Dams made from concrete need to be able to withstand huge amounts of pressure from the volume of water they hold. They also need to handle temperature variations – hot on the outer surface and cool on the inner surface that is under the water. These two factors, pressure and temperature, are the main causes of cracks in concrete.
Cracks may develop internally or externally on the dam. External cracks pose more of a danger to concrete dams than internal cracks. They allow water to penetrate the concrete and can spread more easily across the surface of the dam, weakening it as the external cracks spread. Luckily, there are ways to prevent cracking in concrete dams.
Prevent cracking in dams by using low heat cement
The reaction that occurs when concrete dries and hardens is exothermic – meaning that it produces heat. In mass concrete structures, like dams, the heat released during the curing process can cause the internal temperature of the structure to rise. This can cause thermal cracking.
To prevent thermal cracking, cement with low heat of hydration can be used instead of regular cement Low heat cement keeps the internal temperature of the structure cool as the hydration reaction takes place during the curing process. However, the rate of hardening is slightly slower with low heat cement.
Prevent cracks by pre-cooling the aggregates
Cracking in dams can be prevented by pre-cooling the aggregates – the sand and crushed stone. This can be done by spraying the aggregates with water or by placing large fans nearby to blow a cool breeze over them. The chilled aggregates are then added to the concrete mix, which will help the concrete to remain cool. This will offset the heat generated during the hydration process. Cold water can also be used in the concrete mix. Cement should also be stored in a cool, dry place.
Prevent cracking by post-cooling concrete
Post-cooling concrete can also prevent cracks from forming. This is done by embedding pipes in the concrete when it is poured. Cold water is then pumped through the pipes to keep the internal temperature of the dam to a minimum. The water is pumped through the pipes until the curing process is complete – generally for around 28 days.
Preventing cracks by reducing cement ratio
Cement is what causes the heat generation in concrete during the curing process. Using less cement in a concrete mix can help to lower the heat generation. Contractors often use lower cement ratios for the concrete that is poured for the internal sections of the dam. The external sections can use more cement as the heat is dissipated more easily at the surface of the structure. In general, contractors use 20% less cement for the internal segments of a dam wall.
Prevent cracking by making control joints
Control joints can be made in concrete dams to provide points of expansion and contraction under temperature changes. This helps to prevent thermal cracking. The control joints can be made horizontally and vertically across the surface of the dam wall. The gaps between control joints should be equally-spaced and form a brick-like pattern across the dam wall.
Preventing cracks by controlling the layers of concrete
In mass concrete structures, like dam walls, the concrete is poured in layers. Each layer should be left to set before the next layer is poured on top. This allows the lower layers to harden sufficiently and gain enough strength to hold the weight of the concrete layer above it. A layer that has not hardened enough can crack under the weight of the layer above it. A time period of four to seven days is recommended between concrete pours.
The height of the concrete sections in a dam wall should also be limited. It is recommended to keep each layer below 1.5 metres to prevent stress cracks from forming. These tips will help contractors build a strong and durable dam wall. Cracks are serious problems in high-pressure structures, like dams. Any cracks need to be addressed immediately.
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