Pouring new concrete over exisitng

[SteelPE]

I have a client that wants to pour a 3" slab over the top of an existing slab on grade. The new slab is for a change in elevation only (no structural purpose). The existing slab has been in place for 30+ years and is currently used for industrial purposes.

I am going to have them clean the existing slab with a power washer and then have them apply a bonding agent to the existing slab an pour the new slab over the top.... but I am a little concerned with having the new slab bonded to the existing. It almost seems as if I want the new slab to be free from the slab below (to allow for proper shrinkage). What do others do in this instance.

 

[oldestguy]

A common method that works great for re-finishing a parking garage floor is as follows. Jack-hammer off the deteriorated and contaminated surface. Clean off and use compressed air if u can. Do not wet the old surface. Immediately before placing new concrete apply a "cream" consistency made up of Portland cement and water. Brush it in with a broom. Apply the new surface concrete and cure properly. I've cut cores from the completed job and with a cold chisel try to break at the bond, Never have had the break there.

So, adopting what you can from this and it will work great. Remember a dry old surface will allow the cream to soak into the old for a better bond.

 

[jike]

Bonding agents can be tricky and can be affected by cleaning, timing and moisture. I prefer doweling one slab to the other for a reliable connection, if you need it.

 

[SteelPE]

The problem is that I don't need the topping at all. The topping is being placed to for a change in elevation only. I am torn between bonding one slab to the next or providing a bond breaker. Everything I have read seems to be split on the subject.

 

[jayrod12]

How large is the area that you are topping?

 

[SteelPE]

The proposed topping is 3" over 2,600 square feet.

 

[jayrod12]

Are aesthetics a concern? If some cracking in areas is not a concern than I would be just cleaning and pouring the topping slab. If they are a concern then I would look into creating a bond break between the slabs.

 

[SteelPE]

It's an wood shop for a vocational school. I don't think cracking is of concern..... but that may be me and the client may have other expectations.

 

[EngineeringEric]

Can you place a layer or poly, then a place concrete with minimal shrinkage WWF and lots of control joints (locate your new control joints on top of old and a new one between each control joint (half the spacing and replicate for reflective cracking)

And get something in writing as to if cracking is a concern.

Also look into topping cements or gypcrete or leveling mixtures that can go to that thickness...?

 

[oldestguy]

Cracking of concrete can get you in trouble if not well controlled. For some reason many laypersons think cracking is bad. If a school board with authority there has such type members, the control of cracks would be your major factor to consider, not so much the bond or bond breaker. If the old concrete has a decent spacing and nice looking joints, you should follow them with the surface. Mark them carefully and saw the new surface accordingly. Imagine someone moving in a new wood planer via a fork lift and new random cracks show up as a result. I'd opt for the roughening of old and bonding. No point in developing a bad reputation to save money.

 

[TXStructural]

There is probably little reason to bond the slabs, since doing so puts a good outcome at risk without providing any benefit. Unbonded overlays simply transmit vertical loads to the new slab directly below. Bonding does not do anything except risk the complications caused by initial slab shrinkage and then thermal cycling.

For a bonded overlay, or one that is not intentionally debonded, make your mix as "non-shrink" as possible (i.e., keep cement content low, keep W/CM low, use a polycarboxylate water reducer, biggest large aggregate as feasible) and make sure the old concrete is SSD or you allow enough water into the mix for what will be sucked out by the old slab.

In any case, be sure to cure the new work well and plan so you do not get curling, which will happen is you put plastic between the slabs (curls up due to exposed surface drying) or if you allow the old slab to suck moisture out of the new one (curls down as the bottom of the new work dries.)

While you could tool joints, it's probably better to do early-entry sawcuts. These cuts will reduce curling related cracks.

 

[oldestguy]

OK now some discussion. Take a typical slab on earth. Typical of highway slabs. If that earth is dry, water leaves more readily than for upper part. Thus a low W/c ratio at bottom as compared to top. Low W/c means less bottom shrinkage later when things dry. This is very common and results in concave up curling, especially noticed at joints. Curling does not take place if you bond a thin overlay to a thicker older slab. It follows what happens from below.

As you see I differ as to what happens if a plastic barrier is at the bottom. Most noticeable for those is the finishers get mad having to wait way past quitting time to do their job. Setting is slower. I have not seen interior slabs on moisture barriers curl.

 

[SteelPE]

One of the concerns I had with installing a plastic layer was slab curling as mentioned which only muddies the waters in trying to make the decision. With proper curing methods, slab curling can be eliminated.

I guess if you asked 10 engineers the proper way to do this you would get 10 different answers.