Evaluation of Bond Strength between Overlay and Substrate in Concrete Repairs (Thomas)

1. Introduction

Most pavement projects today are a rehabilitation or rejuvenation of existing and distressed pavements. We often add more overlay layers or mill out patches for repair material rather than reconstruct the entire roadway. One of the key requirements for those repair systems is to have adequate bond strength between the existing concrete substrate and overlay throughout the service life. When a repair is performed, the differences in the properties of two materials will affect bond strength and stress distribution. Of particular relevance are differences in shrinkage, elastic modulus and thermal movement. Repaired sections of concrete pavement often fail due to debonding or delamination of the top repaired layer. Thus there is a need to improve or to maintain the bond by improved material properties or improved interface conditions.

2. Repairing of concrete structures

2.1 Crack repairs

Hairline cracks, cracks in sidewalks, holes in concrete walls or even small broken corners do not need large scale maintenance. These are mostly due to extra stresses applied to the surface, freezing and thawing, or wearing problems. Normally in these cases simple tools and materials are employed. Crack repairing (Figure 1) is done following this procedure.

a) Cleaning the area, using detergent in case the area is oily
b) Widening and enlargement the cracks by hammer or cold chisel
(the crack must be wider at the bottom than at the top, this is called undercutting)
c) Removing all loose material and brushing the area
d) Moistening the area and waiting for evaporation of surface water
e) Using some resins or adhesive and then grouting a Portland cement mortar
(all the patch must be filled once)
f) Smoothing and leveling the surface with trowel
g) Curing for some days

2.2 Causes of Damages

Concrete may be damaged due to many different reasons of which some of the more important causes are discussed below.

Excess concrete water mix

Excessive water is basically one of the most common problems that cause damages in concrete. It highly decreases the strength and the abrasion and increases the shrinkage and creep. Damages of high water cement ratio are difficult to diagnose since it usually gathers with other problems. The only way to have permanent repair in this case is to remove and replace the new layer instead of the problematic layer, however in some special case sealing compounds and coating the layer with a high solid content mortar can reduce the permeability and increase the strength against thawing and freezing. In addition low ratio also causes damages. Performing the concrete in hot weather sometimes makes the top layer have low water cement ratio therefore the top layer can easily get damages.

Faulty design or construction defects

Placing the bars too close to the surfaces or corners, lack of adequate contraction joints, slabs with insufficient expansion joints, dimensional errors, finishing defects, improper compaction or curing and many other factors in design and during construction, cause damages of concrete.

Alkali Silica Reactivity (ASR)

Some silicaceous minerals, including quartzes and opals, react with water in a high alkaline environment to form silica gel, a material used to absorb moisture. As silica gel swells when it absorbs moisture, the material can cause concrete to crack, and white deposits of silica appear. Figure 2 shows the cracks caused from ASR.

Deterioration through carbonation

Carbonate exists in the air. Although most of the chemical attacks have effects mostly on the surface, carbonation goes deeper and deeper by the time. It decreases the PH value of the concrete and thus it influences on passivation of the steel and corrosion can take place.

2.3 Importance of bonding in concrete repairs

After repairing the concrete structure and replacing a new layer, there should be enough strength in both layers since the damaged part of substrate has been already removed and the new layer has been designed and placed according to the requirements of the work. Despite having adequate strength in both layers, the interface is still vulnerable to damages and could be the most sensitive part of the system. Two layers have different modulus of elasticity, so exposed to the same load each shows different strains. The interface should be able to bear this difference. The same problem exists for the temperature strains. In addition the new layer has shrinkage which is considered as another factor for interface weakness. Since the interface is the plane of discontinuity in the system, it exposes to all these extra forces and it should have enough resistance to hold the integrity of the layers. Thus a key requirement of a repair material is to good adhesion at the interface. There are many factors which have influence on the bond strength and some test methods to figure out the strength and quality of the bond. Next chapters discuss the factors and test methods in details.

