Fresh and hardened concrete

Concrete has two lives. Fresh concrete, for the first few hours, is a plastic mix that must be transported, placed, compacted and finished without separating. Hardened concrete, from setting onwards, must carry loads and resist weather and chemicals for decades. Good concrete needs both lives to go right, and most defects in the hardened state start with mistakes in the fresh state.

The key fresh properties are workability (how easily it flows and compacts), cohesion (resistance to segregation of stone from paste), and bleeding (water rising to the surface). Workability is checked by the slump test for ordinary mixes, the compaction factor or Vee-Bee tests for stiff mixes, and slump flow for self-compacting concrete. Setting time and temperature matter in hot weather and for long pours. Air content matters for freeze-thaw resistance.

Hardened properties include compressive strength, the main specification parameter, measured on cubes in India at 28 days; tensile and flexural strength, roughly a tenth and an eighth or so of compressive strength; modulus of elasticity; shrinkage and creep; and durability properties such as permeability and resistance to chloride or sulphate. Strength depends chiefly on the water-cement ratio and on compaction and curing. Grade names like M30 mean a characteristic cube strength of 30 MPa: the strength below which no more than 5 percent of results are expected to fall.

Between the two lives lies setting. Initial set is when the mix starts to stiffen and can no longer be properly vibrated; final set is when it has hardened enough to resist light pressure. Placing must finish well before initial set, and a fresh layer must be placed on the previous one before it sets, or a cold joint forms.