Fairly and Rapidly Assessing Low Carbon Concrete Made with Slowly Reacting Cements

The CIIIA+SR combination in BS 8500-1 accounts for around 40% of UK ready-mixed concrete production and will normally consist of 50%-55% GGBS. The slower setting times and strength gain, especially at lower ambient temperatures, can cause issues with finishing due to extended bleed and programming due to extended stripping times. Contractors have become accustomed to these challenges, however the performance of the lower clinker cements in EN 197-5 will present an even bigger challenge compared to their binary equivalents. Low carbon alternatives typically require anywhere between 90 and 180 days of standard water curing to reach equivalent standard 28-day concrete maturity. Whilst this has significant implications for programming of construction work, it is also difficult to make material performance comparisons within timescales suitable for clients and designers to make decisions on material specification.

A range of cement type combinations were cast using typical w/c ratio used for structural and non-structural concrete (0.4, 0.5 and 0.6 w/c). Elevated temperature curing regime (at 50°C) was investigated by means of comparative performance against standard water cured samples. Analysis involved comparison of compressive strength development to establish strength-based equivalent maturity. MIP and SEM were done to investigate both porosity and physical microstructure of test specimens.

Compressive strength equivalency was used as a proxy for equivalent degree of maturity of investigated concrete compositions. The research shows that 21-day elevated temperature cured samples are very similar to 90-day standard cured specimens in terms of both compressive strength and internal physical microstructure. The similarities diverge with increasing water-cement ratio with little compressive strength equivalency for concretes cast at 0.6 w/c. However, the microstructure remains similar suggesting equal degree of maturity for both elevated and standard cured samples of corresponding cement type concretes for all investigated water-cement ratios.