Concrete must be placed
carefully to prevent
bleeding. Bleeding is the formation of a layer of water on the surface
of freshly placed concrete when the aggregates and cement separate from
the water. Bleeding reduces the concrete
strength.
Vibration is used to
consolidate stiff concrete by increasing
its fluidity. Too much vibration, especially in wet and highly
workable
concrete mixtures, can cause segregation and a weakening of the
concrete.
Hydration is the chemical reaction between cement and water.
As the hydrating cement paste hardens it forms a bond with itself and
the aggregate. A strong
concrete is formed by cement paste with few pores and low water content
because it has more cement to bond with the aggregate. High heat
of hydration can protect concrete from freezing
when it is
placed during winter. Too much heat of hydration can put excessive
stress on on large concrete structures such as dams.
