Materials used in precast structures
‘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 fck = 45–70 N/mm2 would not be an exaggeration. Table 2.1 lists typical mean concrete strengths fcm (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 fcu (BS EN 197-1 2011). The table shows that even allowing for fcu = fcm − 4 N/mm2, concrete at 20 h just about exceeds the requirements for cement strength at 2 days. Precise delivery of concrete into the mould. (Courtesy of Tammer Elementti Oy, Finland.) Compaction problems in precast concrete columns. Strength requirements for cement based on BS EN 197-1:2011 and typical concrete strengths achieved by precast producers Cement strength class Early strength (N/mm2) Standard concrete strength at 28 days (N/mm2) 20 h concrete Precaster 2 days cement BS EN 197 7 days concrete Precaster BS EN 197 Precaster 42.5N 15–20 ≥10.0 40–55 42.5–62.5 50–70 42.5R 25–35 ≥20.0 50–70 60–90 52.5N 20–30 ≥20.0 50–70 ≥52.5 60–90 52.5R 30–35 ≥30.0 50–80 70–100 Key: BS EN 197 characteristic value for cement (italics); producer’s mean cube strength.