Bonded Concrete Overlay

This chapter describes bonded concrete overlays (BCO) on CRCP, not on CPCD. BCO is not a good option for the rehabilitation of CPCD. In the past, concrete pavements were designed and constructed with insufficient thicknesses for today’s traffic demand. This insufficient thickness often resulted in pavement distresses such as punchouts for CRCP and mid-slab cracking or joint faulting in CPCD.

If the PCC pavement is structurally sound except for the deficient thickness, in other words, if the slab support is in good condition, bonded concrete overlays (BCO) can provide cost-effective rehabilitation strategies to extend the pavement life. In bonded concrete overlays, a new concrete layer is applied to the surface of the existing PCC pavement. This increases the total thickness of the concrete slab, thereby reducing the wheel load stresses and extending the pavement life.


The critical requirement for the success of BCO is a good bond between a new and old concrete layers. If a good bond is provided, the new slab consisting of old and new concrete layers will behave monolithically and increased slab thickness will reduce the wheel load stress at the bottom of the slab substantially, thus prolonging the pavement life. On the other hand, if a sufficient bond is not provided, the wheel load stress level in the new concrete layer will be high and the pavement performance will be compromised.

The design and construction of BCO involves the following procedures:


Evaluate whether the project is a good candidate for BCO.

This task involves the evaluation of pavement conditions to determine whether a BCO is the most appropriate rehabilitation solution.

In general, a BCO could be a good candidate for PCC pavements that are currently structurally sound, but are structurally insufficient to carry future projected truck traffic. A BCO can be used to increase the structural capacity of these pavements to enable them to carry traffic for many more years. In order for BCO to perform adequately, severe distresses need to be repaired prior to BCO placement; potentially the required repairs will be too expensive for the BCO to be economically feasible. In that case, other alternatives, such as an unbonded concrete overlay should be considered.

A feasible alternative is the one that addresses the cause of the problem motivating the rehabilitation; therefore, the pavement condition must be investigated before making these decisions. The reason for the rehabilitation may be the structural or the functional condition of the pavement. Structural condition refers to whether or not the pavement is fit to support current and future traffic loads over the desired design period. The functional condition encompasses pavement characteristics related to the way the user experiences the safety and comfort of the road, such as skid resistance, roughness, appearance and hydroplaning.


The evaluation of the structural condition involves studying the distress nature and history of the pavement, which will provide information on the impact of past traffic loadings. This is assessed by means of a visual condition survey. Historical information on patching, slab replacement and other repairs are other valuable sources for structural condition assessments. Finally, destructive and non-destructive testing (NDT) methods are helpful in determining the structural integrity of the pavement. Among the NDT procedures, the most common is deflection testing using the Falling Weight Deflectometer (FWD). Destructive testing implies the extraction of samples from the pavement to evaluate their properties in a laboratory. The evaluation of the functional condition requires the measurement of roughness and skid resistance and an assessment of the present serviceability.

Develop adequate slab thickness and steel designs.

In the design phase, the necessary thickness of the overlay is determined. For this, several variables and properties of the existing pavement, as well as parameters determined by the overlay purpose, are analyzed. Various design procedures are available to determine the design thickness of the overlay. Most of them are based on the principle that the overlay is being placed for the purpose of structural improvement (most cases); the required thickness of the overlay is a function of the structural capacity of the existing pavement under current conditions and the structural capacity necessary to fulfill future traffic demands. In order to assess the structural capacity of the existing pavement, a comprehensive evaluation of its condition should be conducted, including visual surveys, and field and laboratory tests. If the overlay is being placed only to remedy functional deficiencies, normally a thinner overlay would suffice. However, two inches is the minimum practical constructible thickness for an overlay. Alternately, a thin hot mix asphalt wearing course may be considered.

As for the steel design, if the overlay thickness is more than 40 percent of the existing CRCP, longitudinal steel needs to be provided for the overlay. If the steel is not provided, the longitudinal steel in the existing CRCP will be subject to much higher stress, diminishing its ability to restrain concrete volume changes. Also, the distance between the overlaid concrete surface and the existing steel will be increased and the ability of the existing steel to control concrete volume changes at the surface will be diminished, resulting in more concrete volume changes and larger crack widths at the surface. The amount of steel needed should be sufficient to control the overlaid concrete volume changes. Guidelines to be developed in a current research study are expected to address the steel design.


Prepare surface of existing pavement for overlay.

As described above, the critical factor for good performance of the BCO is the bond between existing concrete and overlaid concrete. An insufficient bond will compromise pavement performance. If a good bond is established, the overlaid concrete will be for the most part in compression when subjected to traffic loading. In other words, the strength of the overlaid concrete shouldn’t be of great concern as long as a good bond is established and the durability of the overlaid concrete is not compromised. One of the requirements for good bond is to ensure adequate surface texture on the existing concrete pavement.

Surface preparation encompasses the operations conducted on the existing substrate to roughen its texture sufficiently, enabling the new concrete layer (BCO) to become bonded to it allowing both strata to behave as a monolithic structure.

