Hollowcore slab with alternative cementitious material for summer conditions

The stringent early-age strength requirements for production of prestressed hollowcore (HC) slabs have restricted the use of alternative cementitious materials, such as ground granulated blast furnace slag (GGBS). In the present paper, an experimental study conducted in a precast factory is described in which GGBS-concrete HC slabs with 30 and 50% GGBS are compared with concrete slabs made from control samples (using 100% CEM II A-L cement, as used in summer mixes). The use of a novel admixture, thermal curing and a new early-age strength determination technique enables confirmation of the development of the required very early strength, such that cutting and lifting of the HC slabs can occur within 20 h of pouring. The HC slabs are instrumented to study the internal temperatures in the different slabs, leading to early-age strength development whereby these slabs are tested for immediate transportability and ultimate capacity. Further, the control and 30% GGBS slabs are tested for early-age creep under a flexural test in the factory. At later ages, using the new technique for making cube specimens, cubes and cores cut from the actual HC slabs are shown to have similar 28-day strengths and density.