Repair driveway slab. 3 square ft broke off along the side.

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Old 08-07-14, 11:03 AM
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Repair driveway slab. 3 square ft broke off along the side.

I had a new concrete driveway installed in 09. Within 2 years i had a piece of one of the slab break off. Frustrating because I thought rebar and 5 inch thickness would be be a good idea but the installed convinced me it wasn't necessary. I want to cut out around 2-3 sq ft where the piece broke off the side of the drive and replace the concrete. Just not sure how to attach it to the rest of the slab and if a piece that small would handle the weight if a vehicle ran over it.
 
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Old 08-07-14, 11:10 AM
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I'm not a concrete expert but I think if the proper base and drainage stone is put in then it should not make a difference.
 
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Old 08-07-14, 11:16 AM
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Apparently, the base under slab was not compacted enough and the weight of a vehicle cause the corner to crack along the line of least resistance.

Dig out the base about 6" under the broken area and compact some new material well with a hand tamper. You may to add a small amount of moisture to get it compacted. Clean washed stone would be a waste of time, effort and money. Doweling in some rebar might help, but if there is nothing substantial under the replacement area, oy will crack again and settle.

A corner is a prime weak area because there is no continuity(stress transfer) to adjacent since it is on a corner. High strength concrete will not help, but could cure faster.

Dick
 
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Old 08-07-14, 11:21 AM
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Like Norm said. It's not the concrete but it's base. Think of concrete almost like an eggshell. It makes a nice hard, durable surface but the ability to carry the weight of a vehicle comes largely from it's foundation. Without a proper foundation/base it will likely crack & sink.

Depending on the shape and size of your repair it will not need to be "attached" to your existing driveway if it has a good base. For added assurance you can use a hammer drill and bore holes horizontally into the old slab and insert pieces of rebar or concrete anchor bolts with the end protruding out into the new concrete.
 
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Old 08-07-14, 01:05 PM
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I can compact the surface and make the patch 6" thick but being only 2-3 sq ft I'm concerned the patch will sink under load. I could use some rebar to attach it to the rest of the slab but under load this could cause more cracking to the slab. I thought of using an earth anchor under the patch to help support the patch. http://www.amazon.com/Tie-Down-Engin...=earth+anchors Not sure if this would help much or not.
 
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Old 08-07-14, 01:18 PM
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You can go either 5" or 6" on the concrete - That is not the cause of the crack.

Make sure you have 6" of WELL compacted base under the concrete. The problem is not the concrete thickness, but the strength of the base under it. Don't be tempted to just dump in 3/4" or larger clean rock, because that does not compact in the long term.

Dick
 
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Old 08-07-14, 01:48 PM
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I understand its the base. I'm thinking compacted earth will only handle so much load. The problem is the small surface area of the patch. Its on the side of the driveway but at some point a tire will run over it. This situation reminds me of the landscaping bricks around my yard-light. I have to mow around them and then weedwack. If I try to mow up against the bricks the back wheel of the mower runs over them. Over time they are even with the ground and the grass grows over them. If the ground doesn't provide enough support the weight would go on the rebar if used and the adjacent slabs. This I fear could cause more damage. May not be a clear answer here. Best answer is either a larger patch or replace the slab.
 
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Old 08-08-14, 06:13 PM
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You really don't have to replace an entire slab. But I'd suggest installing a few mechanical anchor bolts in holes drilled horizontally into the adjacent concrete, for the simple purpose of keeping the new concrete attached to the old. Even with a base compacted until the cows come home, separation and horizontal movement can take place once the bond at the interface is broken, allowing water into the resulting crack. When it freezes and expands, it can push the smaller piece of concrete away from the larger. Your profile doesn't say where you live, but if you're a Southerner, the anchors will still help in preventing lateral wheel loads from displacing the repair concrete piece.

The corner cracked because it lacked steel shear reinforcement, and was subject to a load greater than its inherent shear strength (usually less than 80 PSI for even higher-strength concrete). Also, if you live in a cold climate area, the steel earth anchor you linked is likely to cause more problems than it solves. That being because the surrounding concrete will heave when the ground under it freezes, but the anchored patch will be restrained from doing so, and subsequently causing it to break free from the moving slab around it.
 
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Old 08-09-14, 04:06 AM
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you had rebar + 5" of conc or not ? ' but the installed convinced me it wasn't necessary ' compacted granular base is the answer ( not dirt ) + tie bars to hold it tight to the existing good conc,,, you gotten good advice so far,,, just pay attn to them & don't use too much wtr in the mix

knowing you location often helps
 
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