(In the Summer 1997 issue (Vol. 7, No. 2) of the "Nevada Milepost," the Nevada T² Center at the University of Nevada Reno published several articles describing the qualities of concrete pavement and giving information on its maintenance. With the permission of Ms. Maria Ardila-Coulson, T² Center Director, we have reproduced eight of those articles here.)

I. DETECTING AND TREATING CONCRETE PAVEMENT DEFECTS

The first step in repairing concrete flat surfaces is a basic understanding of the product.

Concrete is a rigid paving material and as such requires a uniform base to rest on. Concrete can be used to span openings, but when designed for that purpose it usually contains steel reinforcement.

Generally concrete's compressive strength (its resistance to crushing) is seven to 10 times greater than its flexural strength (its resistance to bending). Concrete's tensile strength (its resistance to being pulled apart during the cure cycle) may be only about 5 percent of its final compressive strength, so concrete design engineers tend to use plain concrete when there is uniform base support and add reinforcement where more flexural and tensile strength is required.

In the mixing and placement of concrete, more water is needed for workability than is required to hydrate the cement.

A short time after placement the temperature of the concrete begins to rise from the cement hydration. The temperature peaks a few hours after placement and the slab begins to cool. This cooling and evaporation of the excess mixing water causes the slabs to shrink.

When stress from the shrinkage (resisted by subbase friction) exceeds the tensile strength of the concrete, the slab will crack (typically at intervals of 2 feet for 1 inch of pavement thickness). That would be 15 feet for a 71/2-inch-thick pavement.

To control shrinkage cracking and provide straight, durable joint faces that will hold up under use, either saw or form joints at regular intervals during or immediately after the placement of concrete.

The proper construction and maintenance of the joints are one of the most important factors in good concrete pavement performance.

During the service life of the pavement these joints allow for thermal expansion and contraction as well as curing and warping movements of the slab. Also critical are proper mix design, mixing, placing, finishing and curing of the concrete at time of construction.

Once the pavement is in place, contractors are forced to deal with distress that occurs either from errors during construction or from use that exceeds what the pavement is designed for.

The best time to repair concrete is early in the failure mode. Most distress starts out minor and becomes major if ignored. Sort of like failing to change the oil in your automobile.

A distress survey of your pavement every one to two years, and follow-up repairs as needed, will help control deterioration and add many years of service life.

Cracks in concrete

Ignore small hairline cracks with openings smaller than 3 millimeters wide. They generally only penetrate the surface 15 millimeters to 20 millimeters and usually do not get worse over time.

Cracks that are "working" like joints should be saw cut with a small-diameter diamond saw blade mounted on a crack saw. They should then be sealed to prevent debris from entering the crack when the pavement is cold and causing "point bearing" damage when the temperature rises.

During the curing/warping and contraction/expansion cycles, random cracks will generate their own damaging debris from the fragmented edge--if left untreated.

Cracks that have faulted (differential settlement between adjacent slabs) or have spalled (broken chunks from point bearing greater than 50 millimeters) might need restoration of load transfer and/or partial depth repair in addition to the saw-and-seal mentioned for working cracks.

Cracks that are multiple and intersecting with one another normally will require a full-depth patch that extends beyond the distressed area.

Full-depth patches are most successful when they are constructed a full lane wide; at least 1 1/2 meters long, and are doweled into the adjacent slabs.

Detecting and treating voids

Voids beneath the slab are troublesome because frequently structural damage takes place before voids are noticed.

When voids result from base being "pumped" from beneath the slab by repeated loading during wet weather, they can be detected by stains on the surface.

A couple of diamond core holes through the pavement and any underlying stabilized base allows the injection of pozzolan or urethan grout beneath the slab, which fills the void and provides a non-erodable even support to the slab. Care must be taken not to lift or break the slab.

Voids from settlement over utility cuts or errors during construction might be much larger and can be filled with flowable fill in conjunction with other needed repairs at that location.

Accurately locating the void, filling without raising the slab, and getting the fill material in the proper location presents the greatest challenge. When the pavement is sunken, but still intact, it can be "jacked" with great care back to the proper elevation.

First, the slabs must be isolated by saw cutting full depth around the perimeter. Then, while checking with a string line, the slab can be raised a little at a time by pumping in the proper sequence through several injection holes. This is a sensitive process but can produce cost-effective repairs if handled carefully.

