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Finding buried treasure with diamond grinding

By John H. Roberts

With an unprecedented number of roadways in need of repair, the need for cost-effective pavement rehabilitation techniques remains one of the most pressing issues for roadway engineers and owners alike. In the past, many transportation agencies covered structurally sound concrete pavement with an asphalt overlay to improve the ride, friction or reduce tire pavement noise.

With the recent increase in asphalt prices, the mill and overlay option is becoming far too expensive. Diamond ground pavement surfaces often meet and exceed the smoothness, friction and noise characteristics of the best asphalt surface treatments and can be half the cost of an asphalt overlay. 

Provided that the underlying concrete pavement is still structurally sound, diamond grinding a previously overlaid pavement becomes a cost-effective solution and allows for the recycling of the asphalt millings as a gravel base or to be sold as reclaimed asphalt pavement (RAP) by the ton for a future asphalt project. This innovative process is frequently referred to as “Buried Treasure.”

How it works

The first step in implementing the Buried Treasure concept is to conduct a pavement evaluation to ensure that underlying concrete pavement is worth investing in. This is accomplished through the use of core samples and other technology as necessary to determine whether the underlying concrete is structurally functional.

Once the asphalt overlay is removed, a variety of Concrete Pavement Restoration (CPR) options can be used. CPR is a series of engineered techniques used to manage the rate of pavement deterioration in concrete streets, highways and airports. It is a non-overlay option used to repair areas of distress in concrete pave­ment without changing its grade. This preventive procedure restores the pavement to a condition close to or better than original and reduces the need for major repairs in the future. It also lasts significantly longer than an asphalt overlay. Available CPR techniques include slab stabilization, full and partial depth repair, dowel bar retrofit, diamond grinding and joint resealing.  

After the structural repairs have been completed, diamond grinding – the removal of the thin surface layer of hardened Portland cement concrete using closely spaced diamond saw blades – begins. The process results in a smooth, level pavement that has a longitudinal texture with desirable friction and low noise characteristics.

By applying the Buried Treasure concept, the value of asphalt millings can be used to offset the cost of removal and some of the diamond grinding cost. Every inch of asphalt generates approximately 0.05 tons of RAP per square yard. Depending on the oil content of the RAP and the proximity to a project location, this equates to real value in these times of ever increasing asphalt prices. Innovative owners and contractors alike can save money and help the environment at the same time.

Case in point

A case in point is the busy McCarter Highway (Highway 21) in Newark, N.J. The pavement was between 50 and 80 years old and consisted of a jointed reinforced concrete pavement design comprised of 73-foot mesh reinforced panels utilizing stainless steel dowels. In 1993, after a series of wet-weather accidents, a 9.8-mile stretch of the road received a micro-surfacing treatment. By 2001, the treatment had begun to delaminate, and a second micro-surfacing treatment was applied.

When this treatment failed in 2008, the New Jersey DOT decided to use CPR for a longer-term repair solution. Recognizing that the existing concrete pavement was structurally sound and in good condition, the NJDOT decided to remove the asphalt overlay and diamond grind the underlying concrete pavement. To minimize traffic interruption, construction could only occur Monday through Saturday nights between the hours of 8 p.m. and 6 a.m.

Because the soil beneath the pavement was sandy and therefore susceptible to washouts under the transverse joints, the slabs had to be stabilized. Polyurethane grout was chosen to fill voids under suspect areas of roadway. Four small holes were drilled on each side of the transverse joint, and the polyurethane grout was pumped into the voids.

Where full-depth repairs were needed, precast panels were constructed in three standard sizes, with extra panels always on hand for flexibility. All of the precast panels were cast with ¼-inch thick strips of foam on the bottom so the panel could be embedded into the recycled granite sand sub-base material. The panels were lowered by a crane and the eye hooks were removed after placement of the panel.

Partial-depth repairs had to be able to carry traffic in a very short amount of time, so the design called for the use of a hot-pour patching material that bonds to the concrete surface, yet remains flexible to allow for movement without cracking or de-bonding. After the patching material was placed, a friction layer of black granite sand was added to the surface.

After all the repairs were complete, diamond grinding could begin on the pre-existing concrete pavement. Because of the proximity of the steel mesh to the surface of the concrete, all of the milling operations had to be exact, in order to leave as much of the old concrete as possible. After the asphalt overlay was removed, the drainage inlet structures were lowered. After all repairs and diamond grinding were complete, the joints were resealed with a hot-pour material.

Proven solution

Although the McCarter Highway project was just completed, the diamond grinding has already improved the ride on the roadway by approximately 30 per cent. Not only is the ride superior to the previous micro-surface treatments, but the concrete pavement still has the structural ability to carry today’s heavy traffic load. As long as the underlying pavement is structurally sound, CPR and diamond grinding can be cost-effective tools to restore ride and longevity to an old concrete pavement.

According to Robert W. Sauber, supervising engineer, Bureau of Materials for NJDOT, this project validates that an asphalt overly of an existing concrete pavement can be effectively removed and replaced with a diamond ground surface texture. While this project presented may challenges, it proved that an urban freeway with curb and gutter drainage can be rehabilitated with CPR under a night-only construction schedule. Further, this project demonstrates that the life-cycle cost of diamond grinding is lower than an asphalt overlay when the concrete contains hard, durable aggregates.

Buried treasure benefits

John H. Roberts is executive director of the International Grooving & Grinding Association.
www.igga.net.