Influence of water absorption of coarse recycled concrete aggregates on the performance of concrete.ing. Joseph Miquel1, ing. Boehme Luc1, prof. dr. Ir. Vandewalle Lucie2

1 KU LEUVEN, Department of Civil Engineering, Technology Cluster Construction @Kulab, Faculty of Engineering Technology, Belgium2 KU LEUVEN, Department of Civil Engineering, Division of Building Materials, Faculty of Engineering Science

Recycled Concrete

Introduction:

The “ValReCon20 program” has proven the possibility to replace up to 100% of the coarse virgin aggregates by coarse recycled concrete aggregates (CRCA) in concrete of strength class C25/30. At this moment most uncertainties about recycled concrete concern durability in aggressive environments. This PHD researches destructive mechanisms that use water as transportation for harmful reagents.

The physical phenomenon that describes water transportation is known as “Water Permeability” This is often linked to Porosity (the total amount of voids). This study takes a closer look at how water absorption of aggregates affect the porosity of recycled concrete.

Coarse Recycled Concrete Aggregates (CRCA)

Recycled concrete from building demolition is a heterogeneous material with many different properties. One of the important properties of CRCA, needed to design concrete mixes, is the water absorption. High water absorption leads to a higher water demand to obtain the same workability and, once the excess water evaporates, to a higher porosity.

The total amount of attached mortar is one of the important differences between CRCA and natural aggregates. This parameter affect the density, water absorption and overall strength of CRCA.

An other important parameter is the particle size. Small particles contain more mortar, which leads to a higher average water absorption. Small particles reach the phase of saturation faster than coarse aggregates. This is important during the mixing process.

Hydration rate of recycled concrete aggregates 4/8Hydration rate of recycled concrete aggregates 8/20

Influence on Mixture Design

The variation due to the heterogeneous nature of CRCA makes it harder to predict the actual amount of water that will be absorbed by CRCA during the mixing process. The following results compares the effects of over- and underestimation of the water absorption of CRCA. Various properties are investigated. The results show large differences in workability and density of the fresh concrete, water absorption by submersion and capillarity of the hardened concrete, dynamic modus of elasticity, mechanical strength properties.

Influence density of concrete on water absorption

The use of CRCA leads to concrete with a lower density, due to the higher amount of pores. This part of the study links the density of concrete to parameters like water absorption by submersion and water absorption by capillarity. The results show that density of concrete can be used as a good indicator to predict the overall water absorption of concrete.

Density [kg/m³]

Rate of water absorption by capillarity in function of density

Rate of water absorption by submersion in function of density