Q.: Two similar questions have been raised regarding placement of concrete on top of recently completed slabs and footings. In one case the contractor wanted to form and place his basement walls the day after completing the footings, but was being required by the architect to wait 7 days. In the other case, the engineer was denying permission to place column concrete above a fully shored and supported slab that had been placed 5 to 7 hours earlier the same day. The columns in question were centered above those on the lower level. What are the rules that limit the timing of concrete pours under these conditions?
A.: After much searching and inquiry, we've concluded that there aren't any written rules to cover these cases. The concrete pretty much makes its own rules in terms of setting and hardening time, and builders respecting these natural limits have not had problems. For example, the contractor will not go on a finished slab to set formwork for columns until concrete is hard enough to remain undamaged by the activity. Formwork and shoring supporting the slab are normally designed also to carry loads from construction work on the slab.
Basement walls: It is common practice to pour walls the day after the footings are poured but you're not likely to find a reference that says either you can or can't do this. One reason this is comparatively safe is that footings are frequently sized to meet local code minimums that are actually oversize when compared with the load that the wall will place on the footing. Remember also that at this stage of construction the only load on the footing will be from the weight of the wall since the structure above for which the footing has been designed is not yet in place.
Placing columns on the new slab: After checking more than a dozen books, technical reports, and standards dealing with formwork and concrete construction (from both the United States and Europe), we found no statement limiting time of placement of columns on top of a slab.
What we did find, however, are restrictions on the related condition of placement of concrete in slabs and beams on top of a deep lift of fresh concrete in walls or columns. The American Concrete Institute (ACI) Manual of Concrete Inspection, the ACI "Specifications for Structural Concrete for Buildings (ACI 301-84)," and the ACI Building Code (ACI 318-89) all have statements similar in intent to the following Section 8.3.2 from ACI 301-84: "Placing of concrete in supported elements shall not be started until the concrete previously placed in columns and walls is no longer plastic and has been in place at least two hours."
Not one of the three ACI documents sets any limit for the related condition of placement of concrete above the supported element (beam or slab).
We discussed the question with the former chairman of ACI Committee 301, DavidGustafson, technical director of the Concrete Reinforcing Steel Institute; and with the current 301 chairman, Timothy Moore of Gilbert/Commonwealth, Inc. Both said they knew of no rules or limitations on time of placement of column concrete on top of recently finished slabs. Moore further stated that no such provisions are being considered among the many changes the committee is considering for future revisions of ACI 301. He said that the only other provisions of ACI 301 which might have any bearing would be those for construction joints. Section 6.1 of ACI 301 (construction joints) has provisions for location of construction joints and bonding at construction joints where required or permitted, but nothing on timing of placement.
We also queried Randy Bordner, former chairman of ACI Committee 347, Formwork for Concrete. Bordner is a professional engineer and a specialist in form design and construction for multistory buildings. He stated that on his jobs column concrete has routinely been placed above slab concrete placed on the same day, the only concern being satisfactory hardness of the slab for attachment of any necessary templates and bracing.
Of related interest, P. Kumar Mehta of the University of California, Berkeley, says in his book Concrete Structure, Properties, and Materials regarding the setting and hardening of cement paste in concrete (page 191): "The time taken to solidify completely marks the final set, which should not be too long in order to resume construction activity within a reasonable time after placement of concrete."
This statement implies that resumption of construction activity could take place at the time of final set.