How Long Should Concrete Cure Before Setting Anchor Bolts?

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Anchor bolts--used to secure heavy structural beams or machinery to a concrete slab foundation--are installed in a concrete slab during placement of the concrete, before it cures, so that they become integral with the slab. Anchor bolts are selected for the loads and stresses they will endure during the life of the construction. But the stability of an anchor bolt is guaranteed only if the concrete around it cures properly.

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Function

Building codes will specify the number and size of the anchor bolts required for the construction, as well as how deeply the bolts must be embedded in the concrete. When many anchor bolts are required, engineers design a template, calculating the exact location of each anchor bolt. The template is positioned over the slab location prior to pouring the concrete. A rigid framework is constructed to hold the template and anchor bolts in place. When just a few anchor bolts are needed, they can be positioned with chalk string, tape measure and a surveyor's transit. But, the precision of placement is still crucial.

Time Frame

Concrete is poured around the securely positioned anchor bolts. The concrete will set in two to four hours, depending on the mix, and attain 75 per cent of its ultimate strength in about 7 days.Concrete cures to 90 per cent compression strength in about 28 days. The anchor bolts must remain absolutely still while the concrete cures. Any stresses placed on the anchor bolts during this time may loosen the anchor bolts.

Significance

Anchor bolts bear directional forces of pulling, pushing and twisting, in addition to pressure. So the security of anchor bolts depends not just on the compression strength of the concrete, but also upon the concrete's tensile strength. Tensile strength of concrete is much lower than compression strength. This makes the proper curing of the concrete even more important. Loss of potential concrete strength due to improper curing could result in catastrophic failure of the anchor bolt.

Effects

Curing conditions for concrete are as crucial to strength as adequate curing time. Temperature fluctuations, or temperatures below 4.44 degrees C, or above 32.2 degrees Celsius can inhibit concrete curing, ultimately causing anchor bolts to fail. Plan concrete placement during favourable weather. Keep concrete surfaces wet while curing, to dissipate heat and prevent surface evaporation.

Considerations

Wedge anchors and adhesive anchors can be installed in concrete that has already cured, but these may not have the same pull-out strength as set-in anchor bolts, and they may not meet all building code requirements. Wedge anchors and adhesive anchors require drilling, and should be installed after concrete has cured fully. However, some foundations cannot be drilled without seriously compromising their structural integrity. Always consult your home builder or an expert contractor before drilling your foundation.

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