Mass Concrete Consultation for Cut-and-Cover Tunnel

CTLGroup was asked to help the client understand and work within project specifications to place mass concrete successfully and cost-effectively.
Project specifications called for:

  1. Initial concrete temperature up to a maximum of 80°F
  2. Maximum concrete temperature (during curing) of 125°F
  3. Maximum temperature difference of 35°F
  4. No (or minimal) thermal cracking

Typical mass concrete elements consisted of walls, slabs-on-ground, and roof slabs with dimensions of:

Walls - 10 to 25 ft (high) x 25 to 50 ft (long) x 3 to 4¾ ft (thick)
Slabs-on-Ground - 90 ft (wide) x 45 ft (long) x 5 to 7½ ft (thick)
Roof Slabs - 90 ft (wide) x 45 ft x 3 to 3½ ft (thick) placed on precast concrete decking

For this project, CTLGroup helped the contractor select cementitious materials and concrete mix designs. We performed thermal analyses to determine concrete temperatures and the potential for thermal cracking. We worked with the contractor and design engineer to show that:

  1. Thermal cracking is acceptable to widths up to 0.012-in.
  2. Initial concrete temperatures during placement can be increased to 90°F provided that maximum in-place concrete temperatures do not exceed 155°F.
  3. The allowable temperature difference within concrete should be based on calculations rather than the generalized 35°F "rule-of-thumb" from the project specifications.

The owner's representative accepted all of CTLGroup's recommendations and the tunnel was constructed based on those recommendations.