Effect of cement-bound sub-bases on cracking

Paper by SPRIHGENSCHMID SONNEWALD from ISCR 5th 1986 Aachen Germany

Cement-bound sub-bases (CaSBs) are constructed ~ithout joints. During and after hardening, cracks may be formed in consequence of drying and/or cooling, even if the sub-base has noc yet been subjected co any traffic load. It would be desirable to have no cracks or only very fine ones (capable of eransmieting force across chem). Wide open cracks are liable to impair the functioning of the pavement structure as a whole. The laboratory investigations so far carried out have been confined mainly to measuring the unrestrained deformation of test specimens due co shrinkage or thermal expansion. The visco-elascic properties (e.g., relaxation) of the specimens are not taken into accounc in such tests. Longitudinal scresses due to drying and temperature change in conjunction ~ith virtually complete restraint of the specimen can be measured ~ieh the aid of a testing device (cracking frame). Measuring begins immediately after che fresh conseruction mix has been placed in the device. The hardening conditions (temperature, atmospheric humidity) can be varied within wide ranges. The cross-sectional dimensions oi the specimens (150 mm x 150 mm) are large enough to allow the use of 32 mm maximum aggregate particle size. For studying the cracking susce~tibility of cementbound sub-bases the follo~ing influences have so far been more closely investigated: type of aggregate (round,crushed), grading, content of particles under 0.063 mm, ~ater content in the as-placed condicion, type of cement, cement content, and type of curing applied. The results indicate how the stresses due co 'temerature and shrinkage can be reduced in a controlled manner; using a cement with appropriate strength development and/or little s~elling, together ~ith adequate curing, keeps the stresses in the hardening cement-bound subbase do~ to low values. In this way the cracking temperature of a caSB mix installed at, say, 20 ·c can be lo~ered to below O'C, ~hereas it would be about 10'C in the absence of this precaution. Tests in the cracking frame also allow a better assessment o.f the mechanica~ properties of CBSB mixes made wi th aggregates derived from hitherto unutilized sources, e.g., by-products of che gravel indusery (fine sands). Fureher experimental results relaeing to such materials ~ill be available at che time of presentation of this paper.

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