Repair of cracks in PQC in concrete pavements

By

P. Gupta, 

Technical Advisor, COWI – DIPL Consortium, Naini Bridge Project, Allahabad, India

And (Ex) Material Consultant Allahabad By-Pass Project, Allahabad

Large scale construction of Bridges & Highways is going on in India and World over.

• They are mostly made as rigid pavements using Pavement Quality Concrete (PQC). On one hand, concrete is a very good and strong material, but it develops cracks due to temperature, shrinkage, creep, etc. Until enough precautions are taken, some cracks do come up on the concrete surfaces.
• Prominent cracks should be repaired for better performance and longer life of Highways/pavements. To repair these cracks, several materials and procedures have been developed. The efficiency of these materials and procedures depends on the type of crack, weather conditions, and applicator efficiency. 
• In this paper, various procedures of crack repair are described. However, any procedure, in general, should be first tried on a pilot basis, and then depending upon its success, repairing of cracks be done at the site.
• A procedure of injecting low viscosity Epoxy Material is an excellent material. It is discussed here and can be used for the repair of cracks.

First, it is necessary to understand why the concrete cracks. There can be many reasons why the concrete cracks. These need to be understood from its reaction and setting pattern point of view. The main reason is improper concrete mix design and laying/jointing practice related to the inherent nature of concrete to change volumetrically due to moisture, reaction and thermal effects, etc. The incidence of cracks can be minimized by improving concrete mix design, laying time weather condition, and jointing practices with timely saw cutting and properly managed to cure.

The relevant pavement is surveyed and the affected portion is marked with a marker. The area can be struck with a handheld hammer to further qualify and quantify the crack. The dull and hollow sound is emitted from the unsound concrete whereas metallic sound is emitted from sound concrete. The boundary of the area to be treated should be marked and as far as possible rectangular and normally about 50 mm should be taken into the sound concrete to make sure that all unsound concrete is included. Cracks and their location can be identified from the top surface of PQC and some from the sides. Then judge the extent of cracking in concrete by any one of the following methods.

The cracks may be classified in following categories:

Such Surface Cracks develop in PQC due to temperature difference, late joint cutting, defective curing of PQC, etc. The cracks will allow water/mud/debris to go into the cracks and widen them further.

Some typical Medium or Full-depth cracks are shown in Figure 1.

Such cracks normally develop because of poor compaction or poor Concrete Mix Design. Some typical cracks are shown in figure2.

Figure 2: Typical Panel Settlement or near panel Cracks

Visually examine all such cracks and decide which crack is to be repaired on the basis of width, depth, and pattern. In general hair or micro-cracks do not affect much the structural integrity of PQC. Thus remedial works or repair of such cracks may not be necessary. However, partial/half depth cracks should be repaired. About full depth cracks and many checker cracks, it must be decided on a case-to-case basis after looking at the site and analyzing the condition depending upon its width, length, and a number of cracks in a panel as to what treatment is to be given.

• The material, techniques / System for repairing the PQC cracks should be such which will have the durability of a minimum of 10 – 15 years or as per the designed life of the pavement. Otherwise, water may penetrate from these cracks and reach sub-grade and embankment levels causing settlement.
• This will ultimately damage the pavement. Hence it is necessary that a methodology and guidelines for the repairs & rehabilitation of such cracks in PQC be developed.
• Normally the repair material should remain flexible enough to accommodate anticipated expansion & contraction, water-tightness, etc. Repaired cracks should not be rigid otherwise the cracks may appear again in the same panel at adjacent panels.

Following are some of the material used in crack repair.

Epoxy mortar is a common material used for the repair of concrete surfaces in general, especially for the potholes in rigid pavements. However, 100 % epoxy-based mortar is a little brittle in the long run; hence in the case of repairs of pavements, its modified products should be used such that the resultant material is not rigid.

As epoxy mortars are non-UV resistance, but still it is a good high strength material, so use a polyurethane-based coating on the repaired surface of mortar that is exposed to UV or direct sunlight. This coating will protect the existence of epoxy mortar till the time mortar remain intact & not start to lose its property of non-brittleness. PU coating should normally be of about 300-micron thickness.

This is an epoxy modified cement concrete used in repairs not only in PQC crack repairs but also in structures and concrete surfaces. For PQC Crack repairs Elastomeric concrete is useful material.

• Fiber-reinforced concrete is a concrete having small size aggregate (say 6 mm or so) or Cement sand mortar. It is mixed with any one of the different types of fiber given below which are generally used in construction.
• Poly propylene Fibers are more appropriate for this purpose.
• Though, there are several types of fibers available as given below.
• Normally fiber concrete can also be used as a thin top layer or bearing surface over the cracked PQC pavement after treatment of cracks.

These are very low-width cracks or hairline cracks. Such cracks generally do not affect the performance of road surfaces. Thus these cracks, unless the severity level increases, need not be repaired. Further, observe these cracks over a period of time for enlargement.

If cracks are not moving then a very low viscosity epoxy resin may be used for its repair with or without fine sand to bond the crack faces. V-shaped ‘joint wells’ are made with 3 to 5 mm saw blades. Also, make 8 to 10 mm diameter grout holes at regular intervals. Then pump the epoxy bonded into holes using a grout gun.

• The spalling of concrete occurs near the joints mainly due to adjacent slab load transfer-related movement at the joint, particularly in summer.
• The spalling will occur frequently in mid slab locations and other locations mainly due to high slump due to w/c ratio, poor construction, or failure of earlier done repairs.
• Spalls are a localized problem and can be easily repaired with partial depth repair methods.

The full depth repairs are necessitated due to Transverse Crack in the slab in full panel width. This crack may be in one lane or both lanes. The reasons for such crack are:

In all such cases where cracks are more harmful, then the QPC Panel should be examined carefully if the panel is to be repaired or removed. If it has to be repaired then use the technique of cross-stitching method. However, if that portion of the panel is to be removed and then recast the panel as indicated by Removal of Existing Unsound Concrete.

Depending upon the method of repair the following equipment may be used.

A minimum of 50 mm deep rectangular cut should be made at the end of the panel which is to be removed. Use a saw cutting machine with vertical faces. Inside this panel mark rectangle, the smaller square of size 100 mm × 100 m and 25-30 mm deep.

Report on Defects and Distress of Rigid Pavement Sections:

The survey, Rating, Monitoring, Causes and Repair By Jose C. Horta