Michael Anson

Residual strength and pore structure of high-strength concrete and normal strength concrete after exposure to high temperatures

Abstract Based on normal strength concrete (NSC) and high-strength concrete (HSC), with compressive strengths of 39, 76, and 94 MPa respectively, damage to concrete under high temperatures was identified. After exposure to temperatures up to 1200 °C, compressive strength and tensile splitting strength were determined. The pore structure in HSC and in NSC was also investigated. Results show that HSC lost its mechanical strength in a manner similar to that of NSC. The range between 400 and 800 °C was critical to the strength loss. High temperatures have a coarsening effect on the microstructure of both HSC and NSC. On the whole HSC and NSC suffered damage to almost the same degree, although HSC appeared to suffer a greater worsening of the permeability-related durability.

Impact of high temperature on PFA concrete

After being subjected to high temperatures, the residual properties of pulverized fly ash (PFA) concrete have been investigated. Both mechanical and durability properties of concrete were tested on concretes made with different water to binder ratios and PFA contents. Microscopic techniques were then employed and the pore structure and microhardness values of hardened cement paste (hcp) were determined. The results of rapid chloride diffusion tests revealed that concrete durability deterioration commences after exposure to temperatures which are lower than those at which compressive strength deterioration commences. The rise in compressive strength, which occurs after exposure to 250°C, may be largely due to the hardening of cement paste caused by drying and the further hydration of cementitious materials. The simultaneous loss of durability, however, can be explained by a weakened transition zone between hcp and aggregate, and by the concurrent coarsening of the hcp pore structure. When PFA is included, an improvement of fire resistance as characterized by the residual compressive strength was observed, and this relative improvement over non-PFA concrete was the most pronounced for maximum exposure temperatures of 450°C and 650°C.

Measurement of the performance of ready mixed concreting resources as data for system simulation

Concrete is a very important construction industry material, and this is especially true in Hong Kong. The efficient organization of the supply of concrete from ready mixed concrete plants is beneficial to both concrete companies and contractors. It is also essential for an efficient construction industry, since a very high propor1 tion of the in situ concrete in Hong Kong is supplied ready mixed from an off-site plant. Whether concrete companies in Hong Kong are using their resources well and are able to satisfactorily match the timing of their deliveries to construction site needs are questions discussed in this paper, based on data collected by the authors. The data are planned for use in validating a simulation model of ready mixed concrete plant operations in Hong Kong. The third author spent 20 weeks in 1999–2000 at concrete batching plants in Hong Kong, studying their operations. Data on a sample of 15 typical operational days at four plants, which included 295 pours on sites and 1677 truckmixer trips, are presented. Truckmixer journey times for delivering concrete and journey times for returning to plants and their distributions have been found. Queuing statistics for truckmixers on site waiting to be unloaded, placing times when unloading concrete, washing out and waiting times on sites after unloading, and delays in the continuous delivery of concrete are also described. Service has been studied in terms of the matching achieved between site delivery timing require1 ments and the need to use truckmixers efficiently. For the 295 pours, the relationship is shown between gaps in the supply of concrete to site and the extent to which truckmixers are bunched in a queue on site. Matching was found to be good for only 17% of the 295 pours, the variability in service received by different sites is considerable, and the importance of studying the truckmixer scheduling problem is highlighted, so that this variability might be reduced and extreme cases eliminated.

Comparison of the concreting productivities in Hong Kong and Beijing and a proposed comparison methodology

The ready mixed concrete (RMC) industry is an important sector of the construction industry in many places. Its level of development in terms of size and performance may be taken as an index representative of the development level of a particular construction industry taken as a whole. Therefore comparisons between different places are of interest, and of potential economic benefit. As part of a larger research study in Hong Kong on the productivity of concreting, data have been obtained on the RMC industries in Hong Kong (HK) and Beijing (BJ). Both are large cities at different stages of economic development. The study involved close observation in the early 1990s of 154 pours on building construction sites in HK and 38 days spent at 38 RMC plants, one day at each, together with 34 site pours and 20 days at RMC plants in BJ. Much detailed performance information has been derived, and a comparison methodology is proposed which could have general application in this industry. It is demonstrated that the RMC industry in HK is more productive than the industry in BJ.