Cast-in-Place Concrete Construction

Cast-in-place (CIP) concrete construction builds the structure by constructing formwork on site, placing reinforcing steel and/or post-tensioning tendons, pouring concrete into the forms, and stripping the forms after the concrete achieves adequate strength. CIP concrete can create virtually any structural shape and is the structural system of choice for many building types — particularly high-rise residential buildings, parking garages, hospitals, and any structure requiring monolithic construction, inherent fire resistance, or architectural concrete finishes.

This lesson covers the three essential components of CIP concrete construction: formwork, reinforcement, and concrete placement.

Formwork is the temporary structure that shapes and supports the concrete until it achieves sufficient strength to support itself. Formwork is typically the single largest cost component of CIP concrete construction — often 40–60% of the total concrete cost. Efficient formwork design and reuse are critical to project economy.

Wall Formwork

• Gang forms (panel forms): Large, pre-assembled form panels (typically 8'×20' to 8'×30') made of plywood or steel sheathing on a steel frame. Gang forms are assembled on the ground, lifted into place by crane, and stripped as a unit after the pour. They are designed for multiple reuses (20–100+ cycles with proper maintenance). Gang forms are the standard for walls in multi-story construction.
• Modular forming systems: Proprietary systems of interlocking aluminum or steel panels (e.g., PERI, Doka, EFCO) that can be configured for any wall shape. Quick to assemble and strip, with very high reuse counts.
• Single-sided forms: Used when one side of the wall is against earth (basement walls, retaining walls). Ties to the opposite side are replaced by bracing to the floor slab or other structure.

Floor (Slab) Formwork

• Shoring and reshoring: Vertical supports (posts, typically aluminum or steel) that carry the weight of the freshly poured floor slab and the forms above until the concrete cures. Shoring supports the slab directly; reshoring is the reinstallation of shores under previously stripped floors to share the load from upper floors being poured. Multiple levels of reshoring may be required for multi-story construction (typically 2–3 floors of shores/reshores below the floor being poured).
• Flying forms (table forms): Large, pre-assembled floor form assemblies (deck + beams + shoring) that are stripped as a unit and "flown" by crane from one floor to the next. Extremely efficient for repetitive floor layouts (high-rise residential towers with identical floor plans). A flying form cycle can achieve one floor per week.
• Flat plate forms: Simple plywood decks on joists supported by adjustable shores. The most basic and flexible system, used for irregularly shaped floors.

Specialized Formwork

• Slip forms: A continuously moving formwork system used for tall, uniform structures (elevator cores, silos, chimneys, towers). The form moves upward at a rate of 6–12 inches per hour, with concrete continuously placed at the top and hardened concrete emerging at the bottom. Slip forming can construct a concrete core at a rate of one floor per day.
• Climbing forms (jump forms): Formwork systems that are attached to the previously cast concrete and jacked or crane-lifted to the next level. Used for high-rise core walls and other tall vertical elements. Climbing forms provide a protected working platform for workers.
• Self-climbing forms: Climbing forms with integrated hydraulic jacks that lift themselves — no crane required for form movement. Used on super-tall buildings where crane access to the core is limited.

Concrete is strong in compression but weak in tension (approximately 10% of its compressive strength). Reinforcing steel (rebar) is embedded in the concrete to carry the tensile forces that the concrete cannot.

Rebar Basics

• Material: ASTM A615 Grade 60 (yield strength 60,000 psi) is the standard rebar in the United States. Grade 40 is available but rarely used. Grade 80 is used for special applications.
• Bar sizes: Designated by the number of 1/8-inch increments in diameter:

Bar Size	Diameter (in)	Area (in²)	Weight (lb/ft)
#3	0.375	0.11	0.376
#4	0.500	0.20	0.668
#5	0.625	0.31	1.043
#6	0.750	0.44	1.502
#7	0.875	0.60	2.044
#8	1.000	0.79	2.670
#9	1.128	1.00	3.400
#10	1.270	1.27	4.303
#11	1.410	1.56	5.313

• Deformations: Rebar has raised ribs (deformations) on its surface that mechanically grip the concrete, creating the bond that transfers forces between the steel and concrete.

Rebar Placement

Rebar must be placed in the exact location shown on the structural drawings. The critical dimensions are:

• Cover: The minimum distance between the rebar surface and the nearest concrete surface. Cover protects the rebar from corrosion and fire. Typical minimum covers: 1.5" for beams and columns, 0.75" for slabs, 3" for concrete cast against earth.
• Spacing: The distance between adjacent bars, both for structural effectiveness and to allow concrete to flow around the bars.
• Lap splices: Where two bars overlap to transfer force between them. Lap splice length depends on bar size, concrete strength, and bar spacing — typically 30–60 bar diameters.
• Development length: The minimum embedded length of a bar needed to develop its full strength in the concrete.

