used as a standard of length against which the readings of the gauge can be tested. this enabling corrections to be made for any changes in the dimensions of the apparatus between successive measure- ments of a test specimen.
(d) Drying oven. A drying oven shall be used which complies with the following require-
ments:
(i) It shall have an internal volume equivalent to not less than 0.3 ft3 per specimen, with a minimum total value of 1.8 ft3.
(ii) It shall be reasonably air-tight and shall be provid- ed with a fan to keep the air circulating effectively during the drying of the specimens. (iii) It shall be maintained at
a
temperature of 50 1:1°C (122 [2°F).
(iv) The humidity of the air in the oven shall be controlled at approximately 17% relative humidity by means of saturat- ed calcium chloride solution. Suitable dishes or trays con- taining this solution shall be provided to give an exposed area of solution not less than 0.3 ft2 for each cubic foot of volume of the oven. The dishes or trays shall contain sufficient solid calcium chloride to show above the surface of the solution throughout the test.
MATERIALS
(a) Coarse aggregate. The coarse aggregate shall be sieved into two sizes, in. in. and in. 3/16 in.
re-
(b) Fine aggregate. The fine ag- gregate shall be used as ceived from the source except that it shall all pass ain. sieve. A sieve analysis shall be carried out.
(c) Cement. Ordinary Portland cement complying with B.S. 12: 1958 shall be used.
PREPARATION OF
shall be spread out on shallow trays and dried for 1 hour in the oven described above. (c) Proportioning. The amounts of cement, aggregates and water required to make the three prisms shall be: Ordinary Portland cement
550 FL g
Coarse aggregate
in. in. 1466 1 g
Coarse aggregate
in.-3/16 in. 7341 g
Fine aggregate
3/16 in. down 1100±1
g Water
3301 g (d) Mixing. The concrete for the three prisms shall be mixed together. The mixing shall be performed on a non-porous surface which has been wiped over with a damp cloth. The cement and fine aggregate shall be mixed dry for one minute with two trowels. The coarse aggregate shall then be added and mixed dry until the mixture is uniform.
The water shall then be added and the whole mixed for three minutes with two trowels. The concrete shall be transferred to the gang mould in two approximately equal layers, each of which shall be given at least 35 strokes of the standard tamping bar distri- buted in
in a uniform manner over the cross section of the mould. After each layer has been tamped, the mould shall be jogged over the edge of the mixing bench until the air bubbles stop breaking the sur- face of the concrete and moist film of laitance is brought to the top of the layer. Finally the top of the concrete shall be
be smoothed with a trowel.
a
STORAGE OF SPECIMENS
the
Immediately after making, prisms shall be covered over with a flat impervious sheet (e.g. thin rub- ber, polythene or steel) making con tact with the upper edge of the mould. After 24 h the prisms shall he marked for identification, re- moved from the mould and weighed. Indentations about in. deep and i in. diameter shall be made at the centre of each end of the three specimens and -in. stainless-steel balls shall be cemented in ordinary Portland cement. The prisms shall then be covered over with damp sacking for a further 24 h when the surface of the balls shall be wiped clean of cement and the prisms placed in air at a temperature of 14 19°C (58 66°F) for a fur- ther 26 days.
PROCEDURE FOR TESTING
When the prisms are 28 days old they shall be immersed in water at a temperature of 1419°C (58- 66°F) in such a way that one of the larger faces just breaks surface in the water and left so immersed for four days. They shall then be re- moved from the water, the balls wip- ed with a clean dry cloth and the length of each prism measured in the apparatus described above. Im- mediately after the prisms are mea. sured the length of the Invar rod shall be measured.
The prisms shall then be dried in the oven
described above at the specified temperature and humidity for a period of three weeks. The prisms shall then be removed from the oven and cooled for at least 4 h in a desiccator containing solid cal- cium chloride in a saturated solu tion of calcium chloride. The length of the prisms and the Invar rod shall then be measured as described
Table 2
DENSE AND NO-FINES CONCRETE
Shrinkage (%)
Aggregate
Dense concrete
No-fines concrete
Rock
1:2:4:0.6
1:6:04 1:8:0.4
1:10:0.4
SPECIMENS
Control, quartz
gravel
0.000
0.025
0.012
0,014
0,012
(a) Number of specimens. Three
Dolerite
prisms shall be cast.
0,04 I
0,079
0.071
0.073
0.064
Dolerite
0,065
0,095
0.084
0.079
0.073
(b) Aggregates.
The aggregates
(Turn to page 128)
120
THE HONG KONG & FAR EAST BUILDER-VOLUME 18, NUMBER 2
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