Concrete surface defects

Scaling of the surface can result from poor placing or curing practice or from use of deicing procedures, particularly if the pavement does not have proper air entrainment. The addition of a liquid hardener or submerging the pavement in water have produced favorable results.

If the problem is restricted to the 3 millimeters of surface, diamond grinding can remove the weakened surface and leave a durable uniform surface in its place.

Treating a rough surface

When the pavement is structurally sound but simply is rough, it can be corrected by diamond grinding. This is a wet abrading process and presents some water control, ventilation and maneuverability problems when working inside, but it is commonly used to improve riding quality on streets, highways, bridges and airfield pavement.

Joints in floors used by high-speed material-handling equipment might deteriorate and become rough from use.

One successful repair technique is to diamond grind a 100-millimeter-wide slot 3 millimeters deep and backfill the slot with a wear-resistant material that will withstand future deterioration. The joint is reformed and filled with a suitable sealant.

Treating slippery surfaces

Diamond texturing, diamond grooving, shot abrading or skid-resistant coatings are techniques for correcting surfaces that become slippery when wet. This is commonly needed on bridges, airfields and parking garages, etc.

Modifying the surface with diamonds is the gentlest of the processes and can produce strong uniform surfaces that are durable and skid resistant. The diamond blades abrade right through the sand particles and top surface of the paving aggregate without damage to the remaining pavement, leaving a skid-resistant surface that looks dry in the rain.

Load transfer

Loads are transferred from one slab to the next by subbase support, aggregate interlock (the bottom portion of the joint that breaks around the rock) and the addition of load transfer devices such as tie bars or dowel bars.

When joints or cracks have a deterioration in load transfer ability, they will begin to fault, the slabs will rock and eventually cracking or slab break up will occur. Load transfer can be restored by cross stitching longitudinal cracks or joints and dowel retrofitting transverse cracks or joints.

Adapted with permission from Pavement Maintenance & Reconstruction, January 1997.

II. CONCRETE OFFERS GREATER RETURN ON THE DOLLAR

In life-cycle costing, concrete shows a better return on the dollar spent than asphalt for the same paving projects, according to Jim Mack, American Concrete Pavement Association director of engineering and rehabilitation.

While operation and maintenance costs account for maintenance contracts, materials and equipment, much of this information is not known or is hard to get. But when comparing concrete to asphalt paving, according to Dr. Stephen J. Kirk and Alphonse J. Dell'isola in their book Life Cycle Costing for Design Professionals, the amount of maintenance for a concrete road per year is 50 percent less than the amount of maintenance required for an asphalt pavement.

Mack says the number of times the pavement is under repair is less than that for asphalt pavements. Also deterioration of the roadway must be considered in life-cycle costing, and since concrete roadways deteriorate slowly, rebuilding and maintaining roadway systems with long-lasting pavements can significantly reduce the user's cost due to maintenance and deterioration.

Cost factors

Concrete comes out ahead of asphalt in life-cycle costing due to the following factors:

When using life-cycle costing to determine the best use of money over a period of time, higher initial costs for concrete may be less than the extra costs for asphalt maintenance, repair and replacement of the surface. If possible, it sometimes is advantageous to go beyond the initial low bid and consider long-term costs of any project, whether using concrete or asphalt, and determine what is really the least expense for the project over its expected lifetime.

Adapted with permission from Better Roads, January 1997.

III. THE PROPER CURE FOR CONCRETE

Concrete is created when water and cement react to build a hardened paste which binds aggregate together into the familiar rock-like mass. To ensure that hydration continues, especially at the surface, the concrete must be cured. Curing means water at the surface of the concrete is retained to allow the concrete to hydrate to a point where it has a strong, durable structure. Generally, curing takes four to seven days. If curing is inadequate, the water evaporates and hydration stops, resulting in a low-strength concrete.

Surface drying may even affect the underlying concrete as water will be drawn from the lower levels into the dry surface concrete. Any significant internal drying also will slow or stop hydration and the structure may not gain adequate strength.

Advantages of proper curing

Methods for curing concrete

Curing should begin immediately after the finishing operation. Minimal delay is especially important in hot and/or dry weather to avoid rapid evaporation from the concrete surface. The benefits of curing concrete are significant, as can be the problems if curing is not performed as detailed above.

