Formwork and Reinforcement, Concrete

Listen

Formwork Design and Safety Requirements

Concrete formwork is a temporary structure designed to contain fresh concrete until it reaches self-supporting strength. Under Cal/OSHA Title 8 Section 1717, formwork must be designed, braced, and maintained to safely support all vertical and lateral loads. For projects where the shoring height exceeds 14 feet or where individual form panels exceed 600 square feet, the design must be prepared by a California registered civil engineer. The lateral pressure exerted by fresh concrete is a critical calculation dictated by ACI 347R. This pressure is influenced by the rate of placement, the temperature of the mix, and the weight of the concrete (typically 150 pounds per cubic foot). Contractors must ensure that form ties have a factor of safety of at least 2:1. When using wood for formwork, the National Design Specification (NDS) governs the allowable stresses for species and grades. All forms must be coated with a release agent before reinforcement is placed to ensure clean removal without damaging the green concrete surface. Proper alignment is verified using string lines and plumbs, with tolerances usually restricted to 1/4 inch over a 10-foot span.

Vertical Shoring and Form Removal

Proper shoring and reshoring are vital for the structural integrity of multi-story concrete buildings. Cal/OSHA Title 8 Section 1717(b) requires that shoring equipment be inspected prior to use and that any damaged components, such as bent jacks or cracked timber, be removed from service immediately. Sills for shoring must be sound, rigid, and capable of carrying the maximum intended load; they should never be placed on frozen ground or uncompacted fill. When stripping forms, contractors must follow CBC Chapter 19 and ACI 318 guidelines. Forms supporting the weight of concrete, such as those under beams or slabs, must remain in place until the concrete has attained sufficient strength, generally at least 70% of the specified 28-day compressive strength (f'c). For a standard 3,000 psi mix, stripping should not occur until the field-cured cylinders indicate at least 2,100 psi. Reshoring is required when the slab being stripped is not yet capable of supporting the construction loads from the floors above. This process ensures that the load is distributed across multiple levels of the structure.

Reinforcing Steel Placement and Cover

Reinforcing steel, or rebar, provides the tensile strength that concrete lacks. Steel placement must strictly adhere to the structural drawings and CBC Chapter 19. The most common grade of rebar used in California is Grade 60, signifying a yield strength of 60,000 psi. Positioning is critical; reinforcement must be secured against displacement by using wire ties, chairs, and spacers. For concrete cast against and permanently exposed to earth, such as footings, a minimum of 3 inches of concrete cover is required to protect the steel from corrosion. For concrete exposed to earth or weather, like exterior walls, the cover requirement is 2 inches for #6 through #18 bars and 1.5 inches for #5 bars and smaller. In protected environments, such as interior slabs, the cover may be reduced to 0.75 inches for #11 bars and smaller. These clearances ensure the alkaline environment of the concrete protects the steel from oxidation and provides the necessary fire rating according to CBC Table 721.1. Any movement of the rebar during the pour can lead to structural failure or premature spalling.

Splicing, Ties, and Seismic Detailing

Splices are necessary when the length of a structural member exceeds the standard 60-foot length of a rebar shipment. ACI 318 defines two types of lap splices: Class A and Class B. A Class A splice requires a lap length of 1.0 times the development length, while a Class B splice requires 1.3 times the development length. In no case shall a lap splice be less than 12 inches. For #11 bars and larger, lap splices are generally prohibited in tension zones; mechanical couplers or welding must be used instead. If welding rebar, California contractors must comply with AWS D1.4 standards, and the carbon equivalent of the steel must be verified to ensure weldability. Stirrups and ties are used to provide shear strength and to confine longitudinal bars. For columns, #3 ties are required for longitudinal bars up to #10, and #4 ties are required for #11 and larger. The tie spacing must not exceed 16 longitudinal bar diameters, 48 tie diameters, or the least dimension of the column. This confinement is essential for seismic resistance in California's high-risk zones.

Pre-Placement Inspection and Preparation

Before the concrete pour begins, the contractor must perform a final inspection of the forms and reinforcement. This includes removing all debris, such as sawdust, tie wire scraps, and wood chips, from the inside of the forms using compressed air or water. CBC Section 1905 requires that reinforcement be free from mud, oil, or other nonmetallic coatings that decrease bond strength. While light rust may actually improve the bond between the steel and the concrete, heavy scale that flakes off must be removed. The moisture content of the formwork is also important; wood forms should be wetted down before pouring to prevent them from absorbing water from the concrete mix, which can weaken the interface. Vapor retarders, if required under interior slabs-on-grade, must be at least 10 mil thick per ASTM E1745 and have all seams lapped at least 6 inches. Verification of anchor bolt placement for sill plates is also vital; per CBC 2308.3.1, bolts must be at least 1/2 inch in diameter, spaced no more than 6 feet apart, and embedded at least 7 inches into the concrete.

Concrete Placement and Consolidation

Effective concrete placement techniques are necessary to prevent segregation of the aggregate from the cement paste. According to ACI 304R, concrete should be placed as close to its final position as possible and should not be moved horizontally over long distances with vibrators. The maximum vertical drop for concrete should not exceed 5 feet; for deeper forms, a tremie or "elephant trunk" must be used to direct the flow. Consolidation is typically achieved through mechanical vibration. Internal vibrators should be inserted vertically at uniform intervals and penetrate at least 6 inches into the previous lift to ensure a monolithic bond. The vibrator should be withdrawn slowly once air bubbles stop appearing and the surface takes on a glistening appearance. Over-vibration must be avoided as it causes heavy aggregates to sink and grout to rise, leading to "honeycombing" or sand streaks. In California, special inspections are often required for concrete with a design strength over 2,500 psi, ensuring that the water-cement ratio and slump remain within the limits specified by the engineer.