The demolition phase of the North Wing Addition project is complete, and now we begin to construct what will become 26,000 square feet of new gallery space for contemporary glass. That means we’re pouring a lot of concrete in the middle of a New York winter. Here’s how it’s done.
First, soil tests determine the soil’s bearing capacity. The engineer determines the size of the concrete footing by calculating the sum of the building’s various loads. The lower the bearing capacity of the soil and the larger the loads, the larger the footing area must be.
Column Footing
The engineer also calculates the needed compressive strength of the concrete and the appropriate reinforcing bar.
Once the strength of the concrete is determined, the order is sent to the supplier. The supplier determines the proportions of the sand, aggregate and concrete. The engineer may specify additional ingredients for example, pozzolans like fly ash. A pozzolan is a siliceous and/or aluminous material which in the presence of water reacts chemically with calcium hydroxide to form compounds possessing cementitious properties (ASTM C618). Plasticizers may be added to make placement easier with less water. Air-entrained concrete has microscopic bubbles to increase freeze-thaw resistance. The American Society of Testing and Materials sets standards for concrete.
Heated water prevents freezing.
Winter’s sub-freezing temperatures add problems for concrete placement. The Portland Cement Association has recommendations and information about cold weather concreting. Concrete must be protected from freezing for at least 24 hours or a strength of 500 psi. Early freezing disrupts the cement paste matrix and can reduce the ultimate strength by 50%. Several things can be done to ameliorate this problem. The water for the Museum’s concrete was heated by the plant’s boilers to 156 to prevent freezing.
The form before concrete placement. The vertical rebar in the center will become part of a column. The horizontal rebar resting on the top of the form will be incorporated into the floor.
One method of determining how much water the mix contains is the Slump Test (ASTM C143). The concrete is carefully placed in a cone. The cone is removed, a straight edge is placed across the cone and the slump measured The Museum’s engineer specified a 6” to 8” slump. If the slump is too great, it is an indication that there is too much water and the load may be rejected. Generally, concrete is strongest if the minimum sufficient amount of water is used.
The slump test
The concrete tester also places samples of the load in cylindrical molds which is cured under lab conditions. After it is cured for the specified time (7 days or 21 days) the cylinder is placed in a press and subjected to increasing pressure until it fails. The psi achieved in the test should meet or exceed the strength the engineer has specified.
The concrete is vibrated to ease placement and to eliminate bubbles.
Concrete does not dry out to attain strength, it completes a chemical reaction. As long as the concrete is not allowed to be diluted, concrete can set under water. The reaction takes a long time, reaching perhaps 60% of its final strength after 7 days and close to full strength after 28 days. Ca3SiO5 + H2O → (CaO)·(SiO2)·(H2O)(gel) + Ca(OH)2 .
The concrete’s curing is an exothermic reaction. That means that the concrete produces heat as it cures. The concrete’s internal heat plus the blankets above will protect the concrete until it is sufficiently strong.
