My suggestion is that a concrete infill is not a good idea. The paper on
the existing GWB will get wet and lose its strength, and the gypsum core
material will soften. Ordinarily the GWB would dry out, but the real
problem is the hydrostatic pressure of the water laden concrete. The
pressure in the lowest 1 ft square area would be 62 lbs per sq. ft.
times 8 for the 8 ft. wall. Thats an enormous force, and is sufficient
to burst the GWB. Placing the conc. in small pours to allow curing in
stages could work, but the hardened material at the top opening would be
difficult. The existing GWB will warp when wet due to the pressure.
Instead, apply 1 or 2 layers of either 1/2" or 5/8" GWB to either or
both sides of the wall with screws to located studs. That will be
faster and stronger. One layer of 5/8" GWB on one side should give a
considerable reduction of sound transmission. 3-5/8" of concrete would
provide far more result than is really needed.
The fundamentals of sound transmission are simple. One, have no openings
in the wall, e.g., duct terminations or convenience outlets. Place SPF
in the wall around the J-boxes. Use acoustic caulk at the top, bottom,
and side edges or the GWB panels, and tape the joints. Two, the mass of
the wall determines the sound absorbing efficiency of the wall. In that
respect the concrete is appropriate, however, its placement is difficult
though not impossible.
Sound can travel in the under-floor spaces or above-ceiling spaces
between joists. Mineral wool insulation between the joists can add mass
and limit sound travel., and there are more than one ways to place that.
SPF can be optionally sprayed in. Recessed lighting fixtures between the
joists should not be covered with any type of insulation.
Three, carpet and pad the rooms on either side. That will lower energy
of the frequencies of intelligible speech that may be transmitted.
If you have to use concrete, a retarder in small batches should extend
the 20 min. working time. I'd pour only 6-10" at a time in each bay.
Small drilled holes at measured heights or measured quantities of mix
would work. Larger entry holes for a large funnel at 2 or 3 heights at
each bay may work. I would use sand or gravel fines to make a mortar
grout and no stone. Several materials can be used to reduce the weight
and density of the mortar, e.g., expanded rock or microspheres. Use an
exhaust fan to remove water vapor from the rooms and wall for 1-2 weeks.
Several bags may not be enough, and the required volume would need to be
calculated. If the cement is old or has ever been wet due to bags
standing on a garage floor it may not work chemically and not harden.
Make a small test batch. All in all, the concrete approach is messier,
and that would ultimately require patching of holes. GWB, mud and tape
may be less expensive than additional cement. A cost calculation may be
Four. A different approach is to place SPF (sprayed polyurethane foam)
in the wall cavities. That is an expensive remedy. That doesn't add much
to the mass and doesn't lower the total energy transmitted that much,
however, it would lower the transmitted sound frequencies making them
If a music room, for example, is being isolated appropriately add layers
of GWB. One or two layers should work.