Potholes and Patches: The Ups and Downs of Pavement.
The basic types of on-grade pavements are rigid, Portland cement concrete and flexible, bituminous concrete. A more recent trend is the use of loose-laid, fired-clay, or concrete pavers. Some of the problems that all of these pavement types have include settlement, frost heave, freeze-thaw durability, and poor surface drainage.
Slabs on grade settle because the unconsolidated soil subgrade on which they are built consolidates under the weight of the pavement or loads imposed on it. If settlement occurs, rigid pavements made with Portland cement concrete may crack. If the pavement is made of relatively flexible material, such as bituminous concrete (asphalt) or masonry payers, it may simply deform the surface. Either condition will affect appearance and may result in serviceability issues and possible liability for the property owner.
The only thing that will prevent settlement is to have a stable sub-grade. During site-grading operations, if the sub-grade consists of undisturbed and well-consolidated soil, it must be protected from disturbance prior to placement of the pavement sub-base and the pavement material. If the existing soil is unsuitable to support loads, because, for example, it has high organic content or is uncontrolled backfill, that soil may have to be removed. Any backfill used to achieve the desired final grade should be clean, granular soil or crushed stone and should be compacted in maximum 12-inch thick layers to a strict standard (e.g., 90-percent modified Proctor density test of the sub-grade material as verified by the American Society for Testing and Materials test [ASTM 1557]). Sub-grades for pavements are commonly proof rolled or otherwise mechanically compacted to achieve the proper density. If the sub-grade is stable, the pavement will have much better serviceability. Any reasonable expense to achieve a good s ub-grade is money well spent.
If an existing pavement has minor cracks, routing the cracks and filling them with an elastometric, traffic-grade sealant that is formulated to be compatible with the pavement material can repair them. As bituminous pavement starts to weather and dry our, the surface can be sealed to extend its service life. However, in the case of existing, unacceptably-heaved or potholed pavements, removal of the pavement may be the best option. Removal could be followed by mechanical compaction of the existing subgrade material, followed by installation of new pavement.
Installation of new pavement over an existing settling pavement, while commonly done, and immediately less expensive, will not cure the cause of the problem and is not cost effective in the long term because the pavement is likely to keep moving and deteriorating.
A common source of degradation of existing pavements is ongoing construction of underground utility lines. When these lines are cut and trenched through pavement, care should be taken to ensure that the existing adjacent pavement sub-grade is protected from being undermined and that the new back-filled sub-grade is adequately compacted. If not, it will not be long before settlement of the sub-grade occurs followed by deformation and cracking of the pavement and the development of potholes.
In the northern and Midwestern part of the U.S., repetitive freezing and thawing temperatures play havoc with on-grade pavements. Typically, water that becomes trapped under the pavement in the sub-grade and sub-base course of the pavement freezes, causing upward expansive forces that result in heaved pavements. Repetitions of this process along with subsequent vehicle traffic over buckled pavement will cause progressive damage in the form of cracks, which let more water under the pavement, and finally in sinkholes and potholes. The pre-emptive action is to make sure that the sub-base of the pavement is a granular soil or a crushed-stone drainage layer that can lead water out and. away from the underside of the pavement slab.
Another problem in the Northeast is damage that results when porous pavement material becomes saturated with water and freezes. The resulting expansive pressure in the pavement can break down the paving material. This problem is more prevalent in concrete pavement, where it takes the form of scaling and spalling of the surface. The most important way to avoid these problems is to use a pavement material with a low rate of water absorption. Less water absorbed into the pavement material means less damage from freeze-thaw expansion and contraction. For concrete, this low-water absorption may be achieved by initially using a low water-to-cement ratio when the concrete is first placed, proper curing to maintain the availability of water for hydration of the cement, and the periodic use of a surface sealer such as one of the siloxane products that are available.
For bituminous concrete, a similar result may be achieved by using a very dense bituminous concrete that is made from well-graded aggregates, resulting in fewer voids to retain moisture. New admixture products on the market are available to improve the physical properties of bituminous pavement. An asphalt sealer should periodically be applied to the surface to close the pores at the surface, thereby minimizing water entry into the pavement.
