Overview
Pavement structures with a cement-bound granular material (CBGM) road base and an asphalt wearing course are relatively rigid and suitable for roads with heavy traffic. The road pavement type known as cement bitumen pavement (CB) is common in many European countries and was previously also widely used in Sweden, but has been used very little here in recent years. One reason for this is that many roads have had problems with reflection cracks in the asphalt due to crack formation arising in the CBGM. This crack formation consists mainly of thermal cracks that continue up though the asphalt layers. Crack formation in the cement-bound layers arises at different distances, with wider cracks at a distance of 15–20 metres, and it is these that continue up through the overlying asphalt layers.
This report examines seven well known road objects that have been paved with CB and that have been trafficked for between 2 and 30 years. Because the roads have different traffic loads, the thickness of the CBGM layer has varied from 160 mm to 240 mm. Depending on when the roads were built, they have been subject to different building requirements for CB. The strength requirement for CBGM has varied from 8.5 MPa to 17 MPa. The thickness of the asphalt binder course between CBGM and the asphalt wearing course has varied from 50 mm to 115 mm and the type of binder course has varied from bitumen stabilized gravel (BSG) to various types of softer or harder asphalt types, and overlying this there has always been a 40 mm asphalt wearing course.
Results
For some roads with wide cracks different repair methods have been tested. On Ring Road East Kristinehamn tested reinforcing with glass fibre mesh alternative to the fractured coating before a new surface course was laid. This was shown not to work over time. The use of mesh or nonwoven layers between asphalt does not seem to delay or prevent cracking.
On the outer ring of the Malmö had broad reflection cracks different tests made, including mesh reinforcement, but the only method that in the current situation seems to work is sealing the cracks with grout.
For CBO stretch of the E6 at Fastarp repaired the cracks of various asphalt mixes before a new wearing course was laid.
The results of the performed repair actions show that thin cracks need to be addressed. Something wider cracks can be filled with bitumen or mastic asphalt, while the wider cracks should be repaired with any type of sealant. The action selection depends on how wide the cracks are.