International Concrete Abstracts Portal

Quality of concrete with recycled aggregate is generally lower than that of virgin aggregate. The main reason is that, recycled aggregate with its higher water absorption property has porous mortar matrix around than that the virgin aggregate and hence develops an inferior bond. In order to improve the quality of recycled aggregate concrete, an innovative method is proposed in this paper. High quality recycled aggregate concrete HiRAC can be obtained through a decompression and rapid release (DC-RR) procedure applied after normal mixing of concrete with recycled aggregate. Through the DC-RR procedure, the quality of transition zone between aggregate and cement matrix can be dramatically improved. In this paper, experimental studies are described on the effectiveness of DC-RR procedure on some of the mechanical and physical properties of recycled aggregate concrete. It was found that, by applying the DC-RR procedure, compressive strength of recycled aggregate concrete can be increased by about 20%, creep and carbonation depth can be reduced by about 20% and 30%, respectively.

The use of recycled-aggregate concrete is increasing faster than the development of appropriate design recommendations. This paper reports limited experimental data on the shear capacity of reinforced concrete beams recycled-aggregate. Twelve beams were tested to determine their diagonal cracking and ultimate shear capacities. The variables in the test program were shear-span/depth ratio a/d =1.5, 2.0, 3.0, and 4.0; aggregate types; and shear reinforcement ratio ps = 0, 0.089, 0.244, 0.507, and 0.823 percent. Six of the test beams had no web reinforcement and the other six had web reinforcement along the entire length of the beam. Test results indicate that the ACI Building Code predictions of Eq. (1 l-3) and (1 l-5) for recycled aggregate concretes are unconservative for beams with a tensile steel ratio of 1 .11 percent, and a ld ratios greater than 3.0.

Normal portland cement concrete is the most widely used material in the construction industry in Singapore. Under normal curing conditions, it can take one day or more before the concrete can be safely handled without damage. In precast production, it is desirable for the concrete to attain sufficient strength within a short period of time so that moulds and other resources can be used more efficiently. A revolutionary method of curing using microwave heating utilizes the internal energy dissipation associated with the excitation of molecular dipoles in electromagnetic fields. This method enables faster and more uniform heating and has been found to shorten the process time necessary to achieve high early strength. At present, the use of microwave curing on an industrial scale is still in its infancy. The Prefabrication Technology Centre in Singapore has developed the first prototype mechanised industrial microwave curing system in this region. This paper will discuss the use of this system for the production of ferrocement secondary roofing slabs on an industrial scale. It is believed that this technology has great potential to revolutionalise curing of precast concrete.

The speed of curing is often a critical issue in the manufacture of precast concrete elements. For some products, the curing cycle consumes up to 70% of the total production cycle. To improve the speed of production, heat curing is often used to accelerate the hardening of precast concrete. Conventional heating techniques rely on thermal conduction. Microwave energy offers a potential to increase the rate of bulk heating in precast concrete through its relatively deeper penetration, which allows quicker through-depth heating and maturing. Research on microwave curing of concrete has been ad hoc in the past and a wide range of issues remain unresolved. These encompass materials-microwave interactions, process design and control, hardware and logistics, as well as the impact of microwave curing on concrete properties. In this paper, progress on research on microwave curing is described with reference to work carried out at CSIRO. In particular, results from pilot-scale heating of slab-type elements are discussed in relation to heating characteristics, process control, set acceleration, strength development and process efficiency. Our results show that for the same bulk heating rates, microwave heating produces significantly lower temperature gradients when compared to steam heating. Using rapid curing cycles of less than six hours, compressive strengths in excess of 25 MPa can be achieved in high quality precast concrete. Doubling the bulk heating rate using microwaves does not result in any deterioration in near-surface quality as was the case with conventional steam heating.

The feasibility of using crushed post-consumer glass as a partial replacement of sand in concrete has been studied. To suppress the deleterious reaction between the alkali in cement and the silica in crushed post-consumer glass (ASR), a Class F fly ash was used in the experiment with the cement replacements of about 15, 30, and 45 percent by mass using a ratio of fly ash inclusion to cement replaced of about 1.25. Therefore, actual fly ash to total cementitious materials ratio was 18, 35, and 5 1 percent by mass. For each combination of cement and fly ash, 15%, 30%, and 45% volume of SSD sand were replaced with crushed glass. The compressive strength and splitting tensile strength of concrete were determined at specified ages for each mixture. Alkali silica reaction was evaluated according to ASTM C 1260 (Mortar Bar Method). Test results indicate that both compressive strength and splitting tensile strength of concrete decrease slightly with an increase in the replacement rate of sand with crushed glass. At lower replacement rates (less than 45%), the Class fly ash could only delay the onset of expansion, while with high amount of fly ash concrete was immune to ASR.