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A closure strip, otherwise known as a pour strip, is typically a temporary gap between two separate concrete slab placements. The closure strip is subsequently infilled with concrete at a later date. Closure strips are common with staged construction where there is a construction sequence involved in casting the slab. Applications of closure strips can be found in bridge deck construction, post-tensioned (P/T) slab construction, and normal two-way slab building construction.
BRIDGE DECKS
Closure strips in a bridge deck are required when an existing deck is replaced under staged construction or bridge widening. More than likely, one-half of the deck would be closed per construction phase, while traffic is maintained on the other half. The closure strip is located near the center of the deck, transversely, and extends the full length of the bridge. It serves to reduce the effect of adjacent traffic live load vibrations during concrete curing. The joint is usually left open for as long as possible to permit transverse shrinkage of the deck concrete to occur.
Representative requirements from the Nevada and South Carolina bridge manuals indicate that closure strips should have a minimum width of 3 ft (0.9 m) and contain the lap splices for the transverse reinforcing steel.1,2 The width may be greater, however, to account for the anticipated relative dead load deflection between Stage 1 and 2 concrete placements, across the closure strip. Both bridge manuals indicate two very useful purposes of the closure strip: 1) it delays final connection of the two stages of concrete work until deflection from the deck slab dead weight has occurred; and 2) it provides the deck width necessary to facilitate differences in the final deck grade resulting from theoretical deflection calculations and/or construction tolerances.
P/T Slab s
Closure strips in P/T slabs allow the separate slab sections to have ample time to shorten, and thus minimize any restraint cracking. Shortening has many components in a P/T slab and can be attributed to:
* Elastic shortening due to precompression;
* Creep shortening due to precompression;
* Shrinkage of concrete; and
* Temperature gradients.
The Post-Tensioning (PT) Manual3 provides guidance for the use of closure strips and the length of the slab between closure strips:
* If the slab length is less than...