A composite, structural concrete topping is commonly used in floor construction with hollowcore slabs. The composite action is desirable to add stiffness and strength for gravity loads and may also be required for load transfer within a diaphragm. When a composite topping is used, consideration must be given to its strength, detailing and quality assurance.
The required compressive strength of the topping may be determined from the hollow core slab design requirements. Load tables provided by local producers will normally indicate that either a 3000 psi or 4000 psi concrete is required. Diaphragm requirements may necessitate a higher strength topping concrete.
Since the composite topping and hollow core slabs interact to create the final structural element, it is imperative that the topping bond well with the slabs. While the building designer may only be interested in the final product, the process of achieving a well bonded, composite topping is very important. The hollow core producer is dependent on a properly bonded topping, yet is not involved in specifying, designing or installing the topping. The hollow core producer is responsible for supplying a slab that is capable of bonding with a topping. The installer of the topping is responsible for surface preparation, topping concrete mix design and curing to assure proper bond.
Design of hollow core slabs for composite action is usually limited to a horizontal shear strength of 80 psi according to section 188.8.131.52 of ACI 318-95. Through limited published and unpublished testing, the machine finished surface has been found to meet the requirements of that section. The horizontal shear check should be based on the shear diagram rather than using an average horizontal shear over the distance from zero moment to maximum moment when checking compliance with the 80 psi limit.
Design Assist is the procurement method by which, prior to completion of design, a construction contract may be awarded on a best value basis pursuant to which a contractor/subcontractor provides design assistance to the design team and ultimately the owner.
In recent years, a trend toward early integration of the design/build process has developed into what is now called design assist.
In this process, the precast concrete producer is selected based on qualifications for executing the demands of the specific project, PCI Certification, and the capability for technical expertise to assist the design and construction team in the development of the project. A contract is then awarded and should, at a minimum, include compensation for the design assistance. Significant benefits can be achieved if the contract also includes the supply and installation of the final products.
Benefits of design assist include development of concrete mixtures and finishes during the development process rather than going through a sample approval process after bid and award. Requests for information (RFI) are eliminated with the precast supplier as a participating member of the design team. Precast erection drawings are prepared as design development nears completion leading to a much smoother shop drawing review process. The structural design of the precast panel connections are completed in collaboration with the Engineer of Record to assure coordination with the design of the supporting and bracing structural system. The construction schedule is coordinated with the precast production schedule to assure timely delivery and installation. Any activity that normally occurs after award in a typical bid/build arrangement would be coordinated much earlier when the total precast contract is executed shortly after the design and construction team is assembled.
Precasters can offer detailed expertise that allows for expedited development of the design with engineering innovations and scheduling improvements while enhancing aesthetics and controlling budgets from conceptual design to project completion. Each element can be made as cost-effective as possible, taking advantage of the inherent performance characteristics of precast concrete. The precaster will be able to contribute design and detailing suggestions to ensure that maximum efficiency is achieved at the lowest erected cost. The result should be a functionally efficient and aesthetically pleasing enclosure that meets or exceeds the expectations for the project.
Finally, the precaster should be considered as a partner on the design team. This will impose a responsibility on the precaster to understand related construction materials that must interface with the precast concrete so details appropriate to all the materials can be developed.
-Courtesy of PCI