I wish to raise two points in connexion with the Paper which presents some much needed data obtained from field measurements.
62.The calculation in Q 46 on the critical percentage of reinforcement must have been made by many engineers but the fact remains that adequate crack distribution can be achieved with lower percentages. The reason may be connected with the fact that plane sections normal to a bar do not remain plane immediately adjacent to it.The stress-raising effect of the bar may, therefore, propagate cracks at lower percentages. This is a problem that might usefully be examined using the dynamic -relaxation technique described in Paper 7033.63.. Secondly, it seems likely that the recommended precautions will not prove to be very practical and the proposal to add ice to the water could be deleterious should some of the ice fail to melt prior to placing. Nevertheless the object of preventing a iemperature rise by heat of hydration is a very desirable one.
The following contribute towards this:(U) Avoiding excessively thick sections, in excess of 12 inches.(b) In hot weather, stripping shutters as soon as possible.
65.In fact there appears to be a case for re-examining the CP 20078 requirement for limiting the uncracked concrete stresses. This leads to thick sections with, in some cases, a relatively low percentage of reinforcement. There is a theoretical advantage in having thinner, more heavily reinforced sections.
M r 1. B. Aitken, Construction Manager, Jarnes Miller & Partners LtdThe Authors appear to have dismissed the shrinkage characteristics of concrete rather lightly-aggregate shrinkage is very much in vogue and blamed for a number of badly cracked structures. Knowledge, therefore, of the drying shrinkage values of the aggregates used would be of interest.67. The subject in general raises the question of the designer in this connexion and his responsibility. Surely, for a good design of a concrete structure, the designer must accept the deficiencies of the material he will be using and design accordingly, viz. the designer should design the reinforcement to suit his, and only his, positioning of construction joints. He should also position narrow construction gaps (with reinforcement lapped therein.) which, from his own constructional experience, he knows will suit the contractor's method of working. He should then virtually guarantee that what cracks there are will occur at the construction joints and that these, particularly in buildings, will not be detrimental both to appearance and efficiency.68. My overseas experience with cracks, in walls 5-6 ft thick, up to 0.50 mm showed that cracking was eliminated when reinforcement was lapped in contraction gaps and that the temperature rise (35°C) was exactly the same in summer and winter (cf. 0 50) but with a delay of 20 hours in the winter hydration temperature peak.Paper published: Proc. Znsrn ciu. Engrs, 1968,39 (Jan.) 111-125.
