Properties of concrete incorporating fly ash and recycled demolition waste

Transcription

1 Properties of concrete incorporating fly ash and recycled demolition waste K. Sagoe-Crentsil, A. Taylor and T. Brown Division of Building, Construction and Engineering, Commonwealth Scientific and Industrial Research Organisation (CSIRO), Melbourne, Australia Abstract High level replacement of natural virgin resources with secondary materials at all levels of construction provides a unique platform for the efficient management of the solid waste burden. This step, however, requires the development of binders that are tolerant to chemical and physical contaminants characteristic of the solid wastes. The properties of blended cements indicate that given appropriate mix designs, such binders can significantly enhance the properties of concrete and concrete products derived from demolition waste. Blended cements may comprise of Portland cement and industrial wastes, typically blast furnace slag, silica fume or fly ash from coal-fired power stations. Performance tests have therefore been carried out on the properties of fresh recycled aggregate concrete incorporating fly ash. The results indicate significant improvement in both concrete slump retention and setting characteristics. Also, concrete durability, including permeability and compressive strength, are marginally affected. These findings suggest that higher levels of fine and coarse recycled aggregate replacement in new concrete are achievable with the use of secondary cementitious materials for non-structural applications. Keywords: Concrete; aggregate; recycling; fly ash; durability; compressive strength; fines.

2 1 Introduction The effective use of recycled materials and products remains central to current efforts aimed at optimising available natural resources in building and construction. Globally, the prospects of replacing natural quarry products with recycled concrete aggregates (RCA) in premix concrete production, for specified applications, is rapidly gaining acceptance [ 11. This strategy offers clear economic and environmental benefits. In recent years, several studies have been conducted to demonstrate the viability of RCA as a source of quality construction material for use in concrete production [2, 31. Furthermore, with the fines fraction accounting for up to 40% of the total weight of crushed rubble, there is an obvious need to use as much of this material as possible. However, the implications of RCA properties on mix design and concrete performance appear to be much less established. This paper evaluates both rheology and the performance of RCA concrete containing significant loadings of RCA fines and fly ash. The ultimate goal, firstly, is to ensure satisfactory use of RCA and, secondly, that concrete structures are designed and constructed in such a way as to serve as a vital source of valuable recyclable material. 2 Experimental Two separate batches of RCA fines and a single source of commercial-grade coarse RCA were used in the concrete mixes. The mix proportions were designed to provide concrete mixes with a nominal Portland cement content of 325 kg/m3. The fly ash component was batched as a supplementary addition to the fine sand content rather than as a binder supplement. Fly ash was added to mixes to assess possible improvements in fresh concrete cohesion and workability. The water/binder ratio of all mixes were adjusted to achieve comparable consistency and, hence, equal nominal slump of 80-t-10 mm. Reduction in the water requirement of mixes was attained by using a water-reducing admixture at the manufacturer s recommended dosage level. Specimens were cast for compressive strength and shrinkage and cured as required under standard conditions. Table 1 shows the mix details of concrete specimens prepared, with the reference mix designated C2108A. Two sets of specimens were investigated - the first to evaluate the effect of natural fine sand replacement with recycled fines, and the second to evaluate the role of fly ash addition on concrete properties. I 2.1 RCA properties The properties and requirements of crushed concrete for use as concrete aggregate are similar to those of natural aggregates. This includes quality and grading requirements, as well as cleanliness and limits on deleterious matter that could affect the setting and durability of the concrete. Compliance of RCA to strength requirements, flakiness and permissible fines content was closely monitored to ensure satisfactory aggregate performance. Tolerance limits of chloride and sulfate are, however, yet to be established, as well as the potential for alkali-silica reaction. Table 2 shows results of sieve analyses of RCA fines used in the study.

