Concrete pavements without and with dowels

The deformation behaviour of a transverse joint, without and with dowel bars, under the passage of a wheel, as calculated with the aid of Westergaard's formulae /1/ is represented in fig. 1. This shows that when there are no dowel bars and no aggregate interlock across the crack (efficiency index W ~ 0%) the difference in deflection ~ y of the joint edges is about ten times greater then wben dowel bars are provided (W - 90%). For load acting at the centre of the slab and causing an in-situ deflection of 0.1 to 0.2 mm (22 cm thick concrete pavement on a stabilized sub-base) the difference in deflection in undowelled construction is 0.35 toO.70 mm, as compared with 0.035 to 0.07 mm with dowelled construction. This is confirmed by measurements performed by the Testing Laboratory for Highway Construction, Technological University of Munich /2,3/ . The deflection curves for the centrally loaded slab and at transverse joint without and with dowel bars are represented in fig. 2. It shows the increase and decrease in the deflection of the two slab edges which occur under the action of a wheel crossing the joint. Thus at an undowelled joint without shear transfer there occurs an abrupt increase and an abrupt decrease. Under frequent load repetitions this produces the familiar "pumping" phenomenon, leading to "faulting" (or "stepping") at the joints, with a deterioration in riding comfort and worsening of service conditions of the pavement. On the other hand, with dowel bars the deflection curve deviates only little from a continuous curve • so that favourable conditions e~ist for a good long-term behaviour. It must furthermore be considered that without shear transfer the flexural stresses at the slab edges are about twice as great. This necessitates making the concrete slab about 40% thicker in order to avoid premature cracking. Despite such thickening, the difference in deflection of the edges at a joint is still about six times as great as that in a thinner pavement with dowelled joints. This is so because, as a first approximation, with regard to the flexural tensile stress the slab thickness decreases in proportion to the square, whereas with regard to the deflection calculation the slab thickness decreases in proportion to the 1.5tb pow-er. According to available measurements, the efficiency index for a new- concrete pavement is about 90 to 95% witb dowel bars and about 80% without dow-el bars. Under frequent repetition of traffic loads, bow-ever, the efficiency index declines in the undowelled pavement and attains in the most unfavourable case a value of 0% (wide open joint). In this context it must be borne in mind that under Central European conditions the contraction joints in a concrete pavement built w-ithout expansion joints will become 0.5 to 0.7 mm w-ider over a period of 3 to 5 years /4/ • From the experience that has been gained it appears that the transverse joints should be provided with dowel bars in order to obviate "pumping" which adversely affects the long-term behaviour under frequent traffic load repetitions (fig. 3) .