How Concrete Overlays Maximize Performance
By Eric Ferrebee, Contributing Author
Concrete overlays are a pavement rehabilitation solution that works in a wide range of situations. Overlays offer excellent service life, typically being designed to extend a pavement by about 30 years — although they can be designed to last even longer.
Ongoing maintenance costs are limited, and advancements in overlay technology and construction are making installation times comparable to those for asphalt.
Concrete overlays play a vital role in asset management by providing a cost-effective solution that preserves existing investments, enhances sustainability and improves resilience against extreme weather.
Concrete overlays can be applied to existing asphalt, composite or concrete pavements. They can be used whether the existing pavement is in good or poor condition, and they are suitable for city streets, county roads, highways and parking lots.
Concrete overlays have been used by state and local agencies since the early 1900s, but their use has expanded rapidly since the late 1990s and early 2000s. More than 100 million square yards of concrete overlays have been built with more than 90% being paved in the last 25 years. This expansion has led to concrete overlays regularly making up more than 10% of all the concrete pavement paved every year.
When considering a concrete overlay, there are two main questions to address. First, what is the current condition of the road? Second, what is the desired service life of the concrete overlay?
If the existing pavement is in good or average condition, the overlay can bond to and work with the existing surface; if it is in fair or poor condition, the overlay will utilize the existing pavement as a strong, durable foundation. The desired service life will be a deciding factor in the thickness chosen for the overlay.
To assess the current condition of the road, the design team should reference historical construction records and perform site inspections to determine the pavement's structural integrity.
The team should identify layer thicknesses and composition, number and types of distresses, drainage issues and support conditions. Obtaining core samples and doing material testing can also provide valuable information.
During the pavement evaluation, a critical step is to ensure good drainage and consistent subgrade support. Overlays do not solve drainage issues, so it is important to identify and address problems with the pavement’s drainage system and/or drainage-related subgrade damage.
Indicators of isolated spots where subgrade support has been lost include rocking or cracked panels, areas of thin pavement after milling, areas of existing pavement that have undergone extensive full-depth repairs, etc. Pre-overlay repairs should be performed in such areas to restore structural support.
Once current conditions are understood, the concrete overlay type can be selected. Overlays can be bonded or unbonded. Bonded overlays adhere directly to the existing pavement and require existing pavements to be in good to fair condition.
Unbonded overlays are not intentionally adhered to the existing surface and are designed such that the concrete can withstand the applied stresses independently, treating the underlying layers as a support system.
Overlays are categorized into four main types based on bonding and the underlying pavement:
- Concrete on Asphalt – Bonded (COA-B): Suitable for structurally sound asphalt pavements, these overlays are thinner due to the bond's structural contribution.
- Concrete on Asphalt – Unbonded (COA-U): Used for deteriorated asphalt, these overlays can bridge over existing distresses.
- Concrete on Concrete – Bonded (COC-B): COC-B overlays are used infrequently. Their usefulness is restricted to situations in which load capacity must be increased to meet an increase in traffic demands on a pavement in otherwise relatively good condition.
- Concrete on Concrete – Unbonded (COC-U): COC-U overlays feature a separation layer (asphalt or geotextile) to isolate the overlay and prevent reflective cracking from the underlying pavement.
Once the existing pavement has been assessed and critical design variables have been identified, overlay thickness can be calculated. Software is available for designing concrete overlays. The design is driven by bonding condition, in addition to typical design factors such as traffic, material properties, pavement geometry and environmental factors.
Bonded overlays are typically 6 inches thick or less, since they work in conjunction with the existing pavement layers. For unbonded pavements, typical thicknesses are 6 inches or greater, as the existing pavement essentially serves as a strong foundation for a new concrete pavement.
If the unbonded overlay is being placed on an existing concrete pavement, a separation layer should be utilized and may factor into the design (this layer is typically not necessary on unbonded overlays on asphalt).
Panel sizes are designed with a focus on reducing slab stresses and minimizing early-age debonding, with smaller panel sizes generally reducing stresses the most. Proper joint placement and sawcut depth are additional design and construction considerations, since jointing will help prevent cracking and ensure durability. Macrofibers are often added to thinner overlays to improve crack resistance, minimize slab migration and improve overall durability.
