Although no comprehensive study of the effects of air entrainment has been made, when small quantities (2 or 3%) of air were entrained in the concrete made with the aggre gate of medium shrinkage referred to above, cracking was eliminated even at the high water/cement ratio of 0.70. Air-entrained concrete made with a more highly shrinkable dolerite aggregate from two sources has been subjected to exposure and in all cases there was a distinct im- provement in its durability.
For
specimens of 1:2:4 dolerite con- crete made with water/cement ratios of 0.5, 0.6 and 0.7, the 'damage index' fell sharply as the volume of entrained air was increased from 0 to 5%; further increases in the volume of entrained air from 5% to 20% resulted in only slightly in- creased resistance to damage by ex. posure. The 'damage index' based on the number and size of the cracks developed in specimens under
test.
Appendix II
was
DETERMINATION OF DRYING SHRINKAGE OF CONCRETE
of
This method of test covers the standardized produre for deter- mining the drying shrinkage concrete prisms made with the coarse and fine aggregate to be test ed.
APPARATUS
(a)
Mould. One gang mould suit- for casting three 8 in. x 2 in. x 2 in. prisms.
(b) Tamping bar. The tamping
Table I
bar shall be a steel bar weigh- ing 4 lb., 15 in. long and shall have a ramming face 1 in. square.
(c) Measuring apparatus. A mea- suring apparatus shall be used which incorporates a micrometer gauge reading_ac- This curately to 0.0001 in. gauge shall be rigidly mount- ed in a measuring frame and shall have a recessed end which can be located upon a 1-in. diameter stainless-steel ball cemented in the specimen as described below under 'Storage of specimens. The other end of the frame shall have a similar recessed seat- ing which can be located up- on a second ball in the speci men. An Invar steel rod of 8 in. length with -in. diameter hemispherical ends shall be
RECOMMENDATIONS FOR USING CONCRETES OF VARIOUS SHRINKAGE LEVELS
Drying shrink-
age of concrete
118
%
0.000 0.025
0.025-0.04
0.04 0.06
0,06-0.08
0,08-0.10
Over 0.10
Remarks
Possible only with non-shrinkable aggregate of high elastic modulus such as quartzite gravel
Produced with hard, dense aggre- gate of non-shrinking or very low shrinking types, e.g. granite, mountain limestone, blast furnace slag and some dolerites
Produced with hard, dense aggre- gates of low shrinkage and softer non-shrinkable aggregates
Concretes
at this and higher shrinkage levels can be produced with aggregates covering a wide geological range,
At this and higher levels of shrink- age, durability is likely to be- come affected
Severe warping and widespread deterioration has taken place in concretes of this shrinkage level
Use
Suitable for all concreting purposes, and essen- tial for precast products where very low values of shrinkage are demanded by British Standard
Suitable for all purposes except precast pro- ducts of very low shrinkage unless these can be cured with high pressure steam
Not suitable for precast products of low shrink- age and thin reinforced sections such as clad- ding panels and cast in-situ floors, particularly if heated, unless special precautions are taken in design. For concretes with this or greater shrinkage an extra allowance should be made for loss of prestress
Suitable for all general structural purposes except thin reinforced members as above, plain concrete exposed to the weather unless air en- trainment is used, and no-fines concrete
Suitable only in positions where complete drying out never occurs, for mass concrete provided ! the surface layers are air-entrained, and for heavy symmetrically reinforced members not exposed to the weather
Suitable only for concrete kept in a permanent- ly wet condition
No. of occasions each shrinkage level had been met in the present investigation
4
34
26
13
9
3
Note: 1. Different batches of ordinary Portland cement may be expected to give rise to variations of the
order of 0.005% in the drying shrir kage of the concrete
2. In general, the coarse aggregate plays the major role in establishing the shrinkage of the concrete but the level does increase as the grading of the sand becomes finer. This effect is more pronounced with highly shrinkable aggregates and in extreme cases the change in grading trom Zone 1 to Zone 4, B.S. 882:1954. may increase the shrinkage of the concrete by 0.020% 3. it is assumed that the normal recommendations for quality control given by the appropriate
Code of Practice for the particular job will be observed.
THE HONG KONG & FAR EAST BUILDER—VOLUME 18. NUMBER 2
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