrete exposed to air borne chloride but not in direct contact with sea water: Coastal (Based on distance from coast)

The distance from coast for this classification has been divided into three portions in Table 4.

1) Portion up to 10 km from coast has been classified as D1.2) Portion beyond 10 and up to 50 km classified as D2.3) Portion beyond 50 km classified as D3.

IS 456-2000 does not specify the limit of distance from the seawater front to be treated as coast. Since CPWD Specifications categorically state that distances up to 10 km be treated as coast, the same is proposed in the above table with classification as D1. The portion beyond 50 km is treated as inland and classified as D3 (On the lines of AS 3600-2009). The portion between 10 and 50 km is classified as D2.

Most international codes prescribe concrete grade of M 40 for the type of exposure classification represented here as D1. Hence M 40 is suggested as the concrete grade for D1. The water to cement ratio and cement content are suggested in line with other codes.

Comparing the above values with the requirements provided

Distance from coastExposure

ClassificationMin. grade of concrete

Min. Cementi-tious Content

(kg/m3)

Max. w/cm

Min. clear cover (mm) Remarks

Up to 10 km from coast D1 M 40 360 0.40 50Based on CPWD Specifications (Distance up to 10 km to be treated as coast)

Beyond 10 km and up to 50 km D2 M 30 320 0.45 40

Beyond 50 km (Inland) D3 M 25 300 0.50 30Based on AS3600 (Distance be-yond 50 km to be treated as inland)

Table 4. Exposure classification for corrosion in concrete due to air borne chloride but concrete not in direct contact with sea water

Exposure classifica-

tion

Min. grade of concrete

Min. Ce-mentitious

Content (kg/m3)

Max. w/cm

Min. clear cover (mm)

SW1 M 40 360 0.40 50

SW2 M 50 400 0.40 75

Table 5. Exposure classification for concrete in contact with sea water

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for the “Very Severe” classification in IRC 112:2011, it can be seen that the category D1 matches all the IRC requirements with the exception of the cementitious content, which is lower by 20 kg/m3. However, the IRC criterion of 380 kg/m3 cementitious content is for a suggested design life of 100 years, whereas design life criteria have not clearly been incorporated in the suggested classifications in this paper. In the extreme case, the values suggested may conform to the design life of 50 years, in line with the BS8500 recommendations.

2. Exposure classification for concrete in contact with sea water

As the severity of exposure conditions is going to be different for concrete completely immersed in sea water and the concrete in spray / tidal zone, two classifications of SW1 and SW2 are proposed for concrete in contact with sea water as shown in Table 5.

SW1: Concrete completely immersed in sea water

SW2: Concrete in spray / tidal zone.

In this case, SW1 would fall under the “Severe” category of IRC 112, while SW2 would come under “Extreme” category. The maximum w/c suggested for SW2 is 0.40 as against the value of0.35 suggested by IRC112. However, the higher strength requirement suggested here would make up for the difference in w/c. Cementitious content and clear cover are the same in both cases. SW1 in Table 4 is more stringent than the requirements for “Severe” condition in IRC 112.

3.Exposure classification for concrete exposed to sulphate

Since sulphate attack depends on the concentration of SO3 in the ground water and soil, the classification of concrete exposed to sulphate is categorized as S0, S1, and S2 based on the sulphate concentrations, as shown in Table 6. The minimum cement content, cement to be used, water / cement ratio are selected in line with other codes. An additional clause (S3) is included to account for Magnesium attack, in a magnesium sulphate environment, which is known to be more severe than other forms of

sulphate attack.

S0: No risk: SO3<0.2% (soil), <300 ppm (water)

S1: Moderate risk: SO3: 0.2% to 1.0% (soil), 300 to 2500 ppm (water)

S2: Severe risk: SO3>1% (soil), >2500 ppm (water)

S3: Severe risk with magnesium sulphate SO3>1% (soil), >2500 ppm (water)

Compared to the clauses pertaining to sulphates given in IS456:2000 and IRC 112:2011, the following aspects can be noted:

(i) Only three levels of severity are considered here as compared to five levels in IS 456 to maintain simplicity. An additional clause for magnesium sulphate has been added to address the issue of magnesium attack.

(ii) OPC in combination with slag and silica fume as mineral admixtures is permitted for even the higher levels of severity, as opposed to IS 456 where OPC is not permitted for sulphate contents in soil above 0.5%.

(iii) Concrete grade and cover requirements have been built in as part of the table, rather than providing a separate table for cover as in IS 456.

(iv) Supersulphated cement has not been included here as it is not readily available in the Indian market

4.Exposure classification for corrosion in concrete due to carbonation

The effect of carbonation on concrete is dependent on

Exposure clas-sification


Min. Cementitious Content (kg/m3) Max. w/cm Min. clear cover (mm) Type of cement

S0 M 25 300 0.50 30 OPC

S1 M35 340 0.45 40SRC, PPC, OPC with slag or

silica fume.

S2 M50 400 0.40 50SRC, PPC, OPC with slag or

silica fume.

S3 M50 400 0.40 50 SRC, PPC

Table 6. Exposure classification for concrete exposed to sulphate

Exposure classifica-

tion


Min. Ce-mentitious

Content (kg/m3)

Max. w/cmMin. clear

cover (mm)

C0 M25 300 0.50 30

C1 M30 320 0.45 40

C2 M34 340 0.40 40

Table 7. Exposure classification for corrosion in concrete due to carbonation

Concrete


many factors like humidity level, wetness of concrete etc., hence three classifications are proposed as shown in Table 7.

CO : No risk of carbonation (i.e.) concrete which will remain dry during its service life or concrete permanently submerged in water.

