fore, the internal shear forces should always be resisted by shear rein. forcement whatever type of aggre gate is used. In general this is not necessary for subsidiary members if the calculated shear stresses are low- er than the permissible shear stresses given in Č.P.114: 1957, nor is it usually required for reinforced con- crete slabs in which actual shear stresses are normally low.
of
Tests have been made at the Building Research Station on beams without shear reinforcement made both of gravel concrete and various types of light-weight aggre- gate concrete. Modulus of rupture and cylinder splitting tests had in- dicated that, for the same compres- sive strength, there was no im. portant difference between the ten- sile strengths of the gravel and lightweight concretes; it might have been expected, therefore, that beams made of the two types of concrete and having identical crushing strength would have a similar resis- tance to diagonal tension cracking. In fact the loads at which diagonal cracking occurred in the lightweight concrete were between 80 and 95% of the average loads at which dia- gonal cracking occurred in the gravel concrete. It follows from this that the permissible shear stress for lightweight concrete should be 20 to 25% less than that for concrete made with gravel aggregates.
The method of design for flexural members with special web reinforce- ment given in C.P.114: 1957 leads to shear reinforcement that is amply sufficient to ensure
that the beam develops its full bending strength before failure in diagonal tension can occur. The use of these recom- mendations for lightweight concrete beams would therefore be safe.
be
Clause 6 of the recommendations of the L.C.C. on the use of light- weight concrete for reinforced con- crete structures requiring shear rein. forcement (see Appendix) can interpreted as dealing with the shear resistance of main beams only. For flexural members of subsidiary im- portance, such as lintels or floor and roof slabs, the shear resistance of the concrete may still be taken into account provided that the permis sible shear stress in the concrete used in design is reduced to comply with Clauses 5a and 5b.
110
Bond strength
ness,
For adequate strength and stiff- the internal forces in rein- forced concrete structures must be transmitted heween the steel and the concrete by bond resistance acting on the surface of the reinforcing bars. Bond resistance depends main- ly on adhesion between the steel and the concrete, on frictional forces at the contact surfaces. and on mechanical anchorage forces. In general, frictional forces cannot be come operative until adhesion is overcome and, in turn. mechanical anchorage along the bars or at the ends of bars will only become effec- tive after the frictional resistance has disappeared.
at
Bond resistance should be such that the load factor against failure due to a breakdown in bond is least as high as the load factor against bending failure, to that sudden collapse does not occur.
ensure
Tests at the Building Research Station have shown that, for plain round mildsteel reinforcement, the bond strength obtained is variable and that it is, in general, somewhat lower for lightweight concrete than for gravel concrete having the same cube strength. This reduced bond strength is not inherent in the ma- terial itself, since with the bars held vertically during casting the hond strength was found to be of the same order whatever the type of aggre- gate used; it appears to be due to the formation of shallow but some- times widespread spaces in the con- crete cover, next to the horizontal steel bars. Their formation is prob- ably connected with the higher water content of lightweight con- crete mixes as compared with those. of gravel concrete; loss of excess water may lead to sinking of the concrete beneath the reinforcement and thus to some separation of the cover from the steel. To a much smaller extent, these shallow spaces ('cavitation') also occur in gravel concrete. Cavitation can be reduced considerably by careful mix design and good workmanship, but at pre- sent it is difficult to ensure that con- crete made on the site or in the precasting factory attains its maximum bond strength consistent-
ly.
For plain round mild-steel re- inforcement in lightweight concrete. therefore, the permissible bond in C.P.114: 1957 given in
stresses
would appear to be too high and should be halved. as reflected in Clause 5a and 56 of the L.C.C. re- commendations.
Tests on beams containing de- formed high-yield reinforcement showed that the failing loads were of the same order for both grave! concrete and lightweight concrete beams, but that, for the same bond stress, the slip at the ends of the reinforcing bars was greater
for the latter: in lightweight concrete beams a slip of 0.001 in. was reach- ed at a bond stress that was only half to three-quarters of the stress needed to produce the same slip in gravel concrete beams. For the time being, therefore, for deformed bars as well
as plain mild-steel round bars. lightweight concrete members should be designed with permissible bond stresses that are one-half those recommended in C.P.114: 1957. With deformed bars, how- ever, the bond strength is not likely to be critical and. with improved mix design and site control, light- weight concrete with greatly im- proved bond characteristics may be generally available in the near future. Inflexible values for permis- sible bond stresses should not there- fore he established for lightweight concrete and permissible stresses might be based
in future on some form of bond test.
of
In the U.S.A., where experience with structural grade lightweight concrete is greater than in Britain. the addition of air-entraining agents in the mix has been found to lead to a very great im- provement in workability and dur- ability as well as in the bond strength of reinforced lightweight concrete. No reduction in permis- sible bond stresses has therefore been introduced for lightweight concrete in the American building code recommendations.
In lightweight concrete members containing reinforcement held ver- tically during casting such as columns the permissible bond stresses laid down in C.P.114: 1957 for
dense aggregate concrete can
THE HONG KONG & FAR EAST BUILDER - VOLUME 18, NUMBER 1
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