Early Cracking of Concrete Pavement - causes and repairs

Binod Kumar, Scientist, and Dr. Renu Mathur, Head, Rigid Pavements Division, CRRI, New Delhi

Hundreds of kilometers of concrete pavements are being constructed in the country under National Highway Development Programme (NHDP). Some of the concrete pavements have already been opened to traffic while some others are due to be completed soon. The construction of these concrete pavements in different parts of the country has been carried out by the best available construction agencies using most modern construction equipment, the best possible materials, and under the supervision of various national and international consultants and experts. Despite adopting good construction techniques and quality control measures, many kind of premature distresses have been observed in these newly constructed concrete pavements. The most common distresses have been observed in the form of full depth transverse and longitudinal cracking, settlement cracking, corner cracking, cracks over dowel bars, cracks over slab culverts, and blow ups etc. These distresses do indicate the failure of concrete slabs but more than that they indicate the human failure to understand the few basic and fundamental things related to concrete material and pavement. It is the ignorance and negligence on the part of those associated with the construction of concrete pavements that most of the time leads to the occurrence of such premature distresses. This paper discusses the different kinds of distresses that have been observed on newly constructed concrete pavement slabs under different projects. The causes of distresses and repair techniques that have been adopted at some of the projects have also been discussed.

Introduction

It is to the credit of the concrete that not many complaints are received on the vast amount of concrete laid for the construction of concrete pavements. While it is easy to determine the properties of hardened concrete that will be suitable for the pavement quality concrete, great care is required throughout the entire construction process to ensure that the laid concrete attains all its desired properties without leading to any premature distresses. Any negligence during the construction process may lead to premature distresses in concrete pavement slabs. These distresses may appear in the form of uncontrolled cracking, plastic shrinkage cracking, settlement cracking mainly over pipe culverts and near slab culverts, popouts, crack over dowel bars etc.

Many such type of cracks have been observed in concrete pavement slabs that either have been completed in recent past or are being constructed at the projects under National Highway Development Programmes. All kinds of distresses observed in concrete pavements happen due to some specific reason associated with the behavior of either concrete as a material or concrete pavement slab as a structure. Sometimes, it is very confusing and difficult to ascertain the correct cause of a particular kind of distress but it is possible to find it out by carrying out an investigation coupled with proper understanding of concrete pavement response to environmental and traffic loadings. In any case, it is very essential to fix the cause of distress before selecting the most suitable repair methodology.

When it comes to the repair of distressed concrete pavements, very limited options are available. It is very difficult and time consuming to repair hardened concrete. Full depth and partial depth repair, slab replacement, crack sealing, crack stitching, staple pinning are some of the repair techniques that can be used for repairing the distressed concrete pavements. The causes and repair techniques for these distresses are discussed in the following paragraphs.

Uncontrolled Shrinkage Cracking

Like all materials, concrete also expands and contracts with variations in temperature. Concrete shrinkage starts as it cures. The temperature and moisture gradient that exists between top and bottom of the concrete pavement slabs causes the curling and warping of the slab. The natural response causes the concrete pavement to crack at regular intervals. A fundamental feature of concrete pavements is to introduce a jointing system to control the location of these expected cracks. Contraction joints which are generally provided at each 4.5m interval, are specifically meant for controlling the location of these type of cracks.

The contraction joint system assures of crack control in new concrete pavement. However, certain design or construction factors may influence the effectiveness of a contraction joint system. Unexpected changes in the weather during and after construction can induce uncontrolled cracking despite the adoption of proper jointing system. Because of the complexity of interrelated factors, uncontrolled cracks will occur in some concrete pavements. Theses cracks generally develop within the first thirty to forty five days.

Prevention of Uncontrolled Cracking

Concrete slabs crack when tensile stresses within the concrete overcome the tensile strength. At early ages, the tensile stresses develop from restraint of the concrete's volume changes or slab bending from temperature and moisture gradient through the concrete. Each transverse and longitudinal saw cut induces a plane of weakness where a crack will initiate and then propagate to the bottom of the slab. Uncontrolled cracking can be controlled by adopting following precautionary measures.

Timing of Sawing Joints

There is an optimum time to saw contraction joints in new concrete pavements, which is defined as the sawing window. It represents a short period after the placement of concrete within which concrete can be cut successfully before it cracks in an uncontrolled manner. If the sawing of the joints is started too early then it may lead to raveling along the cut. The jagged, rough edges arte termed as raveling. Some raveling is acceptable if the widening of saw cut for filling joint sealant would remove the ravel edge. If the raveling is too severe, it will affect the appearance and ability to seal the joint. If sawing of joints is delayed beyond a certain period when significant concrete shrinkage occurs then it may induce random cracks within the pavement (Photograph 1).

