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There are numerous types of joints utilized in concrete pavements and all of them serve a specific purpose. All joints are designed in some way or another to help the pavement achieve its design life. Jointed plain concrete pavement (JPCP) is most representative of how concrete pavements utilize joints but all concrete pavement types, including jointed reinforced (JRCP) and continuously reinforced concrete pavement (CRCP), use joints for a number of reasons. This page looks at the purpose and history of joints as well as their proper design and construction.
Why Joint Concrete Pavements?
The first reason for jointing concrete pavements is because concrete shrinks. This happens under a few different mechanisms. Drying shrinkage is the result of water being used for hydration while it is also lost to the atmosphere through evaporation. Thermal shrinkage is the product of the heat of hydration of the concrete. As cement is mixed with water, the mixture gets hot. As the concrete sets and cools, heat is given off and the pavement shrinks. Chemical shrinkage occurs because the products of the cement hydration (concrete) occupy less volume than the reactants (cement and water in addition to aggregates).
Shrinkage itself is not an issue and ultimately wouldn't be an issue if pavements were not subject to restraint. If the concrete is free to shrink, as it would be if it were constructed on a frictionless base, then shrinkage does not cause any issues. However, friction from the subbase or subgrade provides resistance to the shrinkage forces, which in turn builds up internal stresses. As shrinkage progresses over time, the internal stresses build up, eventually reaching the tensile strength of the concrete and the pavement will begin to crack. Without joints, the concrete will naturally begin to crack at about a 40-80 ft interval to relieve the built-up internal stress. As shrinkage continues to progress, cracking will continue at an interval of 15-20 ft. This interval will occur not only transversely, but longitudinally as well because the restraining forces applies to all directions. This cracking can occur completely due to shrinkage, without any load being applied.
To control the cracking, concrete pavements can be jointed at an interval less than what the concrete would crack naturally. This cuts the pavement into slabs or panels that are small enough that the internal stresses are minimized and random or mid-slab cracking is eliminated. The joints can be sawed to make slabs about 15-18 ft long and typically around 12 ft wide. The timing of the saw cut is important as sawing too early results in ravelling and sawing too late results in natural cracking beginning.
While controlling cracking due to shrinkage is the primary reason we joint concrete pavements, there are a number of other reasons. One such reason is that jointing allows division of the pavement into construction lanes or increments. This can be through longitudinal joints that occur at the edges of a slipform paver as it passes or at a header at a transverse joint at the end of a day's paving. This helps the contractor construct the pavement. Since many times longitudinal joints happen to occur at the same location as the edges of lanes, they often help delineate lanes for motorists. However, delineating lanes is not one of the reasons to joint pavements. It can be beneficial to place longitudinal joints in the center of lanes if it eliminates a pass of the paver or moves the loading away from the edge of the slabs.
Another reason to joint concrete pavements is to accommodate slab movement. Isolation joints provide room for a concrete pavement to expand and contract without pushing up against adjacent structures. The expansion and contraction of concrete occurs in cycles as weather fluctuates with humidity and temperature.
Joints can also provide load transfer through placed dowels because joints can be sawed over the location of the dowels. The dowels can be placed with baskets on grade or with a dowel bar inserter (DBI). Such load transfer is not provided over a crack because cracks occur at unplanned locations. If a pavement is not jointed, then planning where the dowels need to be placed becomes difficult. To have restore load transfer over a crack, dowel bars can be retrofitted into the pavement if aggregate interlock will not provide enough load transfer.