these units is subject to special Codes of Practice. Precast prestressed beams of various sizes with post- tensioned reinforcement have found increasing use in the United States.
The use of prestressed gravel concrete beams limited above
certain spans not merely by the increasing dead weight which ab- sorbs a large proportion of their loadbearing capacity, but
also by
the facilities required for transport- ing such heavy precast members. By using lightweight concrete a considerable reduction in weight can be achieved, so that available lifting and transport facilities
can
be used. or. for a given weight. larger spans become possible. Be- cause of the greater shrinkage and creep of lightweight concrete and its greater elastic deformation. the losses of initial prestress are also greater with prestressed lightweight concrete than with ordinary gravel concrete, and this must be taken in- to account in design.
or
Lightweight concrete is particul- arly suitable for composite types of construction, used extensively both for bridge-deck systems and floor systems, combining steel girders and reinforced concrete slabs prestressed precast concrete girders and cast in situ slabs; prestressed gravel concrete girders have recent- ly been used with a lightweight con- crete filling in motorway bridges in the United Kingdom. thus reducing dead weight and also improving the impact characteristics of the system. The use of compacted lightweight concrete in steel and concrete slab- and-girder systems will also increase their useful loadbearing capacity. In such systems, the top slab and girders are combined by means of different types of shear connector or by prestressing. In one type of construction. precast lightweight concrete slabs are attached to the main girders by means of highten- sile steel bolts: the friction between the members due to the pressure of the steel bolts suffices to resist the horizontal shear force. By using composite methods of construction. stiffness and ultimate loadbearing capacity are greatly increased and, since the deflections are small in any case, the use of lightweight concrete has little untoward effect on stiff. ness. Owing to the lower modulus of elasticity of lightweight aggregate
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concrete compared with dense gravel concrete, the neutral axis of light- weight concrene flexural members tends to be lower and thus allows slightly higher stresses to be reach- ed at the top flange of the steel girder. Normally with composite slab-and-girder systems. the top flange of the steel girder is not fully utilized. leading to inadequate use of the steel section, particularly at working loads. under concentrated loading. Lightweight concrete can thus lead to a more efficient use of the compressive flange.
DESIGN CONSIDERATIONS
The design of reinforced concrete structures is governed in this coun- try by B.S. Code of Practice 114: 1957, which provides for the use of reinforced lightweight con crete for subsidiary members and indeed even for principal members. subject to the engineer's discretion. Research in recent years suggests, however, that in some of its re quirements for lightweight concrete this Code is perhaps too restrictive while in others it may not be adequately safe. To remedy these difficulties and in anticipation of a revision of the Code of Practice. certain recommendations have been made by the Building Research Station, most of which have been incorporated in tentative. mendations made by the London County Council for the structural
use
recom-
of lightweight concrete within their jurisdiction. These recom- mendations are set out in the Ap pendix of this Digest, and some of the clauses are referred to in the following discussion.
Reinforced lightweight concrete is not fundamentally new; it is merely a variant of an established and well-tried method of construc- tion. Its acceptance is based on practical experience obtained in Europe and U.S.A., as well as on laboratory researches,
In general. well compacted structural lightweight concrete, even when of comparable compressive strength, differs from gravel con- crete in its density, modulus of elasticity, shrinkage. creep and thermal expansion, bond strength and shear strength. These differ- ences should be taken into account in the design of structural elements.
without
but only rarely can they lessen the safety of lightweight concrete struc- tures appreciably. Test results in- dicate that where these digerences are disregarded in design. and the design rules laid down in C.P.114: 1957 are used modification, the resulting struc- tures usually possess an adequately high load-factor though some of their functional characteristics may be different from those of reinforced gravel concrete structures.
concrete
Some doubts have been expressed by engineers as to the durability of reinforced lightweight structures, and in particular, the protection afforded against corro- sion of the steel reinforcement embedded in a concrete which con- tains such porous lightweight ag- gregates. This subject of corrosion has been dealt with in Digest 25 (second series). Doubts have also been expressed about the stiffness of such structures in view of the reduc ed modulus of elasticity of light- weight concrete compared with that of gravel concrete of equal com- pressive strength. This and some of the other more important design considerations are discussed below.
Some doubts have been expressed by engineers as to the durability of reinforced lightweight concrete structures. and in particular. the pro. tection afforded against corrosion of the steel reinforcement embedded in a concrete which contains such por- ous lightweight aggregates. This subject of corrosion has been dealt with in Digest 25 (second series). Doubts have also been expressed about the stiffness of such structures in view of the reduced modulus of elasticity of lightweight concrete compared with that of gravel con- crete of equal compressive strength. This and some of the other more im. portant design considerations are discussed below.
Shear strength
According to modern design con- cepts. the load-factor against failure in shear should be higher than that against failure in bending. Where no special shear reinforcement is used in the form of stirrups or dia. gonal bars, diagonal cracking can occur as a result of unforeseen cir- cumstances and weaknesses in the concrete. For major structural ele- ments such as main beams, there-
THE HONG KONG & FAR EAST BUILDER
VOLUME 18, NUMBER 1