Placing New Concrete Over Old

by Steve VandeWater

 

 

This stamped concrete patio and steps were poured atop an existing concrete patio

So you’d like a new stamped concrete patio, but already have a plain gray one.  Does your contractor need to tear yours out before installing the new concrete?  Maybe and maybe not.  If done properly, new concrete can often be poured right over an existing slab.  For this to be feasible, the contractor needs to pour at least 2 inches thick, use smaller aggregate, and incorporate reinforcement such as welded wire mesh or fiber mixed into the concrete.  Steel rebar is also a good idea if the slab is thick enough to allow it.  To clarify the point, this article deals with pouring concrete over concrete, not applying a thin polymer-modified stampable overlay, which is another option.  For more on thin overlays, please click here

Make no mistake…there are certain situations when it’s best to tear out existing concrete and start anew by pouring on a compacted stone or sand base.  For instance, if you have large working cracks in your slab and the concrete has settled to the point that it’s at several different levels, you should remove it.  Likewise, if tree roots are causing your existing concrete to heave, then it’s best to tear out the concrete, resolve the root problems, and start fresh.  Perhaps door thresholds or stair riser heights are an issue and you simply can’t pour any higher without creating problems.  Any of these are good reasons to remove the existing concrete and start from scratch.

However, if your concrete is relatively sound and raising its elevation a few inches would not create problems, then you can pour new concrete directly over the old.  Well, maybe not directly over the old.  You’ll want to install a bond breaker so the two slabs do not bond to each other

“Why?” you may ask.  “Don’t you want the new concrete to bond to the old?”  The answer is usually no.  If you bond new concrete to old, any cracks in the existing slab will transfer up into the new slab, and usually within a day or two.  These include cracks located within sawed or tooled crack control joints.  However, if you use a bond breaker such as plastic sheeting, roofer’s felt (tar paper), or a layer of sand or stone between the new and the old concrete, the existing cracks will not transfer through.  The slabs will remain separate.  You must, however, place crack control joints where needed to eliminate random cracking.  In thinner concrete slabs, crack control joints should be spaced closer together than with full-depth (4 inch +) concrete.

Roofer's felt (tar paper) was placed over the existing concrete as a bond breaker prior to pouring a 2 inch cap

 

Some people contend that if they aren’t bonded together, the two slabs will somehow separate.  Considering the enormous weight of the material, there is little to fear in that regard.  The new concrete certainly isn’t going to float up in the air or slide off.  Skeptics fail to recognize that pouring over the top of sound concrete is no different than pouring over a super-compacted stone base.  Over the course of my 20+ year contracting career, I’ve poured countless 2 inch caps over existing concrete with no problems whatsoever.  The photos included in this article are just a few of the many jobs I’ve completed using this method, and one of them is at my own home.

2 inch thick stamped concrete cap over an existing concrete patio

 

 

Pouring atop existing concrete exposed to vehicular traffic is also being done on highway, driveway, and parking lot projects.  It’s called whitetopping, and it is growing in popularity due to decreased expense and a proven track record.  However, with slabs which will be exposed to vehicular traffic, the concrete is thicker and no bond breaker is used so the top slab is bonded to the existing slab.  This seldom works with residential driveways leading into a garage because the added height makes the driveway taller than the garage floor.

 

The existing cracked pool deck was capped with 3 inches of stamped concrete atop a bond-breaking membrane

 

 

Don’t let the cost, mess, or headache of a complete tearout stop you from considering stamped concrete.  Maybe you just need a cap-over! 

 

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