1985 Ed.] Building (Construction) Regulations
[CAP. 123
B 123
[Subsidiary]
(a) specified strength;
(b) durability:
(c) degree of control;
(d) age at which a particular resistance to crushing is required:
(e) size of members;
(f) type of cement;
(g) spacing of reinforcement:
(h) maximum size of aggregate;
(i) shape and grading of aggregates available:
(j) available means of compaction:
(k) ratio of water to cement ratio; and
(l) ratio of aggregate to cement ratio.
2. (1) The designed mean resistance to crushing of special mixes shall initially exceed the specified resistance to crushing by a margin of not less than 14 MPa.
(2) Where the works tests show to the satisfaction of the Building Authority that the standard of acceptance can be maintained with a smaller margin, the mix may be redesigned with a margin of not less than 10 MPa.
3. (1) The designed mean resistance to crushing of designed mixes shall exceed the specified resistance to crushing by a margin of not less than 2 times the designed standard deviation.
(2) Where there is no previous information on the standard deviation which may be expected under similar conditions, the designed standard deviation used initially shall be not less than 7 MPa for the purpose of ensuring that the margin initially taken shall be not less than 14 MPa.
(3) Where there is previous information on the standard deviation which can be expected under similar conditions or where the standard deviation can be calculated from the results of at least 40 works cube tests, such standard deviation may be used as a basis for designing or redesigning the mix:
Provided that the standard deviation used in designing or redesigning a mix shall be not less than 3.5 MPa.
4. (1) For concrete which is protected from the weather or from corrosive conditions, the cement content and the ratio of water to cement will generally be selected on the basis of the required resistance to crushing and workability.
(2) For exposed concrete the maximum ratio of water to cement may be limited by the requirements of durability rather than strength. The Table hereto sets out the maximum ratio of water to cement for various types of exposure. The cement content should be selected so that the required workability can be obtained without exceeding the maximum ratios of water to cement.
TABLE
Maximum Ratios of Water to Cement (BY MASS) FOR DURABILITY OF DIFFERENT TYPES OF STRUCTURE UNDER VARIOUS DEGREES OF Exposure
Criteria for special mixes, L.N. 294/76. Criteria for designed mixes, L.N. 294/76. Durability L.N. 294/76. Type of structure234 Conditions of exposureMediumHeavyThin sections sectionssections Protected concrete0.550.600.65 Normal outside exposure0.500.550.60 Frequent wetting and drying in fresh water or continuously wet in sea water0.450.500.55 Frequent wetting and drying in sea water or spray0.400.450.50