By Eric Ferrebee, Contributing Author
Jointed plain concrete pavement (JPCP) is the most used type of concrete in the United States. Concrete is desired for its durability and minimal maintenance requirements, and JPCP utilizes various types of joints to reduce slab stresses and improve performance.
Proper jointing design, followed by proper joint construction and activation, is crucial to achieve a long-life pavement.
The most prevalent reason to joint concrete pavement is that it shrinks. This happens through different mechanisms such as thermal shrinkage, which happens as the heat of hydration of fresh concrete is lost over time, drying shrinkage, which occurs as the mix water hydrates and evaporates out of fresh concrete, and chemical shrinkage, which exists because the reaction products of concrete have less volume than the individual materials that go into the mix.
These shrinkage factors combine with restraint from the underlying subbase or subgrade to develop stresses within the concrete that will lead to cracks forming in the concrete.
A typical pavement will crack at intervals of 40–80 feet, and that will progress to approximately 15-20 feet over time. Since cracks cause maintenance and preservation issues, proper jointing can control where these cracks develop, or activate, and can reduce maintenance issues while also providing a more aesthetic surface.
While controlling cracking is the most prominent purpose of jointing, it does have additional benefits. Jointing divides the pavement into construction lanes or increments, which allows for paving, slipform or otherwise, to be done in a number of passes, affording contractors flexibility in phasing their operations.
Jointing also allows for slabs to expand and contract with thermal and moisture cycles throughout the life of the pavement.
Placing joints at intended locations allows for the use of dowel bars, which mechanically transfer the load from one slab to the next and provide a smoother ride long term relative to pavements that rely on aggregate interlock at the joints.
Joint Types and Functions
There are three main types of joints: construction, isolation and contraction. Each can be used in the transverse (perpendicular to traffic) or longitudinal (parallel to traffic) direction.
Construction joints are placed between paving operations and feature a flat joint face. In the transverse direction, these joints also are known as headers, and they often will require dowel bars to transfer loads between slabs due to the flat joint face.
When construction joints lie in the longitudinal direction, tie bars are used to keep them tight. Tie bars do not transfer load, but load transfer is not essential in the longitudinal direction.
Isolation joints are utilized for separating two sections of pavement or separating the pavement from structures or in-pavement objects, such as utilities or drainage structures.
There are numerous configurations of isolation joints, such as a thickened edge, sleeper slab system or even having a doweled isolation joint with an expansion cap.
The most common type of joint is the contraction joint, which is sawed into freshly set concrete.
In the transverse direction, these joints will use dowel bars if the pavement is greater than 7 or 8 inches thick, as that thickness tends to correspond with higher levels of truck traffic where aggregate interlock will not be sufficient to transfer the load long-term. In the longitudinal direction, contraction joints utilize tie bars.
Contraction Joint Activation
When paving a road, a long stretch of concrete is often placed in a continuous operation. This means many contraction joints will need to be sawed to divide the pavement into the smaller slabs that are more resistant to random cracking.
The joints must be sawed at the appropriate time and depth to make sure that cracks activate from the bottom of the saw cut.
The first key to joint activation is sawing the joints at the appropriate time. This brings in the concept of a “sawing window.” The start of the window occurs when the concrete has developed enough strength such that the saw can cut into the matrix and not just ravel and scar the surface by pushing coarse aggregates around.
The end of the window occurs when the internal stress of the concrete, which is developing due to the aforementioned shrinkage mechanisms, exceeds the strength development from the concrete hydration. This is when random cracking will occur.
The sawing window can vary based on many factors. The first is that concrete mixtures have various development rates in terms of strength and stress.
The weather also can play a big role in the concrete’s development, with temperature, humidity, cloud cover and wind impacting strength gain and stress buildup.
The subbase and subgrade also will impact the sawing window due to the friction and restraint placed upon the concrete in addition to the moisture of the underlying layers.
Curing is the last major factor impacting the sawing window. Curing helps keep heat and moisture within the concrete mixture, which in turn helps the cementitious materials react and hydrate.
This allows for earlier strength gain while delaying the development of internal stresses. When done properly, curing will widen the sawing window and result in better, more durable concrete.
