Q.: A large number of 2- and 3-foot diameter, 18-foot-high concrete columns were placed on caissons without the required reinforcing steel that connects the caissons and columns. To repair this, the contractor had to drill holes in the caisson outside each column perimeter, insert rebar dowels into the holes and bond them with an epoxy adhesive, tie vertical steel to the dowels, and build forms for a concrete jacket around each column. The jacket increases the column diameter by 18 inches, so the rebar is at the center of a 9-inch-wide annular space into which we will place new concrete. Placing concrete from the top of the columns isn't feasible, so we're planning to pump the concrete from the bottom up. There isn't room to insert a vibrator without the risk of getting it snagged in the rebar and embedded in the pour. Can we get adequate consolidation from the bottom-up pumping alone, without using a vibrator?

A.: In the bottom-up pumping jobs we've read or heard about, vibrators were used to consolidate the concrete. External vibration doesn't seem to be an economical alternative to internal vibration because of the column height. However, adding a polycarbonate admixture used to produce self-consolidating concrete might allow you to achieve adequate consolidation without vibration. We recommend a test placement on one column using the high-slump, very flowable concrete that can be produced with the admixture. Surface appearance and, perhaps, a nondestructive test for internal voids could confirm that the concrete was adequately consolidated.