Property development and cracking tendency in hardening concrete: Effect of cement type and fly ash content.FA 3 Technical performance. SP 3.1 Crackfree concrete structures

Abstract

The concrete mix design influence the risk of thermal cracking in the hardening phase. Here the effect of binder composition has been investigated, where the variables are cement type and fly ash dosage. The fly ash was added during concrete mixing. Seven concretes (fc28 = 60 - 80 MPa) with a water-tobinder ratio of 0.40 (silica fume not used) have been tested with regard to properties relevant for thermal cracking: Hydration heat, free deformation, mechanical properties and restraint stresses have been measured at 20 oC and semi-adiabatic conditions (relevant for a 1 m thick wall). Based on compressive strength tests the activation energy has been determined. 1-dimensional stress calculations were performed and a relative ranking of the concretes with regard to the tendency of through-cracking was made.

The results show that among the three cement types the crack index varies around 15%. There is a systematic reduction of the crack index with fly ash dosage; about 20% lower crack index for the concrete with 35% fly ash compared to the reference without fly ash.