C1:Moderate to high humidity (i.e.) concrete inside buildings with moderate to high humidity, exposed concrete sheltered from rain.

C2:Cyclic wet and dry (i.e.) concrete exposed to rain and not sheltered.

In the above table, the category C0 corresponds to the “Moderate” condition in IRC 112:2011. With the exception of the grade of concrete, all other requirements are more stringent in the IRC document. On the other hand, categories C1 and C2 correspond to the “Very Severe” and “Extreme” environments as per IRC 112. On the whole, the prescriptive values suggested in Table 6 are significantly different from the very stringent guidelines presented in IRC 112 with respect to carbonation. Possibly, a greater understanding of this issue is required with respect to carbonation in Indian service environments. This can be made possible by long term studies, which are not available currently in India.

Summary

Some ideas on the efforts needed for the development of performance specifications for concrete in India are presented in this paper. The shortcomings of the current codes with respect to durability are highlighted briefly and aspects that need to be worked on are discussed. The shift towards performance specifications in India can be a reality given the availability of a vast amount of data from work conducted worldwide. However, a step by step approach would need to be followed to bring this into practice. Some part of the work carried out at IIT Madras to achieve these objectives are presented in the final part of the paper.

References

1. AASHTO TP64-03, “Standard Method of Test for Prediction of Chloride Penetration in Hydraulic Cement Concrete by the Rapid Migration Procedure,” American Association of State Highway and Transportation Officials, Washington, D.C., 2003.

2. ACI Committee 318, “Building Code Requirements for Structural Concrete (ACI 318-05) and Commentary (318R-05),” American Concrete Institute, Farmington Hills, MI, 2005, 430 pp.

3. AS 3600-2001, “Concrete Structures,” incorporating Amendments No. 1 and 2, Standards Australia, Sydney, NSW, 2001, 165 pp.

4. AS 1379-1997, “Specification and Supply of Concrete,” incorporating Amendment 1-2000, Standards Australia,

Sydney, NSW, 2000, 35 pp.5. ASTM C457 / C457M - 10a (2010) Standard Test Method for

Microscopical Determination of Parameters of the Air-Void System in Hardened Concrete, ASTM, Philadelphia, USA.

6. ASTM C1202 (2010), Standard Test Method for Electrical Indication of Concretes Ability to Resist Chloride Ion Penetration, ASTM, Philadelphia, USA.

7. ASTM C1556 - 04 (2004) Standard Test Method for Determining the Apparent Chloride Diffusion Coefficient of Cementitious Mixtures by Bulk Diffusion, ASTM, Philadelphia, USA.

8. ASTM C1585 - 04e1 (2004) Standard Test Method for Measurement of Rate of Absorption of Water by Hydraulic-Cement Concretes, ASTM, Philadelphia, USA.

9. Alexander, M.G., Ballim, Y. and Mackechnie, J.R. (1999), Research Monograph No. 4, Department of Civil Engineering, University of Cape Town, 33pp.

10. Alexander, M.G., Mackechnie, J.R. and Ballim, Y. (2001), ‘Use of durability indexes to achieve durable cover concrete in reinforced concrete structures’, Chapter, Materials Science of Concrete, Vol. VI, Ed. J. P. Skalny and S. Mindess, American Ceramic Society, Westerville, pp. 483 – 511.

11. Ballim, Y. (1991), A low cost falling head permeameter for measuring concrete gas permeability. Concrete Beton, Vol. 61, pp. 13-18.

12. Bickley, J. A., Hooton, R. D., and Hover, K. C. (2006), Performance Specifications for Durable Concrete, Concrete International, No. 9, pp. 51 – 57.

13. CAN/CSA-A23.1-04/A23.2-04, “Concrete Materials and Methods of Concrete Construction/Methods of Test and Standard Practices for Concrete,” Canadian Standards Association, Toronto, 2004, 516 pp.

14. Day, K. W. (2005) Prescriptive on Prescriptions, Concrete International, No. 7, pp. 27 – 30.

15. EN 206 (2000), Concrete – Part I: Specification, Performance, Production and Conformity.

16. IRC 112 (2011), Code of Practice for Concrete Road Bridges, Indian Roads Congress, New Delhi.

17. IS 456 (2000), Plain and Reinforced Concrete – Code of Practice (Fourth Revision), Bureau of Indian Standards, New Delhi.

18. Kulkarni, V. R. (2009), Exposure classes for designing durable concrete, Indian Concrete Journal, Vol. 83, No. 3, pp. 23 – 43.

19. Lobo, C., Lemay, L., and Obla, K. (2005) Performance-Based Specifications for Concrete, The Indian Concrete Journal, Vol. 79, No. 12, pp. 13 – 17.

20. Ronne P., Alexander M. G. (2002), ‘Quantifying the effects of steam curing on concrete durability’, Proceedings, Concrete for the 21st century: Progress through innovation, Midrand, South Africa.

21. RILEM Report 040 (2007), Non-Destructive Evaluation of the Penetrability and Thickness of the Concrete Cover - State-of-the-Art Report of RILEM Technical Committee 189-NEC, R. Torrent and L. Fernandez Luco (Eds.), RILEM, France.

22. Saravanan, R., and Santhanam, M. (2012), Environmental Exposure Classifications for Concrete Construction – A Relook, Indian Concrete Journal, Vol. 85.

23. Taylor, P. (2004) Performance-Based Specifications for Concrete, Concrete International, No. 8, pp. 91 – 93.

Concrete

Publishers Note: This paper was presented at the One Day National Colloquium on Concrete Construction for Coastal Conditions Causes, Concerns and Challenges ( 7-Cs) Held in Kochi, Kerala.The Masterbuilder was the official Media Partner for the above event.

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