How to Manage Concrete Road Life Cycles
Research points to using excellent mixes as a starting point.

by Ruth W. Stidger, Editor-in-Chief

Concrete road life cycles range from 20 to 60 or more years. Often, well-designed roads make it to 40 or more years with minimal maintenance. It’s all in the basics of design and construction, the experts report.

When restoration is needed, some techniques extend the life of concrete pavement up to nine times their original design life, according to the Northeast Chapter of the American Concrete Pavement Association.

With good management, concrete roads cost 25 to 50% less to maintain than asphalt, according to the same group.

Design and life

Many road designers expect a 30- to 35-year life cycle for concrete pavements. Working with the American Concrete Pavement Association, the Innovative Pavement Research Foundation set up a seven-year plan to improve both the concrete road design life and maintenance to extend the complete life cycle. Some projects began in 1998 and have been completed. The entire program will be finished in 2005.

Research areas include pinpointing today’s best practices, reducing initial costs without compromising concrete pavement performance, reducing road user delays during construction or maintenance, developing cost-competitive concrete pavement options, and increasing concrete pavement design life.

Finding the right mix greatly affects design life. This includes portland or blended cement and supplementary materials such as fly ash, chemical admixtures, aggregates, and water.

Good quality aggregates in a variety of sizes let the designer use less water and cement, yet improve concrete life, says mix expert, Jim Shilstone from Dallas.

Most U.S. departments of transportation use only local aggregates, but Australian agencies often haul good aggregates 100 miles or more, offsetting hauling costs with the resulting lower cement expenditures.

Traffic and life

Heavy traffic, particularly heavy truck traffic, can affect pavement life. In Canada, the Nova Scotia Department of Transportation and Public Works carried out a five-year study to determine which concrete technologies provide the best pavement life for residential streets, collectors, arterials, expressways, and freeways.

The study started by comparing the structural characteristics of rigid (concrete) and flexible pavements.

Flexible pavements rely on the asphalt, base, and subbase layers to transfer the applied load, the NSDOT reports.

Rigid or concrete pavements do not require the base or subbases for structural strength and subgrade strength is not a critical element in the thickness design, according to the study. Subgrade has minor impact on overall thickness in terms of structural design, but is a consideration for drainage.

The study, which compared adjoining portland cement concrete pavement and asphalt pavement, looked at profile ride index, riding comfort index, surface friction, noise data, cost of repairs and maintenance, and distress survey information.

Quick reconstruction

When concrete pavement does need repair, it can be done quickly, contrary to popular beliefs that concrete materials are slow curing.

Researchers at the University of California in Berkeley documented a reconstruction project on Interstate 10 in California — 1.6 miles of concrete pavement was replaced in 55 hours over one weekend by the Morrison-Knudsen Corporation. The work included lift-out and haul-away of slab segments previously sawcut; clean cement-treated subbase; installing tiebars and dowels; placing, finishing, and texturing the new concrete; curing for four hours; sawing joints; and opening to traffic, according to the Innovative Pavement Research Foundation, who reported on the project in conjunction with the Federal Highway Administration.

The segment of I-10 running through southern California, commonly called the San Bernardino Freeway, was built in the early 1960s for a 20-year design life. Traffic volumes in this stretch of freeway are high, as much as 240,000 average daily traffic with about 9% heavy trucks. Time and traffic had taken their toll on the old pavement, resulting in extensive slab cracking and joint faulting.

To document the effort, the Innovative Pavement Research Foundation hired researchers at the University of California at Berkeley. The research objectives included the documentation of the traffic management plan and construction process for both nighttime and weekend closures, to track the techniques the contractor used to reconstruct the urban concrete pavement, and to identify which construction areas were constraining the overall project productivity. Identification of the constraining activities could help improve future construction productivity by allowing contractors and agencies to focus on innovations in a particular area.

The replacement of 1.7 lane miles used fast-track concrete with four-hour curing time and was completed successfully in 55 hours. This indicates that state agencies should be confident of at least that amount of pavement reconstruction during a weekend.

Concrete delivery to the site was found to be the constraining factor. The overall progress of the project was found not to be controlled by the demolition activities, the maximum amount of the demolition under the contractor’s process for 55 hours of work could have handled a 3-lane-mile project. Likewise, concrete cure time was not a significant limiting factor.

If the initial paving progress had been maintained throughout the project with no breakdowns, the maximum amount of rehabilitation which could have been completed within a 55-hour weekend would be 2.1 instead of 1.7 lane miles.

Opening it up

Concrete roads subject to heavy rains can have increased life cycles with pervious concrete use. This material is a porous concrete surface that lets rain and stormwater through it rather than flooding the road, surrounding area, or storm drains.

Pervious concrete is only one element of a porous paving system, warns Charger Enterprises, a pervious concrete contractor in Seminole, Florida. Other needed elements include a suitable base soil, proper preparation of a compacted, well-drainable subbase, a suitable mix design, and correct mixing, placement, finishing, and curing.

What makes a good installation:

1. Soil evaluation, according to Charger Enterprises. Test borings will establish whether soil will support the subbase and pervious soils that contain significant levels of silt or clay that are either highly compressible, lack cohesion, or expand or contract with moisture absorption. These may not be suitable for concrete pavement unless remediation steps are taken to stabilize the soil.

2. Base preparation. Before concrete is placed, the subbase must be compacted using a vibratory or other suitable piece of equipment to achieve a minimum density of 90 to 95%. The compacted base must be done prior to the placement of paving material, but no puddles of free-standing water can be tolerated.

3. Mix design. According to Charger Enterprises, pervious mixtures must use high-quality control. They contain portland cement, a nominal 0.375-inch Florida limerock aggregate, and admixtures as designed. Almost all fine aggregate is eliminated from the mix to provide the necessary voids to allow penetration of water. Typically, pervious concrete has about 70% of the density of standard concrete paving mixtures. The amount and timing of the addition of water to the mix is critical. Too much water will carry the cement slurry to the base of the pavement, creating a barrier that prevents water from passing into the subbase and substrate. Too little water prevents bonding of the aggregate to the cementitious mixtures.

4. Replacement. The subbase must be compacted to specification. Ruts created by equipment or vehicles prior to placement must be eliminated and recompaction used. Forms must be built to specified grades. Rail-mounted leveling equipment should be aligned and placed prior to beginning placement. The material should be discharged from the trucks as rapidly as possible and feasible. In leveling, the paved area must be rolled immediately, using a full-length roller and supplemental compaction if needed.

5. Curing. Following placement, the pervious concrete should be covered with polyethylene film, which should be held down securely. Curing time without traffic is seven days.

Air-entrained concrete, where thousands of microscopic air bubbles are introduced, lets the pavement material withstand the effects of freeze/thaw cycles.

Also called foamed or cellular concrete, the material is made from mixing preformed stable foam to a slurry of cement, sand, and water to create enclosed air bubbles.

The material is especially suited for road bases and bridge abutments.

FASTFACT

Concrete pavements can be reused both for road surfaces and bases, with the entire operation accomplished on site.

FASTFACT

Because of its light color, concrete reflects from 33 to 50% more light than asphalt and can lower a city’s temperature by several degrees.

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