3. Bond in concrete

The bond strength is the adhesion between overlay and substrate which can be the weakest link of the system. Good bond strength is a key factor to have a monolithic system [1]. Bond can be expressed by shear resistance or tensile resistance. It is important to select the one which can better state the stresses subjected to the structure in the field. While in many cases the stress in the field is of shear type it is more practical to use the tensile strength for bonding in structural works. In practice the usual way to determine the tensile bond strength is the pull off test, in which the tensile force applies perpendicular to the overlay till the failure occurs. The bond strength can be easily defined as maximum force divided by the interface area, in the condition that failure occurs completely at the interface. Failure in the substrate indicates that the bond strength is greater than the tensile strength of substrate and a failure in the overlay indicates that the bond strength is greater than the tensile strength of the overlay [2]. Normally in repair works failure in the substrate is preferred, since it shows the overlay has been done properly. Figure 3 shows the three different failure types in the system.

4. Main factors influencing bond

Several elements are particularly important in ensuring a good bond between the concrete overlay and the underlying concrete pavement. There are issues related to the concrete mixture, surface preparation and cleaning, curing, sawing, and strength measurement. While several of these factors are currently considered in conventional concrete paving, specific emphasis should be placed on certain key factors of these elements, since bonded concrete overlays are particularly sensitive to volumetric changes.

4.1 Concrete mixture

Concrete mixtures used in bonded concrete overlays should shrink as little as possible and behave thermally as close to the existing concrete pavement as possible. This typically means optimizing the cement content to minimize shrinkage while maintaining an adequate strength. Many researchers and building clients have pointed out that shrinkage is the single most important factor determining the service life of an overlayed structure, e.g. Granju et al [3], Rahman et al [4], Weiss et al [5], Yuan et al [6]. Shrinkage in the newly cast overlay causes normal tensile stresses to develop as the contracting movement to some extent is restrained by the substrate. If the stresses reach the strength of the overlay material cracks will start to propagate through the overlay, see Figure 4.

4.2 Surface preparation

Surface preparation of the existing concrete pavement is accomplished to produce a roughened surface that will promote bonding between the two layers (see Figure 5). A variety of surface preparation procedures may be used, including shotblasting, milling, and high water pressure blasting (see Figure 6 and Table 1).

The importance of surface preparation and the effect of roughness have been emphasized by many researchers. Sholar et al. [7] and West et al. [8] found that surface texture of the lower layer significantly affects the interface bond. They showed that laying a new asphalt mixture over a milled existing asphalt pavement significantly enhances the shear strength of the interface between those layers. An investigation by Partl et al. [9] on the effect of contact surface roughness showed that the shear strength at the interface increases as the contact surface roughness increases. Ferrotti [10] demonstrated the effect of interlayer roughness on the measured bond strength of the interface. The interlayer roughness was quantified by performing an image analysis on the images obtained using the x-ray computed tomography equipment. Silfwerbrand [11] suggested using automatic laser equipment for quantifying the surface roughness in his study of the effect of roughness on bond of repair materials.

4.3 Cleaning

Following surface preparation, the surface should be thoroughly cleaned to remove all loose material to ensure adequate bonding. Cleaning may be accomplished by sweeping the concrete surface, followed by cleaning in front of the paver with compressed air. Airblasting (Figure 7) and waterblasting (Figure 8) should be used only as supplementary cleaning procedures to remove loose material from the surface after shotblasting and milling. In no case should standing water or moisture remain on the pavement surface when the overlay is placed. Paving should commence soon after cleaning to minimize the chance of contamination.

4.4 Curing and temperature effect

Curing also can have a pronounced impact on concrete overlay bond. The diligent and thorough application of curing compound is an effective method to control moisture loss and thus lower shrinkage and early-age cracking potentials. This is particularly true for thin bonded overlays. More “extreme’ measures, such as wet or blanket curing, can help to minimize the risks of poor performance when proper curing compound application is either difficult or doubtful. These types of more “extreme” curing regimes are typically only applicable for short paving sections such as intersection rehabilitation. The other important aspect of curing is temperature; the concrete must not be too cold or too hot. As fresh concrete gets cooler, the hydration reaction slows down. Hot concrete has the opposite problem: the reaction goes too fast, and since the reaction is exothermic and produces heat, it can quickly cause temperature differentials within the concrete that can lead to cracking. Effect of curing temperature on compressive strength development is presented in Figure 9 [12].

5. Test methods to evaluate bonding strength

In some cases, given the critical importance of proper bond development to good bonded overlay performance, states are relying on actual bond measurement to increase their confidence level in the quality of the bond. Many tests have been developed to evaluate the bond strength in the interface. Figure 10 shows various test methods to evaluate bond strength [13].

