Prewetting for structural grout 1

[sdz]

Cementitious grouts generally seem have application instructions that require prewetting, such as
"All prepared surfaces must be saturated with water several hours prior to grouting, ensuring it is free of any surface water or puddles."

How important is this? Does it get done in practice? Can prewetting be avoided?

 

[judie]

Answers for your...

Question 1: It is important as it enhances bonding between the old and new concrete. Prewetting is normally required if there is no chemical bonding agent that will be applied prior to grouting works. I think you need to establish first whether you need the bond or not before deciding whether to prewet, although the applicators/suppliers/manufacturer's representative will require it anyway to safeguard their product against claims later on.

Question 2: Yes, it is done, but not as long and frequent as indicated in the material data sheet. This usually happens if QA/QC is not that good.

Question 3: Yes. Alternatively you can use chemical bonding agent as mentioned above. Better bonding but expensive.

Hope these help.

 

[concretemasonry]

If you are using the grout in reinforced masonry, the prewetting should not be necessary unless the storage of the units and the and construction conditions are highly unique.

The purpose of the grout in structural masonry is to transfer the loads from the wall into the reinforcement and is rarely used to increase compressive strength of the wall since far and away the strength of the units controls and obtaining higher strength units are possible and can be much more economical.

The grout for masonry must have a slump of 8" to 11" to provide proper filling of the voids and the units absorb the excess water during the consolidation process.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[paddingtongreen]

I was taught that wetting was necessary because, otherwise, the cement/water mix would migrate into the old concrete, leaving a lean grout.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[dik]

Testing has indicated that effective pre-wetting can provide equivalent bond similar to some of the bonding agents.

Dik

 

[KeerthiR]

Dik,
Is there any possibility that I can get hold of these test reports? That will be very helpful.

 

[dik]

I'll see if I can find the backups... I had them on a machine that the harddrive went south about two weeks back...

Dik

 

[sdz]

concretemasonry, sorry I wasn't clear, I meant grout under column base plates.

 

[Ron]

Michael's point is almost right on target with grout bonding.

Further, it depends a bit on the substrate material as to how effective the bond can be. If the surface is formed, grout bonds are weakened because of less mechanical interlock. For unformed surfaces, the interlock occurs, but you have to be careful that you don't "suck" so much water out of the grout that proper hydration is inhibited at the bond interface. This phenomenon occurs right at the bond interface, so bond is compromised when that occurs. It doesn't reduce the amount of water enough to compromise the overall hydration and strength of the grout, but does do so in a localized condition at the interface. That helps create a bi-lateral shrinkage plane that you don't want at that location.

If you saturate the surface to receive the grout, the opposite can occur. The water-cement ratio can get quite high, so strength at the localized plane is compromised. Ideally you want enough wetting to maintain a stable condition within the grout as it hydrates, and to allow comparable cross-migration at the interface to achieve mechanical and chemical bond. Easier said than done!