Rebar Size, Grade, and Placement. Reinforcing steel adds the vital property of tensile strength to a concrete wall (concrete by itself is not very strong in tension). There are two grades of steel covered in the ACI 332 tables: Grade 40 (with a yield strength of 40,000 psi) and Grade 60 (with a yield strength of 60,000 psi). If you use Grade 60 rebar, you can install fewer pieces to gain equivalent strength—“and there is virtually no cost difference between the two grades,” says Bartley. Labor is the key factor when you’re installing rebar; you save money by using fewer pieces, which you can do by increasing the diameter, the grade, or both. At diameters above 3/4 inch (#6 bar), the steel gets harder to bend around corners; but most contractors can readily bend any size or grade of bar using mechanical equipment. (Note: For all basement walls, ACI 332 requires three bars of rebar placed horizontally along the length of the wall at the top, center, and bottom to limit shrinkage cracking.)
Of course, not all home buyers want their house designs squeezed to the economic minimum. In fact, in parts of the Washington, D.C., vicinity, says Bartley, customers are insisting on basement ceilings 10 or even 12 feet high. It’s a little extreme, he admits, but he says, “It’s not my job to tell people what to do.” In cases like that, or for homes built in the very active “marine clay” soils found in parts of the Virginia lowlands, Bartley usually finds himself building to a specific engineered design—and charging accordingly.
But for the average house on good soil, Bartley’s generic solution is an 8-foot or 9-foot basement wall, 10 inches thick, poured with 3,500-psi concrete using four horizontal runs of rebar—and, in most cases, no need for vertical rebar. That’s actually pretty conservative, he says. “We’re conscientious, and we don’t want to get anyone in trouble.”
For Bartley, the new ACI 332 standard is by no means an excuse to get away with doing less. On the contrary, he says, the new rules fill a void in the residential construction industry. “We created a standard that the residential industry should be living up to—something that would represent value to the customer as well as a reasonable standard for the contractor himself to meet.”