April_1967 — Page 30

and the flexural tensile strength (modulus of rupture) of lightweight concrete compare favourably with those of normal-weight concrete, if determined with moist-cured speci-

mens.

As an approximation, the direct tensile strength may be taken as

the 5V/ and

flexural tensile strength as 81/s, for both types of concrete, where ca is the cube strength of the concrete.

When lightweight concrete dries up, internal stresses are induced by non-uniform shrinkage. The effect may cause a reduction in the tensile strength as much as 40 per cent.

Bond strength:

same.

Pull-out tests have shown that the bond strengths of lightweight concrete and normal-weight con- crete are practically the However, poorer bond is expected in lightweight concrete beams owing to the greater tendency for tiny air and water bubbles to accumulate beneath the horizontal bars during concreting.

Tests on special bond-beams have indicated that the bond strength of under this lightweight concrete

condition is 30-50 per cent lower than that of normal-weight concrete.

Modulus of elasticity:

The modulus of elasticity of lightweight concrete, ranging from 1.5 x 106 lb./sq. in. to 3.0 x 106 lb. sq. in. is 2 to 2/3 of the modulus of elasticity of normal- weight concrete. It increases with the unit weight and the compres- sive strength of the concrete. Two formulae of correlation have been derived from experimental results:

(i) Ec 33w2/3 √c

d.com

=

3w2 √//c

u

(ii) Ec where E, denotes the modulus of

elasticity in lb. sq. in.

w denotes the unit weight

in lb cu. ft.

cy denotes

the cylinder strength in lb. sq. in. cu denotes the cube strength

in lb./sq in.

Granite

Fig. 2. Exposed-aggregate surfaces

50

Shrinkage:

Shrinkage of lightweight con- crete varies between wide limits .0005 to .0011. An average of .0007 is generally adopted for de- sign purposes. Though shrinkage strains are large, the stresses asso- ciated with these strains are most cases comparable to those en- countered in normal-weight con- crete, owing to its low modulus of elasticity.

Creep:

in

Creep of lightweight concrete varies considerably, depending very much on the quality of the aggre- normal- gates. Compared with weight concrete, lightweight con- crete generally creeps more under the same condition.

The unit creep strain lies within the range of 0.4 x 10-6 to 0.9 x 10-6 per lb./sq. in., and decreases with increasing compressive strength. If the compressive strength is above 5000 lb./sq. in., the unit creep strain is less than 0.6 x 10-6 per lb./sq. in.

Basis of Design

Laboratory research, as well as site experience, has shown that conven- tional design methods are applicable to reinforced or prestressed light- weighweight concrete, provided that the basic properties of the material are taken into account.

In the American Building Code ACI 318-63,(1) lightweight concrete is treated on an equal basis with normal-weight concrete, except for shear and diagonal tension for which separate provisions are laid down for the two types of concrete.

The British Code of Practice for Prestressed Concrete, CP 115:1959(2),

though allowing the use of lightweight aggregates in building construction, is in fact not meant to cater for light- weight concrete.

The LCC Specification on light- weight concrete, (3) based on the re- commendations of the Building Re- search Station (4) constitutes a set of comprehensive regulations govern- ing the design of reinforced light-

Lytag

weight concrete structures. The main provisions in this specification are re- ductions in the permissible shear stresses, bond stresses and span/depth ratios, as compared with the corres- ponding values for normal-weight

concrete.

Similar clauses are included in the amended Code of Practice for Rein- forced Concrete, CP 114: 1957(5) and the new Code of Practice for Precast Concrete, CP 116:1965(6).

Application and Typical Examples

Lightweight concrete is often used not as a substitute for normal-weight concrete but one that is especially suited in its own right for particular purposes. Its low unit weight leads to a considerable reduction in the dead load of the structure, and also results in lower handling and hauling costs.

Other advantages of light- weight concrete include a high degree of thermal insulation, acoustic insula- tion and fire resistance.

Apart from its use in building con- crete barges. lightweight concrete was mainly applied to load-bearing block walls during its early years. In the 1950's, pumice, foamed slag and ex- panded clay were widely used in the precast concrete industry in various parts of Europe. Prefabricated rein- forced lightweight concrete floor and roof slabs, as well as wall panels of storey-height, were not uncommon.

In the USA, owing to the high costs. involved in delivering natural aggre gates from far-off quarries to building sites, it is often more economical to use locally-produced lightweight ag- gregates. Among the early examples of lightweight concrete structures is the Statler-Hilton Hotel in Dallas; in this 18-storey building of flat-plate de- sign, the use of light-weight concrete instead of gravel concrete resulted in much lighter column loads.

Another typical example is the Greyhound Bus Terminal in Pitts- was built under and burgh, which around an existing railway plate-gir- der bridge; the need for large col- umn-free areas, as well as a high de- gree of fire resistance, dictated the use of lightweight concrete for the building.

In a 24-storey apartment house in Dallas, high strength lightweight con- crete (6000 lb./sq. in.) was used in conjunction with high tensile steel bars to keep the column sizes within certain architectural limitations. To quote the consulting engineer: "The saving of nearly 8,000 tons of dead load by the use of expanded shale rather than crushed rock as an ag- gregate was one of the factors which allowed the extraordinary design of the building to be expressed in con- crete."(7)

Application of lightweight concrete to shell structures has two advantages: (a) It reduces the total load trans- ferred to supporting elements and to the foundations; (b) It enables the use of larger and fewer precast elements for each shell. The most notable

Far East Architect & Builder April, 1967

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