Dealing with Precast Concrete

Precast concrete is a construction product produced by casting concrete in a reusable mold or “form” which is then cured in a controlled environment, transported to the construction site and lifted into place. In contrast, standard concrete is poured into site-specific forms and cured on site. Precast concrete is being used more and more in home and building construction because it is more readily available than ever before, and provides quality and uniformity that may be lacking in on-site concrete work. Because the plant specializes in concrete, they can usually offer a variety of shapes, sizes, thicknesses, and finishes for your concrete structure. When you consider the cost of labor for a team of skilled workers pouring concrete on-site, precast concrete may save significant money on the cost of the project. Precast concrete is not a new concept, as Ancient Roman builders made use of concrete and soon poured the material into moulds to build their complex network of aqueducts, culverts, and tunnels.

By producing precast concrete in a controlled environment (typically referred to as a precast plant), the precast concrete is afforded the opportunity to properly cure and be closely monitored by plant employees. Utilizing a Precast Concrete system offers many potential advantages over site casting of concrete. The production process for Precast Concrete is performed on ground level, which helps with safety throughout a project. There is a greater control of the quality of materials and workmanship in a precast plant rather than on a construction site. Financially, the forms used in a precast plant may be reused hundreds to thousands of times before they have to be replaced, which allow cost of formwork per unit to be lower than for site-cast production. 

The strength of precast concrete gradually increases over time. Other materials can deteriorate, experience creep and stress relaxation, lose strength and deflect over time. The load-carrying capacity of precast concrete is derived from its own structural qualities and does not rely on the strength or quality of the surrounding backfill materials.

Studies have shown that precast concrete products can provide a service life in excess of 100 years. For severe service conditions, additional design options are available which can extend the life of the precast concrete product. This is extremely important when calculating life cycle costs for a project. Precast concrete also resists most substances. While no material is completely immune to attack from aggressive chemical agents, precast products can often be designed to resist corrosion in specific applications. Because precast concrete products typically are produced in a controlled environment, they exhibit high quality and uniformity. Variables typically affecting quality on a job site such as temperature, humidity, craftsmanship, are closely controlled in a plant environment.

Because of the joints between the sections, this type of subfloor requires finishing with a concrete topping before resilient flooring is installed.

The topping prevents the finished floor from cracking or loosening from the subfloor due to movement of the concrete. Trowelable underlayments are not satisfactory for smoothing preformed concrete subfloors. Concrete toppings on precast concrete must be tested for moisture as for ordinary concrete floor slabs. Structural Topping thicknesses vary with a minimum of 2″ to 2 1/2″ depending on code requirements. Structural topping must be bonded to the planks, continuous from support to support uninterrupted by walls or expansion joints. 

Precast concrete should be tested for moisture the same as regular concrete. When testing per ASTM F2170, the depth of the probe should be located 20% of the slab thickness (not the overall system thickness) since it is drying from both sides. 

Double Tees

Double tees are an excellent choice for parking decks and can also be used in floor and roof applications where a long, clear span is required such as gyms and pools. Spans up to 70′ can be achieved using Precast Double Tees. Double Tees are plant-fabricated, which provides excellent quality control and speeds up the construction process.

Advantages of Precast Double Tees:

  • Low initial project costs and little maintenance throughout the life of the product.
  • Fabrication and erection happen much faster, speeding up the construction process compared to cast-in-place concrete structures.
  • Precast Double Tees are extremely durable, sound resistance and can withstand earthquakes and fire. 

Precast Concrete

 

Hollowcore is a pre-stressed concrete slab manufactured with continuous voids to reduce weight and cost. It is primarily used as a floor and roof deck system. Hollowcore flooring systems provide safe, solid and secure flooring for your needs. This unique product is both strong and durable allowing for increased floor load capacity.

Hollowcore floors are the key element when it comes to architectural freedom and design: their load bearing capacity has a direct influence on the need for partition walls and other structural elements of a building. Hollowcore slabs are prestressed floor elements with voids. The excellent load-bearing capacity and structural efficiency allows you to build large areas with fewer partition walls.

Advantages of using a Hollowcore Flooring System:

  • Hollowcore provides clear spans of up to 60’ while maintaining very thin cross sections of

8″, 10″, 12″ and 16″. 

  • Hollowcore is naturally fire resistant, offering 2+ hour fire ratings with no additional fireproofing needed, resulting in lower construction and insurance costs.
  • Hollowcore plank is sound resistant providing STC ratings of 50 or more.
  • Being very economical and providing unlimited design flexibility, makes it possible for Hollowcore to work with unusual building shapes and cantilevers. 
  • Hollowcore plank is compatible with steel, wood, masonry and other precast products.
  • Hollowcore is manufactured indoors while the site is being prepared, installation begins as soon as the site is ready.

Flooring over Precast Concrete

 

Glue Down Vinyl/Linoleum/Rubber Flooring 

Vinyl floors require relatively flat and level floors and should not be applied directly to the untopped precast planks. In areas limited in size, such as bathrooms and kitchens or commercial service areas, a leveling system can be used. Large areas such as classrooms, assembly areas and open offices should have a composite concrete topping applied to the plank. Like other adhered systems the materials should be tested to insure the compatibility of the glue with the concrete substrate.

Hardwood

Hardwood floors are typically set as floating or sleeper systems. For sleeper systems set upon a grillage of wood nailers, there is little preparation required. The installer should check the entire floor area for levelness prior to setting the first nailer. The plank camber will create a curvature in the floor and the nailers will require shims to set the system level. The high strength concrete in the plank will require the use of hardened nails or power actuated fasteners if they are attached to the floor.

Floating Hardwood floors require leveling materials so that the finished system does not reflect the plank irregularities. These irregularities can cause uneven wear in the flooring and damage the locking system where pressure points occur. Hardwood floors glued directly to the plank system are not recommended. This will require a leveling system.

Ceramic Tile

Ceramic Tile requires the most attention when preparing the plank. Tile suppliers do not recommend applying tiles directly to the plank system whether it be topped or untopped. A membrane should be used between the tile and slabs for both thin set and full depth mortar beds.

Carpet 

Carpet systems are the easiest to use with untopped plank floors, but a proper leveling material must be used in order to reduce differences in elevations. Regardless of how the carpet is installed, directly adhered or set with tack strips, a pad of sufficient thickness and density will minimize the irregularities in the untopped plank system. For systems utilizing thinner padding it is best to provide a leveling material either in spot locations or throughout the carpeted area. For directly adhered systems, the installers should test the compatibility of the adhesives with the concrete. With tack strip systems the installer should be aware of the higher strength concretes associated with hollowcore planking. Hardened nails and/or powder actuated fasteners may be needed for the installation of these elements.

Floating Floor Systems (Avaire/Cork/LVT/Laminate)

Floating floors will require leveling materials so that the finished flooring does not reflect the precast plank irregularities. These irregularities can cause uneven wear in the flooring and damage at the locking systems where pressure points occur.

Hollowcore precast facility and mold

 

Flooring Expert