Cast-in-place concrete is concrete that’s poured onsite and is found in foundations, slabs, beams, columns, walls, parking lots, and other areas in the construction industry. Forms are constructed to create the desired shape and support the fresh concrete until it’s gained enough strength to remove the formwork. Reinforcing steel, such as bars or welded wire mesh, is placed within the formwork to add tensile strength and help reduce shrinkage, but must have adequate cover of concrete to protect the steel. There are many concrete mix design choices for an array of properties and needs.
Placement
Fresh concrete should be placed in a manner that’s well consolidated within the forms and around the reinforcing steel. Rock pockets, segregation of aggregates, and over and under vibration lead to premature deterioration or failure.
Movement Joints (Control and Construction Joints)
Concrete materials by nature shrink, so control joints (partial-depth cuts) are necessary and should be installed in the fresh concrete while it’s still green – timing is critical. Joint spacing and cut depth should be calculated and planned based on the size and thickness of the concrete. Shrinkage cracks form at these notches to control where cracking occurs. These joints can be filled with materials suitable for the application so that edge raveling is not created from areas that are loaded (for example, a warehouse with forklift traffic). Construction joints allow for large portions of concrete to have some movement and are formed full depth. Cold joints, on the other hand, are not considered movement joints but rather are the bonded plane between 2 separate pours of concrete.
Finishing
Timing and technique for finishing concrete is important so that the top surface is durable. Over finishing can trap bleed water and lead to scaling. Textures, such as broom, float, or trowel can be achieved. Flatness and moisture content should meet the requirements of the applied finish materials, if applicable.
Aesthetics
Rubber templates or stamping can be used prior to initial set of the concrete to alter the texture and make the concrete look like brick, stone, wood, or other materials. It should be noted, however, that while concrete can be made to look like other masonry materials, its performance and long-term durability vary greatly from unit masonry systems. Pigments or dyes can also be added to concrete mixes to create a variety of shades and colors; topical stains can be applied when hardened.
Curing
Curing procedures commonly include wet methods or a spray applied coating application. Moist curing helps limit the amount of moisture in the fresh concrete from evaporating quickly and creating shrinkage cracks. It also enables the mix to mature at a rate that develops higher long-term strength. This step for concrete work is critical.
Concrete Coatings and Traffic Bearing Membranes
Coatings are often applied to concrete as added protection from the environment and traffic. Vertical surface coatings can range from acrylic, polyurethane, mineral silicate, silane/siloxane, to cementitious. Horizontal, often high foot or vehicular traffic areas are coated with traffic bearing membrane systems, which can be polyurethane, polyurea, epoxy, methyl methacrylate, or cementitious. Interior concrete flooring surfaces may also use any of the above listed coating types, however, it’s critical that moisture content and vapor drive be tested in conjunction with interior coating applications. Mockups and pull-off testing are recommended as well as quality assessment during the project.
Precast concrete is a product often poured into reusable forms at a manufacturing facility and transported to the project site after it’s been cured and developed strength. These products are used as decorative and functional exterior wall panels, which are installed on the structure with anchors. Joints between panels are often sealed with a sealant joint.
Often, mix designs for precast use quicker setting portland cement so that forms can be stripped every 12 hours or less. Aggregates are smaller to be appropriate for the form thickness. Mixes can be wet-cast or dry-cast. Dry tamp cast stone has zero slump, while the wet cast mix includes a higher water content with 3 to 4 inches of slump. Because dry cast concrete mixtures have a minimal amount of cement paste, shrinkage potential is reduced.
Applying fresh concrete or mortar mixtures to a substrate through a nozzle with pneumatic pressure is known as shotcrete. This application of concrete is done in place at the project site and used to line walls, stabilize excavations, underground parking structures, and other structural walls. Mixes can be applied both pre-mixed with water (wet-mixed) and dry materials mixed with water at the nozzle (dry-mixed).
The mix design should be designed to be appropriate for the project and installation should be performed by a qualified applicator. The pressure, distance from the substrate, angle direction, and moisture content are all critical in making sure that the mortar or concrete material achieves consolidation, bond to the substrate, and long-term durability. Mockup shotcrete panels are critical for these projects to meet the project goals.
