Repair of Popcorning Spalling Slab

[MumPE]

I am working on the renovation of a building that is structural steel frame with cast-in-place concrete slab. The building was built in the 1920s. The concrete slab mix must have been quite poor as the appearance is almost of popcorn or "fireproofing" slab in nature.

Needless to say, the slab is spalling and falling out in popcorn like chunks. Some areas the WWF is exposed, perhaps due to inadequate cover but most likely due to just the poor concrete mix. There is no sign of rust on the WWF.

I am thinking that in the areas that are particularly bad I should specify a thin layer of grout? Any tips or references I should look into?

Should I recommend a section be tested for compressive strength?

 

[JedClampett]

First make sure that this isn't some lightweight or gypsum mix. Usually concrete installed a long time ago is pretty sound, as they were very conservative in their mix designs, plus the 90 years of curing tends to solidify it. You might have a case of alkali silica reactivity (ASR).
But what to do? I'd call you local Sika representative. They have a lot of repair materials for this type of situation. Grout would probably work, but it might not adhere and then you've wasted all that effort. Thin layers of material can be tricky to put in, so invest the money in a speciality repair.

 

[Ron]

Agree with JC...find out what you have before you take a chance on how to fix it!

Have a petrographic examination done on the concrete to characterize it.

If you have ASR, then putting a topping on it won't help.

 

[msquared48]

Is the slab internal, or extermal where it could be exposed to water and salt?

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[MumPE]

I am pretty sure it is not ASR but insufficient fine aggregate in the original concrete mix (no-fines). It appears as though there are large interconnected voids. The large voids are causing the spalling and popcorn like appearance. The slab is definitely getting worse with age and normal loading.

Mike the slab internal.