Design and build imperishable concrete reservoirs, concrete specialist tells built environment professionals
There is no reason that an appropriately designed, built and maintained concrete reservoir should not continue adding value for at least 100 years. This is considering the durability of concrete which enables it to resist many deleterious effects while maintaining its desired engineering properties.
So said renowned materials specialist, Dr Rod Rankine, when addressing cement and concrete industry stakeholders at Cement & Concrete SA’s (CCSA) Concrete Working for Water Roadshow. The well-attended events in Johannesburg, Durban and Cape Town brought together leading local built environment professionals to find solutions to South Africa’s dire water crisis. Also offering cost-savings for the initial build and maintenance later on, concrete will no doubt continue to play a central role in the country’s water-augmentation strategies.
However, Dr Rankine noted that it is important that South African engineers, contractors, sub-contractors and participants in the civil-engineering construction supply chain ensure that concrete water infrastructure realises its full potential. This relies on best practice in terms of construction; quality control; structural design; and detailing.
“When we design and build new reservoirs, it should be done so well that they last as long as possible, which is at least a century. This is not unusual for an appropriately maintained concrete reservoir that has been delivered to the highest possible quality standard,” Dr Rankine said.
He expressed deep concerns about a rapid decline in the quality of the design and construction of concrete water retaining structures in municipal jurisdictions. He showed delegates examples of concrete reservoirs in various stages of construction that would never be able to function. In some instances, concrete construction design and application was so poor that these partially completed and final structures would have to be demolished and rebuilt entirely. Others required large concrete repairs, also costly and disruptive to service delivery. The situation places additional strain on already-stretched municipal resources, exacerbating the serious water crisis with which the country currently grapples.
Dr Rankine noted that appropriate concrete design and construction, which included adequate measures to prevent corrosion of steel reinforcing, would save asset owners significantly in major repairs further down the line.
He cited Prof Walter de Sitter’s, “Rule of Five” to demonstrate his point. According to De Sitter’s Rule of Five, R1 invested during the design and construction of reservoirs is equivalent to R5 after the structure has been built, but corrosion is not yet evident; R25 when corrosion has started at some areas; and R125 when corrosion has become widespread, and rehabilitation is required. For example, a concrete reservoir that has been built with reinforcing steel that has been hot dip galvanised according to the SANS 121 specification will last three times longer than a similar water-retaining structure that has been constructed with black rebars. This justifies the additional expenditure in hot dip galvanising. There is currently no other method that safeguards against reinforcement corrosion, the leading cause of failure of reinforced concrete structures, than hot-dip galvanising.
Despite the availability of such state-of-the-art technologies that improve the performance of concrete and the already many proven benefits of concrete, defective reservoirs are still being constructed on behalf of South Africa’s municipalities.
The reservoirs inspected by Dr Rankine mainly leak through cold-joint lifts, which he described as the “weakest link” in concrete water-retaining infrastructure and where reinforcing bar starts to corrode first.
Cold-joint lifts are a necessity, but they are inevitably a weak point because the concrete at the top of the previous lift is of inferior quality as a result of bleeding. Honeycombing at the bottom of walls, adjacent to joints between form panels and at cold-joints is a problem is a common cause of inherent weakness and deficiency which continues to bedevil this industry. It can all be attributed to a poor understanding of concrete technology and poor site practice.
Appearing as cracks, honeycombing and areas with exposed aggregate and reinforcing of various sizes, they prevent a strong, water-tight bond between the two layers in the reservoir wall.
The large areas of honeycombing that Dr Rankine has observed time and again are as a result of poor compaction practices and incomplete placement of concrete in formwork.
