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2024.8.14-16 Shanghai New International Expo Centre,China

World Of Concrete Asia

2024.8.14-16 Shanghai New International Expo Centre,China
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Home > > A Solution for a Crumbling Concrete Deck

A Solution for a Crumbling Concrete Deck

MRTWISTER / ALAMY STOCK PHOTO

Tim Ulher and his team recently framed a house on a sloped site with a walk-out basement and a garage on the main level. Instead of leaving the area underneath the garage unexcavated and pouring a slab on grade, they decided to take advantage of this real estate and create a roughly 24-by-24-foot living space below the garage slab.

Q. My client has a failing concrete porch deck with a crumbling surface. An effort to fix it with a layer of stamped concrete failed, and the owner is looking for advice on what to do next. Would tile be a valid solution?

Engineering considerations. The slab and supporting structure needed to be able to support the weight of the materials and vehicles, but that’s not all. The footings and foundation also would need to resist soil lateral loads on the 10-foot-tall foundation walls.

Floor design. They had already planned on using Roseburg I-joists for the home’s floor system, and our engineer was able modify his design to support the slab using 11 7/8-inch I-joists 12 inches on-center that sat in joist hangers attached to a 5 1/4-by-11 7/8-inch RigidLam LVL located midspan. This LVL was then supported at each end by a 6×6 column with an expanded footing and by one column in the center of the room below the garage on a 36-by-36-by-12-inch footing. The subfloor itself is 3/4-inch AdvanTech. While they would have loved to eliminate the center column, the depth of the beam needed to provide an unsupported span would have presented headroom issues.

An unbonded topping is basically a completely new slab placed on top of the old slab. Since it’s atop concrete, it has a nice strong subbase, but that base must be relatively flat and stable, and there must be a definitive way to prevent bonding. The bond breaker could be a vapor retarder, which would both prevent bonding and act as a slip sheet for the new topping to help prevent random cracking from shrinkage. The unbonded topping needs to be designed as if it were a new slab, which means thick enough to manage the loads and properly jointed.

Despite the failure of the decorative topping, a bonded topping could still be a good solution. Obviously, the substrate concrete must be sound—any loose or spalling surface must be removed, and there must be no contaminants on the surface and no moving cracks. Since a bonded topping doesn’t carry any significant load on its own, it must be fully bonded to the base slab. To assure that bond, a good, rough surface is needed, which can be accomplished with a sandblaster. Next, you must thoroughly clean the surface, typically by vacuuming the surface to remove all dust, prior to installing the overlay. Then pre-wet the surface to prevent it from absorbing water from the topping mix. Finally, a bonding grout can improve your success—either a cementitious material or an epoxy. Finish by cutting joints into the topping to exactly match those in the base concrete; otherwise, the joints will reflect through.

To read the rest of this case study from JLC click here.

From the photo of this project (above), it’s clear that the attempt to fix the surface with a thin decorative topping failed miserably. That was likely due to insufficient surface preparation. Either that or the underlying concrete is soft or defective and is simply incapable of supporting the loads. In that case, one solution would be to isolate the original base material and place an unbonded topping, although that would need to be at least 4 inches thick, which might not work with existing doors and steps. Another solution would be to use an isolating membrane and install tiles.

To learn more about successfully installing toppings, get a copy of the recently published “Guide to the Design and Construction of Concrete Toppings for Buildings” from the American Society of Concrete Contractors (ASCC) (ascconline.org).

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