By Matthew J. Zeller, Contributing Author
Temperature extremes are becoming more pronounced, and road damage caused by severe weather patterns threaten our mobility. Our infrastructure faces excessive expansion and contraction in pavements.
Vehicle load is another stressor for pavements. For example, Minnesota experiences steady truck traffic associated with farming. The state’s range in temperature—close to 100 degrees in the summer to more than 20 degrees below zero in the winter—also places stress on the state’s pavements. Thanks to the Concrete Pavement Association of Minnesota (CPAM), the state is a leader in determining best practices for the installation and maintenance of concrete pavements.
The work of the association has demonstrated that, despite environmental and other stresses, concrete pavement can be designed to last 50 years or more, and has a record of success achieving this goal with proper maintenance.
As with any other asset—such as the automobiles we drive—establishing a routine of proper maintenance should be a given. Owners should have pavement management systems in place that define trigger values, which guide pavement maintenance decisions and ensure that preservation occurs in a timely manner.
Concrete pavements can endure a certain amount of truck traffic and other heavy loading before the pavement “fails.” But what is failure for a concrete pavement? Cracks, roughness and shattered panels are commonly recognized indicators of distress. Yet to understand which of these indicators truly represents a problem and to determine the most effective means of repair, it is important to examine individual areas and assess root causes. Combinations of distresses that present together can provide clues about underlying causes and should be analyzed during visual inspection.
Repairing Cracked or Buckled Pavement
Some cracks may not need to be repaired. This is because concrete pavement is designed with joints to control, or constrain, the normal cracking that occurs with drying and shrinkage. If a pavement displays a transverse crack that appears likely to lead to faulting or a rough ride, a simple, targeted solution is to retrofit the crack with dowel bars to improve load transfer at the location.
Longitudinal cracks usually occur because of movement or settlement in the structure below the pavement. Often these cracks do not require repair unless assessment reveals there is a likelihood of the crack opening to allow the pavement to separate. In this case, tie bars are the solution, and they can be cross stitched into the pavement to arrest movement.
Special attention should be paid to crack networks adjacent to joints. These are typically related to joint failure, which occurs when joints become “locked” (i.e., unable to expand and contract with seasonal temperature changes). For a 100-degree change in temperature, concrete will expand or contract about two-thirds of an inch over a 100-foot stretch of pavement.
In Minnesota, most pavements are designed with joints that divide the pavement into 15-foot-long panels, which translates to one-eighths of an inch of movement—an amount that is easily accommodated when pavements function as designed. However, when joints fill with incompressible material (debris that occupies space, causing the void below the joint to become rigid), the situation can change.
The problem is exacerbated if not all joint locations crack as intended. Having uncracked joints makes the effective length of a pavement panel longer than 15 feet and this causes joint cracks to widen where they do occur, sometimes to as much as half an inch. These wider-than-designed-for cracks fill with too many incompressible items. Minnesota can experience this situation in its rural areas, where agricultural activities result in a lot of dust that settles into the joints. When this prevents joints from expanding back into the space they formerly occupied, the pavement is prone to buckling as a result of compressive forces building up.
With buckled pavement due to joint failure representing a significant portion of repair work on concrete pavement, repair strategies should effectively address root causes, as well as fix surface issues. One mistake that is often made is to repair a buckle with an asphalt patch, but asphalt patches should only be used as a temporary solution.
Full-depth asphalt repairs will work like an expansion joint, allowing the concrete pavement to expand on either side of the patch and allowing the joints in nearby areas to move and fill with additional incompressible material, resulting in a cycle of damage. If the asphalt repair is only on the top portion of the pavement, the lower section of concrete is exposed to much higher stresses due to the smaller surface area, creating a section prone to more buckling.
Example of an asphalt patch that allowed concrete to shift several inches in each direction (over a five-year period during which the patch was in place).
Repairing a buckle or cracked joint in concrete pavement should always be done using concrete. The proper repair technique is to remove all deteriorated concrete and use full- or partial-depth repair (depending on the depth of deterioration), often with dowels installed to achieve load transfer.
The Overlay Option
Thin concrete overlays are an increasingly popular option to extend the life of an existing concrete, asphalt, or composite pavement. As a naturally long-lived material, concrete can deliver longevity in overlay applications just as it can in new construction. Keep in mind that an overlay—whether asphalt or concrete—is a new surface over an existing road. If used to cover structural issues in the foundational roadway, the overlay may then itself get damaged.
Because thin concrete overlays are only 4-6 inches thick, they need to be designed to avoid curling (which happens as the top of the concrete dries faster than the bottom) with joint spacing closer than traditional concrete pavement—typically 6-feet long by 6-feet wide. Closer joint spacing also helps minimize the opportunity for random cracks.
Thin concrete overlays do not expand or contract significantly during temperature swings due to their short panel lengths. However, they should still be monitored for joints that do not crack along the sawn lines, leaving other joints susceptible to excessive widening. In Minnesota, research is commencing to see if running heavy loads (for example, full water trucks) over overlays shortly after joints are sawn will help activate the joints.
Smooth Pavements Last Longer
Today, concrete pavements can be constructed with International Roughness Index (IRI) smoothness values of 30 inches per mile or less. But over time, due to curling or other pavement distresses, roughness can creep up.
Fortunately, diamond grinding offers a simple and economical solution to restore rideability and smoothness, easily lowering an IRI of 90-100 to 40 or 50 inches-per-mile. Investment made in this repair technique by the public is recouped in just a few years by the fuel savings associated with driving on smoother pavements. And smooth roads stay smoother longer, minimizing the overall amount of impact loads the concrete pavement is subjected to and extending the life of the pavement.
This helps lower long-term repair costs for the road owner, and it improves mobility for the country.
Since diamond grinding is minimally invasive and easily constructed under traffic, and costs about the same as a seal coat for an asphalt pavement, it should be considered as a frequent (every 7 to 10 years) preservation technique for concrete pavement.
Skipping a vehicle’s oil changes or brake repair leads to larger—and more expensive—problems in the future, and the same is true for pavements. The concrete paving industry has many contractors who focus solely on repairing concrete pavement. These contractors not only perform repairs efficiently and economically but can help agencies plan and prioritize work.
Fixing a small number of issues every few years, using proper concrete pavement preservation techniques that address root causes of distress, will save time and money over the life of a pavement. R&B
Matthew J. Zeller is the Executive Director of the Concrete Paving Association of Minnesota
