Section 4: Bonded Concrete OverlayAnchor: #i1010926
This section describes the construction of bonded concrete overlays (BCO) on both CRCP and CPCD pavements.
Many of the older concrete pavements in Texas were designed and constructed with insufficient thicknesses for today’s traffic demand. This insufficient thickness often results in pavement distresses such as punchouts for CRCP and mid-slab cracking or joint faulting in CPCD. However, if the Portland cement concrete (PCC) pavement is still structurally sound, does not show significant signs of distresses, but has insufficient thickness, BCO can provide cost-effective rehabilitation strategies to extend the pavement life.
When constructing a bonded concrete overlay, 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. There are BCO projects in Texas that have provided an additional 20 yr. of service. At the same time, there are BCO projects that did not perform well for various reasons.
The most important factor for the success of a BCO is having a good bond between a new and old concrete layers. If a good bond is provided, the new composite slab, old and new concrete layers, will behave monolithically as a thicker slab. On the other hand, if the two layers are not properly bonded together, the two layers will behave as independent slabs, which will result in high wheel load stress within the new concrete layer, and the pavement performance will be compromised.Anchor: #YRWMPXFM
4.2 Bonded Concrete Overlay (BCO) Procedures
The construction of bonded concrete overlay (BCO) involves the following procedures:
- Anchor: #ISKXRPKS
- Repair distresses in the existing pavement. Anchor: #ROWANRKS
- Prepare surface of existing pavement for overlay. Anchor: #FWDKCVXP
- If needed, place steel. Anchor: #POOKVEQX
- Place and cure concrete. Anchor: #OJYRNXQN
- Saw-cut and seal joints.
Each step is explained in more detail.
1. Repair distresses in the existing pavement
All the major distresses present in the existing pavement should be repaired prior to the overlay placement. The main guideline to follow when performing this work 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.
Deep spalling, delaminations, punchouts, and deteriorated patches must be repaired. Existing asphalt concrete (AC) patches should be removed and replaced with PCC patches so the existing pavement is made structurally sound. Concrete repairs should be performed in accordance with Sections 2 and 3. Working longitudinal cracks may be repaired by stitching, as described in Section 6, Stitching.
It is common practice to remove and replace the large deteriorated areas when structural distresses are extensive. When the distress is caused by a localized foundation weakness, it is necessary to ensure that the weak base layer materials are removed and the remaining base is well compacted during FDR, as detailed in Section 2. When voids are detected under existing slabs, grout should be injected to stabilize the pavement.
When constructing a BCO over a CPCD section, it is necessary to ensure that the sections have adequate load transfer efficiency. CPCD sections built without dowels will need to have dowel bar retrofits done prior to constructing the overlay. Section 7, “Dowel Bar Retrofit,” details these requirements.
2. Prepare surface of existing pavement for overlay
As described previously, the critical factor for the good performance of a BCO is the bond between the existing concrete and overlaid concrete. One of the requirements needed to ensure a good bond is to provide adequate surface texture of the existing concrete pavement.
Surface preparation encompasses the operations conducted on the existing substrate to roughen its texture in such a way that enables the new concrete layer (BCO) to become bonded to it as if both layers were a single structure.
There are several surface preparation methods that can be used to achieve a roughened surface. The most common are:
- Anchor: #IVPUDBSE
- shotblasting, Anchor: #WRSLRBMS
- sandblasting, and Anchor: #KRXBEOLR
- cold milling and shotblasting.
Shotblasting involves a spinning drum equipped with compressed air that blasts tiny steel balls (shot), which impact the surface at an angle to scarify the surface. A vacuum collects both the shot and the dust. The shot is separated from the dust by magnetic action for continuous reuse. Regulating the speed allows the level of scarification to be controlled. Slower speeds yield a higher depth of scarification. Shotblasting 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 shotblasting, but instead of shot, sand particles are used. However, unlike shotblasting, sandblasting generates airborne dust, and sand may remain on the surface after it is scarified, making it necessary to clean the surface to remove debris prior to paving. The sandblasting surface finishing is not as uniform as shotblasting 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 10-14. 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 requiring deep scarification. However, it generates a high amount of dust and contamination, which must be removed prior to overlaying.
Figure 10-14. 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 micro-fractures are detrimental to its structural integrity. Shotblasting is often required after cold milling to remove the loose fractured remnants on the pavement.
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 shotblasting 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 1/4-in. deep into the coarse aggregate. It can also be specified in terms of some standardized texture test method, such as Tex-436-A, “Measuring Texture Depth by the Sand Patch Method.” Typical texture readings from this test are between 0.050 in. and 0.095 in.
If the pavement has been overlaid with HMA layers, these layers should be completely milled off during the surface preparation. Remnants of HMA will hinder the bonding of both PCC layers and are likely to trigger delamination. Complete milling of these layers will ensure that all surface contaminants such as oil, carbonates, and acids are removed.