To achieve joint activation, the saw cut must not only be done at the appropriate time within the sawing window, but it also must be cut to the proper depth.
Joints activate because the saw cut creates a weakened plane in the concrete. To achieve an appropriate weakened plane, joints need to be cut between one-fourth and one-third of the thickness of the pavement for transverse joints.
Unstabilized bases can be cut to one-fourth of the thickness. Stabilized bases usually have more friction with the concrete and thus require a deeper cut to ensure the joint activates.
Longitudinal joints can be cut to one-third the thickness as there is less need to maintain aggregate interlock for the purpose of load transfer.
When Joints Don’t Activate
Even when joints are saw cut at the proper time and depth, it may take some time for the joints to activate, and some may never activate.
While having some joints that do not activate is not of significant concern, having many joints like this can be problematic. If numerous sequential joints remain uncracked, the result is that an area which would be comprised of a number of small slabs is instead comprised of only one large slab.
Since large slabs tend to expand and contract more than smaller ones, this can lead to joints that open wider than expected. Commonly referred to as dominant joints, these wider joints can have reduced load transfer due to loss of aggregate interlock. They also can allow for an increased amount of moisture and incompressible materials to infiltrate the joint, which can lead to distresses such as spalling and buckling.
Dominant joints have always existed within concrete pavements, but they may have seen an increase in recent years due to a number of factors. The first is the increased push for optimized concrete mixes. These mixtures tend to be more durable and sustainable and may also result in less shrinkage.
While these factors are good for long-term performance, the reduced shrinkage may mean the shrinkage stress development is not great enough to activate the joints.
To address this, one easy strategy would be to ensure the joints are cut to the proper depth; they also might require a slightly deeper cut.
The increased popularity of concrete overlays also can contribute to more dominant joints.
Concrete overlays are a great strategy to rehabilitate or preserve old asphalt or concrete roads as they typically last more than 30 years.
They also tend to be economical because they can be designed to be thinner than conventional concrete pavements (due to having a robust existing support structure and, often, an optimized joint pattern).
While a short and optimized joint pattern can reduce stresses, it also can lead to an increased number of joints and to more that are not activated. Again, cutting the joints to the proper depth can help reduce the prevalence of unactivated joints.
Another strategy that can minimize the development of dominant joints is to saw transverse joints successively. This means ensuring that each saw cut is placed in order, beginning at the start of a day’s paving operation and not skipping cuts. (Skipping cuts may be unavoidable in extreme cases, where random cracking is imminent).
For concrete overlays with shorter joint spacings, sawing transverse joints successively may mean having more saws on a project than would be typical in a concrete pavement or overlay with conventional joint spacings.
Innovative Strategies
Many strategies for joint activation have been attempted. The conventional methods of sawing joints have been the most effective. However, some recent research has examined supplementing this approach with other strategies to improve joint activation.
Research on early loading of concrete pavements has been performed at the Minnesota Department of Transportation’s MnROAD facility. Researchers evaluated the impact of loading the pavement at early ages when the strength of the concrete was still lower than a typical opening strength.
Experiments involved loading the pavement at approximately 3 hours, 5.75 hours, 8.33 hours and 10.5 hours, which corresponded to flexural strengths, estimated through maturity testing, of 73 psi, 196 psi, 267 psi and 318 psi, respectively.
Some of these strengths are well below levels where agencies would load a concrete pavement; however, the experiment did not result in cracking. These results indicate that placing loads on pavements at lower strengths could help activate joints by providing some additional stress at the weakened plane of the saw cut.
Opening pavements at slightly lower strengths also could help construction productivity and provide better access to surrounding communities.
As concrete pavements continue to evolve with sustainable mixtures and optimized designs, it is important to revisit the basics of jointing and understand the reasons behind current practices.
Engineers and contractors continue to strive for activated joints with minimal dominant joints. Paying attention to the sawing window and depth should be monitored and adjusted as needed to achieve appropriate performance. RB
Eric Ferrebee, P.E., is a senior director of technical services at the American Concrete Pavement Association.
