STUDY OF SOME ASPECTS OF STRENGTHENING THIN CONCRETE PAVEMENT WITH FLEXIBLE OVERLAYS

Reported in this paper is a study on the strengthening with flexible over-lays of thin concrete highway pavements existing in India. Concrete panels of 10 ft. by 10 ft. by 4 in. were cast indoors with typical ground support conditions. Strain gauges were fixed to their under-faces by mounting the gauges on precast concrete blocks which were made integral with the slab concrete. Granular overlays of 3, 6 and 9 in. W.B.M. were constructed successively on one panel for testing, and similarly bituminous overlays of 2, 4 and 6 in. A.C. were laid on the other. Static load tests were carried out at interior, edge and corner positions using 12-in. diameter steel plates with loads applied up to 9000 lbs. Strains and surface deflections were measured. The load test data show that flexible overlays do bring about reduction in load stress in concrete slab but that reduction is relatively limited. Broadly speaking, 9-in. W.B.M. overlay or 6-in. A.C. overlay reduces the load stress to about 60 percent for all the three load positions. Analysis of the pavement as an elastic layered system corroborates the test measurements and indicates that even 4-in. concrete slab acts as quite a rigid base. Test data show that the equivalent angle of load dispersion for W.B.M. overlays increases from 33 deg.-34 deg. for interior loading to 37 deg.-38 deg. for edge loading to 45 deg.-47 deg. for corner loading. The corresponding values for A.C. overlays are 45 deg., 49 deg.-51 deg. and 51 deg.-53 deg. One inch of A.C. overlay is seen to be equivalent to 1.3-1.7 in. of W.B.M. overlay. Test data were also developed through measurements on outdoor sections on the effect of flexible overlays on temperature differential in concrete slab. The data show that the differential is reduced to about 50 percent by plus 4 in. A.C. overlay and to about 40 percent by plus 6 in. W.B.M. overlay. The study indicates that 4-in. concrete pavement cannot be saved from distress with reasonable thicknesses of flexible overlays under long term application of 9000-lb wheel load. Where thick subbases already exist, as for most of such roads in India, it would appear desirable to provide the flexible overlay from the point of flexible pavement requirements, keeping reflection cracking, etc. in view. Measures to take the operation of loads away from the inside of the concrete edges would go to enhance the life of concrete slab. Reported in the paper are also broad indications available from 5-year performance study of about 50 different specifications of flexible overlays laid as an experiment on sections with cracked and uncracked concrete slabs. In the case of uncracked slabs, reflection cracking has developed in overlays of up to 6 in. thickness. There is as yet no reflection cracking in plus 4.5 in. overlays on cracked slabs. Asphaltic concrete, with a high binder content, has shown good resistance to reflection cracking. /AUTHOR/