3 Mix Cement W/B Natural Natural RCA RCA RCA FlY designation loading coarse fines coarse fines 1 fines 2 ash wm3> v> 0 v> 0 m V)0 * 0 v> 0 ( /> 0 C2108A C2108B C0712B CO712C Fly ash trials Cl 101A Cl 101B Cl 101c Cl 101D Table 1. Mix details of concrete specimens Fine aggregates Percentage of weight passing through sieve Max. size (mm) pm Table 2. Sieve analysis of RCA fines 3 Results and Discussion Significant variations in the fresh and hardened properties of concrete were observed with changes in the proportion and type of RCA fines introduced to the mix. The extent of bleeding of fresh RCA concrete mixes was significantly lower compared to control mixes of equivalent workability, although setting characteristics remained comparable. Fresh RCA concrete properties and concrete consistency, as determined by the slump test and setting characteristics, have been investigated and found to be generally comparable to control mixes. For instance, laboratory test results on the setting behaviour of RCA based on the penetration resistance tests (Proctor needle) give values of initial and final set as 3 12 and 427 minutes respectively, corresponding to 3 18 and 406 minutes for control specimens made with natural aggregates. Whilst changes in fresh concrete properties were relatively less pronounced, corresponding modifications to the hardened concrete were more readily measurable. Results obtained from concrete compressive strength and shrinkage characteristics have therefore been used to index the impact of RCA proportioning on concrete performance. 3.1 Compressive strength Compared to the control mix made from natural materials, a reduction in the compressive strength of RCA concretes was observed at all ages. The compressive strength of concrete mixes up to 91 days are shown in Figure 1. Generally, the rate of strength gain is similar for all concrete mixes regardless of the source and type of aggregate used. The absolute strengths of the different mixes are, however, dependent on both type and

4 C2108A 0 C2108B I C0712B 0 CO712C FOG CURING TIME (days) 91 Figure 1. Compressive strength of control and RCA concrete \ quantity of aggregate incorporated in each mix. This trend in strength development is further dependent on the proportion of RCA fines. As shown in Figure 1, a 12% strength reduction is observed at 91 days when natural fines are replaced with RCA fines 1, i.e. mixes C2 108A and C2 108B. Only marginal differences in strength were observed between concretes made with RCA fines 2 and RCA fines 1. The compressive strength of concretes made with 100% coarse RCA, but containing 50% natural fines, 50% RCA fines, and 100% natural fines respectively were relatively similar. Previous tests, however, have shown that this difference can be as high as 40% [3]. This latter result suggests that the hardened properties of concrete may be significantly influenced by both quality and grading of RCA fines. The overall strength reduction of the RCA concretes range from about 13% for concretes containing coarse RCA and natural fines, to 32% for mixes containing only RCA materials. Such characteristic strength reductions are in line with published results [ 1,4]. This trend is to be expected given that the water demand of RCA concrete often exceeds the requirements of normal aggregate concrete, particularly for mixes including RCA fines, as shown in Table Correlation between RCA properties and concrete strength The strength of RCA concrete primarily depends on the strength of the cement paste and on the bond between the paste and aggregate. The strength of the aggregate also exerts some influence on the strength of the concrete [2], but this is nominal for most normal weight aggregates. The bond between RCA and the surrounding matrix is influenced by the mineral composition, cleanliness, surface texture, and particle size and shape of the aggregate. Given that cement paste normally bonds better to a rough surface than a smooth or friable surface, residual coatings of clay or carbonated cement dust which adhere to RCA after mixing will alter bond properties. Surface coatings on RCA generally tends to increase the quantity of fines in the mix and hence water requirements. Furthermore, adhering coatings impair the bond between

5 aggregate and the cement matrix, and consequently the load transfer characteristics, resulting in overall strength reduction. The quantity of residual fines in the mix, generated either from abrasion of the RCA during the mix or due to improper grading, has the potential to increase water demand. Therefore, within limits, mix proportions should be adjusted to compensate for potential changes in the grading of RCA fines during the mixing process. 3.3 Partial replacement of RCA fines with fly ash Figure 2 shows plots of the compressive strength development of concrete containing different proportions of RCA fines and additional loadings of fly ash. At 10% additional fly ash loading to the fines content, only a nominal 7% drop in the 9 l-day compressive strength is obtained compared to the control made from totally natural materials. At 20% fly ash loading, the corresponding strength reduction is of the order of 25%. The pozzolanic properties of fly ash significantly contribute to the later-age strength properties of the concrete, as indicated by the relatively rapid rise in concrete strength between 28 and 9 1 days (Figure 2). Additionally, other durability properties of RCA concrete are likely to be enhanced with the introduction of fly ash. In particular, reduced permeability may be expected [4]. The role of RCA fines in concrete strength development requires further examination with partial replacement of RCA fines with fly ash. In general, significant improvements can be achieved with the inclusion of fly ash as part replacement of the binder. Thus, when fly ash is included as part of the total binder, the optimum loading characteristics would be expected to increase significantly beyond 10%. 4 Shrinkage The dimensional stability of RCA concrete is critical to satisfactory design in most areas of concrete construction. Figure 3 shows the dependence of concrete drying shrinkage z 5 40 E (3 Z 2 30 l- CJI 9 g 20 2 CL > 10 0 l.4/ P I I Ol 7 0 C2108A I CIIOIA o CIIOIB 0 C1101C A C1101D 1 I I FOG CURING TIME (days) Figure 2. Compressive strength of RCA concrete containing fly ash

