Frost-induced deterioration of concrete in hydraulic structures : Interactions between water absorption, leaching and frost action

Sammanfattning: Frost deterioration of concrete can result in severe consequences to the safety, durability and functionality of a structure since it impairs the strength of the concrete. Frost damage may occur to structures that are in contact with water and subjected to frost action. Different types of concrete damage, which are suspected to be frost related, have been observed in concrete structures in Swedish hydro power plants. Questions have consequently been raised regarding the durability of concrete in hydraulic structures in cold climates. “Hydraulic structure” is the umbrella term for structures in contact with water – hydro power stations, dams, locks, canals, harbours and bridge piles.Different types of suspected frost related damage to concrete structures in hydro power plants are identified and categorised in this thesis. The underlying causes of damage are experimentally investigated by means of laboratory and field studies. Knowledge about the deterioration of concrete is important in order to secure long service life for hydraulic structures. The obtained results can be used as input for repairs of existing structures and in the case of new builds. The results may also be applicable to hydraulic structures in other countries where similar environmental conditions are present.The environmental exposure conditions of most Swedish hydraulic structures can be considered harsh. Most hydraulic structures are in contact with water all year round and are subjected to frost action in winter. The water in Swedish rivers is generally considered soft due to the geology of Sweden. Ice floes and driftwood may also cause abrasive wear to some structures. Progressive deterioration of the concrete surface occurs at the waterline of many hydraulic structures. Despite the fact that these structures are exposed to fresh water, the damage is similar in appearance to salt scaling of concrete. Surface deterioration results in exposure of the coarse aggregate and eventually the reinforcing steel.Analysis of the chemical composition of the cement paste shows that long-term exposure to soft water causes leaching of the concrete surface. Leaching leads to a decrease in the resistance against frost action and abrasion. The surface of frost resistant concrete may also suffer from frost damage if subjected to leaching prior to freezing. This thesis experimentally demonstrates that different degradation mechanisms can boost each other since the amount of superficial damage caused by the combined effects of leaching, frost action and abrasion exceeded the total amount of damage separately caused by these mechanisms.Progressive deterioration of the concrete surface at the waterline begins with leaching during the spring, summer and autumn months. Hence, the concrete surface becomes susceptible to frost action and is damaged in winter. During the spring, the damaged surface layer falls off and the process begins again.In some cases, internal damage has been observed at and just above the waterline of hydro power structures built prior to the 1950s. Experimental results show that internal frost damage may occur above the waterline in non-frost resistant concrete. Determination of saturation levels in concrete in existing structures and in specimens after laboratory tests showed that the risk of internal frost damage at and above the waterline is low in frost resistant concrete.Another type of internal damage has been observed far below the waterline of thin water retaining concrete structures. The damage can be characterised by spalling of large concrete pieces. All damaged structures were subjected to long periods of unidirectional freezing in winter. The studies performed show that poor concrete quality and the effects of ageing and imperfections in the concrete may facilitate growth of macroscopic ice lenses, which causes spalling.In mechanically sound concrete with a water to cement ratio (w/c) lower than 0.9, the risk of macroscopic ice lens growth is low. In concrete with internal cracking due to frost damage, spalling may occur within a few days regardless of the w/c-ratio. Concrete spalling may occur also in concrete with cavities and other imperfections. However, the higher the quality of concrete, the longer the freezing time required to facilitate macroscopic ice lens growth. Since unfavourable temperature and saturation conditions may exist in winter, the risk of macroscopic ice lens growth in thin water retaining concrete structures cannot be overlooked.Determination of saturation levels in concrete from a 55-year old dam showed that high saturation levels are to be expected so that even frost resistant concrete is at risk of frost damage. Microstructural observations of concrete from the dam further showed that the frost resistance of concrete can be reduced due to the fact that air voids fill with ettringite and calcium hydroxide crystals over time. In spite of high saturation levels in the concrete, the risk of frost damage can be minimised by preventing thin water retaining structures from freezing in winter.