Saw Cut depth

The influence of the saw cut depth on early cracking primarily depends upon the time of sawing. Early sawing of the joints may require lesser saw-cut depth for preventing random cracking. Generally the saw-cut depth is kept as 0.25 to 0.33 times the depth of the slab. If the depth of the saw-cut is less than the required depth then it may not sufficiently weaken the concrete at that location and it may ultimately lead to cracking of concrete elsewhere.

Weather and Ambient Conditions

The weather almost always has a role in the occurrence of uncontrolled cracking. Air temperature, wind, relative humidity, and sun light all influence concrete hydration and shrinkage. These factors may heat or cool concrete or draw moisture from exposed concrete surfaces. Concrete paved in early morning will often reach higher temperatures than concrete paved during the late morning or afternoon because it receives more radiant heat. As a result, concrete paved during the morning will generally have a shorter sawing window, and often will exhibit more instances of uncontrolled cracking.

Joint spacing

Theoretical and practical studies have shown that the optimum joint spacing depends upon the slab thickness, concrete aggregates, subbase, and climate. Pavement with long transverse joint spacing may crack at locations other than the saw cuts due to tensile stresses from temperature curling. Most of the time the spacing of transverse contraction joints in plain pavement are kept 4.5 m to 6.0 m. It is also important to check the transverse and longitudinal contraction joint spacing to see if it is within the limits as described in various codes and specifications.

Curing conditions

The internal temperature and moisture of concrete also influence the time available for joint sawing. The temperature relates to the strength gain of the concrete and partly controls the time of initiating the saw cut and final time before the onset of cracking. The sawing of joints should be completed before the concrete surface temperature begins to fall since thermal contraction begins as soon as the concrete temperature falls. Heat development profile of a concrete mix can be obtained by using concrete maturity meters. Monitoring of concrete surface temperature will let know the concrete strength and also the point when surface temperature begins to decline and sawing should be completed.

Misalignment of dowels

The formation of initial cracking will not be influenced either by the presence or misalignment of dowel bars. The alignment of dowel bars only becomes a factor of restraint if a crack extends below the joint saw cut and misalignment exceeds a tolerance of 3 percent. If a crack exists below the saw cut, and an uncontrolled crack occurs nearby, then it is possible that dowels are misaligned and have caused locking of joint i.e. not allowing joint opening or closing (Photograph 2).

Blowups

Blowups are compressive joint failures brought about by excessive expansion related to high temperatures, high moisture contents, or a combination of the two. Blowups may occur gradually or may be sudden and dramatic. Failures are full depth and full lane width and can present serious hazards to traffic.

Blowups become likely when normal joint movement is restricted by infiltration. Increase in concrete volume brought about by elevated temperatures and moisture contents create longitudinal thrust that may overcome the compressive strength of the weakest joint in the section. Blowup tendency is more pronounced on pavements with long slabs where individual joint movements are greatest. Joints typically fail in the lower portions first. This failure provides an inclined plane for the slabs to slide upward when further expansion occurs. A sudden and dramatic blowup can occur when the upper portion shears off with little or no warning. Most blowups occur during a significant hot spell and usually in the afternoon. Blowups seldom occur where joint spacing is less than 6 m, with no intermediate expansion joints, even where joints are not sealed. Blowups almost never occur in new pavements. If the pavement is susceptible to blowups, they begin to occur after 3 to 5 years of age. The blowups usually occur at joints or cracks in the pavement and the concrete at the blowup appear to be weak or deteriorated at that point.

Settlement Cracks

Settlement of the subgrade and subbase can cause the cracking of the concrete pavement. Cracks resulting from settlement of subgrade are normally variable in direction but most commonly they appear diagonally and extend continuously to many slabs. Repeated heavy truck loads may further cause breaking of slabs into several pieces due to loss of support beneath the slab. Locations of underlying pipe culvert and slab culvert are more prone to settlement cracking.

Premature Distresses in Concrete Pavements: Causes and Repair

Settlement of subgrade and other pavement layers over pipe culverts and in the vicinity of slab culverts mainly during and after rainy season can cause the full depth cracking of overlying concrete slabs. Photograph 3 shows the shattered slabs due to settlement.

