The basic hardening process
When the constituents of concrete (cement, aggregate and water) are brought together, the cement and water chemically react and eventually set to form a hard mass which adheres to and binds together the aggregates to form concrete. Setting takes time, approximately two hours after which the hardening process (strength development) can be irrecoverably damaged by re-working.
Setting time is reduced if the weather is hot as heat speeds the chemical reaction. As the concrete sets (‘goes off’), the workability reduces and the concrete steadily becomes more difficult to compact and mould, however it is still inherently weak and can be easily damaged.
Generally speaking, concrete will reach a useful strength in about 3 days although this does depend on the mix design and constituent materials. The majority of strength is gained within a month. It is important to remember that concrete will reach its maximum strength only if moisture is present during the hardening process.
The hardening process is therefore not dependant on the concrete ‘drying out’, and it is normally important that the concrete is properly ‘cured’ to maintain the moisture in the concrete (especially at vulnerable surfaces) while the cement water reaction is active. Early loss of moisture will cause a reduction in strength and lead to poorer durability.