IV. HOW TO REPAIR SPALLS

What potholes are to asphalt-surfaced roads, spalls are to concrete roads.

The question then is why potholes receive so much more negative attention than spalls? One reason is that there are more potholes to go around. There are many more miles of asphalt-surfaced roads than concrete ones--close to 3.5 million miles versus fewer than 100,000 miles. Further, most of the concrete roads are on the Interstates compared to asphalt, most of which--71 percent--are on local roads.

Does this mean that spalls are less critical than potholes? Indeed not! Any type of distress left unattended will accelerate pavement deterioration and reduce the service life of that pavement.

How spalls are formed

Spalls are the cracking, breaking and chipping away of concrete at or around unsealed joints or cracks. They occur when the incompressibles, such as stones, sand or dirt, become lodged in joints and cracks when the joints are open as a result of cooler temperatures. During warmer temperatures the joints close and the incompressibles prevent these joints from closing and causing the concrete to crack and chip away at the top and bottom of the slab.

Spall repair

There are many different ways to repair spalls in concrete pavements. In most cases, there is an optimum combination of materials and preparation procedures. The Strategic Highway Research Program manual Materials and Procedures for Rapid Repair of Partial-Depth Spalls in Concrete Pavements provides guidelines for making cost-effective repairs.

Material selection

Several materials are available to repair spalls. Material should be compatible with existing pavement and climatic conditions. If the pavement is scheduled for rehabilitation within 18 to 24 months, spray injection or some proprietary cold mix may be a more cost-effective alternative.

Patch preparation

Several techniques--saw and patch, chip and patch, mill and patch, waterblast and patch, clean and patch--are available for preparation of the area that is to be patched, and each has advantages and disadvantages associated with its use. The major point to consider in selecting a preparation procedure is the availability of proper equipment and workers.

Installation

Proper installation steps should be observed to obtain longer-lasting repairs. The following steps are important to good repair:

More information

More information on this repair procedure is contained in the Manual on Concrete Spall Repairs available from the Nevada T² Center.

V. LINSEED COMPOUND PROTECTS CONCRETE

Linseed anti-spalling compound protects concrete surfaces in two ways: by penetrating the porous surface of the concrete to a depth of approximately 1/8 inch and by combining with atmospheric oxygen to form a protective coating through which destructive moisture and salt cannot penetrate.

The oil is most effective if applied to new concrete upon completion of the initial curing period, usually considered to be about 28 days after placement. However, it has been successfully applied to new concrete after two weeks of curing.

Preapplication conditions:

Application: Two coats are recommended, applied as follows:

Complete drying should be permitted between coats. At temperatures of 70 degrees F or above, drying is complete within a few hours. At lower temperatures proportionately longer drying times are required.

Care should be taken to cover the concrete surface completely, including all edges which are sometimes missed in spraying. Maximum protection is afforded only when coverage is complete.

CAUTION: Linseed anti-spalling compound has a flash point around 120 degrees F. While not dangerously flammable, it should not be heated.

Adapted with permission from New Hampshire Road Business, Spring 1994.

VI. THE CONCRETE WHIZ

A computer program called HWYCON can help public works employees to diagnose problems and plan repairs of concrete pavements. In addition, departments that rarely work with concrete can use the program to review their specifications for concrete work and determine whether specifications should be updated.

Developed as part of the Strategic Highway Research Program, HWYCON incorporates knowledge from literature searches, interviews with concrete experts and reviews of published guidelines, standards and practices. Using this knowledge HWYCON emulates the problem-solving approach of a human expert.

User-friendly program

The program runs in a Microsoft Windows environment on IBM compatible 386 or 486 computers, including laptops. It can run on a system with only two megabytes of random access memory, but four megabytes is desirable. If the entire HWYCON system is loaded onto a disk, it will occupy about 15 megabytes of storage space. A reasonably high-quality monitor is needed to view black-and-white photographs and drawings that depict various types of concrete distress.

HWYCON can perform three different functions: diagnostics, materials selection and pavement repair and rehabilitation. Diagnostics and materials selection apply to pavements and structures, but the repair module applies only to pavements.

Materials-related specifications

In addition to diagnosing existing distresses, HWYCON can help highway agencies determine whether the materials selected for constructing or reconstructing a concrete structure will comply with materials-related specifications, standards and guidelines.