Cracks Over Dowel Bars

Fine hair line to moderately wide cracks may develop sometimes over the dowel bars (Photograph 4). The length of the cracks may be as long as the length of dowels. These cracks are mostly surface cracks with 25 to 40 mm depth from the slab surface. However, in some cases, these cracks may penetrate up to the surface of dowel bars. These cracks may not affect the load transfer capacity of the dowel bar assembly, but gradually the spalling of these cracks may lead to deterioration of slab surface over the dowel locations. Possible reasons of such cracks are too little or too much vibration of dowel bar inserter unit, stiff concrete mix, shallow depth of dowel bars, and natural phenomenon of settling heavy solids in a liquid medium around dowels. Inadequate vibration of DBI unit and stiff concrete mix may leave a dowel trail in which the concrete is not compacted properly. Subsequently concrete settles down within the trail and creating a crack over dowel location. Too much vibration of DBI unit may create segregation of aggregates in the dowel trail resulting in too much accumulation of slurry and water in the trail over dowel. This may cause the excessive shrinkage of concrete at these locations leading to cracking. Also, the surface concrete over dowel locations becomes weak due to high water cement ratio leading to abrasion of mortar from the surface.

Misplaced Saw Cuts at Transverse Joints

Saw cut of transverse joint at proper location with respect to position of dowel bar assembly is a common construction practice. But some occasional mistakes may happen resulting in the misplaced saw cut. Proper location of saw cut over dowel assembly, not only improve the joint load transfer efficiency but also ensures better performance of the pavement through-out its life. Tolerance for sawing transverse joint i.e., the allowable translation of saw cut from the middle of the dowel assembly, depends upon the length of the dowel. It has been found that 15 cm of dowel embedment is all that is necessary for effective load transfer under highway loadings. Thus, for a commonly used 50 cm long dowel bar, the available tolerance is 20 cm, i.e. 10 cm either way from the centre of the dowels.

Cracks Over Slab & Box Culverts

If concrete pavement slabs are constructed over an underlying slab or box culvert and the transverse joint locations do not match with the boundary of underlying slab of the culvert, then it is most likely that full depth transverse cracks will develop in the concrete pavement slabs just above the extreme boundaries of culvert slab on both sides (Photograph 5). Many such instances have been observed in recently completed concrete pavements in the country. Occurrence of such cracks in more prevalent where pavement quality concrete (PQC) and dry lean concrete (DLC) layers are laid directly over the culvert slab without any intermediate layer of granular sub-base. If a granular layer is placed over culvert slab before laying PQC and DLC, then this layer acts as a crack arresting layer and possibility of developing transverse cracks in pavement slab is reduced if not eliminated completely.

Plastic Shrinkage Cracks

The weather almost always play an important role in the occurrence of uncontrolled cracking of concrete pavement. Air temperature, wind velocity, relative humidity and sunlight influence the hydration and shrinkage of concrete. These factors may heat or cool concrete or draw moisture from exposed concrete surface. Plastic shrinkage cracking is a result of rapid drying of concrete pavement surface due to either high ambient temperature, high wind velocity, low humidity or a combination of these factors. These cracks are generally tight and appear in the form of parallel groups perpendicular to the direction of the wind soon after the placement of concrete Adequate curing measures are necessary to prevent their occurrence (Photo 6).

Surface Popouts

A pop out is a conical fragment that breaks out of the surface of the concrete leaving a hole that may vary in size. Usually a fractured aggregate particle will be found at the bottom of the hole, with part of the aggregate still adhering to the point of the popout cone. The cause of a popout is a piece of porous rock having high water absorption and relatively low specific gravity. As the offending aggregate absorbs moisture, its swelling creates internal pressure sufficient to rupture the concrete surface. Pyrite, hard-burned dolomite, coal, shale, soft fine grained limestone, clay lumps, or chert commonly cause popouts. It may also be caused by water uptake of expensive gel formed during the chemical reaction between the alkali hydroxide in the concrete and reactive siliceous aggregates. Most popouts appear within the first year after placement. Popouts caused by alkali-silica reaction may occur as early as few hours to a few weeks, or even a year after placement of concrete. Popouts caused by moisture induced swelling may occur shortly after placement due to the absorption of water from the plastic concrete or they may not appear until after a season of high humidity or rainfall. Popouts are considered a cosmetic detraction and generally do not affect the service life of the concrete.