One of the most critical facets of BCO construction is surface preparation, because it is highly accountable for the bonding of the overlay to the existing concrete. The bond quality of a BCO usually determines the success or failure of the rehabilitation. The bond at the interface between the BCO and the existing concrete is subject to considerable stress from concrete volume changes, and loading. The goal of surface preparation is to provide a rough surface that favors the bonding of the BCO to the existing pavement by facilitating the bonding mechanisms. One important aspect of surface preparation is that, once the surface is roughened, the section should not be opened to traffic until the overlay is completed and cured.

There are several surface preparation methods to achieve a roughened substrate. The most common are:

    1. Shot Blasting
    2. Sandblasting
    3. Cold milling

For shot blasting, a spinning drum equipped with compressed air blasts tiny steel balls (shot) that impact the surface at an angle to scarify it. A vacuum collects both the shot and the dust. The shot is separated from the dust by magnetic action for continuous reuse. Regulating the blaster speed allows control of the level of scarification, wherein lower speeds yield a higher level of scarification. Shot Blasting removes the matrix surrounding the coarse aggregates in a uniform way, but keeps the aggregate intact. It is a clean procedure that minimizes dust and air pollution.

Sandblasting is similar to shot blasting, but instead of shot, sand particles are used. However, unlike shot blasting, sandblasting generates airborne dust, and sand may remain on the surface after it is scarified, making it necessary to air blast the surface to remove debris prior to paving. The sandblasted surface finish is not as uniform as that executed by shot blasting operations.

Cold milling removes the top of the substrate to a specified depth by the chipping action of rotating mandrels with sharp tips mounted in a machine like the Rotomill as shown in Figure 1. As a result of its action, the surface texture after cold milling is rougher and more angular than after sandblasting or shotblasting. Cold milling is the most widespread method for large areas of concrete surface preparation requiring deep scarification. However, it generates a high amount of dust and contamination, which must be removed prior to overlaying.

Teeth of the Rotomill
Figure 1: Teeth of the Rotomill

Cold milling, while being an efficient way to remove the grout matrix, has the drawback of fracturing the exposed aggregate, because the procedure relies on breaking the surface. The microfractures of the exposed substrate may be detrimental to its structural integrity. Shotblasting is a more economical and efficient alternative.

The scarification depth and texture should be specified for each project, depending on economical considerations as well as the materials properties, for both the existing pavement and the new overlay. For instance, if the substrate grout paste is relatively soft and the coarse aggregate is especially hard, a light shot blasting will be sufficiently strong to remove the paste to reach the specified depth, while the aggregate will remain intact, resulting in a good surface texture. Normally, the depth of surface removal is about ¼ in. deep into the coarse aggregate. It can also be specified in terms of some standardized texture test method, such as by the “Sand Patch Method” (ASTM E965 or equivalent). Typical texture readings from this test are between 0.050 in. and 0.095 in.

All the major distresses present in the existing pavement should be repaired prior to the overlay placement. The main consideration is to assess whether the distress is likely to affect the performance of the overlay within a few years. If that is the case, the distress has to be repaired before the BCO is built.

Concrete repair must replace the damaged concrete and reinforcement and restore structural functionality; the repair must protect itself, the adjacent concrete, and the underlying subbase from aggressive environmental elements. Areas of localized breakup are more susceptible to these elements; therefore, the repair should address any possible attack from extraneous factors.

Deep spalling, delaminations, punchouts, and deteriorated patches must be repaired. AC patches should be removed and replaced with PCC patches, because AC prevents bonding of the PCC layers. Also, no AC should be used for any of the pre-overlay repairs. Most distresses require FDRs, which is described in another section. These procedures will preserve the load transfer capability of the slab, in which reinforcement in the repair must be properly tied to the reinforcing steel in the existing concrete. Partial-depth repairs, in which the distressed concrete is removed by a combination of sawing and chipping, or by cold milling, are suitable only when the deterioration is limited to the surface of the concrete. Working longitudinal cracks may be repaired by stitching, as described another section.

It is common practice to remove and replace a deteriorated section when the presence of structural distresses is extensive. When the distress is caused by a localized foundation weakness, it is essential to remove and replace the weak material and to stabilize the foundation before replacing distressed slabs. When voids are detected under existing slabs, grout should be injected to stabilize the pavement.

Surface cleaning refers to the removal of dust and debris after the surface preparation is complete and prior to the placement of the BCO, to ensure that no foreign elements interfere with achieving adequate bonding between the concrete layers.

After the surface preparation operations are finalized, and the reinforcing steel is in place, a final cleaning of the surface is done just before concrete placement by airblasting as shown in Figure 2.

Airblasting to clean the concrete surface
Figure 2: Airblasting to clean the concrete surface

Air blasting is not capable of removing paint stripes, tire marks or grout matrix. Therefore, it should be used only as a supplementary cleaning procedure to remove loose material and debris from the surface after milling, shot blasting, or sandblasting. Airblasting is to be used just before overlaying to thoroughly remove debris from milling or shot blasting operations. It is important not to leave a large time lag between the final surface cleaning and paving in order to prevent the contaminants from resettling.

