huangyhg

concrete topping slab
i have a project with an existing lightweight concrete (3000 psi) slab on metal deck on steel beams. the existing slab has 2 large areas of depressed slabs. one is a 2" depressed slab (11,000 ft^2) and the other is a 3/4" depressed slab (2500 ft^2). currently there is tile and grout in the 3/4" depression and grout and stone in the 2" depression. both of these are coming out and being leveled. both areas are interior office space.
for the 2" slab i am showing a 3000psi n.w. slab infill with 6x6-w2.1xw2.1 wwf and contreol joints centered directly over each of the beams below (~10'-0" o.c).
for the 3/4" slab i am calling for a prepackaged polymer modified mortar (sikatop 122+) with 1/4" aggregate added to help reduce shrinkage. this area will be carpeted.
for both infills i was calling for the existing slab to be prepared with a scabbler to roughen the surface. then to clean the surface of all dust, oil, laitance, and bond-inhibiting materials. then adding a latex bonding grout on the surface prior to pouring the new slab topping.)
does anyone see anything i should be concerned with? should i use a stronger epoxy bonding agent?
your approach is good. you might consider the following as well...
for your 2-inch slab, use a smaller wire and make sure it is delivered in the form of a mat, not rolled wire.
make sure your control joints also correspond to existing control joints. one easy way to accomplish this is to place flatbar in the existing joints, then place the concrete. the reduced concrete section over the flatbar will crack and it will be a straight, narrow crack, not quite as clean as a sawn joint, but much better than a "random" crack. it will be a little less visible than a sawn joint and you will have no dust or residue cleanup.
i would wet the existing concrete prior to placing the bonding grout for the 2" infill. the concrete should be 'damp' but not 'wet'.
this will help prevent the existing concrete from sucking all the water out of the new concrete and grout.
fwiw, we have found that scrubbing a concrete paste with sand aggregate into the existing concrete prior to placing the topping concrete is just as effective as the bonding grouts available on the market. this is the 'old school' way of doing this kind of work.
usually lightweight slabs are used for fire rating. you might check to make sure you are maintaining the fire rating.
do not use air entrainment.
while a scabbler may provide a decent bonding surface, shot blasting may be more economical. pre-wet the surface (damp, no standing water), scrub in an ordinary cement paste immediately before placing the infill. mist constantly and do not let any area dry out. then cure properly - preferably constant misting.

ralph
structures consulting
northeast usa
thanks for all of the great replies! would anyone use an epoxy bonding agent instead of the cement paste for a stronger bond? if so, would you also wet the slab before applying the epoxy bonding agent.
in response to jike's response about the fire rating, the depression was to allow floor materials. this building only requires a 1 hour rating and the existing slab covers that requirement. thanks.
epoxy bonding agent if not applied and concrete poured within the specified amount of time, it can act as a bond breaker. the good thing about epoxy is that it generally is not affect by moisture.
so you would still dampen the floor priot to placing the epoxy bonding agent.
we just did a bonded topping - tried multiple methods (rubbed grout, bonding agent, no bonding agent) and did many (whew - many) pull tests on it. the rubbed grout didn't work as well as the bonding agent, which we ended up using.
the big issues to watch carefully are:
1. per jike above - make sure the bonding agent is put on right before the topping.
2. damp surface (not wet, just damp)
3. make very very sure that all loose debris, cement powder, etc. is totally removed from the surface prior to applying the bonding agent.
4. cure the topping very well...preferably with fog or some type of moisture cure....vs. spray-on.

a re-imulsifiable bonding agent works great for having more working time but is affected by moisture (that is how it works!). you do not want it to re-activate. the moisture in the wet concrete activates its bonding characteristics.

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