Curb Cracking

Curb cracking mainly occur wherever the curbs are cast monolithically with concrete pavement slab. It may also be observed, though not so predominantly on the curbs laid cast-in situ with curb casting machines but not casted monolithically with the slabs. In both the cases, cuts are provided into curb stones just opposite to the transverse joints of the pavement to allow the expansion and contraction of the curbs. If the joint of these curbs is blocked by soil, stone grits and other material then the expansion of curbs along with concrete slabs becomes difficult and due to excessive compressive stresses curbs may crack.

Repair Methodology

Full Depth Repair

Full-depth repair (FDR) is a concrete pavement restoration (CPR) technique that can be used to restore the structural integrity and reliability to concrete pavements having certain types of distress. It involves making lane-width, full-depth saw cuts to remove the deteriorated concrete down to the base, repairing the disturbed base, installing load-transfer devices, and refilling the excavated area with new concrete. It is an effective, permanent treatment to repair pavement distresses particularly those that occur at or near joints and cracks. By removing and replacing isolated areas of deterioration, full-depth repairs may delay or stop further deterioration and restore the pavement close to its original condition. The distresses that can be addressed using full-depth repairs include transverse cracking, corner breaks, longitudinal cracking, deteriorated joints, D-Cracking, blowups, and punchouts.

Selection of patch size

It is important that the boundaries be located so that all significant distresses are removed. Deterioration near joints and cracks may be greater at the bottom of the slab than at the top of the slab. Therefore, further investigation should be performed. The location of patch boundaries also depends on the level of load transfer which is to be provided. The patches must be of sufficient size to eliminate rocking and longitudinal cracking of the patch. A minimum patch length of 1.75 m and full-lane patch width of 3.5m is recommended to provide stability and to prevent longitudinal cracking. For the same reason, the minimum remainder of the slab must be at least 1.75 m for a 3.5 m wide slab. Combining two smaller patches into one large patch often can reduce repair cost. However, the longest patch length should not exceed the pavement's slab length. Following guidelines may be followed for locating repair boundaries.

• The recommended minimum repair length for 3.5 m wide slab is 1.75m for repairs provided with mechanical load-transfer devices, and 2.4 - 3m for repairs with aggregate interlock joints. All repairs should be full-lane width.
• The minimum recommended distance from the full-depth repair joints to the nearest transverse crack or joint is 1.75m.
• A boundary that would fall at an existing doweled transverse joint should be extended 0.3 m to include the existing joint.
• If distress is present on only one side of an existing non-doweled joint, that joint may be used as a boundary.
• On multiple-lane highways, it is generally not necessary to match joints in adjacent lanes, as long as the minimum length requirements are met, all of the deteriorated area has been included within the repair boundaries, a separation fiberboard has been placed along the longitudinal joint, and the patch is not tied to the adjacent lane.

However, if the distressed areas in both lanes are similar and both lanes are to be repaired, aligning repair boundaries to avoid small offsets and to maintain continuity may be desirable.

Removal of distressed concrete

The outer boundaries of a repair should be cut by diamond blade saw cut machine. Deteriorated concrete from the repair area may be removed either by lifting out or by breaking up. It is preferable to lift the deteriorated concrete whenever possible. Lifting the old concrete imparts no damage to the subbase and is usually faster and requires less labor than any method that breaks the concrete before removal. For lifting out, holes are drilled into the old concrete surface, then lift pins are inserted into holes and concrete is removed with the help of chains fastened to a crane. Deteriorated concrete may also be removed by breaking it into small pieces. The drawback of this method is that it often damages the subbase.

When using mechanized breaking equipment like drop hammers or hydraulic rams, operators must exercise control on the equipment's break energy. Operators should begin breaking the concrete in the center of the removal area and move outward toward buffer cuts. Buffer cuts are made about 0.3 m away from the perimeter of saw cuts within the patch. The operator should reduce the break energy (drop height) before starting on the area outside the buffer cuts. Then there will be less chance of damaging concrete beyond the patch perimeter.

If subbase has been damaged during removal operation of old concrete then it would be necessary to repair it by adding and compacting new subbase material.

