wet to dry epoxy at construction joints

[sybie99]

Hi Guys,

Would like to hear your opinions on use of wet to dry epoxy to bond new concrete to existing.

When for example you are casting a suspended slab, the pour is too big to do in one hit, now you just let the concrete run off at 45 degrees at the line where you want the construction joint. Can you just clean this concrete surface and brush it off and cast next slab or must you use a wet to dry epoxy? I am not relying on any tensile strength in the concrete, only compressive, so why use wet to dry epoxy?

In the UK it was normal to just jet wash the existing pour end to remove loose aggregate and laitance and then cast next slab (of course you have rebar going through the joint). In South Africa engineers often use an epoxy.

Why use an epoxy, what is its purpose?

Thanks

 

[hokie66]

Firstly, I don't know where the 45 degree "run off" comes from. Construction joints are normally cast vertically near the location of minimal shear.

Preparation of a construction joint for the subsequent placement only requires the joint to be clean, and preferably slightly roughened (sounds like the UK practice). Epoxy bonding compounds are not necessary, and without close control can act as debonding agents.

 

[Ron]

As usual, good advice from hokie66. Don't introduce another material if you don't absolutely need it. If you feel you must have some "bonding" agent, after cleaning the cold joint, apply a thin paste of portland cement and water, brushed on with a moderately stiff bristled brush. As it dries, place the other concrete.

 

[sybie99]

Thanks once again chaps, can always rely on sound advice from you guys!