LIGHTWEIGHT CONCRETE BLOCKS
The following article is a summary of information on the above subject abstracted from the monthly bulletins Nos. 6, 32 and 52, issued by the Building Research Station at Garston, Watford, Herts.,
and reprinted by permission.
The four principal ways in which blocks may be employed instead of bricks in traditional type construc- tion are:-
1.
as internal partitions:
In the many non-traditional forms troubles that occurred when the USE OF BLOCKS of construction that have been deve- blocks were first used are now known loped for housing in recent years, and can. to very large extent, be considerable attention has been given avoided. Other essential require- to materials and methods for building ments for good blocks are that the the external and party walls. Con- concrete should be correctly propor- crete, both dense and lightweight has tioned and the curing and drying of been used extensively and many the blocks before delivery to the specialised systems of construction building site should be adequate. In have been developed. Such systems specifying or have generally differed appreciably should be remembered that, although from traditional housing construction, but one that involves no radical departure from traditional brickwork is the substitution of precast light- weight concrete blocks for bricks. either for a part or for the full thickness of the walls. Block con- struction has been used for housing in England to a limited extent but it is much more common on the
Continent and in America, particular- ly for the full thickness of external walls. In the inter-war years the use of lightweight concrete blocks for internal partitions became tablished practice in the United Kingdom, and their use for the inner leaf of external cavity walls has gained some popularity in the last
five or six years.
an es-
One of the obstacles to a greater use of the blocks in England has been the varying interpretation of similar bye-laws by local authorities. The new Model Series clarifies the situa- tion and the proper use of lightweight blocks should no longer be subject to restriction.
Lightweight concrete block con- struction is not a new and untried method. There is a considerable background of research and develop- ment which has provided the infor- mation necessary for the production of suitable blocks and for their proper use in buildings. Studies of the rate at which the blocks can be laid have indicated that a saving of building labour can be achieved by substitut- ing blocks for bricks. Advantages that result from the use of blocks include:-
a) Economy of labour and in- creased speed of building: b) Reduction in the cost of
walling:
C)
An alternative to bricks and consequently, in a period of shortage, greater availability of bricks for work where blocks cannot be used.
d) Improved heat and sound in-
sulation.
In the manufacture, handling and use of these blocks, it is essential, as with all building materials, that their physical characteristics be under- stood so that any necessary precau- tions can be taken to avoid possible defects occurring in the building structure. The causes of most of the
external walls, the outer leaf being in 2. as the inner leaf of cavity blocks it
brick, stone or dense concrete blocks: adequate strength is necessary, ex- 3. as both leaves of cavity exter- cessive strength may mean a loss of nal and cavity party walls; other valuable characteristics. To ensure that blocks are of specified quality a certificate should be ob- tained from the manufacturer OF tests should be made.
The amount
turer.
external or party walls.
4. forming the full thickness of
For load-bearing walls of two- storey houses, the recommendation in the Code of Practice CPIII that of shrinkage that lightweight concrete blocks should takes place when lightweight con- have a minimum compressive strength crete dries is a major factor to con- of 400 lb. per sq. in. based on the sider. It is important that blocks gross area including core space should
be should well made, cured and be complied with; the thickness of adequately dried by the manufac- walling in relation to the strength of Under normal circumstances the blocks is dealt with in the Tech- shrinkage cracking is important only nical Appendices of the Housing because of its effect upon appearance. Manual. Where concentrated loads It does not affect structural stability occur, as for example at a wide open- nor, in the case of cavity wall con- ing, it may be necessary to use a struction, does it affect the exclusion spreader or to introduce a thicker of rain. Subsequent decorations may, block or a pier. It should be em- of course, conceal the cracking.
phasized that not all lightweight
blocks It is extremely difficult to avoid all concrete
are of adequate shrinkage movement in the structure strength for load-bearing purposes. and the possibility of this movement and selection must be made from must be taken into account in the those having strengths above the design and in construction of the minimum referred to above. building. Four important recom- The use of blocks for internal mendations in this respect are:-
1. Long uninterrupted lengths of lightweight concrete block walling should be avoided by providing breaks or flexible mastic-filled joints in the construction at intervals of about 20- 30 feet in the length, and at points of marked change of section.
2. "Weak" mortars should be used so that any shrinkage cracking will tend to occur in the mortar joints
rather than in the blocks.
3. Where the blocks are used externally and rendered, a "weak" porous rendering should be used. A technique of applying a cement paint by "scrubbing" it into the blocks has been used in America to provide a satisfactory measure of protection against rain penetration for external block walling.
4. Low-absorption type of blocks should not be wetted when being laid in the wall.
TYPES OF BLOCKS
Lightweight concrete blocks may be made with a lightweight aggregate or by asration. The principal light- weight aggregates used are clinker, foamed slag, and more recently, expanded clay, but other aggregates such as pumice are also used.
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partitions or for the inner leaf only of a cavity external wall presents little or no difficulty if good tradi- tional practice of cavity construction is followed, if suitable blocks are used and protected against moisture.
Use of blocks for both leaves of a cavity wall construction also involves no radical departure from cavity brick walling technique, but with most blocks an external rendering is required to limit moisture penetra- tion through the outer leaf and thereby to reduce excessive moisture movement.
The fourth way of using blocks is to build the full thickness of the wall in one leaf of cored or hollow blocks. approximately nine inches thick overall.
THE AVOIDANCE OF CRACKING
With any brick or block construc- tion a certain amount of cracking is almost sure to be caused by the shrinkage that takes place as the work dries out. Even where the units themselves (ie. the bricks or blocks) are of a type that has a low drying shrinkage, the mortar usually shrinks enough to cause fine hair cracks. These fine cracks due to mortar shrinkage are. however, generally confined to the mortar joints, and
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