6 T IJJ E T co I I I I I DRYING TIME (days) Figure 3. Drying shrinkage of RCA concretes on RCA fines loading and, hence, water demand of the respective mixes. Compared to the control, shrinkage values recorded for concrete made with RCA coarse and fines at 91 days were 50% higher than control. Part replacement of RCA fines with natural fines marginally affected the shrinkage levels. These findings highlight the sensitivity of concrete drying shrinkage to RCA fines loading in the concrete mix. It is evident from the results obtained that RCA fines have a significant influence on the drying shrinkage of concrete. This influence largely depends on the extent to which aggregates alter the water content of the concrete. Other RCA properties such as surface texture, grading, particle shape, proportion of fines and maximum aggregate size contribute to different extents to the drying shrinkage of concrete. It is essential also to monitor the absorption of RCA, since excessive absorption rates may produce concrete with very high shrinkage, especially for aggregates contaminated with clay fines. Figure 4 shows drying shrinkage plots of the control mix and mixes incorporating fly ash. Comparing the shrinkage values of Figure 3 to those of Figure 4 indicates that variations in mix constituents have negligible impact on the overall magnitude of shrinkage for mixes containing RCA fines. It must be recalled that only marginal differences exists in the water to binder ratios of the mixes (Table 2). This trend suggests that while fly ash inclusion in mixes significantly alters later-age compressive strength, there appears to be no corresponding beneficial effect with regard to the drying shrinkage of RCA concrete. From the shrinkage plots, it is further apparent that significantly higher shrinkage values are obtained for the RCA mixes with or without fly ash. The higher shrinkage values may be linked to the water content of RCA mixes, owing to the presence of excess fines. Thus, it is essential to establish an optimum fines content. Exclusion of RCA fines in concrete mixes has also been proposed in order to control excessive shrinkage [l]. However, the exact role of RCA fines on concrete performance may require further study, specifically the implications of grading and quality of such fines on concrete drying shrinkage and durability.

7 0 C2108A 0 C2112B 0 CO CO712C W L z z I u-l I I 1 I I DRYING TIME (days) Figure 4. Drying shrinkage for RCA concrete containing fly ash 5 Conclusions It is concluded from this study that although several fresh and hardened properties of concrete are altered with the use of RCA in concrete, the potential exists to offset some of these deficiencies through proper selection and grading of recycled materials and concrete mix design. The results further indicate that the compressive strength of RCA concrete may be enhanced with the addition of about 10% fly ash to the concrete mix. No corresponding benefits in concrete drying shrinkage were observed. The extent of bleeding of fresh RCA concrete mixes was significantly lower compared to control mixes of equivalent workability, although setting characteristics remained comparable. However, owing to higher absorption rates of RCA fines, and consequently higher concrete drying shrinkage values, the use of RCA fines should only proceed with due caution and perhaps be limited to specified applications. 6 References 1. Hansen, T.C. (1992) Recycling of Demolished Concrete and Masonry, RILEM Report No. 6, E&FN Spon, London. 2. Frondistou-Yannas, Y.A. (1980) Recycled concrete as new aggregates, in Progress in Concrete Technology, CANMET, Energy Mines and Resources Canada, Ottawa, pp Sagoe-Crentsil, K.K., Brown, T., Mak, S.L. and Taylor, A.H. (1996) Engineering properties and performance of concrete made with recycled construction aggregates, Proc. National Symposium on the Use of Recycled Materials in Engineering and Construction, Sydney, Australia, May 1996, pp Mehta, P.K. (1986) Concrete, Structures, Properties and Materials, Prentice Hall Inc., Englewood Cliffs, New Jersey, pp

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