DOING THE JOB 25 How to keep basements dry Watertight advice on construction options designed to protect structures from groundwater QUALITY STEWART TENNANT Leaky basement problems are usually due to poor workmanship, wrong assumptions on groundwater levels or poor detailing by designers – in particular around joints and junctions with other basements or structures. The standard governing this area is BS8102, the code of practice for “Protection of structures against water from the ground”. It classifies basements into four grades, with varying levels of protection. These range from grade one, suitable for car parks, to grade four, where the end user requires a totally dry environment, such as an archive. The code also identifies the three different forms of construction. Type A is tanked protection, with either an internal or external membrane. Type B is structurally integral, where the concrete provides the protection. Type C is drained protection, where a drained cavity and internal vapour membrane are provided to wall and floor. There are pros and cons to each option. Tanked protection (type A) traditionally took the form of mastic asphalt, but today bonded bituminous sheet materials are more common. Other options include waterproof renders or polyurethane resins. It is important to ensure the membrane is fully continuous, which may be problematic when you abut existing structures on infill developments. Any movement joints or discontinuities will need to be detailed carefully and similar issues will apply to pile heads or caps. You also need to provide adequate temporary works and working space to ensure relatively dry and clean conditions for application. External membranes need protection against puncture during backfilling or concreting operations and internal membranes need a load coat or screed, in the case of floors, to prevent them being lifted off by water pressure. With structurally integral protection (type B), grade three protection can be achieved with concrete structures designed to BS8007, without any further precautions (membranes). You may be tempted to use an expensive high-specification concrete with micro silica or other additives An unbonded membrane and drainage system being applied to the external walls of a service reservoir “ Construction joints are always a potential weakness and you should keep them to a minimum” that will impart almost magical waterproof properties. But I recommend you concentrate on the basics, such as a well-compacted dense concrete with a low water cement ratio. Quality control on site is key. Construction joints are always a potential weakness and you should try to keep these to a minimum. One joint you cannot eliminate is the kicker. This must be cast at the same time as the base and be at least 150mm high to ensure compaction. Whether or not to use a waterbar in construction joints always provokes debate, but they aren’t required in BS8007. I prefer not to use them, and I advocate that the joint is prepared by jet washing or light scabbling to remove laitance and expose the aggregate prior to the next pour. If you want to use a waterbar, take care when using hydrophilic materials that are expansive in contact with water. They must be placed in the centre of the concrete section, which must be a least 250mm thick, otherwise there is a high risk of spalling. Drained protection (type C) gives the highest protection and can be used where integral or tanked protection is impractical, such as top-down construction or conversion of cellars to habitable basements. The drained cavity can be formed by a blockwork lining wall or a geocomposite “egg box” material. Relatively speaking, these systems are more expensive and slower to construct, but are arguably more reliable and offer more protection. You may have to provide pumps to remove the collected groundwater. A final issue with new-build basements is the effect on the local groundwater regime. In clay, the excavation acts like a sump and in sands and gravels or on a sloping site may form a cut-off. In these cases, the groundwater conditions that the basement has to withstand are likely to be different from that shown in the site investigation, and external drainage is worth considering. Stewart Tennant is a director at consultant GHA Livigunn MATERIALS Steel yourself for tough times Steel price volatility threatens to damage the industry and hand victory to concrete, warns Chris Bone Like many other sectors, the construction market is in decline. If it weren’t for continued levels of public spending, particularly in the education sector, there would be far less work to go around. But while the price of raw materials is sliding fast, steel is bucking the trend. While steel stockholders are de-stocking, offering short-term price relief, the price of rolled steel has risen by around 30 per cent in the past nine months, and shows little sign of easing. The de-stocking effect on price has created a perception that steel structures can be delivered for less than is the case. To compound matters, steel manufacturers are reducing production capacity to maintain inflated prices. WWW.CNPLUS.CO.UK ‘Contractors under pressure to cut build-cost’ These factors are creating a gap between the reality of steel price movement and the expectations of the market, causing a squeeze on steel contractors. Steel manufacturers must realise the effect of this policy on the marketplace. If they do not, then concrete (steel’s age-old adversary) will gain a competitive edge, and all the work the steel industry has done to promote steel as a sustainable, flexible building material will be undone. There are fundamental questions about the viability of concrete as a material, namely its performance in fire. But money talks, and as long as the concrete industry keeps its eye on the ball, the balance of power will shift. It is reported that steel accounts for 60 to 70 per cent of structural frames in the UK. Conversations with blue-chip contractors suggest this may already have swung to 50:50, if not further. Steel contractors are under huge pressure to cut the build-cost of steel frames in line with concrete, perceived global trends and the de-stocking price effect. That, coupled with reducing industry credit insurance and less demand for buildings generally, provides a dangerous environment in which to operate, with potentially sliding margins and therefore less contingency for risk in an increasingly risky market. If steel manufacturers could offer a degree of pricing support to the industry, this would help steel contractors resist slashing margins further. It would also help them buy more for their pound of credit, alleviate stockholders’ uninsured risk and kick-start the sector. We are all in this together and perhaps a degree of industry solidarity would help us all come through the hard times. The corporate stance (cut capacity to keep prices high) will end up reducing both the demand for steel (in favour of concrete) and potentially its supply, as steel contractors may not be around to buy it. Chris Bone is group chief executive officer of Bone Steel THURSDAY 15 JANUARY 2009