If the pavement has been overlaid with AC layers, these layers should be milled prior to BCO placement and prior to surface preparation and repair of distresses. Remnants of AC will hinder the bonding of both PCC layers and are likely to trigger delaminations, because AC works as a bond-breaking layer between PCC layers. Complete milling of these layers will ensure that all surface contaminants such as oil, carbonates and acids are removed.

A milling machine such as the Rotomill should be used to remove the bulk of the AC overlay; the most efficient method of removing any remaining AC residue is by means of shot blasting equipment.

Place Steel (if needed)

Steel should be placed at a depth that provides a minimum concrete cover of 3 inches. If the overlaid layer thickness is not large enough, reinforcement steel bars can be placed directly over the surface of the existing pavement as shown in Figure 10-11, rather than at mid-depth of the overlay. As a matter of fact, for an overlaid thickness that is not large enough, placing steel at the mid-depth of the overlay may not be feasible if a slip form paving machine is used.

Placing steel directly on top of the surface of the existing pavement has advantages and disadvantages. Advantages include saving construction time and costs since it does not require chairs. Another advantage is that the steel will restrain concrete volume changes at the interface most effectively, which will prevent or retard debonding. The primary disadvantage is the reduction of the bonding interface area between the new and old concrete. Taken together, for overlaid thickness up to about 5 inches, placing steel on top of the existing concrete appears to be a better construction practice.

A research study currently underway will address this issue and guidelines will include recommendations.

Place concrete and provide optimum curing.

The concrete and especially aggregates of the BCO have to be compatible with those of the existing pavement. The basic premise for material compatibility is to use aggregates for the BCO concrete that produce moduli and thermal coefficients lower than those of the materials in the existing slab, which will result in lower stresses at the interface, regardless of the season of placement.

Differences in moduli between layers have a significant influence on thermally induced stresses. The main factor affecting the modulus of concrete is coarse aggregate type. High-modulus aggregate will result in high-modulus concrete.

The type of aggregate used in the mix has a significant impact on the concrete thermal expansion. Normal concrete has a thermal coefficient range from four to six millionths per degree Fahrenheit. Large differences in thermal expansion coefficients between existing and new concrete result in increased stresses at the interface, which will impact the bonding. Therefore, it is recommended that the coarse aggregate in the BCO should have a thermal coefficient that is as low as possible.

The maximum aggregate size of the BCO concrete should be one third of the overlay thickness. This will ensure a uniform distribution of the concrete constituents when placing the BCO. If the aggregate is larger than one third of the BCO thickness, segregation of the oversized aggregates is likely to occur, especially in areas where it cannot mix properly (e.g., under reinforcement bars).

As for the reinforcement steel, the amount should be determined from careful analysis.

Type I cement is most commonly used for general construction where no special properties are needed. If a faster-than-normal strength gain is necessary, Type III cement can be used. Type III cement is typically used for high-early strength (HES) concrete, and class HES concrete has specific curing requirements. For a BCO, it is recommended to use Type I cement, as it produces less heat from hydration than Type III cement and, therefore, reduces the development of thermal stresses. Customary concrete placement procedures for new pavement apply for placement of BCOs. Special attention should be given to adverse environmental conditions during paving. Hot, dry climates pose the most problematic setting for BCO placement, because these conditions favor the loss of moisture from fresh concrete. Excessive water evaporation from the concrete can cause volume changes large enough to cause debonding problems at the interface between old and new concrete layers. A combination of high wind velocity, high air temperature, low relative humidity and high concrete temperature is the most harmful for paving conditions because it favors high water evaporation.

Curing is a key component for the preservation of satisfactory moisture content and temperature in the concrete during its early stages so that desired properties may develop. For a BCO, curing is critical since the surface to volume ratio of the BCO layer is greater than for normal paving concrete thicknesses. Moisture loss and resulting drying shrinkage are approximately proportional to the surface to volume ratio. Curing can be accomplished by a variety of methods, which include the use of membrane curing and wet mat curing.

The duration of construction is critical mostly in urban areas or highways with heavy traffic. A BCO inherently represents a quick construction process, because it requires only a limited number of operations. An expedited BCO takes this concept further; by utilizing special materials, the road can be opened to traffic in a minimal time after placement. An expedited BCO can be opened within 6 to 24 hours after placement. To make this possible, normally the BCO is constructed with a high-early-strength PCC mix, which is attained by using Type III cement, as opposed to the normal cement (Type I). In general, if Type III cement is not utilized, the mix is supplemented by the use of superplasticizers.

As for the required concrete strength, unless it is an expedited BCO, the recommended minimum 7-day average flexural strength of 570 psi (680 psi at 28 days) or minimum 7-day average compressive strength of 3,500 psi (4,400 psi at 28 days) will be adequate. As long as there is good bond to the existing concrete slab, the tensile stress in BCO will be minimal.

References

Original article content and pictures contributed by TxDOT.