Providing load transfer

Dowels at transverse joints are essential for load transfer for most full depth repairs, except for light traffic pavements. Holes are drilled in the vertical faces of the slab, parallel to the surface and sides of the slab. The diameter of the holes should be the minimum that is necessary to accommodate the size of dowel bar that is to be used. The hole diameter also depend on the anchoring material. Cement-based grout requires a hole diameter 5-6 mm larger than the nominal outside dowel diameter. Epoxy anchoring materials only require a hole diameter about 2 mm larger than the nominal dowel diameter. After drilling, the holes are cleaned with compressed air to force out all the dust and debris. Holes are then plugged with some suitable epoxy resin with the help of a long nozzle that feeds the epoxy to the back of the hole. Insert new dowel bars accurately aligned parallel to the surface and sides of the slab. Make sure that the epoxy anchoring material flow forward along the entire dowel embedment length during insertion. Debond the dowel bars with thin, tight fitting plastic sheaths. A bond breaking 5-6 mm thick fibre board should be placed along any longitudinal face with an existing concrete lane or concrete shoulder. It would allow the patch and the old concrete to move independently.

Placing & finishing the new concrete

Place and evenly spread pavement quality concrete to the appropriate surcharge. Thoroughly compact the concrete using internal vibrators and then finish the surface with the help of a screed vibrator. Particular care should be taken to ensure full compaction around the dowel bars and edges of the repair. The patch surface should match the surrounding surface profile.

Texturing and curing

Patch surface may be textured so that it is similar to the surface of the surrounding pavement. The first few hours after pouring the concrete are the most critical for good curing. Therefore, apply liquid-membrane-forming curing compound immediately after texturing over the surface of newly placed concrete. To prevent moisture loss and to protect the surface against the occurrence of plastic shrinkage cracks, polythene sheet may be placed over the patch surface.

Sawing and sealing joint

The final step is to saw transverse and longitudinal joint sealant reservoirs at the patch boundaries. Sealed joints will lower the potential for spalling at the patch joints. The joints may be filled with any suitable joint sealant.

Cross-Stitching

Cross-stitching is a repair technique for longitudinal cracks which are in reasonably good condition. The purpose of cross-stitching is to maintain aggregate interlock and provide added reinforcement and strength. The tie bars used in cross-stitching prevent the crack from vertical and horizontal movement or widening. This technique knits the cracked portions of the slab together and reduces the chances of crack to grow further.

Cross-stitching uses deformed tie bars drilled across a crack at angles of 30-45 degrees. Deformed steel bars of 10-12 mm diameter are sufficient to hold the crack tightly closed and enhance aggregate interlock. Full depth holes of 18-20 mm dia are drilled at a pitch distance of 300 mm with the offset of 150 mm from the crack. The holes are drilled alternately from each side of the crack so that one hole passes through the crack from left to right while the next from right to left. After drilling, the holes are flushed with high pressure air to clean out any residual dust. Then a high strength epoxy gel adhesive is injected into the holes. Immediately after injecting epoxy, deformed steel rods are inserted into each hole. The crack is sealed at the top with a silicon sealant.

Do not stitch a transverse crack which has assumed the role of an adjacent joint. Stitching will not allow transverse joint movement (open and closure). A new crack will likely develop near a stitched working crack or the concrete will spall over the reinforcing bars.

Slab Replacement

In cases where a slab has full depth and intersecting multiple cracks, slab replacement become necessary. It involves the demolition and replacement of affected slab. Prior to breaking out of the affected slab, a full depth saw cut be made around the perimeter of the repair to minimize the damage to the surrounding slab. This should include the existing transverse joints on both sides. Care should be taken to ensure that the saw cut do not extend into adjacent slabs. It accidentally it happens, then the cut into the adjacent slab should be repaired with epoxy mortar. The concrete of the affected slab may then be sawn into smaller pieces before being broken up and removed from the slab. The concrete that remains in the corner of the patch after saw cutting should be broken out carefully to avoid undercutting the remaining slab. Reinstatement of the sub-base, if required, should be done by taking care of full compaction especially in the corners. A plate vibrator should be used to compact the subbase. Fixing of dowels into drilled holes, placing, compacting, finishing, texturing and curing of fresh concrete into the patch will be as described in full depth repair section.

Concluding Remarks

Many type of cracks such as uncontrolled transverse full depth cracks, plastic shrinkage cracks, full depth cracks near slab culverts, cracks over dowel bars etc. have been observed on the concrete road projects that have been completed recently. All such cracks can be prevented or minimized by making aware the site staff about the precautions to be taken during concrete paving. Due care during construction can reduce the troubles which otherwise would be very difficult and costly to remove after the concrete has set.

Acknowledgment

Authors are thankful to Dr. S. Gangopadhyaya, Director, Central Road Research Institute, New Delhi, for his encouragement and kind permission to publish this paper.

References

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