Cost-effective, sustainable, fast and safe airport pavement rehabilitation by rubblization

“Rubblization is an attractive alternative for conventional rehabilitation techniques”
-Fer Mooren, Sr Pavement Specialist, NACO Netherlands Airport Consultants B.V.

Many airports around the world have concrete runways and taxiways. With the enormous growth of aircraft weight and movements over the last decades, their bearing capacity has become increasingly insufficient. At the same time, it is widely recognized that simply strengthening existing concrete with asphalt overlays, with or without reinforcement grids and/or stress absorbing interlayers, results in a significant risk of reflective cracking.

At a first glance, the technique of rubblization to eliminate reflective cracking seems too rigorous due to destroying of the residual strength of existing concrete. However, an in-depth analysis proves that rubblization can be very cost-effective whilst at the same time providing a sustainable, fast and safe solution for concrete airport pavement rehabilitation projects.

Fer Mooren, Sr Pavement Specialist at NACO, Netherlands Airport Consultants B.V., was the principal engineer for the € 220 mln Taiwan Taoyuan International Airport Pavement Rehabilitation and Navigational Aids Upgrading Project. He made an in-depth study for the rehabilitation of the existing one million square meters of concrete pavement, which suffers from an ongoing process of deterioration due to overloading by wide-body aircraft.

Existing condition not fit for future purpose
Taiwan Taoyuan International Airport (TTIA) is the main airport of Taiwan, RoC. In 2007, the airport handled 23.5 million passengers, 1.6 million tons of cargo and 163,000 aircraft movements. The existing airport configuration consists of a parallel runway system at 1,500 m separation with a central terminal area. Runways and taxiways, one million square meters in total, all consist of plain concrete pavement, varying in thickness from 365 mm till 420 mm.

On average, the maintenance department of the airport replaces around 37,000 square meters of concrete every year. Slab replacement is predominantly triggered by corner cracking resulting from widespread joint faulting and pumping.

An extensive non-destructive testing program was undertaken as part of this project to investigate the structural condition of the existing pavements. The most significant conclusions from this program are:

1. The PCN of most pavements does not meet the requirements for today’s and future aircraft (Average PCN 56 /R/B/W/T);
2. The load transfer efficiency (LTEδ) dramatically deteriorates due to traffic loading;
3. The overall LTE in the keel section of the pavement is extremely poor (average 25%);
4. There is widespread (>50%) loss of subgrade support.

Rubblization eliminates risk of reflective cracking
For the rehabilitation of these pavements, a total of eight theoretical rehabilitation options were evaluated against the risk of reflective cracking. A structured decision tree facilitating the proper choice was provided by Von Quintus (AAPTP, 2008). The technique of rubblizing the existing concrete in combination with an asphalt overlay came out as the option with the highest success factor and the lowest risk factor.

Rubblization is an in-situ concrete breaking technique. Concrete is fractured to full depth into particles varying from 25 mm at the top of the concrete to 500-700 mm at the bottom of the concrete. By fracturing the concrete, all horizontal and vertical slab action is destroyed which eliminates any risk of reflective cracking. Especially in situations like at Taiwan Airport, where there is widespread loss of subgrade support, rubblization provides a better guarantee against reflective cracking than the more popular technique of breaking & seating.

The reason of breaking & seating being more popular may be that originally the rubblized concrete was rated with a residual stiffness similar to a crushed aggregate base course (FAA, 2004). This basically means that an original stiffness of 30,000 MPa would be reduced to values of around 500 MPa, whereas cracking & seating would result in stiffness in the range of cement stabilized base courses. Years of monitoring in service rubblized pavements and full-scale testing at the FAA National Airport Pavement Testing Facility in Atlantic City has revealed that the residual stiffness of rubblized concrete is significantly higher than originally assumed (Buncher et al., 2008). This is mainly because of the tight interlock between the interconnected pieces of concrete. Design moduli for rubblized concrete on airfields can go as high as 1,500 – 2,500 MPa for concrete of 35 cm thickness or more. Some studies even suggest a gradual increase in stiffness over time.

Having identified rubblization as the most promising rehabilitation technique for the pavements at Taiwan Airport, a comparison was made with the option of full-depth replacement of the existing concrete with new pavement.

Cost-effectiveness is not the only benefit
There are clear financial benefits in terms of construction costs. A saving of more than 30% can be achieved if rubblization would be chosen in lieu of the only technically feasible alternative of full-depth replacement. Even a conservative approach in a 20 years life-cycle cost analysis presented a 15% benefit. However, not only the financial benefits are in favor of rubblization. On many other aspects, rubblization proves to be an attractive option.

First of all, by rubblization a 100% recycling of materials is achieved, resulting in major savings in natural resources. Secondly, rubblization is an in-situ recycling technique which prevents thousands of truck movements for hauling materials to and from the site and hours of plant operations for producing pavement materials. As a consequence, rubblization results in a significant reduction of carbon dioxide emissions. An analysis revealed that the number of plant and equipment hours can be reduced by a factor of 10. Hence, rubblization is an environmentally friendly and sustainable technique of pavement rehabilitation.

Besides the clear benefits to the environment, avoiding thousands of truck movements also results in a considerable increase in aviation safety when such project has to be undertaken at an operational airport. Many of these movements would require the truck to transit active portions of the aircraft maneuvering area. Hence, avoiding truck movements reduces the likelihood of incursions of ground traffic into operational areas.

Production rates of rubblization as high as 6000-8000m2 on a normal working day can be achieved. Compared to conventional techniques of base course preparation, where materials have to be hauled to and from the work site, there is a clear advantage in the reduction of the construction time, thus reducing the time of interference of construction activities with aircraft operations. Hence, rubblization is a relatively fast technique of rehabilitation.

Rubblization is ready for airport application
In summary, it has to be recognized that rubblization of concrete airfield pavements provides clear benefits over other alternatives that would result in too high a risk of reflective cracking. Benefits not only in terms of costs, but also in terms of sustainability, safety and construction speed. Recently completed studies show that rubblization is fast becoming popular in airfield pavement rehabilitation. The technique is a field-proven success that no longer needs to be considered as research or experimental in nature.

Contact details:
Fer Mooren
E: fer.mooren@naco.dhv.com
T: +31 70 344 6300
www.naco.nl