Perfecting the art of dam wall rehabilitation

resistance. This requirement is satisfied when quartzose sand is used. Calcareous sands, such as dolomite, are acceptable if blended with at least 40% of a suitable quartzose sand.

Perrie says the mixing should be done with clean potable water or other water free of substances that may impair the strength, setting time or durability of the concrete, or the strength and durability of any reinforcement.

Admixtures

Under certain circumstances, the properties of the concrete may be improved by the proper use of chemical admixtures, such as water-reducing admixtures. Their use should be based on an evaluation of their effects on specific materials and combinations of materials, including strength development, particularly within the first 24 hours after concrete placing. This is because certain admixtures may retard the setting and strength development of the concrete, thus delaying joint sawing and increasing the risk of random cracking. “The concrete should be strong enough to ensure a hard, durable, skid-resistant surface and to accommodate the tensile stresses resulting from shrinkage, warping and loading. This requirement is satisfied by specifying a target flexural strength or modulus of rupture of not less than 4.5 MPa at 28 days. The mixes for use in concrete roads should be designed by an approved concrete testing laboratory or CCSA should be approached for advice on materials and mix proportions,” Perrie explains.

Drainage

Proper drainage is also essential to prolong the lifespan of low-volume concrete pavements. Experience has shown that inadequate drainage is probably responsible for more road damage in Southern Africa than inadequate structural or material design.

The design philosophy is to provide effective drainage so that the pavement structure is prevented from becoming saturated. Both the discharge of surface run-off and the control of subsurface water need to be considered. Consequently, effective drainage is essential for good pavement performance, and is assumed in structural design.

“Surface run-off can be controlled in the rural environment by constructing sufficiently wide side drains that discharge at sufficiently frequent intervals into the adjacent land, or into culverts that carry the water to suitable discharge points,” says Perrie.

“In the urban environment, it can be controlled with the use of kerb and channel systems discharging into inlets into a stormwater pipe system. To reduce costs for low-volume roads, consideration should be given to dishing the road surface and carrying the stormwater on the road surface to suitable discharge points,” Perrie concludes.

A detailed CCSA publication, Low-volume Concrete Roads, is available as a free download from the Cement & Concrete SA website – www.cemcon-sa.org.za – or phone +27 (0)11 315 0300 for more information.

The rehabilitation of dam walls usually requires the demolition of redundant portions of monolithic blocks and associated concrete structures. However, the dynamic energy imparted by the demolition process has the potential to cause damage to concrete located just across the demolition boundaries and beyond.

For this reason, expert techniques are required to achieve a safe and cost-effective result, as showcased on the Hazelmere Dam rehabilitation project carried out by Jet Demolition. Here, a combination of explosive, mechanical and diamond-cutting methods were employed.

“Our work at Hazelmere Dam allowed for the compilation of specific demolitioncontrol guidelines to be developed for dam rehabilitation projects,” explains Kate Bester, contracts manager, Jet Demolition.

Piano key weir conversion

Located on the Mdloti River in KwaZulu-Natal, the Hazelmere Dam was built in the 1970s. It was designed originally to accept radial arm gates to raise the full supply level (FSL). However, a subsequent redesign showed that

the FSL would be achieved optimally via a piano key weir.

To clear the way for new construction works, demolition of the existing spillway crest, piers, lintel beam and bridge decks required the controlled removal of 5 300 m3 of concrete up to 3 m in thickness. The key requirements were to demolish the redundant structures in a safe, rapid, cost-effective and controlled manner, without residual damage.

The traditional demolition method for projects with large volumes of mass concrete is explosives, with large hydraulic hammers used for the smaller concrete sections and for secondary breakage. Finishing work is typically undertaken by small hydraulic hammers and handheld breakers.

When considering safe blasting vibration levels for a particular project, it is critical to consider the prevailing site conditions and geometrical configurations. “At Hazelmere Dam, our production blasting programme and demolition methods achieved all of these objectives successfully,” Bester concludes.

Jet Demolition has developed new blasting techniques to retain the structural stability of dams during rehabilitation projects

Mechanical demolition in progress during the rehabilitation of the Hazelmere Dam wall, which was redesigned to incorporate a piano key weir