Concrete pipes (usually buried) may have to withstand very aggressive conditions, indeed some of the most adverse conditions to which concrete structures may be exposed. Externally, they may be subjected to aggressive groundwaters or soil minerals which may destroy the integrity of the pipe. Also, they are often subjected to high soil loads and variable founding conditions which can result in structural damage and, more frequently, joint problems. Externally exposed pipes (e.g. on pipe bridges or partially buried in trenches) are subjected to severe thermal cycles which can lead to pipe fatigue. Internally, conditions may be just as severe, if not more so. Pipes may be subjected to physical and chemical attack, including erosion and abrasion from particulate material and flow, temperature shocks and cycles, and aggressive effluents carried by the pipe. There are also many sewer lines in South Africa in which biogenic acid generation is a major problem.