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Crack Sealing

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Don't Neglect Pavement Cracks

There is no argument that all asphalt cement (AC) pavements crack. Cracks in asphalt pavements are inevitable. Neglect and lack of proper maintenance lead to accelerated cracking and/or potholing, further reducing the serviceability of the pavement.

Typical Asphalt Crack
Crack Sealing with Pour Pot & V-Squeegee

What Causes Cracking

There are numerous reasons for asphalt to crack which can be broadly categorized into four distinct classes.

  • Pavement Construction
  • Traffic Frequency & Load
  • Deterioration of the Asphalt Cement Binder
  • Climatic Conditions
Pavement Construction

Examples in this category include flaws in the selection of the right type of asphalt mix, poor mix design, engineering defects, unsuitable ambient conditions, etc. If all of these factors meet design criteria, poor workmanship may be considered as a cause for premature failure of the pavement.

Traffic Frequency and Load

Fatigue cracking and rutting are the two principal considerations in the pavement design process because they are the primary mode of deterioration of asphalt pavements.

Deterioration of Asphalt Cement Binder

This is primarily due to weathering under the degrading effects of the sun.

Climatic Conditions

These include pavement expansion and contractions due to temperature variations, and freeze thaw cycles.

Once cracks develop, water easily penetrates into the base and sub-base of the pavement and damages the structural integrity of the aggregate materials. Pavement joint and crack sealants are designed to protect the pavement by minimizing water infiltration and by preventing the accumulation of debris. It has been amply demonstrated and documented that sealing cracks in flexible pavements is a sound preventive maintenance procedure, which adds many years to the life of the pavement, especially when used in conjunction with seal coating.

The U. S. Army Corp of Engineers, CRRL, undertook a national survey of the crack sealing practices of the Department Of Transportation of all 50 states. It was concluded that 45 states utilized crack sealing under their standard pavement maintenance program. Even the sates that did not do crack sealing, recognized that cracks in their pavements were a problem. States with numerous freeze-thaw cycles cited that;

  • Crack sealing is an important procedure that must be addressed yearly.
  • Crack sealing is one of the most cost-effective ways to prolong the pavement life, as much as 3- 8 years.

How Does the Pavement Deteriorate?

It has been estimated that ninety percent (90%) of asphalt roads in the United Sates have significant problems due to the deterioration of the base and sub-base material because of water infiltration.

Pavements where cracks have not been sealed, offer many routes for water entry into the base and sub-base courses. Water will penetrate through cracks that are over 1/8th to ¼ inch wide. Finer cracks also allow water infiltration into the pavement through the pumping action of the traffic; the surface water is pushed into the cracks when vehicles pass over the cracks. For cracks larger than ¼ inch, water flows under gravity into the surface and the base course. Once water enters the base, the aggregate absorb the water expand in volume and become soft, thus causing internal stress which accelerates the development of more and larger cracks.

The Economics of Crack Sealing

Life Cycle Cost Analysis (LCCA) is defined as “a process for evaluating total economic worth of a usable project segment by analyzing initial costs and discounted future costs, such as maintenance, user costs, reconstruction, rehabilitation, and restoring and resurfacing costs, over the life of the project segment.”

Crack sealing, like any other maintenance project, has to be done with proper economic justification, using proper materials and procedures. It is an established pavement preservation technology, which is steadily evolving to meet the challenges of modern age traffic requirements, utilizing state-of- the-art materials and tools.

Crack sealing is highly cost effective and a lot less expensive than other preventive procedures e.g. an asphalt overlay which costs 8 to 26 times as much as crack sealing. Furthermore, crack sealing slows the reappearance of reflective cracks in the new surface after an overlay. Reflective cracks may be considered as secondary cracks which are caused by the problems within the base course and can not be eliminated without addressing the initial cause of the problem.

State maintenance departments bear most of the burden of dealing with cracks and out of several options available to them (slurry, chip sealing, overlays, etc.), they commonly exercise crack sealing.

Types of Cracks

Transverse Cracks

These types of cracks extend across the pavement from shoulder to shoulder or shoulder to the center-line at approximately right angles to the pavement center-line or the direction of lay-down. They are typically the first cracks to appear and are usually temperature related. They are generally not load- associated. They are caused by the inability of the asphalt layer to redistribute the pavement stresses that occur along the pavement length and width as the temperature decreases. As the asphalt layer gets older and stiffer with age, it loses its ability to redistribute the stresses even more. The cracks then develop at closer and closer spacing.

Longitudinal Cracks

These types of cracks run the length of the pavement roughly parallel to the center-line or lay-down direction. They may be caused by a number of reasons; poorly constructed paving lane joint, shrinkage of the pavement surface (under low temperatures), hardening of the asphalt, daily temperature cycling, etc. Like the transverse cracks, pavements with longitudinal cracks lose their ability to redistribute the stress, which becomes worse with time. Longitudinal cracks appear later than transverse cracks.

Edge Cracks

These types of cracks appear at the joint with concrete curbs. They rapidly widen and deepen with continuous freeze/thaw cycles.

Seam Cracks

These types of cracks occur along the paving joints and are typically caused by improper hot mix paving procedures.

Block Cracks

These types of cracks appear in a block or square pattern and are usually spaced between 4 to 12 feet. Block cracks generally appear towards the end of the pavement life but they may appear sooner if the base courses were not of the right thickness, were improperly compacted, or lack proper drainage.

Reflective Cracks

These types of cracks do not show a set pattern, they may be of any configuration. They appear when the surface material cannot adjust to changes in the movement of the sub-surface courses. To eliminate reflective cracks, problems associated with sub-surface courses should be corrected.

Alligator Cracks

This type of crack is a series of interconnecting cracks caused by the fatigue failure of the asphalt surface under repeated traffic loading. Such cracks are extensive, close together and go in all directions. Cracking begins at the bottom of the asphalt surface, where the tensile strength is the highest under a wheel load. Initially the cracks propagate to the surface as a series of longitudinal cracks. After repeated traffic loading, the cracks connect, forming many sided, sharp-edged pieces that develop a pattern resembling chicken wire, alligator skin or a road map. Alligator cracking occurs only in areas subjected to repeated traffic loading, such as wheel paths. Alligator cracking indicates a seriously deteriorated pavement, which can not be saved by crack sealing.

Final Thoughts

Crack sealing is a highly cost effective maintenance procedure for asphalt pavement. Most states in the U.S. use crack sealing as part of their standard pavement maintenance program.