Finding carbon and cost efficiencies in the design of RC slabs made from high early strength concrete

In a recent study of 25 projects totalling over 1 million m2 of constructed area, slabs account for around 45% of total project concrete volume. In modern high-rise construction, the thickness of flat plate slab systems is often governed by deflections, which are in turn strongly dependent on the concrete material properties and loading history. Since speed of construction is valued by developers, high early strength concretes are commonly used for slabs, where 75% of the characteristic strength is typically obtained within 24 to 48 hours, allowing for a construction cycle of 3 to 5 days per storey. These concrete mixes have a large carbon and cost impact, and mitigating benefits from their inherent long-term strength gain and superior properties are not typically realized in the design of the slab. This paper examines potential carbon and cost savings in typically-reinforced, cast-in-place, two-way slabs using high early strength concrete. Drawing upon commercial design and construction techniques, over 100 field measurements of concrete strength and modulus of elasticity are analyzed to assess probable vs. design deflections of slabs. The results suggest that thickness reduction and material savings can be achieved due to the unexploited long-term properties of such concrete mixes. Commentary is provided on the practical constructability of these solutions including requirements for embedded services, acoustics, and reshoring.