Concrete overlays use conventional materials and are constructed using standard paving equipment and techniques. Unique aspects of their construction may include some surface preparation (i.e. profile milling for concrete overlays on asphalt or the placement of a separation layer for unbonded overlays on concrete) and special attention paid to curing times, especially for thinner overlays.
Unbonded overlays thicker than 7 inches can be maintained during their service lives using standard concrete pavement techniques, such as partial-depth repairs for minor surface distresses, full-depth repairs for structural damage and diamond grinding to restore smoothness and ride quality.
Optimization of Paving Process
Overlays are faster to build than conventional reconstruction. While some surface preparation is typical, no time is needed for demolition, grade preparation or the construction of subbase layers.
Overlays also require fewer materials and less installation equipment, representing additional time savings. Recent advancements in overlay construction are expediting the process even further.
As documented by Iowa State University’s National Concrete Pavement Technology Center (CP Tech Center), an overlay project on a two-lane state highway in Iowa demonstrates how time savings can be achieved.
Strategies included managing project staging and minimizing roadway closures, a reduction in the required opening strength and the use of the maturity method to determine when the required strength had been reached. No accelerated or high early strength concrete materials or mixtures were needed to meet the accelerated construction requirements.
The Iowa Department of Transportation (IDOT) and Croell, Inc. completed the concrete overlay in 2022, installing it on a nine-mile-long section of Iowa Highway 3 in Plymouth County.
The existing roadway was a full-depth asphalt pavement initially constructed in 1995. In 2009, it had received a 2-inch asphalt overlay. The 2022 overlay was designed as a 6-inch-thick concrete-on-asphalt–unbonded (COA-U) overlay. The overlay included 9-inch-thick, 6-foot-wide integral paved shoulders on both sides of the roadway, along with new granular shoulders.
Most concrete overlay projects on two-lane roadways in Iowa are constructed with full road closure and an end-to-end detour for the duration of the project.
Comparable asphalt overlay projects in the state are typically paved under traffic with single-lane closures and a pilot car, with closure times of 36-90 calendar days (depending on the number of lifts in the overlay and whether or not the project includes cold in-place recycling).
The same staging method can be used for concrete overlay construction in the state, although most agencies in Iowa opt for full closure.
For the Iowa Highway 3 project, IDOT provided Croell, Inc. just 24 calendar days from the first roadway closure to complete the project and fully re-open the roadway. Additionally, while through traffic was detoured end-to-end, only three miles of roadway could be fully closed to local traffic at a time.
In response to these constraints, Croell, Inc. developed a moving closure concept. At any given point in time, the closed portion of the roadway included approximately one-half mile for preparing the shoulders and existing surface for paving, one mile for paving, finishing and sawing operations and one-and-a-half miles to allow the concrete to reach opening strength and to complete joint sealing, shouldering and cleanup.
While contractors often wait to perform activities such as joint sealing and shouldering until mainline paving is finished, on Iowa Highway 3 these activities were conducted as soon as the installed pavement reached opening strength, to allow re-opening to local traffic as soon as possible.
Opening strength was determined using the maturity method, which eliminates many of the variables associated with traditional cylinder or beam testing and delivers fast and accurate results.
This, in turn, supports faster opening to traffic. The maturity method involves using trial batches to correlate the time-temperature factor (TTF) with flexural strength, then using thermocouples embedded in the concrete slab during construction to monitor the in-place TTF. The method has been widely used on concrete paving projects in Iowa since the 1990s.
Helping streamline the Iowa Highway 3 project, profile milling of the existing asphalt surface using a single-lane closure was the one activity performed prior to the 24-calendar day contract period.
Mainline paving operations occurred over 14 working days and 20 calendar days, with an average paving distance of about half a mile per day and a daily maximum of eight-tenths of a mile. The highway was fully re-opened 25 calendar days after the first closure (one day after the target of 24 days).
As of 2024 — two years after construction — the overlay had a pavement condition index value of 87/100 and an international roughness index value of 69 inches per mile. The overlaid pavement is expected to serve the traveling public for over 40 years — a good return on the investment of a 25-day closure.
Eric Ferrebee, P.E., is senior director of technical services at the American Concrete Pavement Association.