The microstructural response of recycled concrete after high temperature exposition and rapid cooling

The recycled concrete is increasingly used in buildings. Lowered properties are claimed from the replacement of natural aggregates with recycled concrete aggregates. However, this reduc- tion depends on the quality of the source concrete and the dosage. This latter should be kept below 50 %, in order to achieve the main mechanical properties, such as compressive strength and modulus of elasticity. In this manner, the cementitious material may be adequate to be used buildings. In the case the old source concrete exhibits a high quality, e. g. compressive strength above 60 Mpa, du- rability parameters can also be attained and the recycled concrete can be used for infrastructures. On the other hand, the behaviour to fire and the cooling process of recycled concrete still needs a detail clarification. The water chemically bond within the old mortar may largely influence the per- formance during fire. Concretes were prepared with a different cement dosage and water to cement ratio. They were crushed and the recycled concrete aggregates were aged up to 2 years. Recycled concrete with a natural aggregate replacement of 25% were prepared. The recycled concretes were o exposed to 500 C air or water quenched. The air coling promoted the formation of fine cracks with the microstructure within the old mortar or along the old mortar-new cementitious matrix interface. The rapid water quenching promoted a general widening of the crack to macrocracking, a partial detachment of the cementitious matrix along both the RCA and NA-cement-based matrix interface. Not rarely grain pull outs and a crack branching was seen in the water cooled specimens. For this latter rapid cooling procedure, the aggregates, particularly the Si-bearing aggregates showed an in- creased expansion and cracking a s compared to the air cooling procedure. A denser microstructure with a lower water to cement ratio and a higher cement dosage created a more susceptible micro- structure with respect to the internal thermal stresses, that causes a widening of the cracking, rather than an increase in the microcracking frequency.