Prediction of Chloride Permeability of High Performance Concrete

Transcription

1 Prediction of Chloride Permeability of High Performance Concrete M. Iqbal KHAN Saleh ALSAYED Assistant Professor Professor King Saud University King Saud University Riyadh 1141, KSA Riyadh 1141, KSA Summary This paper reports the results of an investigation of chloride ion penetration for high performance concrete. Concrete was prepared incorporating pulverised fuel ash (PFA) and silica fume (SF) with various water-binder ratios. Chloride ion penetration was measured at various ages using rapid chloride permeability test in accordance with ASTM C1-91. Based on experimentally obtained results, isoresponse contours for chloride permeability were developed showing the interactive and optimized effect between the various parameters investigated. The results show that the inclusion of PFA and SF reduced chloride permeability of concrete. Keywords: Chloride ion penetration; high-performance concrete, pulverised fuel ash, silica fume 1. Introduction Reinforced concrete structures when exposed to the environment, chloride ion penetration is considered a major cause of corrosion of reinforcing bars. The chloride ions destroy the natural passivity of steel reinforcement and promote the formation of corrosion products which exert tensile forces on the concrete cover and is responsible for causing delamination and spalling to the cover of concrete. Since conventional concretes often fail to prevent the intrusion of moisture and aggressive ions adequately, special concretes with low permeability are needed. This problem can be solved by providing a barrier which can prevent or reduce the ingress of chloride ions to the steel surface. This barrier may either be on the steel, as galvanised or epoxy coated bars, or in the concrete itself [1]. In order to meet this challenge high performance concrete mixes are being developed. The need for enhancing durability performance of concrete is driving the search for the development of highperformance concrete. The use of supplementary cementing materials such as silica fume (SF) and pulverised fuel ash (PFA) have been reported to increase the resistance of concrete to deterioration by aggressive chemicals such as chlorides [,3]. The potential synergy between SF and PFA needs to be investigated in the context of achieving an optimum balance for the development of high performance concrete. This paper reports the results of the chloride resistance of the binary and ternary blended systems investigated and prediction models for chloride permeability.. Experimental Programme.1 Materials Ordinary Portland cement (OPC) complying with BS 1: 1991, PFA complying with BS 389: Part 1: 1993 and SF supplied by ELKEM Chemicals, UK were used throughout the investigation. The SF was obtained in slurry form with solids to water ratio of 5/5 wt/wt. A sulphonated naphthalene formaldehyde condensate superplasticizer (HRWR) was used. Fine aggregate (Derbyshire quarry sand) and coarse aggregate (uncrushed gravel) of 1 mm nominal size, were used. The fine aggregate was of medium grading according to BS 88:199. Both the fine and coarse aggregates were air-dried before use.

2 . Mix proportions The PFA was used at,, 3, and 4% cement replacement levels. To these blends, 5, 1 and 15% of SF were incorporated. Cement replacement was carried out on a weight to weight basis. A water-binder ratio of.7 was used for the main group of mixes. Two other series of mixes were cast with w/b ratios of.4 and.5. The slump for all the mixes investigated was maintained at 15 ± 1 mm. This was achieved through the use of a superplasticizer. It is worthwhile to mention here that the water contents of the superplasticizer and slurry were taken into account when calculating the batch weights for mixing..3 Sample preparation Cylinders of 1 mm diameter 5 mm for the determination of chloride permeability measurements were cast. All the specimens were cast and compacted in accordance with BS After casting, the samples were covered under damp burlap and polyethylene sheets for 4 hours. The samples were demoulded the following day and then immediately kept in a mist room at ± o C and 98± % RH prior to testing. The specimens for the measurement of chloride permeability were taken out of the curing environment at the required testing age. In order to achieve maximum water saturation, for the chloride permeability test, the specimens were vacuum saturated as per the procedure prescribed in RILEM recommendations CPC-11.3: 1984 [4]..4 Testing procedure Chloride ion penetration was measured at various ages using the rapid chloride permeability test in accordance with AASHTO T 77 (recently adopted as ASTM C 1-91). This test does not offer diffusion constant, but rather an index, which has been found useful in comparative studies [1,3]. The total electrical charge passed, in Coulombs, during 6 hours was related to chloride ion penetration and expressed as the chloride permeability index. A ranking of chloride penetrability, based on charge passed in Coulombs, has been proposed and is shown in Table 1. Table 1 Chloride ion penetrability Chloride Ion Penetrability Charge Passed (Coulombs) High >4, Moderate, 4, Low 1,, Very Low 1 1, Negligible <1 The rapid chloride permeability apparatus originally developed by Whiting [1] and later automated by Cabrera and Lynsdale [5] was used in this investigation. The schematic diagram of the experimental set-up is shown in Fig. 1.

