Fillers to Improve Passing Ability of Concrete

Ho J.C.M. (Orcid ID: 0000-0002-2755-907X) Fillers to improve passing ability of concrete M.H. Lai1, L. Hanzic2 and J.C.M. Ho3 1Department of Civil Engineering, Guangzhou University, 510006, PRC 2,3School of Civil Engineering, The University of Queensland, QLD 4072, Australia Abstract Concrete possessing high passing ability needs to be flowable and cohesive. Hence, passing ability cannot be improved by solely adding superplasticiser, which increases both flowability and segregation of concrete simultaneously. Decreasing the maximum size of aggregates so that concrete segregates at lower cohesiveness is a possible but undesirable way as it narrows the aggregates’ grading and decrease dimensional stability of concrete. With the same maximum size of aggregates, passing ability can be improved by raising the concurrent flowability-segregation envelope of concrete. In this paper, fly ash and silica fume (cementitious fillers) and limestone (inert filler) were selected to replace cement partially and subsequently the passing ability of concrete was studied. From the results, it was evident that when either type of fillers were used, the passing ability and maximum limits of flowability and segregation achieved simultaneously increase. It is because these fillers are finer than cement that provides better filling effect to increase packing density and excess water leading to better flowability. Concurrently, the cohesiveness of concrete also increases as the content of fine particles increases. These allow concrete to hold the coarse aggregates more firmly when passing through narrow gaps, after which the concrete will keep flowing rapidly. Keywords: Fly ash; Limestone; Passing ability; Segregation; Slump-Flow; Silica fume 1 PhD, Associate Professor, Guangzhou University. 2 PhD, IZS; Post-doctoral Fellow, The University of Queensland 3 PhD, MHKIE, MIEAust, CPEng, NER, MIStructE, CEng; Senior Lecturer (Corresponding author, email: [email protected]), The University of Queensland This is the author manuscript accepted for publication and has undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. Please cite this article as doi: 10.1002/suco.201800047 This article is protected by copyright. All rights reserved. - 2 - This article is protected by copyright. All rights reserved. 1. Introduction High passing ability is considered one of the essential performance criteria of structural concrete that enables it to pass through narrow gaps of reinforcement and reach the corners of formwork. To achieve high passing ability, concrete needs to be highly flowable to overcome viscous force during flow. This can be achieved by using chemical admixture such as poly-carboxylate based superplasticiser (SP) to better disperse fine particles in concrete. Consequently, concrete flowability can be increased at constant water-to-cementitious materials ratio (W/CM), and concrete strength. Alternatively, it implies less water is needed to produce concrete with similar flowability because of the better dispersion of fine particles and hence higher concrete strength can be achieved. Thus, SP can increase simultaneously the maximum design limits of strength and flowability achieved. However, use of SP can also decrease the plastic viscosity of cement powder paste (and concrete) and increase segregation. It impairs the passing ability of concrete particularly at high dosage. In a segregated concrete mix, the cement powder paste or mortar often flows in the front leaving the coarse aggregate behind to block the subsequent flow of concrete due to its lack of cohesiveness. A mitigation method is to decrease the maximum size of aggregates, however, it is undesirable as it narrows the particles size distribution of aggregates, decreases wet packing density (Wong and Kwan 2008a; Kwan and Wong 2008a) and dimensional stability of concrete (Li and Kwan 2013). To increase the passing ability while keeping the same maximum size and proportion of aggregates, the rheology of concrete and cement powder paste, should be optimised (Wong and Kwan 2008b; Kwan and Fung 2011) to increase the maximum limits of flowability and segregation that can be achieved simultaneously. From the above, it is evident that in order to increase the passing ability of concrete, the flowability should be increased and segregation be decreased (or cohesiveness increased). The design of such concrete mix is not straightforward as - 3 - This article is protected by copyright. All rights reserved. flowability and segregation are two contradictory performance attributes of concrete because any change in the design mix that improves one of these attributes will actually undermine another. For instance, adding water or SP increases the flowability but also increases segregation. Adding fine materials decreases segregation but it decreases flowability. Moreover, shear thickening of cement paste (Barnes 1989; Cyr et al. 2000; Yahia 2011; Felekoglu 2014; Jed et al. 2017) and concrete (Barnes 1989; Feys et al. 2008a, 2008b; 2009) can occurs when passing through the narrow