Performance Mixes with PLC Keep Tilt-Up Construction Moving Along
The Dakota Project, in Hamilton, Ontario, is a large warehouse/distribution facility built using the tilt-up concrete method and construction occurred in both summer and winter conditions. All mixes used portland-limestone cement.
In Canada, performance specifications for concrete mixes are standard practice. Per the concrete specification for this project, a CSA A3000 Type GU or GUL were both allowed. CSA A3000 GU (general use) and GUL (general use portland-limestone cement) are the equivalents of ASTM C150 Type I portland cement and ASTM C595 Type IL portland-limestone cement, respectively. Three mix designs were approved during the submittal process: a moderate strength portland-limestone-cement-only mix for the slabs (25 MPa, or about 3625 psi) and two other slightly stronger mixes with portland-limestone cement plus up to 15% slag cement for the tilt walls (40 MPa, or 5800 psi).
Concrete for the slabs was placed by both chute and belt and only required a slump of 80 +/- 30 mm (about 3 ± 1 in.) with no use of plasticizer. Walls, on the other hand, required chute, belt, and pumping for placement, so a plasticizer was needed to produce a specified slump of 150 mm (about 6 in.) for one mix, and 180 mm (7 in.) for the other. The slab mix did not require entrained air whereas the wall mixes were specified to have 5% to 8 % plastic air content.
The tilt-up method is known for speed of construction. Tilt-up is fast because casting the wall panels in a horizontal position makes it easier to form and finish the panels, as well as maintain position of the wall insulation sandwiched inside. In addition, floor slabs at the construction site are regularly used as the casting beds, also a time saver. After panels have cured, they are tilted (lifted) into position, braced, then attached to the slab and adjoining panels. On this project, the first wall panels were being lifted one week after casting and the contractor was able to place 600 linear ft (180 m) of wall in two days of work. For a number of reasons, larger panel sizes can increase the rate of production. On the Dakota Project, the tilt-up erector broke their own records for panel sizes: maximum weight as much as 201,000 lbs (just over 91,000 kg); maximum height as much as 51 ft 10 in. (15.8 m) tall; and maximum width as much as 28 ft 8 in. (8.7 m) wide. The lower carbon PLC mixes did not impact any aspect of this project, either for strength development, ultimate strength, or durability (a Canadian Standards Association (CSA) F-1 freeze-thaw exposure).