APPENDIX
REINFORCED LIGHTWEIGHT CONCRETE
Tentative specification used by the London County Council in considering on their merits applications made to the Council for modification or waiver of the relevant London Building (Constructional) By-laws, 1952. The use of this specification is subject to the approval of the Council in each case. This Appendix is reproduced by permissioin of the L.C.C.
1. The following requirements apply only to reinforced concrete us- ing Aglite, foamed slag complying with the appropriate provisions of B.S. 877: 1939 or 'Lytag' aggregates.
2. The concrete cover of any re- inforcement in any members exposed to the weather should be not less than 2 in. and the maximum aggre- gate size not more than in. The concrete cover for other members should be in accordance with by law 7.02(1).
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3. The aggregate is to be graded as to enable the concrete to be well compacted and the concrete mix should be not leaner than one part of cement to six parts of aggre. gate (all proportions by volume).
4. The inimum resistance to crushing Uw at 28 days (works test) should be not less than 3000 lb/in.2. The seven-day strength should be not less than two-thirds of this value.
5. Permissible concrete stresses: (a) With normal site contro!:
Resistance
to
crushing
Compression Shear Bond
in
Due to slabs Direct bending only Average Local
UY'
Uw
5.25
4
tiw
53
Uw
Hw
67
43
Where
(b) Special site control: there is a site laboratory installed on the job and clauses 209 (a) to (f) of C.F.114: 1957 are generally corpiled with, except that samples of concrete for testing should be taken daily (in. stead of on the first four days and then at weekly intervals), the follow- ing maximum permissible stresses will be acceptable.
Resistance
to
Compression Shear Bond
in
Due to slabs crushing Direct bending only Average Local
Liw
Hw
3
Hw
40
It w
50
33
(c) Factory-made concrete. Where the standard of supervision and control is not inferior to that requir ed in the preceding sub-paragraph
114
Table A
PERMISSIBLE STEEL STRESSES IN COMPRESSION
1. In longitudinal reinforcements:
(a) in axially loaded columns
(b) in columns other than
axially loaded columns
2. In main reinforcements in a
beam or slab:
(a) When the compressive re- sistance of the concrete is not taken into account (b) When the compressive re-
sistance of the concrete is taken into account
(LB/IN2)
Mild steel complying with B.S. 785:1938
Not exceeding
Exceeding
Reinforcement with minium yield point fy lb/in 2
1 in. dia.
14 in. dia.
18 000
16 000
0.5fy or 23 000, which- ever be the lesser
The calculated compressive stress in the sur- rounding concrete multiplied by the modular ratio but not exceeding:
18 000
16 000
0.5fy or 23 000, which- ever be the lesser
18 000
16 000
0.5fy or 23 000, which- ever be the lesser
The calculated compressive stress in the sur- rounding concrete multiplied by the modular ratio but not exceeding:
(b), the concrete stresses specified in that sub-paragraph will be ac- ceptable.
(d) For columns. the permissible bond stresses may be in accordance with Table 10 of C.P.114.
(e) Where deformed bars. are used, the bond stresses may be in- creased by 25%.
6. In beams, the shear resistance of the concrete is to be ignored and the whole shearing resistance is to be provided by shear reinforcement. 7. Permissible steel stresses in tension:
Mild steel:
20 000 lb/in2 for bars not ex-
ceeding 1 in. diam.
18 000 lb/in2 for bars exceed-
ing 1 in. diam. High-tensile deformed bars:
27 000 lb/in2 in beams.
30 000 lb/in2 in slabs.
If plain bars are used as reinforce- ment, they should be hooked, 8. Minimum depths of slabs and beams.
For slabs where the design imposed load does not exceed 60 lb/ft2, the span / overall depth ratios given in Table 13 of C.P.114: 1957 should not be exceeded hut, for other slabs and all beams. the ratio of span to overall depth should not exceed the following: Beams: Simply supported
Continuous
Cantilever
17
21
9
18 000
16 000
0.5fy or 23 000, which- ever be the lesser
Slabs: Spanning in one direc-
tion, simply supported
Spanning in one direc-
tion, continuous
Spanning in two direc
26
30
tions, simply supported 30 Spanning in two direc tions, continuous Cantilever
9. Permissible steel stresses compression - see Table A.
34
10
in
10. Modular ratio. The modular ratio should be assumed to be 30.
11. Columns:
(i) The
ratio of effective least column length to
lateral dimension should not exceed 15.
(ii) The calculated permissible load on any column should not be greater than that permitted for a column with concrete of equal strength and with aggregate complying with B.S. 882. 12. Reinforced concrete walls. Re- inforced concrete walls should be considered as columns and designed in accordance with the recommenda- tions of clause 339 of C.P.114: 1957 and with the limitations specified in clause 11 preceding.
13. Shrinkage. The drying shrink- age should be taken as being 0.07%. 14. Creep. Creep should be taken as twice that for normal ballast ag- gregate concrete.
THE HONG KONG & FAR EAST BUILDER - VOLUME 18, NUMBER 1
Page 120Page 121
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