The Cast System – a la construction

There are many things considered “cast” in the building industry. The cast system we are discussing is related to construction and in that regard we are talking about materials at the site that are fabricated in situ (in place). So we are looking at those items that are cast-in-place at the project site. There are a variety of items that could potentially be cast-in-place on a project, but by enlarge the most significant cast-in-place product at the site is concrete.

Let’s look at the nature of concrete on the project. The drawings and specifications guide the contractor in the construction of the cast-in-place (CIP) concrete systems. So, we will take a look at the specifications for CIP Concrete which is located in specification Section 033000. At some point for reference we will need to calculate the amount (volume) of concrete in the project to get an order of magnitude of the quantity involved. We are several pours down the road and the amount that has been poured increases weekly.

The Cast-In-Place Concrete Specification covers some of the following:

• Submittals; the contractor must submit proposed design mixtures that have a proven performance record, steel reinforcing shop drawings and construction joint layouts.
• Form Materials; what materials the forms are to be constructed with and concrete ties.
• Steel Reinforcement and Accessories; bars, wire, dowels and supports.
• Concrete Materials; cement, aggregate and water (these are the 3 basic ingredients of concrete).
• Admixtures; air entrainment
• Concrete Mixtures; concrete strengths, water-cement ratios, slump limits
• Installation (execution) Requirements; formwork, embedded items, removing-reusing forms, shoring, concrete placement, finishing, curing, repairing, and quality control ( including testing).

Here are some of key points regarding the CIP concrete at the Visitor Center:

• We have specified 4000 psi (pounds per square inch) concrete strength with a water-cement ratio of 0.45; the psi represents the compressive strength of the concrete.
• Before any concrete is allowed to be poured, the contractor must submit proposed design mixtures for the concrete. The contractor submitted 3 mix designs; slab on grade, elevated slab and all other concrete. The “all other” was rejected for incorrect water-cement ratio, the other 2 were accepted and the slab on grade mix was also approved for the CIP walls. There was actually a 4^th mix design submitted for sidewalk construction and it is an air entrained  mixture which helps concrete stand up better to being exposed to the elements including freeze-thaw cycles.
• A slump test is performed for each day’s pour that is over 5 cubic yards. This tests the stiffness of the concrete and verifies that it representative of the correct water-cement ratio. Test cylinders are taken at the same time for subsequent compressive testing at 7 days and 28 days to assure proper strength is being achieved. We have not received any test results to date but should get some soon.
• The forms can be stripped on foundations and walls 24 hours after the pour.
• The maximum length of wall that can be poured without a cold construction joint is 60 feet.

The following photos show CIP walls that have had the forms stripped. There are loop shaped wires protruding from the concrete which are the concrete ties. The ties hold the forms on each side of the wall at a specified distance from each other to achieve the proper wall thickness.

The following photos show the north side of the north most walls which have been stripped and are now receiving water proofing which I will discuss in another post. The tall element in the photos is the elevator shaft which continues to grow upward.