Roles of water, paste and mortar film thicknesses in performance of mortar and concrete

Due to increasingly stringent requirements on concrete performance and complexity of our infrastructures, concrete with not only high strength but also all-round high performance is called for. This puts forth the notion of the so-called high-performance concrete (HPC). However, one major hurdle in the development of HPC is that the technology is still based largely on empirical approaches. For further advancement of concrete technology into concrete science, it is advocated to adopt a more scientific approach, which can improve our understanding of concrete from a level of “know -how” to a level of “know-why”. The packing characteristics of solid particles have great influence on the performance of a concrete mix, but so far there is no generally accepted method of measurement. Herein, a new wet packing method was developed and applied to blended aggregates and concrete mixes. The results for blended aggregates revealed that whilst the packing density of coarse aggregate is only slightly higher under wet condition than dry condition, the packing density of a blended fine plus coarse aggregate is significantly higher under wet condition. Furthermore, the results for concrete mixes showed that the packing density is substantially higher and the filling effects of ultrafine supplementary cementitious materials are much better revealed under wet condition. Hence, the conventional dry packing method should be abandoned and replaced by the wet packing method. It is well known that the fresh properties of paste/mortar/concrete are governed mainly by the mix parameters: water content, packing density and solid surface area. However, these mix parameters vary simultaneously and therefore their individual and combined effects are difficult to evaluate. It has been found recently by others that the combined effects of these parameters may be evaluated in terms of the water film thickness (WFT), which has the physical meaning of the average thickness of water films coating the solid particles. Herein, it was proposed that besides the WFT, the paste film thickness (PFT) and mortar film thickness (MFT) should also have effects on the performance of mortar and concrete. Extensive tests and correlations of the measured properties of mortar to the WFT and PFT revealed that whilst the WFT is the single most important factor governing the properties of mortar, the PFT also has significant effects, especially on the cohesiveness and adhesiveness. Likewise, the results for concrete revealed that whilst the WFT and PFT have significant effects on the properties of concrete, the MFT also has significant effects, especially on the cohesiveness and passing ability. Based on the above findings, it may be concluded that the major factors governing the performance of concrete are the WFT, PFT and MFT. With the correlations of the various performance attributes of concrete to these factors so established, the door is open for the eventual development of a more scientific “three-tier mix design method” for HPC, by which the concrete mix is designed in three tiers: first the WFT of the paste portion, then the PFT of the mortar portion and finally the MFT of the concrete mix.published_or_final_versionCivil EngineeringDoctoralDoctor of Philosoph