Q: When placing vertical rebar in a residential footing, is it necessary to tie them in place before the concrete pour, or can workers just "stab" them in after the concrete has been placed and struck off?

A: As part of a larger research program ("How Clean Must Rebar Be?"), we did some research on the comparative bond strength of rebar held in place during concrete placement and rebar pushed into position after the concrete had been placed. For the held-in-place condition, we constructed three bond pull-out specimens with a 6-inch rebar embedment depth by using the bottom halves of 6x12-inch plastic cylinder molds with 1/2-inch-diameter holes drilled through the bottom of each.

We placed the molds on 2x6-inch boards that had been predrilled with 1/2-inch-diameter, 1/2-inch-deep holes. We inserted #4, Grade 60 deformed black-steel rebar in a vertical position with 1/2 inch of the rebar protruding from the bottom of the cylinder mold into the hole in the 2x6. This helped keep the rebar vertical during placement of 3 1/2-inch-slump concrete around it.

For the pushed-in-place rebar, we placed the concrete into the 6x6-inch cylinder molds and then stabbed #4, Grade 60 deformed rebar into the concrete and through the hole in the bottom of the mold. We made three bond pull-out specimens by this method. Concrete strength was about 5500 psi when the bars were pulled out.

Technicians measured slip of the 1/2-inch-long free end of the rebar while applying load to the longer end of the bar in each specimen. They recorded slip when the bar yielded and when the bar broke. Neither of the two bars-held-in-place or stabbed-pulled out of the concrete.

The results are summarized in the table. Local building codes may not permit stabbing the bars. However, our test results indicate that the performance will be the same whether the rebar are stabbed into place in consolidated concrete or held in place while concrete is placed around them. Bond pull-out test results (average of three tests).

Condition Slip at yield, in. Slip at ultimate, in. Ultimate stress, psi Failure mode
Held in place 0.0001 177 0.0005 101,100 Bars broke
Stabbed 0.0001 177 0.0004 101,200 Bars broke