Once the surface is roughened, the section should not be opened to traffic until the overlay is completed and cured. Allowing traffic on the roughened surface allows the opportunity for contaminates to be deposited on the new prepared surface, which may cause debonding issues.
Air blasting is not capable of removing paint stripes, tire marks or the grout matrix. It should be used only as a supplementary cleaning procedure to remove loose material and debris from the surface after milling, shotblasting, or sandblasting. Air blasting is also used just before overlaying to thoroughly remove debris from the prepared surface. It is important to minimize the time between the final surface cleaning and paving in order to prevent the contaminants from resettling.
3. If needed, place steel
When constructing a CRCP BCO, reinforcing steel is needed within the new layer of concrete to maintain tight cracks. Steel should be placed at a depth that provides a minimum concrete cover of 3 in. If the overlaid thickness layer will not provide sufficient coverage, reinforcement steel bars can be placed directly over the surface of the existing pavement as shown in Figure 10-15, rather than at mid-depth of the overlay. For thinner overlays, steel placed at mid-depth may preclude the use of a slip-form paving machine due to the use of vibrators.
Placing steel directly on top of the surface of the existing pavement has advantages and disadvantages. Advantages include: saving construction time and costs since chairs are not needed, and the steel will restrain concrete volume changes at the interface, which will prevent or minimize debonding. The only disadvantage is the reduction of the interface area between the new and old concrete. Refer to Chapter 8, Section 7, for the steel percentage and vertical location.
After the surface preparation operations are finalized, and the reinforcing steel is in place, the last cleaning of the surface is done just before concrete placement by airblasting as shown in Figure 10-15.
Figure 10-15. Airblasting to clean the concrete surface.
4. Place and cure concrete
The materials selected for use in the concrete mixture must be carefully selected. The aggregates of the BCO should 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 equal to or 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 also has a significant impact on the concrete’s coefficient of thermal expansion (CoTE). Concrete has a CoTE ranging from 4 to 6 microstrain/°F. Large differences in the CoTE between the existing and new concrete result in increased stresses at the interface, which will impact the bonding. It is recommended that the coarse aggregate in the BCO should have a thermal coefficient that is lower than the CoTE of the existing concrete and must not exceed 5.5 microstrain/°F.
The maximum aggregate size of the BCO concrete should be 1/3 of the overlay thickness. This will ensure a uniform distribution of the concrete constituents when placing the BCO. If the aggregate is larger than 1/3 of the BCO thickness, segregation of the oversized aggregates is likely to occur, especially in areas where it is difficult to consolidate around reinforcing steel.
The required concrete strength of the BCO concrete is the same as the strength required in Item 360. Type I or I/II cements are the 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. Special attention should be paid when Type III cement is used. For a BCO, the use of Type I or I/II cements is recommended, as they produce less heat from hydration than a Type III cement and, therefore, reduce the development of thermal stresses.
Typical placement methods used for concrete pavement construction are also used to construct BCOs. BCOs can either be slip-formed or fix-formed placed. Chapter 9 details these placement techniques. When constructing a BCO over a CPCD, prior to placing the overlay concrete, the location of the existing joints should be clearly marked so that the joints in the overlay concrete can be sawn directly over the existing joints.
Special attention should be given to adverse environmental conditions during paving. A combination of high wind velocity, high air temperature, low relative humidity, and high concrete temperature is the most harmful paving conditions. These conditions promote high water evaporation rates from the 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. Under these conditions, necessary precautions need to be taken to prevent any adverse effects. Chapter 9 details necessary steps to take during adverse weather conditions to protect the pavement.
Curing is a key component for preserving satisfactory moisture content and temperature in the concrete during its early stages so that desired properties may develop. Curing is critical since the surface-to-volume ratio of the BCO layer is greater than normal paving concrete. 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. A fast-track BCO takes this concept further; by utilizing special materials, the road can be opened to traffic in a minimal time after placement. A fast-track BCO can be opened within 6 to 24 hr. after placement. To make this possible, normally the BCO is constructed with a high-early-strength PCC mix, using Type III cement, as opposed to Type I cement.
5. Saw-Cut and Seal Joints
For CPCD sections, thinner overlays have a greater potential for rapid shrinkage and contraction, and, therefore, joint construction should take place a soon as possible. For CPCD, joints in the overlay should be sawn directly over joints in the existing pavement to prevent reflective cracking of the existing joints through the BCO. The depth of saw-cutting for transverse joints should be full depth plus 1/2 in. Longitudinal joints should be full depth.
Transverse and longitudinal joints should be sealed according to the joint sealing standard. ftp://ftp.dot.state.tx.us/pub/txdot-info/cmd/cserve/standard/roadway/js14.pdf.