Hollowcore Frequently Asked Questions

Hollowcore is a prestressed concrete slab with continuous voids provided to reduce weight and cost. Hollowcore is primarily used as a floor or roof deck system.

Hollowcore slabs are produced worldwide using a variety of manufacturing processes. Dry cast system typically uses the extrusion process . Zero slump concrete is forced through the machine. The cores are formed with augers and tubes with the concrete being compacted around the cores using high frequency vibrators.

Hollowcore slabs provide economical, efficient floor and roof systems. Structurally, a hollowcore slab provides the efficiency of a prestressed member for unsurpassed load capacity, span range, and deflection control. In addition, the grouted slab assembly provides a basic diaphragm for resisting lateral loads. Excellent fire resistance is another attribute of the hollowcore slab system. Used as floor-ceiling assemblies, hollowcore slabs have the excellent sound transmission characteristics associated with concrete.

Hollowcore should be erected by personnel skilled in such work utilizing lifting devices and cranes capable of safely handling the slabs. The slabs are to be positioned and connected to the structure in accordance with precaster’s drawings and details. The slabs are erected one at a time with slings and spreader bars specifically suited for hollowcore. PCI qualified erector has to install the planks.

Camber is inherent in all prestressed precast products. It is the upward deflection created by the forces of the prestressed strands located below the center of gravity required to resist design loads. Camber cannot be designed to an exact number. Planks with dissimilar lengths, strand patterns or openings adjacent to each other will have camber differences. The benefits of camber and prestressed concrete products is that it allows for longer clear spans and higher load carrying capabilities with shallower depths of section than what could be achieved with conventional building materials.

Where structural topping is required, a minimum 2" concrete topping supplied and installed by the general contractor or others must be bonded to the plank. In using the load tables in technical information section of this website, it is imperative that the topping be of specified strength and completely bonded to the plank. Topping must be designed for a 28-day strength of 4,000 psi. When placing concrete topping, precautions must be taken to thoroughly clean the plank and thoroughly wet the top surface. Topping must be thoroughly cured. It is further recommended that wire mesh be used in the concrete topping. Mesh size to be determined by architect/engineer. Note that 2" minimum thickness topping is measured at mid-span (high point of camber) therefore additional concrete may be required at perimeter to level the floor. All non-load bearing walls (CMU, Steel Stud or Wood Partitions) should be installed AFTER the concrete topping is placed.

Generally, the holes should be lined up in the plank so they cut the fewest number of strands as possible. All holes should be placed on the shop drawings during the approval process so that the openings can be engineered into the plank for working loads.

Yes, generally they should be aligned in the plank so that the fewest number of strands is cut as possible. All intended openings should be placed on the shop drawings during the approval process so that they can be properly design engineered. It is important that the holes are cut after the plank has been erected and the keyways grouted and cured.

Holes can be cut with a zip saw, though larger openings are cut with a walk behind concrete saw. It is important to cut the holes only AFTER the plank has been erected and the keyways are grouted cured. If the openings are a module of the plank (4'or 8'), they can be framed off with a steel header. Round openings can be core drilled by the trade requiring the hole. Once again, care should be taken to reduce the number of strands that are cut.

Holes smaller than 100 square inches should be core drilled by the trade requiring them after the slabs have been erected and grouted. The drilling should be done only with the express written approval of precast engineering department.

Based on 100PSF design load 

• 6" plank span range - up to 20 FT
• 8" plank span range - up to 30 FT
• 10" plank span range - up to 40 FT
• 12" plank span range - up to 46 FT
• 16" plank span range - up to 55 FT

Contact your local producer for specific span capabilities.

You should consult an architect or structural engineer or the building code that governs your project for specific project information, but the following can be used as a rule of thumb. Garage floors are generally 50 to 60psf, offices are 100psf, general assembly spaces are 100 to 150psf, and residential loading is 45psf.

There are numerous methods to connect the hollowcore slabs to the building frame. The type of connection is dependent on the building material the hollowcore bears on or connects to. Please contact with your precaster to obtain cost-effective connection details for concrete, masonry, steel and precast concrete construction.

The typical CMU wall connection is achieved with a rebar that is placed into the bond beam at the top. This rebar is then bent into the plank keyway or center core on planks and grouted solid. The mason should refer to the hollowcore layout drawings for placement of the rebar.

A weld plate embed will be cast into the plank and welded to the steel beam in the field. It is standard construction practice to only weld one end of a piece hollowcore. When the weld plate is used in this manner it is to reduce the un-braced length of beam and does not have a structural purpose.

