Q.: We observed a concrete pour where concrete was revibrated after it had set up. The vibrator head did not “readily penetrate” the concrete as described in ACI 309. In fact the inspector relayed that “it took 15 to 30 seconds of vibrating at each pass for the probe to enter the older concrete.” The “older” concrete had been placed 2 hours earlier. My questions are:

  1. What happened in the concrete as it was revibrated?
  2. Would it have been better to leave the concrete as it was (no revibrating) and place a cold joint?

You may be interested that this occurred at the intersection of a beam and column. The beam and slab are post-tensioned and apparently no incidents occurred upon stressing.

A.: Revibrating concrete momentarily liquifies the concrete again. The primary chemical process that occurs in the first 2 hours after concrete is placed is the formation of calcium hydroxide, which typically makes up 15 to 25 percent of ordinary portland cement concrete. The other major product of hydration is calcium silicate hydrate, which usually makes up about 50 percent of ordinary portland cement concrete and gives the concrete its hardness and durability. Formation of calcium silicate hydrate begins in earnest only after several hours have elapsed.

Somewhere in that process, the concrete reaches initial set, defined as a compressive strength of 500 psi. After initial set, formation of the more brittle, weaker calcium hydroxide continues but falls behind the calcium silicate hydrate formation, which accelerates dramatically between initial set and final set, defined as 4000 psi. (See “time of setting” in ASTM C125, which covers concrete terminology.)

When revibration occurs after the initial set, it breaks down some of the calcium hydroxide that has already been formed. That allows freshly placed concrete adjacent to the revibrated concrete to join with it, rather than introducing a construction joint, and it again becomes a monolithic concrete structure.

Revibration of concrete has been an accepted construction method for more than 50 years. An article from Concrete Construction in February 1959 provided an overview of the practice and concluded by saying: “Concrete will benefit from revibration at any time provided the concrete is sufficiently plastic to permit the running vibrator to sink of its own weight.” Although we’ve learned more since then about what is going on in concrete as it hardens, the benefits of revibration have not changed.