gap of reinforcement in concrete with SP. Various studies on concrete’s passing ability have been carried out by different researchers in the field of cement paste and concrete. Ng et al. (2006) reported that the passing ability of concrete increases as cohesiveness increases and as aggregate content or size decreases. Passing ability can also be improved by having coarse aggregate content not exceeding that of fine aggregate. Nguyen et al. (2006) proposed a theoretical analysis method of the L-box test to determine the concrete height ratio at the gate and end of horizontal channel by considering the force equilibrium of a sample volume of concrete at stoppage, which was verified by test results. Roussel et al. (2006) showed that passing ability is closely related to the blocking parameter which is a matter of probability. A simple dimensionless parameter was proposed for predicting the blocking probability of concrete and verified by test results. Domone (2006) reviewed the case studies of self-compacting concrete in 11 years and summarised some median values of the key concrete mix proportion (i.e. paste, aggregate, powder, water and SP) in order to have L-box ratio not less than 0.8. Roussel et al. (2006) proposed a single fluid computation model for concrete flow passing obstacles. Soneebi et al. (2007) found that passing ability of concrete as indicated by L-Box test depends on the water, SP dosage and coarse aggregate content, the effect of which can be predicted by the proposed statistical model. Safiuddin et al. (2014) indicated that a minimum L-box ratio of 0.8 and segregation ratio not more than 20% can be achieved by replacing an optimal 20% cement by palm oil fuel ash by weight. Ling and Kwan (2015) proposed - 4 - This article is protected by copyright. All rights reserved. to increase passing ability of concrete by increasing fine content of aggregates using ground sand rather than by increasing paste volume which decreased the dimensional stability of concrete. It is evident from the above that the passing ability of concrete can generally be improved by increasing fine aggregates content in the mix, replacing partial cement with fly ash and decrease the maximum size of coarse aggregates. In order to understand the effect of concrete rheology on passing ability, the fundamental scientific principle that governs the behaviour of fresh concrete, i.e. wet packing density (Wong and Kwan 2008b) and excess water (Kwan and Wong 2008b), should be understood. In this model, the effect of water on the particle packing within concrete is similar to that outlined in a paper published by Iveson et al. (2001). To summarise, it stated that effect of water is to bring together fine particles by forming liquid bridges to overcome strong inter-particle forces. The packing of particles increases until it reaches a capillary state where all the interstitial voids are filled with water. Subsequent additional of water will impose dispersion effect to push the particle apart again. Accordingly, the maximum packing density is attained at the capillary state. When denser packing is achieved, the mechanical properties of hardened concrete improves because of the better structural integrity (He et al. 2016; Li et al. 2017). More importantly, the workability also improves because less water is trapped within the interstitial void and for a given water ratio, more excess water will be available to lubricate the concrete mix. This was first proposed by Powers (1968) but was not successfully implemented because the approach of measuring packing density is not complete (DeLarrard and Sedran 1994; Richard and Cheyrezy 1995; Sedran et al. 1996; DeLarrard 1999). It was because the effect of water and/or SP was not considered and only dry packing density was measured. Wong and Kwan (2008a) proposed the first use of “Wet Packing Theory” to account for the effect of water/SP in the packing of fine powder and concrete. A scientific experimental method was proposed for determining the wet packing density of cement paste with water and SP considered (Wong and Kwan 2008a, 2008c; Kwan and Wong 2008a, 2008b) which is very successful in designing the - 5 - This article is protected by copyright. All rights reserved. strength and flowability of cement paste (Wong and Kwan 2008c), mortar (Kwan et al. 2010) and concrete (Li and Kwan 2015, Hanzic and Ho 2017). One of the important conclusions they get is that substitution of partial cement by fillers can improve the rheology of cement paste/mortar/concrete effectively. In this paper, cementitious filler (i.e. Fly ash (FA) and Silica fume (SF)) and inert limestone filler (LS) are used to replace cement partially in concrete. Four groups of concrete were tested for their passing ability, which have various compositions of powder: (1) Cement only; (2) Cement + FA (15&30%); (3) Cement + SF (10&15%); (4) Cement + LS (15-35%). From the results, it was found that substitution of either type of fillers increases passing ability of concrete as seen by the L-box and J-ring tests.

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