He told delegates during his session at CCSA’s Concrete Working for Water that it was clear that sound concrete compaction practice was largely being ignored by contractors appointed to work on these projects. However, onus also lies with resident engineers supervising the works. “There are some inconvenient truths when it comes to proper concrete placement that need to be heard by the companies that were appointed to build these reservoirs. Firstly, there is no such thing as self-levelling concrete and pumped concrete is certainly not self-compacting. Secondly, there are limits to the amount of concrete that a single concrete vibrator can vibrate in an hour and at least three workers are required to operate a singe poker vibrator (one to hold the poker, one to manage the engine while the third is sitting on the toilet),” he said.
Because the natural packing density is interrupted by shutters, the flat floor and struck-off top, Porosity near the top and bottom of the reservoir walls surveyed by Dr Rankine is 0,30pu. Near the surface, porosity is less than 0,48 due to interruption of the natural packing density by the flat shutter. In the centre portion of the reservoir walls where natural packing density is optimal, porosity is 0,26. Thus, the paste requirement near defined edges is aways higher.
This would not have occurred if greater care had been practiced when placing the pumped concrete in the deep forms by using an elephant truck or hydro-valve, Dr Rankine noted.
Meanwhile, the many vertical cold joints that he had analysed were as result of working in one direction on closed-circuit structures. Rather, seasoned contractors know that best practice is to place concrete by starting with two teams at one position and then working in opposite directions so that when they meet, they can join fresh concrete to fresh concrete, when constructing a reservoir wall to avoid this problem.
Dr Rankine was also concerned about high incidences of inadequate concrete cover that he has noticed. This is considering the very large part that concrete cover plays in concrete durability. He said that contractors had one chance to get it right but, disappointingly, were failing to do so judging by the dismal state of the reservoirs that he was asked to inspect.
Grout loss between adjacent shutters was also a cause of honeycombing on these water-retaining structures. As he explained, a lot of grout was being lost due to “leaky formwork”. Skilled and experienced contractors, therefore, ensure tight joints and sometimes use self-adhesive compressible foam gaskets to successfully retain the grout between the adjacent shutters.
He also identified incorrect application of and, in some instances, over reliance on hydrophilic waterstops as a recurring problem. Installed across concrete joints, they also often become misaligned and are damaged when casting adjacent concrete panels. “For a hydrophilic waterstop to work as intended, it has to be coaxially confined. Many contractors have been coerced into using these just because they exist, whereas an excellent watertight joint can be achieved without a hydrophilic strip,” Dr Rankine said.
Furthermore, he identified problems in the correct design and installation of bearing pads, which play a critical role in transferring loads to the foundation of the structure and facilitating movement. A case in point was the severe load- and movement-related cracking of a reservoir wall founded on a Kilcher-type polytetrafluoroethylene bearing with bandage seal between the inside wall and floor.
Dr Rankine also clarified misunderstandings around the allowable size of cracks in a reservoir wall for autogenous healing purposes. Widely regarded as a conservative standard, EN 1992-3-2006 provides for autogenous healing by limiting the ratio of hydrostatic head-to-wall-thickness to a maximum allowable value of 35 for crack widths not exceeding 0,05mm.
However, as he noted, this is not practical or realistic. Dr Rankine said that cracks that do not exceed 0,3mm in width will almost certainly seal themselves with time. Calcium hydroxide (Ca (OH)2), a by-product of cement hydration, from within concrete is conveyed by leaking water to the dry side of the wall. Thereafter, it evaporates and the Ca(OH)2 is deposited on the dry face as a precipitate. This then reacts with atmospheric carbon to form calcite sealing the cracks.
Dr Rankine concluded by quoting an excerpt from the first edition of Everyday uses of Portland Cement, published in 1914, to support his views that good construction; quality control; structural design; and detailing would ensure top-notch concrete water-retaining structures that continue to add value for many years. “The Romans used lime concrete that has stood for centuries and is doubtless better today than when it was first made. However, concrete made with hydraulic lime or Roman cement, was nothing like so strong and serviceable as Portland Cement concrete. We refer to it merely because it shows the remarkable resistance and life possessed by concrete. If lime concrete has stood unharmed throughout the ages, Portland Cement concrete, which is infinitely superior, must be imperishable.”
