No_10_October_1969 — Page 25

3

16

24

40

TIME FROM BATCHING HOURS.

96

10% OF ORIGINAL

BATCH WATER REMAINING AFTER

TIME SHOWN

20-

30

50

70-

904

100

SURFACE TEMP. 68°F

NIL CURING (SAMPLES 6" x 6" x 3"

EXPOSED ONE FACE ONLY)

*

SURFACE TEMP. 65°F

BOTH SETS WITH WAX MEMBRANE

CURING COMPOUND (CALCURE D)

H°F

FIG. SEFFECT OF HEAT RADIATION ON MOISTURE LOSS.

that the tensile strength of concrete is at an extremely low figure.

Cracking occurs when tensile stress exceeds tensile strength. In this case, the tensile stress resulting from early shrinkage is of a high order. At the same time, the strength is at a mini- mum. Cracking must result.

In Sydney a concrete roof slab was being placed under fairly hot summer conditions. Since one section of the roof was to be subsequently covered by a penhouse, the opportunity was taken to actually measure the effects

65°F

of failure to cure a surface of concrete, with all other factors under control. The severe surface cracking which re- sulted on the uncured surface within 24 hours was very obvious. This crack- ing was almost eliminated with no other step than the application of a sprayed curing membrane within three hours of placing and screeding con- crete of the other section of the roof slab.

Even the cured section showed faint traces of surface cracking. This is related to the actual temperature

effects of concrete setting and harden. ing, apart from loss of moisture. No water spray or ponding was used.

Conclusion

For architects and

contractors

there are a number of basic rules to be followed from this research:

1. Always specify a minimum cement content of between 550 and 650 lb., even for structural concrete of around 2500 p.s.i.

2. Specify and supervise the control of temperature particularly during the first few hours after placing and screeding of concrete. Application of fine mist water spray or ponding will ensure this, regardless of fol lowing curing conditions.

4.

3. Specify and supervise the applica- tion of a sprayed liquid membrane curing compound. This will ensure continued curing and retention of moisture, independently of human factors after its initial application. Specify and insist on the inclusion of an adequate cement gel control type of admixture for the reduc- tion of temperature gel effects. Where concrete is subjected to alternate wetting and drying, it would be wise to consider either an integral water-proofing agent, or a surface acrylic or other type of sealing medium.

5.

Brady custom-made doors for every opening. Proven in use.

Roller Shutters

Steel, wood, or aluminium. Fitted singly or in series. Manual or electric operation.

Write for full literature to:

G. Brady & Co. Ltd.,

Dept. 26. Manchester 4. England. and at London, Bradford, Birmingham, Cardiff, Glasgow, Montreal, Toronto, Halifax N.S., Belgium, Sweden, Hong Kong.

Sliding Shutter Doors

Steel or aluminium, Sizes to suit your requirements. Manual or electric operation. Fitted singly or in series.

Brady also make: collapsible gates, fireproof doors, up and over doors, rubber doors, sliding door gear, gas-tight and water-tight doors, Brady service lifts. Approved makers of Colorastic PVC Roller Shutters.

Rolling Grilles

Strong and elegant. Full protection with good visibility. Made from nylon. Available in steel and aluminium.

THE DOORS COMMANDING THE WORLD'S LARGEST SALE

BRADY

Far East BUILDER, October 1969

25

Page 25Page 26