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sawcutting control joints

Concrete is a material that is inherently strong in terms of compressive strength and relatively weak in terms of tensile strength. Because of this (and other studies in physics) concrete slabs have a tendency to crack every so often (typically every 8-16 feet depending on thickness and mix design properties). Because of this, it has become an industry standard to cut ‘contraction’ joints in concrete slabs to predict and control the natural joints (that almost always form) and conceal their unappealing look from view. Sometimes these joints are scored while the concrete is still plastic, but most commonly with concrete floors they are done after finishing with gas powered or electric floor saws (often equipped with diamond blades). Similar to placing and finishing concrete, there is a critical window of time when concrete floors should be sawcut to maximize the chances of aesthetic appeal and functionality. The standard floor saw equipped with a standard steel blade should be utilized after the concrete is strong enough to receive its weight but before the concrete has lost its plasticity so much that it cannot support its own weight and resist the tendency to crack. Also if the saw begins cutting too early, the slab will not have the strength to resist raveling/chipping/spalling on either side of the intended cut that happens as a result of the blades residual force.

Recently the use of ‘early entry’ saws have allowed slabs to be cut earlier than before. The early entry saw consists of a low eight, dry-cut system (as opposed to the old wet cooled blades), an ‘up-cutting’ blade, and a skid plate applying pressure to the surface. Also many believe early entry saw cuts can be shallower (1″ regardless of concrete depth) under the notion that the cracks will be induced by the timing of the cuts in relation to the semi-plastic state of the concrete.

A concrete floor cutting saw has blade sizes ranging from 5″ to 24″ or more and engines with a horsepower of 5 to 25. Standard floor saws have weights ranging from 150lbs to 600lbs and cutting depths 2″ to 10″. Early entry saws weights can be as low as 100-125lbs. Before cutting the sawcuts will need to be laid out. This is typically done with chalk covered string line measured off each wall and snapped onto the concrete. Some floor saws cannot cut all the way up to a perimeter wall. In these cases it is common for this final 2-12″ be cut with a compact hand saw.

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