THIN BLOCKS RESULT IN INCREASED LEVELS OF PAVING FAILURE :

CMA director, John Cairns, says the cause is the specification of thin (40mm) blocks for applications to which they are not suited.

“We are seeing increasing incidence of 40mm thick paving blocks being used in trafficked areas whereas a thickness of 60mm is the minimum recommended by the CMA.  A block thickness of 50mm may be acceptable for applications where traffic is light, such as private driveways, but anything less than 50mm for trafficked areas is asking for trouble. The use of 40mm should be restricted to patios, paths and pool surrounds. ”

Cairns explains that straight-sided paving blocks have no physical interlocking mechanism which means they rely totally on the frictional forces and jointing material between them to hold them together.

“On a 40mm block those forces are 50% lower than on a 60mm block and 25% lower than on a 50mm block, and the stress on the base material is correspondingly higher. This means there is far greater chance of movement, both horizontal and vertical, with the thinner blocks. Horizontal movement is probably the more common cause of failure because the joints between the blocks are often not filled properly. The chance of failure is even greater on a steep driveway or road where frequent braking and accelerating generates horizontal forces on the paved surface.

“Horizontal and vertical movement leads to the blocks grinding against each other, chipping and block separation. Once this has happened water ingress and ultimately, paving failure occurs.

“Most of the straight-sided paving blocks measuring less than 50mm on the market today have been manufactured using wet-cast manufacturing, a process which requires less capital equipment than dry casting. While it is extremely difficult to manufacture a paving block less than 50mm thick using dry casting, it is easily achieved with wet casting, and we have witnessed high growth in wet-cast manufacturing during the past two to three years.

“There is nothing wrong with wet-casting as a paving block manufacturing technique. However, many wet-cast paving manufacturers, not to mention the paving contractors who use wet-cast pavers, are not aware that a thickness of 60mm should be the minimum standard for paving blocks laid in trafficked areas,” concludes Cairns.

The CMA has manuals on correct paving procedure and runs regular paving seminars. For further information contact the CMA on 011 805 6742, www.cma.org.za or the Brick Paving Institute on www.bpisa.co.za.

PICTURE
A typical example of a paved driveway surface which is failing. Vertical and horizontal (rotational) movement has caused these 40mm wet-cast paving blocks to rub against each, a process which has caused some of the blocks to crumble. Had the process been allowed to continue, whole sections of the surface would have broken up.