Precast concrete materials, manufacture, properties and usage - Chapter 7 pptx

7

ACCELERATED CURING

This chapter integrates the experience of Laing R+D and the British

Precast Concrete Federation (BPCF) on the subject of heat curing; the

purpose is to produce a background of guidelines for the practitioner

wishing to produce high-early-strength concrete using heat as the

accelerator. It is not the purpose of the exercise to summarise each of the

reports; data has been abstracted where particular points need to

be made.

When cement hydrates its speed of reaction is mainly a function of the

starting temperature of the system and the curing regime. Hydration is

accompanied by exotherm so the concrete tends to warm up as hydration

progresses. What this means is that a cold-starting concrete, say 5°C,

warms up and gains strength slowly; a warm-starting concrete, say 25°C,

warms up and gains strength more quickly; and a concrete starting at,

say 40°C, can be handled within a few hours. Any method of accelerating

the early strength of concrete is known to detract from the 28-day

strength—the usual specification age for concrete cube strength.

However, this decrease, more often than not, is within the range of the ±

10% variation one obtains. What is really significant is that heat curing

is carried out to obtain a high early strength, and 28 day strength

specifications are generally exceeded by an excess one does not require.

Research data obtained from both industrial and laboratory processes

show that, although there is a decrease in the 28 day cube strength, at 3–

6 months old the strength is equivalent to that of the normal-cured

concrete.

Flexural strength at 4–24 hours old is the practical consideration as

concrete is subject to bending during demoulding and handling. If, for

example, one aimed at and achieved a minimum 16 hour flexural

strength of 3 N/mm2

the cube strength at that time would be about 15 N/

mm2

and about 45 N/mm2

at 28 days old.

Copyright Applied Science Publishers Ltd 1982