WHAT IS A POUR STRIP?
A pour strip is a leave out/gap in cast-in-place concrete construction between two adjacent slab pours. Typically, this gap, between two concrete pours, is left open for approximately 28 to 56 days. This gap is between 4 to 8 feet wide, across the entire width of the slab section.
WHY DO WE NEED POUR STRIPS?
This leave out is typically between two lateral resistance elements (i.e. walls parallel to long direction of the pour) in the structure that can cause considerable restraint to volume change and can cause considerable cracking if the volume change is not allowed to occur freely. This is of concern in post-tension flat plate construction. Based on published information, the total length change in 200 feet can easily be between 1/2 and 1 inch. Our experience has been that the change in length in the pour strip is more in the order of 3/8 to 1/2 inches for a 200-foot total length of slab between restraints. If this shortening is not allowed to occur freely it will distribute itself in a series of cracks between the restraints with the total width of those combined cracks slightly less than the total restraint free shortening of the slab.
POUR STRIP OPTIONS
2.1 The following are taken from the PTI DC20.2-22 recently published. The document can be purchased on the PTI website. https://www.post-tensioning.org/ The following is a quote from this document.
4.4 – Embedded release devices
Release devices are temporary connections formed of steel dowels encased in a sleeve (Fig.4.24). They can be used at slab construction joints or wall-slab interfaces. The sleeve allows temporary horizontal movement, and the dowel takes the vertical shear transfer at the connection. They can be permanently locked using an appropriate locking device and by filling the sleeve with a bonding filler such as cementitious grouts or epoxy. Once locked, the dowels can transfer tensile loads along the dowels’ main axis. They are typically locked 3 to 4 weeks after pouring of the concrete.
Release devices are usually proprietary products where the forces and movement that the connection can take depend on the system used. They are not fire-rated, and they may require the area to remain shored until the connection is locked and the joint is filled with proper sealing material. Filler material should be evaluated so it meets the project’s fire-rating requirements.
Release devices provide the ease of removing temporary formwork and shoring earlier than pour strips. After the sleeve and dowel have been installed and the appropriate concrete compressive strength has been reached, there is no need for the formwork and shoring to remain in place if the construction loads do not exceed the loading on the structure when in service. The release devices are designed for the loading on the structure when in service. Typically, the loads are determined from the general notes, tables, or details shown in the structural drawings, and feedback from the LDP.
These types of slab shear connectors (i.e. Embedded Release Devices) have been around for a considerable time and with limited use in post-tensioned or mild steel reinforced cast-in-place concrete. These slab shear connectors generally require that each slab, adjacent to the joint be designed as self-supporting usually located at the inflection location (i.e. minimal moment) of the slab. Slab shear connectors only provide for vertical shear resistance between the 2 slabs (e.g. dowel type devices used in slabs on grade also on concrete highways). Any moment capacity is local to the device to this type of slab shear connector. The shear values can be calculated using the code (i.e. ACI 318). Tension values of these types of connector are similar to a headed stud (e.g. an expansion bolt headed stud type anchor), that develops local tension, albeit a non-ductile/brittle failure mechanism, since there is no lap or connection to flexural rebar. It is really up to the engineer of record to decide if the ACI code integrity is achieved. Further, there are only 3 ACI code permitted methods for splicing rebar. Again it is up the engineer of record to decide if continuity of the diaphragm chord rebar is in fact established and developing a ductile mechanism to resist lateral loads, as the code requires.2.2 The NEW PS=Ø® Reinforcing Splice System.
This is a unique specialization/improvement of a standard precast concrete rebar splicing method. The slab and beam rebar is spliced much like the conventional pour strip, thus requiring only minimal reinforcing adjustment and only in the bay of the coupler. This type of rebar splicing has had a long and successful track record in both the precast industry and in coupling congested rebar in cast-in-place columns. The unique design of the PS=Ø® coupler accommodates the dynamic movement caused by the post-tensioning compression forces in the slabs. The major difference between these two types of couplers is the design to accommodate volume change (i.e. slab/beam shortening) that occurs particularly in post-tension concrete slabs. These couplers are typically installed in the initial pour. Then the continuation rebar from the second pour is inserted into the coupler full depth through the form board. Vertical steel pipes are installed in the coupler and protrude out the top of the slab to create ports for the future grouting of the coupler. The adjacent slab is then poured directly up to the first slab with a bond breaker. Over the next 28 to 56 days the gap that starts at zero will increase to between 3/8 and 1/2 inch. This system requires no reforming of the pour strip only shoring. After 28 days or so all the couplers are grouted and the small void or gap between pours can be grouted with the high-strength grout at the bottom side formed with a plastic angle, ethafoam rope, or a wood 2 x 4.
No additional safety equipment nor are additional ramps required to cross over this narrow joint making for easier faster construction. No additional forming is required, nor does it need to be left in place for 28 to 56 days. The real construction schedule again is in all the other construction items allowed to move forward earlier unencumbered by the traditional gapped pour strip.