CIVIL ENGINEERING AND PUBLIC WORKS
S
tructural lightweight concrete is generally taken as having a 28- day cube strength not less than 2,500/ lb. sq. in, and an air-dry unit weight not more than 115 lb./cu.ft. To pro- duce lightweight concrete complying with these requirements, the aggre- gates used must be predominately composed of lightweight cellular and granular inorganic material, and must weigh not more than 65 lb./cu.ft. in the dry loose state.
According to the method of manu- facture, lightweight aggregates suitable for structural concrete are classified into two categories:
1. aggregates produced by process- ing natural materials such as pumice:
2. aggregates produced by expand-
ing or sintering clay, shale, slate or certain industrial by-products, such as blast-furnace slag and pulverized-fuel ash.
Production and application of light- weight aggregates rely on the avail- ability of the raw material. Vast de- posits of pumice occur only in the regions which were subjected to in- tense volcanic activities during prehis- toric times, such as the Rhineland in Germany and certain parts of New Zealand. Japan and Kenya. Con- sequently, utilization of pumice as a building material is mostly confined to these places.
In Europe and America, artificial aggregates are more favoured. Foam- ed slag is produced and used in great quantities in Germany and eastern European countries. Expanded clay. shale or slate has become a common building material in the USA, and is also used to a certain extent in Bri- tain, Eastern Europe, Australia and Japan.
Popularity of the expanded clay and shale products, especially in the USA. is due primarily to the wide-spread occurence of suitable clays and shales. and secondly to the ability of these aggregates to produce high-strength concrete. Nevertheless, their position in the field of concrete construction will be challenged in the near future by a relatively new type of lightweight aggregate produced by sintering pul- verized-fuel ash.
The idea of sintering the residue from the boiler-furnaces in a power generating station dates back more than 20 years, but the first serious in-
*Lytag Ltd. the manufacturer, gain- ed the Queen's Award to Industry, 1966, for the development of new material.
Far East Architect & Builder April, 1967
LIGHTWEIGHT CONCRETE
by Dr. H.W. Chung
Department of Civil Engineering, University of Hong Kong
vestigation was initiated in 1952 at the Building Research Station in Britain. Following the success of small scale trials, a pilot-scale plant was built in 1954. The preliminary success aroused the interest of two industrial concerns which began to undertake independent development schemes, re- sulting in commercial production in 1957 and in 1960 under the trade names of Terlite and Lytag* respec- tively.
The use of pulverized-fuel ash is welcomed and encouraged by the elec- tricity authorities, because it automa. tically releases them from the financial burden of waste-disposal. Further- more, pulverized-fuel ash is generally available in or near centres of popula- tion where the building industry flourishes, thus avoiding the high haulage costs incurred by aggregates which depend on regionalized indus- tries or distant natural deposits.
Structural Properties of Lightweight Concrete
The structural properties of light- weight concrete vary as much as the aggregates. A generalization is given below, which indicates the range of variation rather than any definite mag- nitude of each property.
AGLITE SINTERED CLAY)
Compressive strength and unit weight:
Structural lightweight concrete in general use to-day has a compres- sive strength ranging from 3,000 lb./sq.in. to 5,000 lb./sq.in. The respective unit weights are about 95 lb./cu.ft. and 105 lb./cu.ft. (except for foamed slag concrete which is 20 lb. cu.ft. heavier on the average).
Thus, lightweight concrete is 30- 35 per cent lighter than normal- weight concrete of the same com- pressive strength. With special mixes, higher compressive strength, up to 8,000 lb. sq.in.. can be at- tained; but in such cases, the unit weight may exceed 115 lb. cu.ft. owing to the high cement content.
The relation between the com- pressive strength and the water cement ratio for lightweight con- crete is the same as for normal- weight concrete, provided that the quantity of "effective" water in the mix is considered. Due to the enormous absorption of lightweight aggregates, the total quantity of water in the lightweight concrete mix is much greater.
Tensile strength:
im...i
{
Fig. 1. Medium-size lightweight aggregates
Both the direct tensile strength
LYTAG (SINTERED P-F. Ash)
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