3 Vo ltmeter Power supply, 6V dc, 1A + - Multi-processor unit Precision shunts Computer control unit + - OH Cl Chloride permeability cells Fig. 1 Schematic diagram of rapid chloride permeability experimental set-up 3. Results and Discussion 3.1 Interaction of PFA and Prediction models for chloride permeability of concrete at various ages were developed using the Minitab [6]. Using the experimentally obtained results, these models are based on the quadratic response surface model and permitted the calculation of the isoresponse curves from the parameters under study over the experimental domain and the optimization of their effects. The observation of the responsive variable f (x) is measured at combinations of values of variables x 1 and x using the following model: C ip ( n ) β 1x1 x 11x1 x 1 x1x Concrete specimen = (1) where C ip( n) is chloride permeability (Coulombs) at age n days; x 1 is PFA as partial cement replacement (% by wt.); x is SF as partial cement replacement (% by wt.); β, β 1, β,..., β 1 are the coefficients of the model. The coefficients of the models for 7, 8, 9 and 18 days are shown in Table. Statistically insignificant terms have been excluded from these equations and the equations were plotted as isoresponse contours for prediction purposes as shown in Fig.. Table Coefficients for equations 1 Age Coefficients R (days) β β 1 β β 11 β β

4 SF content, % SF content, % days 4 8 days days 18 days Fig. Isoresponse contours for chloride permeability (Coulombs) of concrete at various ages, w/b ratio of.7 The change in chloride permeability of concrete at various ages caused by the interactive effects of PFA and SF contents is demonstrated in Fig.. The incorporation of both PFA and SF decreased the chloride permeability, in comparison with the control mix, at all ages investigated. The results indicate that permeability values reduced gradually with increased curing age from 7 to 9 days; beyond 9 days the reductions were negligible. Both the increase in PFA and in SF levels were associated with reductions in permeability values. The reductions in permeability as a result of increasing the SF replacement level up to 1% were far greater than those associated with increasing the PFA content, at 9 and 18 days. At these ages, the increase in the SF level enabled the permeability to be reduced to "negligible" levels (in accordance with the rating prescribed in Table 1), whilst this was not possible with PFA. At early ages (7 and 8 days), however, and with SF levels > 7.5%, the increase in PFA content up to 4% resulted in significant reductions in the permeability index with values dropping to 5 Coulombs (i.e. negligible ) at 8 days. Increasing the SF levels above 1% seems to give no benefit in the long run with regards to permeability reductions at early age. Similar trends in results were obtained for porosity of paste systems published elsewhere [7]. The results demonstrate the value of ternary blends over binary blends, especially concerning early age permeability; for example, if "negligible" chloride permeability is desired from as early as 7 days, then ternary blended systems containing 9-13% SF along with 35% to 4% PFA are required. If the "negligible" chloride permeability was only required at later age (i.e.>9 days) then either a ternary blended system or a binary blended system based on OPC and 1% SF can be used.

