Dowels in Jointed Plain Concrete Pavements

The general type of concrete structures on German highways is the Jointed Plain Concrete Pavement (JPCP). Contraction joints are saw-cut with a standard spacing of 5,0 m perpendicular to road axis and sealed by a bitumen based or elastomeric joint filler. International experiences indicated clearly that faulting at undowelled transverse joints develops, regardless on pavement structure and climate. Therefore dowels ? 25 mm, length 500 mm, are set to improve the longterm behaviour. Corrosion resistance of the dowels requires a respective coating of ? 0,3 mm, which also should limit the bond restraint between dowel and concrete to prevent excessive concrete stresses, when the joint is opening due to temperature decrease and/or shrinkage. In quality control of dowel production the permissible extraction forces of a dowel embedded in a concrete prism of less than 24 hours age is limited to ? 18 kN (at 0,25 mm movement) at the first loading and to less than 12 kN at the 5th loading. Longterm experiences indicate that these dowel restraints enable the required uniform joint cracking if the dowels are installed merely parallel to road axis. Standard practice for dowel placing is automatically setting by vibration. The achieved accuracy of alignment depends on the kind of slipform paving in one or two layers. More care is necessary with paving in one layer to prevent irregularities in the concrete structure above the dowels; therefore sometimes dowel baskets are used in this case. On the other hand the position of the dowels can be adversely influenced by the compaction process of the upper concrete layer (thickness ? 40 mm), if paving of the concrete is performed in 2 layers. The permissible position tolerances of dowel alignment are based on the results of theoretical and experimental investigations, performed by ordership of the Federal Ministry of Traffic (BMVBW). A series of test specimens extraction experiments were carried out to determine the effect of skew dowels on the extraction resistance. Each test specimen had two dowels obliquely towards each other with exactly predetermined tilt between 0 mm and 40 mm related to dowel length. On the basis of the test results a skew of maximum 20 mm is allowed in German specifications. Additionally the effect of different dowel embedment perpendicular to the transverse joint (longitudinal translation) was investigated. Only a length of dowel embedment less than 100 mm causes significant increase of stresses and displacements at the transverse contraction joint. Up to now a control of dowel alignment is only possible by taking drilling cores at critical spots, mostly indicated by surface crack initiation or irregularities. The results of running test series indicate that in future it seems to be possible to control the correct dowel alignment either by means of georadar or by electro magnetic induction. The latter method enables to perform the measurements not only at the hardened concrete, but already at the freshly paved concrete.