A review on fly ash characteristics – Towards promoting high volume utilization in developing sustainable concrete

This paper presents a mix design method for ultra-high performance concrete (UHPC) prepared with high-volume supplementary cementitious materials and conventional concrete sand. The method involves the optimization of binder combinations to enhance packing density, compressive strength, and rheological properties. The water-to-cementitious materials ratio is then determined for pastes prepared with the selected binders. The sand gradation is optimized using the modified Andreasen and Andersen packing model to achieve maximum packing density. The binder-to-sand volume ratio is then determined based on the void content, required lubrication paste volume, and compressive strength. The optimum fiber volume is selected based on flowability and flexural performance. The high-range water reducer dosage and w/cm are then adjusted according to the targeted mini-slump flow and compressive strength. Finally, the optimized UHPC mix designs are evaluated to determine key properties that are relevant to the intended application. This mix design approach was applied to develop cost-effective UHPC materials. The results indicate that the optimized UHPC can develop 28-days compressive strength of 125 MPa under standard curing condition and 168–178 MPa by heat curing for 1 days Such mixtures have unit cost per compressive strength at 28 days of 4.1–4.5 $/m3/MPa under standard curing.

Optimization and performance of cost-effective ultra-high performance concrete

A newly developed wet packing method has been applied to measure the packing densities of blended fine aggregates (each a mixture of fine aggregates of different sizes) and mortars (each a mixture of cement and blended fine aggregate) under the wet condition, with or without superplasticizer added, and with or without compaction applied. For the blended fine aggregates, the conventional dry packing method has also been employed to measure their packing densities under the dry condition and the results show that the packing density of fine aggregate is generally higher under wet condition than under dry condition. For both the blended fine aggregates and mortars, the measured packing densities have been compared to the predicted packing densities by two existing packing models. Good agreement between the measured and predicted packing densities has been achieved with the mean absolute error being 2.1% for the blended aggregates and 1.1% for the mortars. This is the first time that the packing densities of mortar samples are directly measured and compared to predictions by packing models to verify the applicability of the wet packing method and the packing models.

Packing density measurement and modelling of fine aggregate and mortar

The interrelationship between surface chemistry and rheology in alkali activated slag paste

In previous studies on the mortar portion of concrete, it has been found that the water film thickness (WFT) and paste film thickness (PFT) have major effects on the performance of mortar. The present study aims to extend the concepts of WFT and PFT to concrete. For this aim, a number of concrete mixes with various paste volumes, water/cement ratios and fine to total aggregate ratios were produced for slump, flow, strength and packing density measurements, and from the results, the combined effects of WFT and PFT on the deformability, flowability and strength of concrete were studied. It was found that whilst the WFT is the key factor governing the above properties of concrete, the PFT also has significant effects and thus is an important factor to be considered in concrete mix design. Lastly, based on the test results, two design charts for concrete mix design were produced.

Concrete mix design based on water film thickness and paste film thickness

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.

H. H. C. Wong

Papers

Rheology of Cement Paste: Role of Excess Water to Solid Surface Area Ratio