AIR-VOID STABILITY IN FRESH SELF-CONSOLIDATING. CONCRETES INCORPORATING RICE HUSK ASH
Department of Civil Engineering, University of Waterloo,
200 University Avenue West, Waterloo, Ontario, Canada N2L 3G1
This paper presents the results of experimental study on air-void stability in fresh self-consolidating concretes. Two series of self-consolidating concrete were undertaken for conducting laboratory tests. Each series of concrete included three different fresh mixtures. The air-void stability in fresh concretes was investigated with respect to post-mixing and agitation. The air content of fresh concretes was determined at various test stages and adjusted considering aggregate correction factors. The flowing ability of the fresh concretes was also examined with regard to slump and slump flow. The entire testing period involved four stages extended to 60 and 90 minutes for series 1 and 2, respectively. Test results reveal that the slump and slump flow of the concrete mixtures were consistent in all test stages, and the loss of air content was minimal. The maximum loss of air content over the period of 60 and 90 minutes was less than 1.0%. Rice husk ash did not affect the air-void stability in fresh concretes. However, it increased the demand for high-range water reducer and air-entraining admixture. The overall test results indicate that the air-void stability in all fresh self-consolidating concretes was satisfactory.
Keywords: Air content, Air-void stability, Post-mixing, Rice husk ash, Self-consolidating concrete.
Self-consolidating concrete (SCC) is a flowing concrete that spreads through congested reinforcement, fills every corner of the formwork and achieves consolidation under its own weight (Khayat 1999). In order to provide better durability performance and extended service life in freezing and thawing environments, SCC must contain an appropriate air-void system. For this, an adequate amount of entrained air-voids with proper specific surface and spacing factor should be retained in SCC. Usually, limits on volume of air-voids or air content are specified although the role of spacing factor is significant. This is because air content can be determined more easily and quickly than spacing factor. Canadian Standards Association (CSA) has specified various ranges of air content between 3 and 9% depending on maximum aggregate size and exposure conditions (CSA A23.1, 2004). These ranges of air content are recommended to create an adequate air-void system required for freeze-thaw and scaling resistance. However, when the specifications do not enforce any given air content but rather a spacing factor, an air content of 6% is usually suggested (Lessard et al. 1995). This air content generally maintains a spacing factor less than 230 pm if an effective air-entraining admixture is used in concrete mixture.
Air-voids are widely used for improving the freeze-thaw durability of concrete. The entrained air – voids improve the frost resistance of concrete in freeze-thaw environments, and thereby increase the service life of concrete structures (Cohen et al. 1992, Hayakawa et al. 1994, Siebel 1989). Although the mechanism of concrete deterioration due to freezing and thawing is still a subject of debate, the need for entrained air-voids has become indisputable. In general, the network of entrained air-voids offsets the dilating pressure posed by freezing water, and thus improves the performance of concrete
Corresponding author: e-mail. jswest@uwaterloo. ca
M. Pandey et al. (eds), Advances in Engineering Structures, Mechanics & Construction, 129-138. © 2006 Springer. Printed in the Netherlands.
in freezing and thawing environments (Neville 1996, Chatterji 2003). The dosage of air-entraining admixture is the most important parameter that controls the air-void system in concretes. A sufficient dosage of air-entraining admixture should be added to the fresh mixture to create the required volume of air-voids in hardened SCC. However, determining the correct dosage is not very straightforward. There are numerous factors such as mixture proportions, aggregate grading, cement composition, type of high-range water reducer, type and composition of supplementary cementing materials, quality of mixing water, mixing or placing methods, and temperature etc., that might affect air-entrainment, and therefore achieving the required air content in hardened concrete becomes much more difficult (ACI Committee 201, 2001; Du and Folliard 2005, Pigeon 1994). Nevertheless, the proper air-void system must be maintained in concrete to ensure a good resistance to freezing and thawing. Proper air-void system means that the entrained air-voids remain stable until the concrete is set, and becomes permanent in the hardened paste. This is particularly important for SCC, as the presence of high-range water reducer tends to destabilize the entrained air-bubbles during transport and placement of concrete (Saucier et al. 1990, Khayat and Assaad 2002). Many research reports indicate that high-range water reducers can cause some loss of air content during intermittent agitation (Jana et al. 2005, Johnston 1994). This is perhaps attributed to the production of greater large-size bubbles, which could easily disappear with time. Also, the large dosage of high-range water reducer could induce excessive fluidity and segregation, and thus may cause some loss of entrained air. Consequently, the air-void system in hardened concrete could be affected, and the freeze-thaw durability would be reduced.
Instable air-voids affect the air present in concrete, and thus air-void stability also influences other concrete properties such as strength and porosity. The air content of concrete is reduced due to the loss of instable air-voids. As a result, the total porosity of concrete is decreased and the concrete gains greater strength. In general, the average gain of compressive strength can be 3 to 5% for each percentage loss of air content (Neville 1996). However, the increase in compressive strength does not produce any significant benefit for air-entrained concrete since the desired level of compressive strength is already considered in mixture design. Instead, the loss of air content results in a significant reduction in freeze-thaw durability that cannot be overcome in any way. Therefore, the air-void stability in fresh SCC is significantly more important for freeze-thaw durability than strength.
Comprehensive studies have been conducted on air-void stability of low to medium slump concretes in presence of high-range water reducer (Baalbaki and Aitcin 1994, Pigeon et al. 1989, Saucier et al. 1990). These studies report the effects of high-range water reducer, supplementary cementing materials, and cement-admixture compatibility on air-void stability in normal and high strength concretes with some contradictory results. In addition, Baekmark et al. (1994) investigated the air-void stability of medium-slump concretes with respect to post-mixing, re-dosing of high-range water reducer and pumping operation. They observed that the air-void stability in concrete is greatly affected by the type of air-entraining admixture. While conducting the air content test in all of the aforementioned studies, the concretes were consolidated by external means, which can affect the air – void system. External means of consolidation, including vibration, was found to produce a detrimental effect on quality of air-void system in air-entrained concrete (Stark 1986). Some air-voids can be lost during consolidation with rodding or vibration. Therefore, the air content obtained from an air-meter test may not represent the actual air content of the parent concrete mixture. This discrepancy is eliminated in SCC since it does not require any external means of consolidation. Still the air-void stability problem could occur in SCC due to other factors such as excessive fluidity and segregation, and destabilization caused by HRWR. Yet very few studies have been carried out to investigate the air-void stability in SCC.
Bouzoubaa and Lachemi (2001), and Lachemi et al. (2003) developed air-entrained SCC and examined various fresh properties including air content. However, they did not study the air-void stability in SCC. Recently, Khayat (2000), and Khayat and Assaad (2002) investigated the air-void stability of SCC. They showed that the air-void stability could be ensured in SCC by a proper mixture composition including a suitable combination of chemical admixtures. An increase in the amount of fine material at a low water-binder ratio or the use of viscosity-modifying admixture can secure the appropriate air-void system during transport, placement and setting of SCC. However, none of the above studies investigated the effect of rice husk ash (RHA). The present study produced two series of SCC incorporating RHA and examined the air-void stability in fresh mixtures with respect to effects of post mixing and agitation. The effects were observed through subsequent determination of air content at different test stages.