Trevor Brown

Performance of concrete made with commercially produced coarse recycled concrete aggregate

Performance tests have been carried out for fresh and hardened properties of concrete made with commercially produced coarse recycled concrete aggregate and natural fine sand. Test results indicate that the difference between the characteristics of fresh and hardened recycled aggregate concrete and natural aggregate concrete is perhaps relatively narrower than reported for laboratory-crushed recycled aggregate concrete mixtures. For concrete without blast furnace slag having similar volumetric mixture proportions and workability, there was no difference at the 5% significance level in concrete compressive and tensile strengths of recycled concrete and control normal concrete made from natural basalt aggregate and fine sand. Water absorption rates and carbonation of recycled concrete and reference concrete were comparable for most applications. However, the abrasion loss of recycled aggregate concrete made with ordinary portland cement increased by about 12% compared to normal concrete, while the corresponding drying shrinkage was about 25% higher at 1 year. The ratio of splitting tensile strength to compressive strength was found to be in good agreement with established values derived for equivalent grade concretes made with normal-weight natural aggregates. One-year test results indicate that incremental improvements in durability characteristics can further be achieved with the use of blast furnace slag cement. Enhanced fresh and hardened concrete properties of the investigated recycled concrete aggregate as compared to aggregate derived from laboratory-crushed concrete arise primarily from improved aggregate grading and quality achievable in plant crushing operations.

Studies on the Effect of Atmospheric Oxygen Content on the Thermal Resistance of Intumescent, Fire-Retardant Coatings

Studies are conducted to measure the rate of thermal degradation and the thermal resistance of three intumescent fire-retardant coatings under atmospheres of varying oxygen content. The degradation trials are conducted using thermal gravimetric analysis and differential thermal analysis in an atmosphere of nitrogen or air. It has been found that a low oxygen content in the atmosphere significantly affects the rate of degradation of the char material at temperatures greater than 540°C. The global kinetics of the thermal degradation is modeled using three or four first-order parallel reactions. The degradation of char is modeled using the method of invariant kinetic parameters. The derived kinetic parameters are reported. The thermal resistance of the materials coated on steel plates is determined using a cone calorimeter with controlled oxygen content in the atmosphere. It has been found that the thermal resistance of two of the coatings has been strongly influenced by the oxygen content of the atmosphere; hence, this factor may be of importance when predicting the performance of intumescent materials in real fire scenarios.

Drying shrinkage and creep performance of geopolymer concrete

The creep behavior and drying shrinkage performance of fly ash geopolymer (GP) concrete mixtures have been investigated using equivalent grade 40-MPa ordinary Portland cement (OPC) concrete as the reference system. Drying shrinkage values, measured in accordance with AS 1012.13-1992, up to one year fell well below the nominal 700 μstrain limit, with GP concrete values typically less than 400 μstrain at 1 year. Variations in basic mean creep coefficient of concrete (Фcc.b), measured as the ratio of the creep strain to elastic strain for a specimen loaded at 28 days under constant stress of 0.4f c (f c – 28 day compressive strength) was monitored for up to 52 weeks. Values of Фcc.b obtained for steam-cured GP concrete was found to be of the order of 45% lower than corresponding OPC concrete. Based on these results, the effects of GP concrete mixtures on load-dependent (creep) deformations appear to be negligible. The paper further discusses factors that contribute to observed GP concrete creep deformation and drying shrinkage performance.

Medium to Long Term Engineering Properties and Performance of High-Strength Geopolymers for Structural Applications

The medium to long term engineering performance of high-strength geopolymer concrete systems are largely dependent on fluid ingress and the transport phenomena that govern permeability of structural members exposed to aggressive environments. For the purpose of analysing durability performance, both high pressure water and gas permeability testing of fly-ash geopolymer(GP) concretes have been assessed for samples cured under ambient and steam exposure conditions at 65OC. The observed mean permeability coefficient values for gas(k) and water(Kw) of steam-cured structural grade concrete was respectively 6.19E-17m2 at 300kPa gas pressure and 1.52E-10m/s at 525kPa water pressure. While mean gas permeability values were comparable to reference steam-cured ordinary Portland cement(OPC) systems, the corresponding water permeability coefficient data for geopolymer concrete was ten-fold higher. The transport properties of OPC concrete systems are typically governed by water-to-cement ratio and the degree of hydration which is linked to the level of porosity and pore interconnectivity. However, corresponding permeability of geopolymer concrete appears to be dictated by an inherent mesoporous capillary pore network structure for which transport properties appear to be partly dependent on mode of concrete curing. The Paper examines global implications of increased permeability and key durability parameters such as chloride diffusion, carbonation rates and steel reinforcement corrosion on long-term engineering and durability performance.