Use of magnetic resonance imaging to study internal moist curing in concrete containing Saturated Lightweight Aggregate

This work is part of a much larger program to evaluate high performance concrete mixtures that can be used successfully in hot dry climates. In this research magnetic resonance imaging (MRI) was used to measure the effectiveness of extending the moist curing period by incorporating some saturated lightweight aggregates into a concrete mixture being placed in hot dry climatic conditions. A series of concrete mixtures were prepared and moist cured for either 0, 0.5, 1 or 3 days, or by using a curing compound, followed by air drying at 38°C and 40% relative humidity. To accomplish this, 11% by volume of the total aggregate content was replaced with lightweight aggregate. Type I white portland cement and quartz aggregate plus the lightweight aggregate were all selected for their low iron content to minimize adversely affecting the MRI measurements. The concrete mixtures were low strength concrete (W/C=0.60), selfconsolidating concrete (W/C=0.33 containing 30% fly ash), and high strength concrete (W/ C=0.30 containing 8% silica fume). Specimens prepared with these mixtures were cast in triplicate. After curing, the specimens were dried in one direction in an environmental chamber at 38oC and 40% relative humidity. As the specimens were drying, magnetic resonance imaging was used to determine the evaporable water distribution. After the drying period, the specimens were conditioned in an oven at 105oC and water absorption tests were undertaken to determine their sorptivity. The profiles obtained during drying indicated a reduced moisture loss with increasing length of moist curing. Also the use of saturated lightweight aggregate does not eliminate the need to provide some external moist curing for a reduced period of time. The results from water uptake experiments indicated that the addition of lightweight aggregate particles substantially increases the sorptivity in low strength concrete while it has only a marginal effect in both self-consolidating and high strength concrete, when compared to the same concrete mixtures containing only normal-weight aggregate.