Reflective cracks

The pattern and direction of the reflective cracks vary from linear in the longitudinal, transverse or diagonal direction to block cracking depending on the old pattern of the cracks that developed before applying the overlay. 

Reflection cracks are caused by vertical and horizontal movements of the underlying parts of the pavement showing discontinuity (broken parts or joints). These movements are induced by traffic, expansion and contraction owing to temperature or moisture changes or swelling/ shrinkage of the subgrade. 

The maintenance of reflective cracks depends on their size (crack width) and severity. The following are the usual techniques used  

Slippage cracking is exclusively caused by lack of bond (or poor bond) between two asphalt layers. Lack of bond is attributed to the absence of tack coat and to the presence of dust, traces of clayey soil material, oil, dirt or even water on the asphalt surface to be paved. 

The maintenance of these cracks is carried out by removing the affected area, cleaning thoroughly the exposed surface and applying tack coat and a surface patch. 

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Crack Reflection. Crack deflection can result when particles transform ahead of a propagating crack. The crack can be deflected by the locali2ed residual stress field which develops as a result of phase transformation. The force is effectively reduced on the deflected portion of the propagating crack resulting in toughening of the part. 

The increased pavement life can be attributed to higher viscosity and impermeability of rubberized asphalt. These properties have decreased thermal cracking, potholing, deformation, and reflective cracking in most states in which tests were performed. Studies by the Alaska Department of Transportation showed decreased stopping distances as a result of rubberized asphalt being more flexible and preventing ice formation (37). 

The addition of fibres takes place at the stage of mix production or directly after spraying the bitumen or the bituminous emulsion on the pavement s surface. The latter creates a stress-absorbing membrane to inhibit reflective cracking (Yeates 1994). 

Yeates C. 1994. An evaluation of the use of a fiber-reinforced membrane to inhibit reflecting cracking. Symposium on Performance and Durability of Bituminous Materials. Leeds, UK. 

OGFCs may also be placed to reduce the tyre-pavement interface noise and may also be placed to reduce the occurrence and severity of reflective cracking. 

The induced transverse cracks should be 3 m apart and must be aligned (maximum 100 mm tolerance) with any induced cracks in the underlying construction. The laying width of HBM is recommended to be not more than 4.75 m. The above minimise the possibility of having contraction or reflective cracks on the surface of the pavement in the transverse and longitudinal direction. 

Finally, cracking in hardened concrete may be caused by propagation of exiting cracks (reflective cracks) owing to lack of subgrade support or swelling/shrinkage of soil material. 

The time of appearance of the reflective cracking, or cracking owing to settlement of the subgrade, as well as cracking owing to improper joint spacing or lack of reinforcement, can be several weeks to months. 

When it is time to replace part of the asphalt layers, or to place an overlay, the use of modified bitumen in the new asphalt hot mix is recommended in most cases. The use of asphalt-reinforcing grids, or other reflection cracking-reducing technique, is also advised to be considered. For details, see Section 15.6. 

Reflective cracks occur in overlays laid to rehabilitate old cracked flexible pavement (Figure 15.5). Typical reflective cracks also appear in asphalt overlays on rigid pavements, flexible pavements with cement-bound bases or asphalt overlays constructed over tied shoulders or widenings. 

Figure 15.6 shows a transverse reflective crack of asphalt overlay over a cement-bound base flexible pavement. The crack was attributed to the absence of joint formation of the cement-bound base. 

Figure 15.5 Reflective cracking initiated over an old paving joint in a flexible pavement.

Figure 15.6 (a) Reflective crack on the transverse direction, (b) Core taken at the reflective crack. 15.5.5.1 Causes... 

RETARDATION OF REFLECTIVE CRACKING TREATMENTS AND ASPHALT REINFORCEMENT TECHNIQUES... 

Retardation of reflective cracking, when an overlay is to be constructed over an area with cracks, can be obtained by applying one of the following treatments (a) use of geosynthetics for asphalts, (b) use of stress-absorbing membrane interlayer (SAMI) and (c) use of fibre-... 

When an asphalt layer with a thickness of 40 mm (or more) is laid on SAM, SAM acts as a stress-absorbing interlayer and the technique is called SAMI. The advantage of this technique is the excellent adhesion achieved between the old pavement surface, the absorbing interlayer and the asphalt overlay. SAMI is a much longer-lasting method of treating reflective cracking than SAM. 

On jointed concrete pavements when hot asphalt surfacing overlay is used for texture restoration, reflective cracks at the joints are expected to appear after a certain period. The same applies when micro-surfacing or surface dressing technique is applied, but reflective cracks will appear sooner. To avoid the above, it is recommended that all joints be replaced after laying the hot asphalt, micro-surfacing or surface dressing. 

The asphalt overlay laid on worn rigid pavements involves (entails) the concept of correcting functionally related surface defects rather than exclusively strengthening the pavement to carry future traffic load. In all cases, asphalt overlay on rigid pavements is very much affected by reflective cracking. 

In order to control or minimise reflective cracking, one of the following techniques are employed, before laying the asphalt overlay ... 

Break (or crack) and seat is a technique to reduce the effective length of the concrete slab so that the horizontal strains resulting from the thermal expansion are reduced and evenly distributed. The above reduces the potential for reflecting cracking to occur. The break and seat technique is normally applied to unreinforced pavements but it may be applied to jointed reinforced pavements as well. 

The provision of crack-relief layer was pioneered in the United States for use in jointed and continuously reinforced concrete pavements to minimize reflection cracking. 

In the United Kingdom, TRL trials using porous friction course mixture gradation as a crack-relief layer on military airfields have shown good performance in resisting reflection cracking, even with a crack-relief layer as thin as 20 mm (Coley and Carswell 2006). 

The use of SBS-modified bitumen in asphalt overlays is expected to delay the propagation of reflection cracking and should always be considered as an option. 

Similarly, the use of SAMI or geogrids/geotextiles between the concrete slab and the overlay is also expected to delay propagation of reflection cracking (Ogundipe et al. 2011). 

In the case of thick concrete overlay, the new slab needs to be free to move horizontally to deter the appearance of reflection cracks. The above can be achieved with the provision of an interlayer, which can be a polythene membrane or an asphalt interlayer. The use of an asphalt interlayer is preferred if the underlying concrete pavement has a significant remaining integrity (Thom 2008). 

Fracture existing pavement if concerns over rocking slabs or reflection cracking exist (poor existing joint LTE). 

Reflection cracking in overlays, total reflected cracked area for month Smoothness (roughness), inches/mile IRI inches/mile IRI ... 

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