This thesis investigated the use of fly ash in concrete by establishing reliable and accurate test data on the properties of concrete incorporating different fly ash content. In addition, the study focussed on typical uses by local government authorities in pathways, such as footpaths and bicycle ways, and also looked at environmental impacts, incorporating durability and construction constraints in the life cycle costing issues. The aims of the project were:
• To obtain experimental data on mechanical properties and durability characteristics of fly ash concrete containing 23-50% fly ash with a 28-day compressive strength of 25MPa;
• To assess the effect of curing on the properties of concrete incorporating different percentages of fly ash; and
• To draw conclusions as to the cost effectiveness of concrete incorporating large amounts of fly ash by comparing life cycle assessments that include durability, construction and environmental factors.
This investigation of the high volume fly ash concrete was for the purpose of extending its application in footpaths and bicycle ways. Several concrete mixes using fly ash at levels ranging between 23-50% as a partial replacement of Portland cement were investigated. All concrete mixes had a 28-day compressive strength of 25MPa, and were applicable to the footpaths and bicycle ways construction. The effect of fly ash on the mechanical properties of the concrete mixes was determined under standard moist curing and ambient conditions. The tested properties included setting time, compressive strength, splitting tensile strength, flexural tensile strength, drying shrinkage and abrasion resistance.
The test apparatus, procedures and testing data are described in detail. Test results are reported and compared. In addition to the experimental work, a simple life cycle assessment (LCA) and life cycle cost analysis (LCCA) were undertaken to evaluate the environmental impact and cost effectiveness of using high volume fly ash in concrete pavement construction. The energy consumption and CO2 emissions were compared between high volume fly ash concrete and the typical commercial concrete in the environmental life cycle assessment. The life cycle cost analysis included the costs of initial material cost, construction, rehabilitation, and residual values in the whole life cycle period of pavement structures.
According to the analysis of the experimental results, it is feasible to produce concrete with fly ash replacement of up to 50%, without significantly altering the concrete properties in the hardened state. However, high volume fly ash concrete with 50% fly ash replacement retarded the setting time by up to 6.5 hours, which should be considered in LCCA. High-volume fly ash concrete gained higher ultimate strength under standard moist curing conditions. In typical pavement construction, it is often difficult for the design engineer to ensure that adequate curing, as per the construction specification, occurs. Indeed, evidence suggests that in many applications curing of concrete pavements does not occur at all. However, without adequate curing, no strength development was observed after 28 days for high volume fly ash concrete. The 28-day abrasion resistance is very similar for all mixes with various fly ash percentages, but use of a curing compound increased the wear and tear properties of fly ash concrete to some extent.
Based on the information provided by the local industry and the results of the environmental life cycle assessment and life cycle cost analyses, the following conclusions are summarized:
• From the environmental life cycle assessment, 50% fly ash concrete is the best alternate for pavement design with 20% energy and CO2 savings compared to typical concrete;
• However, in terms of life cycle cost analysis, when variability is considered in the inputs (cost, setting time, service life, etc.), the typical 23% fly ash concrete would be the most cost effective material in footpath and bicycle ways applications; and
• The results of LCCA are highly dependent on the setting time of concrete. When using setting accelerator admixture in high volume fly ash concrete, the cost effectiveness is dependent on the efficiency of the admixture to reduce setting time. Without an increase in construction cost, the high volume fly ash concrete would have a similar life cycle cost to typical concrete.