Abstract

‘Precast concrete is of the highest possible quality, both in terms of strength and durability’. The essence of this statement is captured in Figure 2.1, where concrete is accurately delivered to every part of the mould, ensuring zero segregation, minimising voids and honeycombing, using minimal vibration. However, contrast this view with the precast column shown in Figure 2.2 and it is easy to understand the problems of workmanship, rather than material defects, that certain parts of the industry endure. Using materials that have passed strict quality control procedures, rapid hardening cement type CEM I class 42.5R or 52.5R according to BS EN 197 Part 1 (BS EN 197-1 2011) is mixed with excellent-quality aggregates of known source and purity, in computer-controlled batching and mixing plant, to produce concrete of specified workability and strength. Even the introduction of small quantities of uncontaminated recycled concrete, usually from the factory's own waste production, superplasticisers and pozzolanic materials (such as pulverised fuel ash), has not reduced this standard. To quote standard deviations (SDs) of less than 2 N/mm² on concrete of the 28-day compressive cube strength of f_ck = 45–70 N/mm² would not be an exaggeration. Table 2.1 lists typical mean concrete strengths f_cm (usually obtained from 2 or 3 cubes) for precast production at 20 h (detensioning prestress or stripping moulds), 3 days and 28 days, and the characteristic values for cement cubes f_cu (BS EN 197-1 2011). The table shows that even allowing for f_cu = f_cm – 4 N/mm², concrete at 20 h just about exceeds the requirements for cement strength at 2 days.