The non-load bearing curtain walls (running parallel) on the exterior of the building are connected with a rebar (placed and grouted into the wall) at third or quarter points of the plank span, and bent into notches/pockets cut into the plank.

In most cases, the core ends do not need to be filled. There are certain applications of extremely high loads and longer spans that do require filled core ends for additional shear strength. Precast plant engineers will analyze any specific application and recommend filled core ends on an as needed basis.

Anchors can be drilled and sleeved into the bottom of the plank. Different connectors can withstand varying degrees of pullout force. Drilling through the plank and bolting completely through the plank can achieve heavier hanging loads.

Not all columns, point loads, and wall loads need to be supported from underneath. Plank is surprisingly strong and in some cases can support many types of loads. Precast plant engineers do need to analyze each case of unsupported loads and will give you a specific recommendation on your project.

Hollowcore slabs will be cambered as with any other prestressed flexural member. The amount of camber is affected by span, prestressing forces, concrete differences and curing variations.

The plank bottoms can be finished to provide a durable, maintenance free ceiling for the room below. Planks are suitable for the application of a medium to heavy textured acoustical material such as USG Imperial QT or equal. Planks may require a Block Filler prior to the application of sprayed textured acoustical material.

Plank can be finished with a minimum ¾” leveling, such as a gypcrete. A 2” + structural concrete topping may also be applied. (This is 4000-psi concrete that must be steel mesh reinforced.) The type of topping for each application depends on the finished flooring material to be used. When planning for a topping slab, there are a number of design considerations: camber, floor decks in which planks change direction, elevations of door openings, etc.

In an instance where a structural topping is required, a minimum of 2” of reinforced concrete designed for a 28-day strength of 4,000 psi must be specified. The top surface of the plank must thoroughly be cleaned and properly prepared prior to the placement of the topping slab to ensure proper composite action. The reinforcing wire mesh in the topping slab is to be determined by an architect/engineer. (Note that 2” minimum thickness is measured at mid-span, or high point of camber, therefore additional concrete may be required at the perimeter.) All non-load bearing walls (CMU, Steel Stud or Wood Partitions) should be installed AFTER the concrete topping is placed.

The hollowcore slabs will be laid out on precaster’s drawings to minimize the requirements for field pour strips. However, in some instances, pour areas adjacent to walls or around openings may be required. These will be clearly indicated on our layout drawings and details and are the responsibility of others.

Plank can be split, or cut, to achieve any design width imaginable. Just because the plank is produced in 4'wide or 8'wide sections, does not mean that you must design in 4' or 8' modules. The rough edge of the split is generally hidden over the adjacent non-load bearing wall or above the drop ceiling.

Precast beams and columns are a cost effective way of providing a fire resistant concrete system that can be used for parking garages, offices, dormitories or almost any construction project. A complete precast concrete beam, column, and hollowcore plank can be used in place of any other structural system.

Precast walls and plank (or in conjunction with beams and columns) can provide the entire structural shell of most any building. Wall bearing with precast walls is a fast and economical means of designing a load bearing wall system. The precast walls are ready for textured paint or paper and backer. Also, the electrical conduit and boxes can be cast into the walls during the precasting of the units.

One of the attributes of hollow core slab construction is excellent fire resistance. More than 30 standard fire tests (ASTM E119) have been conducted on hollow core floor assemblies. The Underwriters Laboratories “Fire Resistance Directory” includes more than 50 design numbers for hollowcore slabs which qualify for ratings of 1, 2, 3, or 4 hours. As an alternative to UL ratings, model codes include prescriptive requirements which can be used to establish fire endurance ratings. Use of such provisions eliminates the need for fire tests or UL ratings.

Precast concrete floors and roofs usually do not need additional treatments in order to provide adequate sound insulation. STC rating for 8” Hollowcore plank is 50. A greater sound insulation can be obtained by using a resiliently attached layer(s) of gypsum board or other building material.

Yes, hollowcore plank can be cantilevered. This will require a special design and it is best to consult with the manufacture. Precast Hollowcore cantilever planks used for balconies are not recommended A solid slab would be a better solution for this condition.

Electrical and duct work can be considered. This will require early coordination with the producer.

Fly ash (SCMs ) can be incorporated into the concrete mix and reduce the amount of cement needed as a binder. When planning on using SCMs it is best to consult with a precast producer to finalize the percentage. The maximum allowable dose will vary based upon the Hollowcore producer's manufacturing process. A typical recommended starting point would be 20%.