In search of smoother roads

In the past concrete was considered too expensive and noisy to be an effective road surface. But that has changed – longer life, less noise is now the aim of researchers around the world.

For example, U.S. Departments of Transportation are trying to determine the rate at which concrete pavements expand and contract with temperature changes that hopefully will lead to smoother, longer-lasting roads.

The thermal coefficient of expansion test involves taking a 160 mm concrete core from pavements and measuring its length using a linear variable differential transducer.

The core is then submersed in a heating bath on a stand heated from a starting temperature of 10 ^oC to 50 ^oC. The core expands under the increase in temperature and is measured following the test to check on its rate of growth.

According to researchers, the new testing procedure will significantly improve the road design process by matching a concrete to its environment. The process could reduce the maintenance required on a stretch of road.

In the U.K. the in-service performance of concrete roads has led to the development of technologies aimed at reducing future maintenance requirements, and making the treatment of rigid pavements economic, without the need to remove the concrete.

Research has also stimulated the introduction of measures during the construction of flexible composite pavements to reduce “reflection cracking” which can allow water to enter the pavement structure and cause long-term breakdown.

Implementation of these research findings has effectively extended the design life for these types of pavements and this may encourage the increased use of concrete in future road construction.

Rigid pavements have a long design life due to their inherent strength and minimal maintenance requirements in their early life (15-20 years). After 20 years, long-term maintenance may be required. For this maintenance, the traditional method in the U.K. is to overlay with a thick asphalt layer, for example 180 mm, to inhibit the development of reflection cracking.

Reflection cracks in the asphalt surfacing, above cracks or joints in the rigid pavement, are caused by the thermal contraction of the concrete. More specifically, there are two methods that are being used by the U.K. Highways Agency in maintenance schemes on trunk roads. These are to “crack and seat” the concrete pavement prior to overlay or to “saw-cut and seal” the overlay after overlaying. Also a combination of the two may be used.

The crack and seat method is to induce fine, vertical transverse cracks in jointed un-reinforced concrete road slabs before overlaying with asphalt. This allows the seasonal contraction to take place at locations other than at the joints and reduces reflection cracking in the asphalt overlay.

The fine cracks retain the aggregate interlock necessary to ensure the satisfactory load spreading ability, while still allowing for thermal movements to take place.

In general, concrete slabs are “cracked” at 1 to 1,5 m centers using a guillotine and then “seated” using a pneumatic-tyre roller. Initially, this technique was developed for the maintenance of un-reinforced pavements but more recently the technology has been applied to reinforced concrete.

In the U.K., concrete is being used as a sub-base, particularly in “rutting” lanes. A section of the M25 was the test for an innovative CRCP (Continuously Reinforced Concrete Pavement). The major innovation here was to design the pavement on flexural strength, as opposed to using compressive strength. In this way, the full benefit of using the specified limestone (as opposed to gravel) aggregate was realized, with a saving of 40 mm of concrete thickness. The concrete was then topped with 12 mm thick stone mastic asphalt (SMA).

In the U.S., white-topping is being examined closely as a means of maintenance. It is a process in which 50 to 100 mm of high-strength, fiber-reinforced concrete is placed over a milled surface of distressed asphalt concrete pavement. This ultra-thin white-topping (UTW), has proved to be a low-cost, effective, and fairly simple solution, and is designed for low-speed traffic areas or areas with a lot of stop-and-go traffic, such as street intersections, bus stops, or toll booths. The best part is that UTW requires significantly less time to construct, and repairs last much longer. Given its success in these limited applications, UTW is now being considered for a range of other applications.