Effects of packing density, excess water and solid surface area on flowability of cement paste

Although it has been postulated for many years that it is excess water rather than whole water that lubricates the cementitious materials and governs flowability of paste (excess water is water in excess of that needed to fill up voids), there has been no detailed investigations to study the actual effects of excess water. This was due to the lack of a suitable method for measuring the packing density and voids content of cementitious materials, To resolve the problem, the authors have recently developed a new wet packing method that was applied herein to measure the packing densities and voids contents of cementitious materials containing different amounts of cement, pulverised fuel ash and condensed silica fume. The flowability properties of the paste formed of the cementitious materials with different water contents were also measured and correlated to the excess water contents, each determined as water content minus voids content. The results revealed that whereas an improvement in packing density would increase excess water content, flowability is governed mainly by excess water to solid volume and excess water to solid surface area ratios.published_or_final_versio

Packing density of cementitious materials: Measurement and modelling

Packing density has great effect on the performance of a concrete mix. However, little research has been carried out on the packing density of cementitious materials owing to the lack of an established measurement method. Herein, a new method, called the wet packing method, is presented. With this method, the packing densities of blended cementitious materials, consisting of ordinary Portland cement (OPC), pulverised fuel ash (PFA) and condensed silica fume (CSF), were measured. The results verified the theory that the packing density could be significantly increased by blending two or even three cementitious materials together. Comparison between the measured results and the predicted values by three existing packing models, together with some additional tests, revealed that in the presence of a third-generation superplasticiser, the packing density of CSF is dependent on the lime content. When the lime-containing OPC and PFA contents are low, the CSF particles would flocculate and pack rather loosely, but at higher OPC and PFA contents, or with lime added, the CSF would pack to a higher density. With the effect of lime accounted for, very good agreement between the measured results and the predictions by the packing models was achieved.published_or_final_versio

Water film thickness, flowability and rheology of cement-sand mortar

It is well known that the fresh properties of cement paste and cement-sand mortar are governed mainly by three parameters: water content, packing density and solid surface area. However, these parameters vary simultaneously upon any change in the mix composition and hence the evaluation of their individual and combined effects has been a difficult task. Recently, the authors have found that the effects of these parameters on the flowability and rheological properties of cement paste may be evaluated in terms of the water film thickness (WFT) of the solid-water mixture. Herein, this concept of WFT is extended to cement-sand mortar made of sand with a maximum size of 1.18 mm. A number of mortar samples proportioned with different water contents and different sand gradings were produced for packing density, flowability and rheological properties measurements. From the results, the WFT of each sample was determined and the effects of the WFT on the flowability and rheological properties of the mortar were investigated. The good correlations between the flowability and rheological properties with the WFT revealed that the concept of WFT is applicable also to cement-sand mortar. © 2010 Thomas Telford Ltd.published_or_final_versio

Reducing drying shrinkage of concrete by treatment of aggregate

Drying shrinkage of concrete has been found to cause cracking, water leakage and other serviceability problems and is thus an important research topic. In early studies, it has been found that the shrinkage of concrete varies with the rock aggregate used. This is partly because the aggregate also shrinks and the shrinkage of aggregate is dependent on the type of rock from which the aggregate is derived. However, there have been few studies on the shrinkage of rock and how the shrinkage of aggregate would affect the shrinkage of concrete. In this study, the shrinkage of the granite rock in Hong Kong was measured. It has been found that the rock shrinks quite substantially upon drying and that its shrinkage is dependent on the initial moisture condition. Based on such observation, two alternative methods of treating the aggregate before use so as to reduce the shrinkage of concrete are proposed. Long-term shrinkage measurement of concrete made with untreated and treated aggregates revealed that the proposed methods of aggregate treatment, which are still rudimentary, can significantly reduce the shrinkage of concrete. © 2010 Thomas Telford Ltd.published_or_final_versio

Shrinkage of Hong Kong granite aggregate concrete

In the design of many concrete structures, it is important to assess the amount of shrinkage that could occur because shrinkage could lead to extensive cracking or substantial prestress loss. A number of shrinkage models have been developed all over the world but they differ widely from each other, indicating that the shrinkage of concrete is dependent on the characteristics of the materials used and the local conditions. Early experimental studies in Hong Kong suggested that the shrinkage of Hong Kong concrete is considerably larger than that estimated using shrinkage models developed elsewhere. This is probably due to the local granite aggregate used. In order to resolve this problem and develop a shrinkage model for Hong Kong, a comprehensive testing programme using fibre-optic strain gauges to measure the shrinkage strain was launched. The shrinkage measurement lasted over 3.5 years and covered concrete mixes with and without pulverised fuel ash added and with paste volume varying from 30 to 40%. © 2010 Thomas Telford Ltd.published_or_final_versio

An apparatus for studying spallation neutrons in the Aberdeen Tunnel laboratory

postprin

Packing density of cementitious materials: Part 2-packing and flow of OPC + PFA + CSF

The wet packing method developed in Part 1 [Wong HHC, Kwan AKH (2007) Packing density of cementitious materials: part 1 measurement using a wet packing method. Mater Struct (Paper No. MAAS3281)] has been successfully applied to measure the packing densities of cementitious materials containing ordinary Portland cement (OPC), pulverised fuel ash (PFA) and condensed silica fume (CSF). The test results for non-blended materials revealed that whilst the addition of a superplasticiser would always increase the packing densities of OPC and PFA, the addition of a polycarboxylate-based superplasticiser could decrease the packing density of CSF. On the other hand, the results for blended materials showed that the packing density could be improved by double blending OPC with either PFA or CSF, and further improved by triple blending OPC, PFA and CSF together in appropriate proportions. A maximum packing density of 0.752 has been achieved and a ternary packing density diagram for determining the mix proportions for maximum packing density has been produced. Furthermore, the positive influence of a higher packing density on cement paste rheology has been demonstrated using the mini-slump cone test. Based on these results, the concept of excess water ratio, which is the major factor governing the rheology of a paste, is introduced. © 2007 RILEM has copyright.link_to_subscribed_fulltex

Rheology of cement paste: Role of excess water to solid surface area ratio

Although many attempts have been made in previous research to identify the various parameters governing the rheology of cement paste, there has been little progress in evaluating the combined effects of these parameters. In this paper, a new parameter, the excess water to solid surface area ratio, is proposed to evaluate the combined effects of water content, packing density, and solid surface area on the rheological properties of cement paste. For the purpose of determining the value of this new parameter, a new wet packing method has been developed to measure the packing density of cementitious materials so that the voids content of the cementitious materials and the amount of excess water in the cement paste can be quantified. A number of cement paste samples containing different proportions of cement, pulverized fuel ash and condensed silica fume, and different water contents have been tested and their rheological properties, as measured by a rheometer, correlated to the new parameter. The correlation revealed that the excess water to solid surface area ratio, which serves as an indicative measure of the average water film thickness, is the single most important factor governing the rheology of cement paste. © 2008 ASCE.link_to_subscribed_fulltex