5 3. Influence of w/b The models for chloride permeability as influenced by w/b and PFA content for concrete containing and 1% SF, are represented by equations. The coefficients for the equations are shown in Tables 3. C ip ( n ) β 1x1 x3 11x1 x3 1 x1x3 = () where x 1 is PFA as partial cement replacement (% by wt.); x 3 is w/b 1 Table 3 Coefficients for equation SF level Age Coefficients R (days) β β 1 β β 11 β β 1 % % The isoresponse contours for chloride permeability for and 1% SF, at various ages are shown in Figs. 3 and 4, respectively. These figures demonstrate the influence of w/b ratio and PFA content on the chloride permeability of concrete. Fig. 3 shows the significant influence of w/b on chloride permeability. At 7 days, the chloride permeability of the OPC control mix increased from to 9 Coulombs as a result of increasing the w/b from.5 to.5. As the concrete was cured, the permeability values were reduced but this was more significant for the high w/b mixes. The incorporation of PFA and the increase in its level of replacement resulted in further reductions in permeability at all ages, with its effect being more pronounced for high w/b mixes. As SF was incorporated at 1%, a significant reduction in permeability was exhibited (Fig. 4), and the whole range of results shown in Fig. 3 was shifted towards smaller values. The effect of w/b was still in evidence, but the effect of curing up to 8 days became more significant, affecting all the range of w/b ratios, in comparison to the % SF mixes (Fig. 3). Curing beyond 8 days (for 1% SF mixes) did not result in further reductions in permeability. The effect of PFA and its level of replacement in the 1% SF mixes, results in slight reductions in permeability but this, similar to curing, was restricted to ages up to 8 days beyond which the isoresponse curves became flat.

6 days 8 8 days days 18 days Fig. 3 Isoresponse contours for chloride permeability (Coulombs) influenced by PFA and w/b ratio (with % SF) at various ages.55 8 days 5 9 days days days Fig. 4 Isoresponse contours for chloride permeability (Coulombs) influenced by PFA and w/b ratio (with 1% SF) at various ages

7 The results presented here, suggest that there is an interaction between PFA and SF, with their level of replacement and the age of curing influencing their effect on permeability. The lower chloride permeability values are as a result of micro-filling and pore refinement of these systems incorporating PFA and SF [8]. 4. Conclusions Based on the results obtained in this investigation, the following conclusions are drawn: Ternary blends enabled chloride permeability values to reduce to negligible levels at early age, with contributions from both PFA and SF. SF inclusion, by up to 1% replacement level, significantly reduced chloride permeability for all levels of PFA replacements. Above 1% SF, the reductions in permeability were marginal. The incorporation of PFA resulted in a slight reduction in the chloride permeability values, in comparison with that observed with SF, especially at later ages. For low w/b mixes with 1% SF, an optimum level of PFA replacement, in the range of 15-%, was found to yield the lowest oxygen permeability values measured. The prediction models and isoresponse contours generated in this investigation can provide reasonable predictions for chloride permeability of concrete. Reference [1] WHITING D., Rapid Determination of the Chloride Permeability of Concrete, FHWA Report FHWA/RD-81/119, Federal Highway Administration, Washington D C, USA, [] MALHOTRA V.M., and MEHTA P.K., Pozzolanic and Cementitious Materials Advances in Concrete Technology, Vol. I, Gordon and Breach, Netherlands, [3] OZYILDIRIM C., Rapid Chloride Permeability Testing of Silica Fume Concrete, Cement, Concrete, and Aggregates, Vol.16, 1994, pp [4] RILEM CPC-11.3, Absorption of Water by Immersion Under Vacuum, Materials and Structures, No.11, 1984, pp [5] J. CABRERA G., and LYNSDALE C. J., Measurement of Chloride Permeability in Superplasticizer Ordinary Portland Cement and Pozzolanic Cement Mortars, Proceedings of International Conference on Measurements and Testing in Civil Engineering, RILEM, Vol. 1, 1988, pp [6] RYAN B., Minitab Handbook, Belmont, USA, [7] KHAN M.I., LYNSDALE C.J., and WALDRON P., Porosity and Strength of PFA/SF/OPC Ternary Blended Paste, Cement and Concrete Research, Vol. 3,, pp [8] KHAN M.I., LYNSDALE C.J., and CHOO B.S., Pore Structure of High Strength Cement Mortar Containing PFA and Microsilica, Sustainable Development and Concrete Technology, ACI, Detroit, 1.