Many factors must be considered when placing and finishing site-cast tilt-up concrete panels, but proper planning and thorough quality-control measures can ensure that this process is completed with ease. Since tilt-up concrete professionals can easily handle both the hottest summer and coldest winter days, there are few constraints on when to place concrete. As in all concrete markets, the contractor must account for panel preparation, performance of materials in the plastic and hardened state, anticipated weather conditions, and a safe jobsite. During the initial planning of any tilt-up project, how the concrete delivery and placement vehicles, as well as other supplies and equipment, will enter and exit the work area must be considered. Preconstruction planning sessions to consider these “routine” factors will not only save you from headaches and costly mistakes but ensure the success of your project.
Much discussion today centers on mix design, which is crucial to success. Simply, the mix should be designed for acceptance at the point of discharge from the transportation unit and at the point of placement to comply with the project specifications. Delivery to the point of placement may be direct discharge from the truck or transported onsite via wheelbarrow, pump, or belt conveyor. Admixtures are used frequently to modify the plastic performance of concrete such as workability, setting times, slump, and air entrainment, and to improve consolidation around embedded items. Some potential mix design problems include poor workability (harsh/stony), inconsistent setting times, wrong mix delivered to the site, and improper testing (cylinders, air, or unit weights).
The concrete for tilt-up panels should develop sufficient strength for lifting according to the erection schedule. The engineers and lifting hardware suppliers for the project will determine the proper concrete strength at lifting.
The minimum aggregate size recommended for tilt-up is typically ¾ to 1 inch. The increasingly popular tilt-up sandwich panel (placing a layer of insulation between a thin exterior layer of concrete and the thicker structural layer), however, requires a maximum ½-inch aggregate for the thin layer. These aggregate sizes are common and contribute to an economical mix that can be placed with any delivery method.
Still, the tilt-up process offers enough opportunity that a wide range of mix designs is possible and plausible. Today's contractor can maximize economy by working with the ready-mix provider while minimizing the potential problems as the concrete matures to lift day.
The placing operation
Casting tilt-up panels is a production process, particularly when many panels are ready to pour on the same day. An experienced crew should check forms, strategically place the necessary equipment, and review safety procedures and any potential safety hazards.
During concrete placement, workers must ensure that concrete gets under and around all reinforcement and embedded items. Vibrating is recommended. Low-slump concrete—desirable for panels because of the resulting higher strength and less shrinkage—may not readily flow into corners and around inserts without the aid of a vibrator. However, care must be taken to ensure that the vibrator does not touch the floor or move the concrete horizontally within the form. Touching the floor can cause the bondbreaker to separate from the floor slab, contributing to panel sticking and resulting in visual imperfections. Excessive vibrating can also cause the coarse aggregate to segregate.
Pumping has become a very popular method of placing concrete into the panel forms. It eliminates congestion of concrete trucks entering and exiting the pour area, and also provides better control of concrete delivery. Pumps are available in lengths to reach the most remote panels on almost any job. Low-slump concrete (4 inches) and larger aggregate (1 inch or more) can readily be pumped. Although pumping adds cost to the project, it can result in significant time savings and better use of the floor slab space.
Planning for the project will have determined the optimum placement for all panels required. This may mean that panels are stacked six or seven high, or cast along the edges of the actual floor slab or site. Using concrete pumps and conveyors has greatly improved the placement process. For economy, however, determining the correct placement order for timing and to minimize the movement of large equipment is essential for economy. Placement problems can include delay of concrete trucks, improper vibration, displacement of reinforcement and embeds, and truck dumping and buggies that cause congestion in the placement area. These can all be overcome with good planning and worker training.
After placement, the panels are screeded with a straightedge or vibrating screed in preparation for finishing. This process is similar to that used for the floor slab or the casting slab used for the panels. The finish crew begins with bull floating to even out any high or low areas, and an initial edging follows. Once the concrete reaches its initial set, dictated by the aggressiveness of the mix and the site conditions, the panels are floated again, either by hand or mechanical equipment, to final levelness. The edges are recut. Finally, as the concrete further sets or stiffens, troweling operations begin, if required.