No_10_October_1969 — Page 23

CRACKING

and shrinkage tests

IT IS an unfortunate fact that in many places in South East Asia a certain amount of cracking in concrete work has come to be accepted as inevitable.

Happily, modern design techniques have advanced to the stage where crack- ing is not necessarily a hazard and even under the worst conditions of shrin- kage the situation need not be termed 'dangerous'.

Shrinkage is not just one process (Fig. 1). It consists of a number of things of which the so-called drying shrinkage is the one which is really referred to in common usage. It would be more correct to call this hydration

DIMENSION CHANGE %

0.10

0.20

10.30

0.02

0.04

0.06

0.08

SHRINKAGE

HYDRATION

HYDRATION

SHRINKAGE

%

by J. G. Haugsted

Author of this article, Mr. Jorgen G. Haugsted, Mal (Dan.)ME, AMACI, is Far East engineer representative, based in Hong Kong, of the Australian manufacturers of concrete construction auxiliary products, Cement Aids International.

a part of the normal chemical reaction; expansion rather than shrinkage can occur while the concrete is kept saturated with water.

However, when the concrete even- tually dries out, shrinkage takes place and can be of the order of .03 per cent. to .09 per cent. This appears to be very little but when it is realised that .05 per cent, a very common figure, is in. in 80ft., is starts to

WETTING

HEATING

DRYING

COOLING

IF ALL COMPONENTS WERE IN PHASE,

TOTAL DIMENSION CHANGE COULD BE 0.30%.

|CARBONATION

(NON-REVERSIBLE) MAY BE + OR -

FIG. 1 COMPONENTS OF DIMENSION CHANGE.

TIME - HOURS

I

2

6

10

12

14

TEMP. 90°F

-

VARYING SAMPLES

CONTROLS IN STABLE TEMP.

AND REL. HUM.

WETTING DRYING WETTING DRYING WETTING

DRYING

|

WETTING

FIG. 2 - EFFECTS OF MOISTURE VARIATION ON DIMENSION OF SAMPLES OF

HARDENED CONCRETE (DAMP CURED 7 DAYS, THEN AIR DRIED 11 DAYS)

assume sizeable dimensions. Hydration shrinkage is irreversible once it has taken place.

In addition to hydration shrinkage, there is a reversible or cyclic move- ment caused by alternate wetting and drying (Fig. 2). Thus, when concrete dries it shrinks and when it gets wet it expands. The figures vary consider- ably, depending on the property of the concrete, and can be of the order of .08 per cent. plus or minus.

A further form of shrinkage is carbonation also irreversible and as high as .08 per cent. in very rich concrete with a high lime content. It is caused by the reaction between carbon dioxide from the air, free lime in the concrete and moisture.

It takes place only in relatively thin concrete sections to a depth of 2 or 3in., because below that depth there is little breathing movement of carbon dioxide in and out of the structure of

concrete.

Like other materials concrete ex- pands and contracts as it heats and cools. Above a temperature of 90 degrees F. an expansion of .05 per cent. would be quite normal. Labora- tory tests have shown figures as high as one part in 100,000 per degree Fah- renheit above 90 degrees F. Again these figures do not appear trouble- some, but they do mean that between 30 degrees and 120 degrees F. the volume of concrete can change as much as from hydration shrinkage.

Under extreme conditions the dimensions of concrete can alter con- siderably. For example samples cured in moist carbon dioxide can show shrinkage as much as eight times those cured under standard laboratory con- ditions. Under some conditions of chemical reaction, carbonation can be an expansive action with certain cements and aggregates.

Not much can be done about car-

Far East BUILDER, October 1969

23