Rebar is held in position using:

• Chairs and bolsters: Wire or plastic supports that hold the rebar at the correct height above the form
• Tie wire: Steel wire used to tie intersecting bars together at their crossing points
• Spacers: Plastic or concrete blocks that maintain the correct cover from the forms

Inspection of rebar placement occurs before the concrete pour. The inspector verifies bar sizes, spacing, cover, lap lengths, tie-down, and support per the approved drawings.

Post-tensioning (PT) is a method of prestressing concrete after it has hardened. High-strength steel strands (tendons) are placed in plastic or metal ducts within the concrete before casting. After the concrete cures to a specified strength (typically 3,000–4,000 psi), the tendons are stressed (pulled) with hydraulic jacks and anchored at the concrete edges. This places the concrete in compression, counteracting the tensile stresses from loading and allowing thinner, longer-spanning slabs.

PT advantages:

• Longer spans (30–45 feet or more) with thinner slabs (typically 7"–9" for spans that would require 10"–14" of conventional reinforced concrete)
• Reduced concrete volume and building weight
• Fewer columns (longer spans = fewer intermediate supports)
• Better crack control (precompression keeps concrete in compression under service loads)
• Reduced floor-to-floor height (thinner slabs)

PT construction sequence:

1. Install formwork and conventional rebar (mild steel)
2. Place PT tendons in their ducts, profiled per the structural drawings (tendons are draped in a parabolic profile, low at midspan and high at supports)
3. Pour concrete
4. Wait for concrete to reach specified strength (typically 3–7 days)
5. Stress tendons from the slab edges with hydraulic jacks
6. Cut excess strand and patch the anchor pockets
7. Grout the ducts (for bonded PT) or leave ungrouted (for unbonded PT — which uses greased, plastic-coated strands)

Critical PT construction warning: Post-tensioned slabs contain highly stressed steel tendons. Cutting, coring, or drilling into a PT slab without locating the tendons first can sever a tendon, releasing enormous stored energy — potentially causing injury and structural damage. PT slabs are typically marked with warning stamps in the concrete and identified on structural drawings.

Placement Methods

• Concrete pump: The most common placement method for commercial construction. A truck-mounted or trailer-mounted pump pushes concrete through a pipeline (typically 4"–5" diameter) to the point of placement. Boom pumps have articulating booms (up to 200+ feet reach) that can place concrete at virtually any location on the floor. Pumping rates of 50–150 cubic yards per hour are common.
• Crane and bucket: A concrete bucket (1–4 cubic yard capacity) is lifted by crane and positioned over the pour location. The bucket is opened by the operator on the ground (via a remote release). Slower than pumping but useful when pump access is limited or for small pours.
• Direct chute: For slabs on grade and other ground-level pours, concrete can be discharged directly from the ready-mix truck's chute into the forms.
• Conveyor belt: Used for high-volume, continuous pours (dams, massive foundations).

Placement Procedures

1. Pre-pour checklist: Verify formwork, rebar, PT tendons, embedded items, blockouts, and MEP sleeves are complete and inspected. Verify the concrete mix design matches the specifications.
2. Pour concrete: Place concrete continuously, in lifts if needed, starting at the farthest point from the pump and working back. Do not drop concrete more than 5 feet to prevent segregation.
3. Consolidation: Use internal vibrators (immersion vibrators, "stinger") to consolidate the concrete — removing trapped air, filling voids, and ensuring concrete flows around all rebar and embedments. Over-vibration can cause segregation (heavy aggregate sinks, lighter paste rises).
4. Screeding: Strike off the excess concrete to the correct elevation using a screed bar, laser screed machine, or vibrating screed.
5. Finishing: Bull-float the surface, then finish as specified — broom finish, trowel finish, or exposed aggregate. Power troweling (using a ride-on or walk-behind power trowel) is standard for commercial floor slabs.
6. Curing: Begin curing immediately after finishing. Methods include wet curing (ponding water, wet burlap, or soaker hoses), membrane curing (spray-applied curing compound), or covering with polyethylene sheeting. Curing must continue for a minimum of 7 days. Proper curing is essential for achieving the specified concrete strength and minimizing shrinkage cracking.