0.02
SHRINKAGE
0.04
0.06
2
AGE-DAYS
14
28
60
CALCURE D
@1 GALL./250 SQ.FT.
FIG. 3 EFFECT OF EARLY HIGH TEMPERATURE
ON SUBSEQUENT SHRINKAGE
= WAX CURING (MEMBRANE)
= NO CURING
bonation, other than ensuring that cement used conforms to BSS stan- dards in respect of lime content, and also specifying a minimum cement content to around 550lb. per cu. yd. of concrete.
A specified minimum cement con- tent will ensure a good dense concrete in which breathing, and resultant car- bonation action, is restricted to the outer inch or so of surface.
Moisture loss
A major cause of shrinkage in concrete is loss of moisture. This is affected a great deal by both ambient temperature and wind winds of 15 m.p.h. giving a moisture loss at the surface of the concrete of ten times the rate of loss at 5 m.p.h.
In recent research the lesser-known effects of surface heating from sun- shine were investigated in a laboratory by subjecting 6in. x 6in. x 3in. samples to surface radiation and measuring moisture loss over periods of up to 96 hours. At the higher temperature, without a curing membrane, more than 70 per cent of the original batch moisture was lost within 40 hours (Fig. 3).
Concrete exposed under these con- ditions can be expected to show a severe loss of strength, 'dusting' of surfaces and extreme shrinkage. In the field, temperatures of up to 130 de- grees have been measured in the top inch of flat concrete slabs exposed to sunshine at ambient air temperatures of around 100 degrees F. The use of a wax-type sprayed liquid membrane is a good insurance against the effects of surface heating.
Knowing the effect of radiant heat
MIXED AT 68°F SET
68°F
INITIAL STORAGE 68°F(48 HRS.) FINAL STORAGE 68°F
➡ MIXED & SET @ 110°F
INITIAL STORAGE 120°F FINAL STORAGE 68°F
MIXING, SETTING, INITIAL & FINAL STORAGE
ALL AT 68°F
MIXED & SET @ 110°F INITIAL STORAGE 120°F FINAL STORAGE 68°F
-
on moisture loss, further tests have been carried out recently in Australia to study its consequent effect on shrinkage. It was soon discovered that the standard rectangular moulds, ac- cording to the ASTM design, would not allow measurements in the very early life of the concrete that is within the first 24 hours. The preci- sion made moulds of heavy steel in- volve setting a small stud in the end of a 3in. x 3in. x 10in. mould, made of a number of steel plates. It was found that demoulding at anything under about 48 hours to take measurements, invariably resulted in a displacement of that end stud due to the low strength of the concrete at that age.
It was necessary therefore to deve- lop some means of taking measure- in some cases as low as 12
—
ments hours and a disposable mould was evolved, consisting of cardboard cy- linders coated with a silicone wax and fitted with metal end caps. Into the end caps were set in. studs of the
0.02
SHRINKAGE
0.04
same type as used in the ASTM mould. With this kit mould and all could be placed in the measuring jig without any need to remove the cap. Mea- surements would thus be taken within 12 hours.
Importance of curing (Fig. 4)
It is a common assumption that shrinkage in concrete is directly pro- portional to the net amount of mixing water per cubic yard of concrete, irrespective of the amount of cement. Investigations have shown that this may not be wholly correct.
Three batches of each of three mix designs, with three samples taken from each batch, gave shrinkage results in- dicating an optimum cement content of around 600 lb. per cu. yd. (C/A ratio of 1 to 5). The average of shrinkage in this case was between .05 and .06 per cent, compared to .09 per cent for the very rich C/A ratio of 1 to 3 and .08 per cent of a C/A ratio of 1 to 7. The mix designs used were supplied by a prominent pre-mix com- pany adn no admixtures were used in these tests.
Quite apart from the effect of moisture loss on shrinkage of concrete, there is a very pronounced effect due to temperature rise in the concrete in the first few hours of its life. This is not related entirely to maturity of the concrete, which is the hydration over a period and which, of course, is related to temperature. Quite apart from that, it was discovered that long term shrin- kage is greatly effected by the tem- perature of concrete in its first 24 to 48 hours.
The tests showed that concrete exposed to air in its very early life shows a vastly greater shrinkage than moist-cured concrete, particularly in the first two to 20 days (Fig. 5). This is of great significance in subsequent shrinkage, as it is during this period
6
14
28
AGE-DAYS
60
WAX MEMBRANE CURED
BOTH SETS MIXED, SET & STORED AT 90oF.
CURED IN AIR AT 20% REL. HUM.
FIG. 4 EFFECT OF CURING ON SHRINKAGE.
24
Far East BUILDER, October 1969