Either Portland cement/sand mor- tars with the addition of a mortar plasticiser, or masonry cement/ sand mortars may be used in place of the mortars suggested above, subject to as to mixes. Plain cement/sand the manufacturers' recommendations mortar should not be used.
(4) Building details should be designed so that the blocks when in position remain as dry as possible. The blocks should not in general be used below d.p.c. level. In cavity ex- ternal walls, cavity flashings or damp- proof courses should be provided over window and door openings ex- cept over window heads immediately under overhanging eaves. The flash- ings or damp-proof courses should be sloped outwards to direct water- away from the inner leaf, and weep holes should be provided to drain the cavity.
movement
(5) In long walls, joints should be provided at intervals of 20 to 30 feet. For small houses built in pairs of terraces where the blocks are used for both leaves of the wall, such joints may conveniently be located in the outer leaf on the line of the party wall. Movement joints are not normally necessary in the inner leaf of a small house: when they are required they may be locat ed at a junction with a partition.
Strength and stability
Cavity external walls of small two- storey houses may be built in blocks having the minimum compressive strength of 400 lb/sq. in. specified for Type B blocks. provided the combined thickness of the two leaves is not less than 8 inches; the necessary lateral support is normal- ly provided in such buildings by the cross and return walls and the floors.
Where in a small house the inner leaf of a cavity wall carries the floor joists, 4-in. solid blocks of Type B should be adequate for normal joist loads, but if the blocks are hollow a spreader such as a wall plate should be provided, un- less the strength of the blocks is well above the minimum.
For general construction, the loading for concrete block walls and the thickness required for dif ferent heights and degrees of lateral
stability are defined in the British Standard Code of Practice 3 (Chap- 5) and 111 and also in Ministry of Housing and Local Government Model Byelaws Series IV Schedule 3.
Non-loadbearing walls and parti- tions built in lightweight concrete blocks should conform with the re- commendations of B.S. C.P.122.
Where walls are built with hollow blocks it is generally advisable to avoid hanging from them heavy in- ternal fittings such as basins or flushing cisterns. These should either be fixed to solid blocks or be supported from the floor.
Finishes
External walls built with light- weight concrete blocks normally re- quire some form of surface protec- tion to make them watertight. As with brickwork, rendering this purpose and is advisable for solid walls. The cement/lime/sand
serves
or masonry cement/sand mixes re- commended above for jointing mor tars are also suitable for rendering.
The watertightness of cavity walls depends, as usual, on careful design and construction with attention to such matters as flashings and damp- proof courses. the bedding and jointing of the blocks, and keeping the cavity clear of mortar drop- pings. Provided that the work is satisfactory in these respects, ren- dering in addition is less essential and it may be sufficient to apply two or more coats of cement paint. Due regard must be paid, however, to the finished appearance; painting cannot be expected to provide an acceptable appearance unless the blocks are of good shape and sur- face texture and are laid carefully with special attention to half blocks and other special sizes for bonding.
Internal surfaces of block walls can be plastered in the ordinary way or finished with a dry lining, as re- quired. For plastering, a
1:2:9
mix of cement, lime and sand pro- vides a suitable backing coat with a finishing coat of lime gauged with a small proportion of gypsum plas- ter. For a stronger finish a neat gypsum plaster finish may be used over a gypsum plaster undercoat.
General
For economy in building, consid- eration should be given at the de- sign stage to the setting out of courses, particularly in relation to the floor height; a room height of 7 ft. 6 in. is convenient for the 9 in. courses obtained with standard blocks.
To eliminate cutting large blocks for use at joints. infilling between joints and rafters, and gable verges. special blocks are the most conveni- ent but small blocks of normal brick size in a suitable light-weight concrete may be used. Blocks can be cast with grooves for easy cut- ting at spacings arranged for the best assortment of sizes. This can effect a considerable saving of both time and material.
SCREEDS
Lightweight aggregate concrete is often used as a screed on flat roofs and on floors, since it can easily be laid in the thicknesses required to provide falls for drainage or to carry services without unduly in- creasing the weight. Advantage can also be taken of its thermal insulat- ing properties. The screed often re- quires a topping of cement and washed sand or fine aggregate on which to lay the roof covering or floor finish. To reduce the risk of cracking the topping mix should not be richer than 1:4.
TABLE I
THERMAL INSULATION OF LIGHTWEIGHT CONCRETE SCREEDS
Thickness of scree:l required to give an average U-value of 0.22 for the roofs consisting of 4 in. concrete slab, with screeds covered with 3-layer bitumen fell
Insulating material
Average thickness (in.)
Application
Thermal resistivity ()
Aerated concrete
Foamed slag concrete
Vermiculite concrete
About 40 lb ft3
About 70 lb ft3
About 70 lb/ft3
1.0
falling to z
0.59
"
4
1.0
2
THE HONG KONG & FAR EAST BUILDER - VOLUME 17, NUMBER
109
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