A bonded topping slab only requires joints where the substrate has them because the topping slab will move and respond to volume changes in the same manner as the substrate to which it is bonded. Further, joints will be ineffective in controlling cracking because the topping slab is bonded to the substrate.
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Do elevated slabs need control joints?
Slabs on grade require control joints to permit horizontal movement (shrinkage) without random cracking. Control joints are permissible because of the subgrade support. On structural slabs it is necessary for the concrete to be continuous between supports, thus making it impossible to use control joints.
How big can a concrete slab be without control joints?
Usually, expansion joints should be no farther apart than 2 to 3 times (in feet) the total width of the concrete (in inches). So for a 4 inch thick concrete slab, expansion joints should be no more than 8 to 12 feet apart.
Do patio slabs need expansion joints?
If there are joints in the concrete slab under the tile, there needs to be an expansion joint in the concrete slab, as well. Otherwise, the movement that those joints allow in the slab could carry up into the tile and compromise your installation.
Does a concrete slab need expansion joints?
Expansion joints are virtually never needed with interior slabs, because the concrete doesn’t expand that much—it never gets that hot. Expansion joints in concrete pavement are also seldom needed, since the contraction joints open enough (from drying shrinkage) to account for temperature expansion.
What happens if you don’t put expansion joints in concrete?
Concrete expansion joints give the slabs just enough room to move which helps prevent cracks & buckling. Without these joints, even a little movement creates pressure and stress on the concrete. Eventually weak spots can crack or buckle. Expansion joints are used to relieve this pressure.
Are control joints necessary?
There is a reason control joints are considered essential for whatever projects you have in design. Thermal expansion and contraction will occur in buildings – that’s a given – and control joints help prevent resulting cracks and damage.
When should control joints be cut in concrete?
Joints should be sawed as soon as the concrete will withstand the energy of sawing without raveling or dislodging aggregate particles. For most concrete mixtures, this means sawing should be completed within the first six to 18 hours and never delayed more than 24 hours.
What happens if you don’t saw cut concrete?
Late sawing can result in random concrete cracks. Shallow cracks aren’t sufficient to prevent uncontrolled cracking while deep cuts are excessively labor intensive and undermine the aggregate interlock in the concrete.
Where do you need expansion joints in concrete?
Concrete expansion joints are particularly important where there have been consecutive concrete pours and are also useful when laying concrete within an area bordered by walls or buildings or if objects such as manhole covers need to be incorporated.
What is the difference between control joint and expansion joint?
In building materials, a control joint is used to control cracking while an expansion joint is designed to handle structural movement.
Is it too late to cut control joints?
Joints should be sawed as soon as the concrete can withstand the energy of sawing without raveling or dislodging aggregate particles. For most concrete work, cutting should take place within the first 6 to 18 hours and never beyond 24 hours.
What should I put between concrete slabs?
An expansion joint is a material placed in the cracks (or joints) between concrete slabs to protect the slabs from cracking when they contract and expand as the temperature changes. This material acts as a shock absorber, absorbing the stress from the slab’s movement.
Should you caulk control joints in concrete?
Caulking Concrete Cracks and Expansion Joints
Caulking the joints and cracks is the best way to stop this from happening. Even expansion joints in your concrete driveway should be caulked. They can be the biggest culprit of water under your slabs.
Does a concrete slab need rebar?
Rebar is not necessary for every concrete project. The general rule of thumb is that if you are pouring concrete that is more than 5 inches in depth, you are probably going to want to add in some rebar to help reinforce the entire structure.
How thick can a concrete slab be?
An article once reported that the thickness of one specified 6 inch slab varied from 2 ¾ inches to 8 inches, but the “normal” range should have been between 4 ½ inches to 7 ½ inches of thickness with an “average” concrete thickness of 5 ¼ to 5 ½ inches.
Do post tension slabs have control joints?
Post-tensioning combines principles of physics with steel cables to strengthen the concrete, which naturally lacks tensile strength. As a result, post-tension slabs do not require contraction joints for reinforcement.
How far apart should the control joints be for a 6 slab?
12 to 15 ft.
Rule #5: For better crack control keep the maximum distance between joints in feet at 2 to 2.5 times the slab thickness in inches. For a 6 in. thick slab, the maximum joint spacing should be limited to 12 to 15 ft. Exceeding a joint spacing of 15 ft.
How do I keep my concrete slab from cracking?
If you’re having new concrete poured consider the following ways to prevent cracking:
- Start with a sound subgrade. Make sure the subgrade is compacted.
- Modify the concrete mix. Use a low water-to-cement ratio.
- Install joints. Be active in deciding where control joints will be placed.
- Properly cure the concrete.
How far apart should concrete joints be?
The following recommended tips should be observed: Maximum joint spacing should be 24 to 36 times the thickness of the slab. Joints should be spaced about 10 feet and a maximum of 15 feet. When using joint groove for contraction joints, the joint should be a minimum depth of 1/4 thickness of the slab.
How many times do you float concrete?
Push the bull float in one direction only across the concrete, keeping its front edge slightly above the surface by raising or lowering the handle. Two or three times is enough. Don’t overwork the concrete or you’ll weaken it by drawing too much sand and cement to the surface.