safely be used (see Appendix. Clause 5d.
Deflection under load
same
When subjected to the direct load or to the same bending moment, structural members made with lightweight concrete tend to deform more than similiar gravel concrete members. This is partly due to the lower modulus of elastici- ty of lightweight concretes and their greater creep. and partly to the somewhat greater extent of cracking under load. In flexural members the deflection may be between 10 and 40% greater, depending on the type of concrete and the reinforce- ment percentage.
Agreed criteria of stiffness are difficult to define for the design of flexural members. The current Code C.P.114: 1957 recommends maximum span/depth ratios for dif ferent types of element. These have been found by experience to be generally satisfactory for gravel concrete beams and slabs designed with somewhat lower permissible steel stresses than those adopted in C.P.114. Partly as a result of the reduction in dead load. lightweight concrete flexural members tend to be somewhat shallower than concrete ones.
gravel
To ensure that the deflections of lightweight concrete members are not excessive. it would seem advis- able to reduce the permissible span / depth ratios laid down in C.P.114: 1957 by at least 10% (see Appen- dix, Clause 8).
The greater flexibility of light- weight concrete members should be taken into account where failure may occur as a result of excessive deflection, for example. in long columns.
When deformed high-yield steel elastic modulus may be a disadvant- age in some cases, though not usual. ly a serious one. Structures for which resistance to blast. impact. vibration, subsidence or earthquake must be considered in design, might be better constructed with concrete having a low elastic modulus and a high capacity for deformation be- fore failure occurs.
112
Permissible stresses in the concrete and the steel
For the same cross-section, the stress distribution in reinforced lightweight concrete flexural mem bers may differ somewhat from that in similar gravel concrete members. For example, owing to the reduced modulus of elasticity of lightweight concrete, under working loads the position of the neutral axis in the cross-section is lower; for the same internal moment the maximum com- pressive stress will therefore be less. while the tensile stress in reinforce- ment will be greater compared with similar gravel concrete beams.
According to C.P.114: 1957. the loadfactor (or safety factor when the elastic method of design is used) against failure of the concrete in compression must be increased from 3 to 4 where the aggregate used for a nominal concrete mix does not comply with B.S.882. Strict inter- pretation of the Code would there- fore lead to the use of lower permis- sible stresses for lightweight con- crete (Appendix. Clause 5a). This recommendation places these light- weight aggregates at a disadvantage compared with possibly inferior aggregates of natural origin. This discrimination does not apply where 'special mixes' designed for strength are used (Appendix, Clause 5b). but the supervision at site or factory should then be more thorough than with nominal mixes. In general. ready-mixed concrete would also fall under Clause 5b.
With regard to the reinforcement. the actual tensile stresses are generally somewhat higher in light- weight concrete beams than in similar beams made of gravel con- crete and, owing to the différent bond characteristics, the width of the cracks in the concrete is likely to be slightly greater. For plain round mild-steel reinforcement the crack widths are not critical in respect of corrosion of the reinforcement damage to finishes; it seems reason- able, therefore, that in bending the same permissible tensile stress (20 000 lb/in2) should be used for the reinforcement whatever the ag- gregate used for the concrete.
or
When deformed high-yield steel bars are used in gravel concrete
beams, a permissible tensile stress of 30 000 lb/in2 is at present re- garded as the acceptable limit in order to control crack widths. For
reinforced lightweight concrete beams, experimental results suggest that a rather lower value, of 27000 Ib/in2, would be reasonable. For reinforced lightweight concrete slabs. on the other hand, the need for such a reduction of the permissible ten- sile stress in the steel from 30 000 lb/in2 to 27 000 lb/in2 would seem less obvious. In slabs the actual stresses are usually lower than the calculated stresses and, in the ab- sence of a more
accurate design method for reinforced concrete slabs, the permissible steel stresses recommended in the Code of Prac- tice are regarded as suitable for lightweight concrete as well. i.e. 30 000 lb/in2 maximum
(see Ap- pendix, Clause 7).
Fire resistance
are
The fire resistance of reinforced concrete must satisfy the require. ments laid lown in building by- laws. in terms of the periods of fire resistance of the various elements of the structure when tested in ac- cordance with B.S.476 Fire tests on building materials and structures'. Various types of aggregate classified in the Model Bylaws ac- cording to the fire resistance of concrete made with them. Light- weight aggregates are Class I ag- gregates. signifying a very high fire resistance. Most dense natural ag- gregates, such as gravel or crushed rock, are Class II aggregates, hav- ing a somewhat lower fire resistance (see Digest 25).
Sufficient test results on the be haviour of reinforced concrete made with all the various types of light- weight aggregate commercially avail. able in the United Kingdom are not available. Definite recommendations distinguishing reinforced lightweight and gravel concretes with regard to fire resistance, in relation to Clause 8 of C.P.114: 1957, cannot be made therefore as yet. Nevertheless, it is clear from the test results that are available to date that the fire resis- tance of reinforced lightweight con- crete members is much higher than that of similar members made of gravel concrete.
THE HONG KONG & FAR EAST BUILDER VOLUME 18, NUMBER 1
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