Problems and Solutions
It is important for an owner or building professional to know about the presence of an alternative concrete system, preferably prior to any construction work or even design. While cutting into a slab is the surest way to verify the presence of an alternative concrete system, it is probably the least convenient – or desirable. Severing the wires in these systems can seriously compromise their structural capacity.
Fortunately, there are some markers to look for that suggest the presence of an alternative concrete system. Building age is perhaps the key initial indicator; any building constructed between 1880 and 1920 has a very good chance of having one of these systems in place, though examples can be found as late as 1950. When looking at the slab itself, the presence of unusual aggregate materials such as sawdust, wood chips, or cinders suggests archaic concrete construction. Damaged areas of the slab may expose reinforcing wires or mesh atypical for modern reinforced concrete, particularly on the underside of the slab where the curvature of the catenary may be visible. Less definitive but easier to observe is slab thickness: an unusually thick slab can be evidence for an alternative system.
These systems can present a number of problems. Relatively thin layers of concrete, like those used in cinder fill decks, can crack easily, and even without cracks the relatively porous concrete mixes used often admit water. Once inside, water can traverse cavities present in the design and saturate areas of loose fill, which then may remain wet for years, conducting water throughout the slab.
When water comes into contact with metal reinforcing materials, corrosion can occur, quickly weakening the slab. Due to their thin gauge, wire- and wire-mesh-based systems are especially susceptible to corrosion damage, which can quickly impact their strength. To make matters worse, chemicals present in cinder fill or aggregate can produce corrosive compounds, such as sulfuric acid, when mixed with water, accelerating deterioration. It is not uncommon to find wires in compromised slabs that have completely disintegrated, leaving only rust. Even in the absence of corrosive failure, cracking can cause the embedding concrete to lose enough integrity that the individual wires are subject to ductile failure, as loads are not effectively distributed.