The diagnostics module can be used to help identify the type of concrete distress and its probable cause. In addition to asking questions about symptoms of distress, the program gives users the chance to compare their problem sections to photographs stored in the computer showing what each distress looks like.

Common distresses include D cracking (a freezing and thawing problem), thermal cracking (caused by not properly controlling the temperature of concrete in the curing process) or alkali-silica reactivity (a chemical reaction). In some cases the program may give a 100 percent definite answer. In other cases, such as when alkali-silica reactivity appears to be the problem, HWYCON may suggest particular field or laboratory tests to confirm the diagnosis.

Materials selection and repair

The second module covers material selection, and the third provides recommendations on repair methods, including partial and full-depth repair, bonded and unbonded overlays, diamond grinding and milling. Each module can be used independently of the other two.

VII. NEW WAYS TO PROTECT CONCRETE PAVEMENT AND BRIDGES SAVE MONEY

Portland cement concrete is one of the most durable construction materials. Yet today many concrete bridges and pavements are in disrepair.

Much of the blame goes to damage caused by deicing salts, repeated cycles of freezing and thawing, chemical reactions in the concrete and other causes. For years inefficient test procedures, a lack of funding for rehabilitation, increased traffic and other obstacles have made it difficult for highway agencies to prevent deterioration of concrete bridges and pavements.

The Strategic Highway Research Program (SHRP) set out to find ways to help state highway agencies keep concrete bridges and pavements in good repair. The Federal Highway Administration contracted with the Nevada T² Center to coordinate and manage the project.

Researchers concentrated on improving the materials that went into a concrete mix and solving the problems caused by corrosion of the reinforcing steel in concrete. The efforts yielded dozens of new test methods and guidelines for increasing the service life of new and existing concrete pavements and structures.

Savings ahead

U.S. highway agencies spend more than $6.5 billion every year on Portland cement concrete bridges and pavements. If the agencies were to adopt just six of SHRP's 44 test methods and guidelines for concrete and structures, they stand to reap substantial savings, according to a recent economic analysis by the Texas Transportation Institute (TTI).

The six new test methods and guidelines will allow highway agencies to:

TTI forecasts that using the new procedures for concrete bridges could save highway departments anywhere from $4.1 million to $15.5 million per year (over a 20-year period), depending on the pace of implementation. These savings would result from lower testing and maintenance costs and extended service life.

SHRP products also can be used to mitigate D-cracking and ASR in existing concrete pavements. Assuming that these products can extend the life of an ASR afflicted pavement by 70 percent, TTI predicts highway agencies can save between $13 million and $48 million annually, depending on the pace of implementation.

Longer pavement life benefits motorists as well. Fewer maintenance-caused traffic delays and less vehicle wear and tear could add up to an annual savings of between $38 million and $143 million in user costs.

VIII. A BETTER WAY TO MEASURE CONCRETE PERMEABILITY

A new portable device allows on-site testing of concrete structures. Measurements of the permeability of concrete over reinforcing steel can be useful in identifying potential problems in a concrete structure. A low-permeability concrete generally has higher strength and corrosion resistance.

Determining the degree of permeability that has developed can help engineers plan preventive maintenance measures to reduce permeability. NDOT [Nevada Department of Transportation] sought an efficient and effective way to conduct this testing.

It decided to test a new device evaluated by the Strategic Highway Research Program for measuring permeability. Besides alerting NDOT to situations that needed attention, this information also would help the department evaluate the effectiveness of various concrete surface treatments.

Called the surface air-flow permeability device, it allows measurement of the permeability of the top layer of concrete without damaging the surface. Tests were conducted at five locations--four bridges and a section of pavement damaged by alkali-silica reactivity.

"The device is easy to use and dependable," according to NDOT's Peter Booth.

It was used to compare the results of four different methods of treating the concrete to lower its permeability. NDOT found that areas treated with silane had the lowest permeability. Litium, methacrylate and linseed oil followed in degree of effectiveness.

The device also proved helpful in analyzing concrete surfaces that were not yet severely cracked. Although readings from the device may vary depending on the condition of the surface of the point tested, the results of various samplings can be averaged to provide an overall measurement.

The advantages of the surface permeability device are: