Combined effects of water film thickness and paste film thickness on rheology of mortar

Materials and Structures, Sep 2012

In the mortar portion of a concrete mix, the water must be more than sufficient to fill the voids between the solid particles of cement and fine aggregate whereas the paste volume must be more than sufficient to fill the voids between the solid particles of fine aggregate so that there will be excess water to form water films coating all the solid particles and excess paste to form paste films coating the fine aggregate particles. Hence, it may be postulated that the water film thickness (WFT) and the paste film thickness (PFT) have major effects on the rheology of mortar. In this study, the combined effects of WFT and PFT on the rheology, cohesiveness and adhesiveness of mortar were investigated by testing mortar samples with varying water, cement and aggregate contents. It was found that whilst the WFT is the single most important factor governing the rheology of mortar, the PFT also has significant effects. Particularly, the PFT has certain interesting effects on the cohesiveness and adhesiveness of mortar, which should be duly considered in mortar design.

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Combined effects of water film thickness and paste film thickness on rheology of mortar

A. K. H. Kwan 0 L. G. Li 0 0 A. K. H. Kwan (&) L. G. Li Department of Civil Engineering, The University of Hong Kong , Hong Kong, China In the mortar portion of a concrete mix, the water must be more than sufficient to fill the voids between the solid particles of cement and fine aggregate whereas the paste volume must be more than sufficient to fill the voids between the solid particles of fine aggregate so that there will be excess water to form water films coating all the solid particles and excess paste to form paste films coating the fine aggregate particles. Hence, it may be postulated that the water film thickness (WFT) and the paste film thickness (PFT) have major effects on the rheology of mortar. In this study, the combined effects of WFT and PFT on the rheology, cohesiveness and adhesiveness of mortar were investigated by testing mortar samples with varying water, cement and aggregate contents. It was found that whilst the WFT is the single most important factor governing the rheology of mortar, the PFT also has significant effects. Particularly, the PFT has certain interesting effects on the cohesiveness and adhesiveness of mortar, which should be duly considered in mortar design. 1 Introduction With the advent of modern chemical and mineral admixtures, it is now possible to produce many different types of high-performance concrete (HPC) with high performance in certain attributes at the fresh or hardened state [1]. However, the mix design of HPC, especially those containing many ingredients, is not an easy task. In this regard, it is proposed herein to develop a three-tier system for the mix design of HPC and concrete in general. The first tier of materials consists of the cementitious materials and microfillers of size smaller than or similar to cement. These materials will, after mixing with the water, form a paste for filling the voids in the next tier of materials. The second tier of materials consists of the fine aggregate particles and fillers of size smaller than or similar to fine aggregate. These materials will, after mixing with the paste, form a mortar for filling the voids in the next tier of materials. Finally, the third tier of materials consists of the coarse aggregate particles. With the concrete regarded as a three-tier system, the mix design may then be carried out by considering successively the paste and mortar portions of the concrete. This paper focuses on the mortar portion of concrete. While considering the mortar portion, it should be borne in mind that a layer of mortar, which has to be sufficiently thick and flowable, should be provided to coat every coarse aggregate particle [2] and the fresh properties of a concrete are closely related to the rheology of its mortar portion [3, 4]. For self-consolidating concrete (SCC), it has been suggested that the mortar portion should be designed first [5, 6]. Since a SCC has to flow a long distance and fill into far-reaching corners without segregation, the mortar portion has to have high flowability [36] and high cohesiveness [57]. Furthermore, the mortar portion needs to have high adhesiveness (the ability to adhere to solid surfaces) so as to avoid separation of the mortar from the coarse aggregate particles or in the case of mortar works from the substrate. However, the desired high flowability, cohesiveness and adhesiveness are not easy to achieve at the same time. One reason is that the addition of a superplasticizer (SP) to increase the flowability would substantially decrease the cohesiveness [8] and probably also the adhesiveness. Besides, although mortar with high adhesiveness has been used in concrete repair and brick works [911], a suitable test method for measuring the adhesiveness of mortar is still lacking. A number of studies have been carried out to identify the main factors affecting the rheology of mortar. Banfill [12] found that both the yield stress and viscosity of mortar decrease exponentially with the water content. In other studies, it has been found that the characteristics of the fine aggregate also have significant effects. For example, De Schutter and Poppe [13] showed that the water demand of a mortar is closely related to the packing density of the fine aggregate. Reddy and Gupta [14] found that generally a mortar made of a finer sand would need a higher water content for a given workability and explained that this is because of the larger solid surface area of the finer sand used. From these studies, it may be inferred that the main factors affecting the rheology of mortar are the water content, packing density and solid surface area of the solidwater mixture. At a smaller particle size scale, Kwan and Wong [15] demonstrated that blending of cement with condensed silica fume could increase the packing density, decrease the amount of water needed to fill the voids and thus increase the flowability of the paste formed. Hence, both the packing density of the fine aggregate and the packing densi (...truncated)


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A. K. H. Kwan, L. G. Li. Combined effects of water film thickness and paste film thickness on rheology of mortar, Materials and Structures, 2012, pp. 1359-1374, Volume 45, Issue 9, DOI: 10.1617/s11527-012-9837-y