A completely different approach that can be equally effective is to use an open-pored bituminous concrete that allows water to percolate down through the pavement to a well-draining sub-grade material. The latter method has the added advantage of allowing surface water to enter the ground faster, minimizing runoff and the need for extensive drainage structures.
When using concrete, brick, or stone payers, care should be taken to use low-absorption units that are marketed as severe weathering" grade. These masonry payers should be set dry in a sub-base of granular material such as "quarry product," which is a by-product of the manufacture of crushed stone. If the pavement will be subjected to regular vehicular loading, it is good practice to place a concrete slab on the soil and then to set the payers in the granular setting bed. This will prevent the payers from moving under the concentrated wheel loads, which would otherwise cause rutting of the pavement surface.
Inadequate Surface Drainage
If the surface of the pavement is not positively sloped to drain surface runoff water, problems of ponding and possible icing of the pavement will result. These conditions are obviously inconvenient and potentially unsafe. Less obviously, water that collects on the surface is likely to find its way into and under the pavement and contribute to the sub-grade settlement and freeze-thaw damage described earlier. These problems may be avoided if an adequate drainage slope to drain the surface and proper drainage piping and structures are added. Of course a very effective way to improve pavement surface drainage is to grade and drain the surrounding landscaped areas to drain water away from paved surfaces.
Maintenance and Repair
Up to this point the discussion has been about the pavement that should be installed on a site. Most often, property managers have to work with the pavement that already exists and maintain it as well as possible. The first step to maintaining any built portion of a site is periodic inspection and maintenance. Properly done, these procedures will go a long way toward keeping pavement deterioration from getting worse.
When maintaining any built portion of a site, conduct an inspection by a professional engineer at least once every five years. The engineer will survey conditions, provide an evaluation, and make recommendations for repair work.
Based on these recommendations, the property manager should meet with the engineer to identify priorities, budgets, logistics, and a plan of action for needed repairs. The engineer should then be asked to develop repair-construction documents and, based on these documents, bids should be solicited from a select list of contractors. The repair documents should include all drawings and technical specifications for the proposed work and requirements for the quality control of the material and the execution of the work. Items of concern that should be addressed are sub-grade and pavement constituent materials and site-work design details. Both new construction and repair construction documents should show estimated quantities for the expected work, so competitive bids may be taken for multiple bidders. It is important that construction documents include contract language to accommodate necessary variations in the work scope at pre-agreed unit prices. This will ensure that the right type and amount of work is com pleted at a fair cost and that the bidders will not have to add contingency costs to their bids to cover the possibility of unexpected work.
It may well be that upon evaluation, it is determined that no corrective repairs are required. At a minimum, a plan for ongoing and preventive periodic maintenance should be put into action by the property manager. This plan should probably include power washing (to remove de-icing salt), surface resealing of bituminous pavement, re-striping of parking and traffic markings, and storm drain cleaning. The plan also should consider the effects of certain procedures, such as snow plowing methods and deicing chemicals, that can wreak havoc on concrete reinforcement and other site-work elements such as handrails and fence posts. It may be beneficial to look into alternate methods and materials, for example, rubber-edged snow plows and chloride-free de-icing chemicals that have the benefit of decreased long-term damage to the property.
Consideration of these recommendations will improve the performance of new site pavements and will assist in cost-effective management of existing site pavement. Like other engineered systems, when fundamental performance requirements are incorporated into the design, installation, and maintenance of pavements, they may be relied upon to provide excellent service for many years.
Robert J. Nacheman is a senior vice president and principal of LZA Technology, a division of the Thornton-Tomasetti Group, which also includes LZA Associates and Thornton-Tomasetti Engineers. The Thornton-Tomasetti Group is an interdisciplinary engineering and architectural firm that provides design and investigative services related to building structures, enclosure envelopes, and mechanical/electrical systems.
Mr. Nacheman is a graduate of Worcester Polytechnic Institute and City College, City University of New York, and is a licensed professional engineer in the states of New York and New Jersey.
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|Author:||Nacheman, Robert J.|
|Publication:||Journal of Property Management|
|Date:||Jul 1, 2000|
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