5.1 Shear test

Several bond tests have been used, most notably the Iowa Test Method 406C (Figure 11). This test involve coring the overlaid pavement and placing the core in a shear testing jig, where a shear load is applied along the bond interface on the core. Early work (research and evaluation) with this kind of testing suggests that a shear bond strength of 200 psi is enough to ensure that the “composite pavement” (i.e., the bonded concrete overlay and the underlying pavement) behaves like a truly monolithic slab.

5.2 Pull-off test

The direct pull-off test (ASTM C 1583) (Figure 12) has also been used for evaluation of bond strength, although most commonly with bridge deck overlays. The general procedure of in situ test can be described as follows; Figure 13 shows the schematic pull off test procedure and equipment.
a) Pointing and Marking the area
b) Surface planning - Preparing the surface with the diamond wheel to obtain a plane surface should be done. Dusts and any powders on the surface remaining from planning the surface with diamond should be brushed and blown away. The corner knob should be removed with a grinder.
c) Attaching the disc to the core - The clean metallic core is glued to the surface using epoxy. The epoxy is high strength and quick setting; it can have tensile strength around 10 MPa when it is fully cured. The curing can be accelerated using heat gun. It usually takes 2 to 5 minutes for the glue to be hardened.
d) Partial coring, the core should be cut perpendicular to the surface and the core should pass the interface around 1 inch or 25 mm. The disc is used as a drill guide.
e) Attaching a load frame to the disc
f) Pulling off - Direct tension is applied to the disc. A calibrated hydraulic machine can be used. The tension force increase continues until the specimen fails. The failure mode and failure force are recorded


As with most tests of this nature that are sensitive to core orientation, asymmetry in testing, and specimen handling, results tend to be variable. However, when these factors are carefully controlled and tests are performed by experienced personnel, shear bond testing can be a useful way to develop confidence that the desired quality of bond has been achieved.

5.3 Slant Shear Test

One of the most common types of bonding tests is “Slant Shear Test” in which the interface is under combined state of compression and shear stresses. ASTM C882-99 provides the procedure of bond measurement with this test method. As in many cases the real stresses in structures have the shear component, this test is representing the situation more close to the construction site. The idea behind the test is the idea for the common compressive strength test. In compressive test, concrete failure happens due to the shear cracks in the incline plane. The angle of failure plane with horizontal direction is theoretically between 50⁰ and 70⁰, so 60⁰ could be a proper assumption. Therefore in this test method the interface is placed inclined with the same angle and a compressive force is applied to the system (see Figure 14). The procedure describes the slant shear test method for measuring the bond strength of the interface. The half section of hardened substrate is diagonally cut at 60 from the horizontal and is bonded to the new material, make a complete cylinder. The cylinder is subjected to compression until it meets the failure [14], [15].
a) Casting the substrate
b) Curing the substrate
c) Sawing the substrate at 30⁰ angle from vertical (at least four days of curing needed)
d) Surface preparation and sandblasting at 14 days
e) Placing the cylinder out of curing room for at least two days
f) Checking the 21 days strength of substrate
g) Placing the repair material

Bond Strength= [Maximum Load]/ [Area of slant surface]
It should be noted that the ASTM C882-99 focuses on bond strength of epoxy-resin system. So in ASTM the test procedure is to place two match cylinder halves and bond them with epoxy. But for testing the bond strength of two different materials the second one must be cast after substrate having been cured and prepared. Furthermore, according to Momayez et al [14], the bi-surface shear testing gave approximately twice as high values as the splitting and pull-off testing. When studying results reported in literature this seems to be a rather common relation achieved when comparing results from direct shear and tensile tests.

6. Summary and Conclusion

Concrete repairing mainly includes removing unsound concrete and replacing it with repair or overlay materials. One of the key requirements for any kind of repair system is to have adequate bond strength between the existing concrete substrate and overlay throughout the service life.There are different factors influencing the bond strength which are categorized into three levels based on their effects on bond strength; some of them are considered as major factors and some others are minor factors. Absence of micro cracks, absence of laitance, cleanliness of the interface, proper compaction and curing are the major factors influencing the bond strength.


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