Basalt mesh reinforced, Ultra-thin Concrete Overlay on Asphalt Pavement takes 1. 5 million tons of traffic p.a.

basalt mesh reinforced
50mm thick 30MPa concrete overlay, reinforced with Stonerod Basalt fibre. Image taken by Gordon Forrester

We can lay thin-skin concrete into milled asphalt roads to replace the material that is constantly under repair. We know it will outlast bitumen.

From the early 1980”s, The Council for Scientific and Industrial Research (CSIR) in South Africa, was actively investigating Ultra-thin Reinforced Concrete Pavement (UTRCP) methods. In 2002, they had started a project at a quarry in the Roodekrantz area. The research included traffic loading simulations using a purpose built load repetition frame and a heavy vehicle simulator. In addition some test strips were constructed and subjected to heavy traffic. E80 loads were common for over eight years and >1 million axles.

In early September, 2016, an ultra-thin basalt reinforced concrete overlay was placed over a worn and pot-holed asphaltic pavement on the egress of the busy sand mine and rock quarry, near Krugersdorp in Gauteng, South Africa. In a traffic survey carried out over a twelve-hour period, on a normal weekday, a total of 919 vehicles passed the survey point (Almost 77 vehicles per hour). 508 of the vehicles crossed the test strip, leaving the area. 62% of the egressing vehicles were heavy-duty loaded multi-axle dumper trucks.

The location of the test site was on Abraham Van Wyk Road, the access road to the Roodekrantz Quarry Area, in Ruimsig/Muldersdrift, Roodepoort, Gauteng, South Africa. [26º04’02”S 27º51’43”E]. This road is in an area that has been used on a number of occasions for pavement tests. The CSIR test site was at the original quarry, as mentioned above.

The 3m x 5m test section was marked out on the old asphalt pavement. The asphalt was washed with water and broomed clean. A 25mm high steel frame was fixed to the asphalt surface in order to provide a screed height limiter. A slab edge-thickening foundation was dug, inside the frame. It was approximately 150mm wide and about 100mm deep. Ready-mixed concrete was supplied from a local batch plant at 30 MPa and about 100mm slump. It was poured into the frame and levelled and tamped.

The concrete encapsulated a “modified” basalt mesh product. The modification aimed to make a reduced weight, (reduced cost) reinforcement for the overlay. The original product chosen for the test was 450 g/m2, 25mm x 25mm Stonerod basalt fibre mesh. The product was reduced to approximately 225 g/m2 by physically removing “+” pieces from the middle of four adjoining 25mm x 25mm apertures. This produced a 50mm x 50mm mesh that was obviously lighter and reduced the load bearing and reinforcement capabilities of the mesh. In spite of the alteration, the mesh has performed admirably.

Basalt mesh
Image: Gordon Forrester

Once the first layer of concrete had been placed, the mesh was thrown over the surface and pulled flat to remove any wrinkles. It was tamped into intimate contact in the wet concrete. Thereafter, a second height of 25 mm was added to the screed rail, making a total slab depth of 50 mm thick. More 30PMa concrete was poured into the frame and the concrete was screeded level. It was then covered with black plastic sheeting to provide curing.

The following day, the steel frames were removed and small on and off ramps were provided by plastering triangular edges around the slab. The slabs were not joined to the main slab in any way. In addition, no shrinkage cracking cuts were provided.

A “worst case” scenario was chosen, to fully assess the system. The test-strip was covered with plastic sheeting, for seven days to allow for curing, and then the 20MPa slab was opened to traffic. (30MPa concrete should achieve 20 MPa in seven days.)

The strip immediately was put into service. Traffic in the area is heavy and the strip has faced a real test. With traffic of +/-1625 axles in a 12 hour period, and an estimated annual traffic mass of 1,5 million tons, the system has proved successful. At least 600 000 vehicles attest to its robustness.

The concrete has cracks. That is the nature of concrete. The only reinforcing is “lightweight modified” Stonerod basalt fibre mesh. The edge thickening has provided stress points and a longitudinal crack above the edge of the beam. Normal shrinkage cracking is present, but if the concrete had been saw-cut, those cracks could have been tucked out of sight. There isn’t a pothole anywhere and no maintenance has been necessary.

Author:

R. Gordon Forrester. A.C.T (MICT Ret.).
Technical Director: Stonerod Technologies (Pty). Ltd.
12 Graniet Street,
Jet Park,
Gauteng,
South Africa.

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