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Chapter 6
Concrete
Master formwork systems, cast-in-place methods, quantity takeoff, and precast construction — from materials to real-world estimating problems.
Formwork Materials & Methods
Understanding the temporary support systems that shape concrete structures
Concrete is a mixture of portland cement, fine and coarse aggregate, water, and sometimes chemical admixtures that solidify through a chemical reaction called hydration. Reinforced concrete adds steel rods, bars, or mesh to increase tensile strength.
Key Terminology — Click each card to reveal the definition
Formwork
The complete system that supports fresh concrete while it hardens.
Formwork
The entire system of support for fresh concrete including the forms, hardware, and bracing. Many methods are used for cast-in-place concrete. The formwork type depends on the overall size of the concrete structure.
Form
A temporary mold for holding concrete while it sets.
Form
A temporary structure or mold used to retain and support concrete while it sets and hardens. Forms may be set on or in the ground for piles, footings, and slabs, or elevated for above-grade slabs.
Footing
Foundation element that transfers loads to the soil.
Footing
The section of a foundation that supports and distributes structural loads directly to the bearing surface of the soil, piles, or rock below the structure.
Waler
A horizontal bracing member for wall forms.
Waler
A horizontal member used to align and brace concrete forms or piles. Walers keep forms aligned and resist deflection during concrete placement.
Strongback
Vertical support attached behind horizontal walers.
Strongback
A vertical support attached to concrete formwork behind the horizontal walers to provide additional strength against form deflection during concrete placement.
Form Tie
Metal bar holding two wall forms apart at a set distance.
Form Tie
A metal bar, strap, or wire used to hold concrete forms together at a predetermined distance and resist the outward pressure of fresh concrete. Types include snap ties, she bolts, and coil ties.
Snap Tie
A form tie designed to snap off after form removal.
Snap Tie
A concrete form tie with ends designed to be snapped off after the forms are removed and the concrete is set. Common in residential and light commercial construction.
ICF
Insulating Concrete Form — stays in place permanently.
Insulating Concrete Form (ICF)
A hollow foam expanded polystyrene (EPS) block used as a wall form. ICFs are created from a series of interlocking foam block forms, stay in place permanently, and provide additional insulation to the concrete foundation wall.
Blockout
A frame creating a void in finished concrete.
Blockout
A frame set between concrete form walls to create a void in the finished concrete structure. Blockouts are placed in wall forms to create openings for later placement of doors, windows, and other building items.
Wall Form Systems
Cast-in-place concrete walls are constructed on footings or slabs. Two main types:
- Job-Built Wall Forms: Constructed with 2×4 framing members faced with ½” or ¾” plywood. Panels typically 4’×8′. Studs spaced 12″ or 16″ OC. Holes drilled at regular spacing for snap ties.
- Patented Wall Forms: Manufactured with metal frames faced with plywood or metal. Available in standard sizes. Tie systems vary by manufacturer. Can be ganged into large panel units for crane placement and reuse.
Slab Types
- Slab on Grade: Placed directly on ground. Minimal formwork (just perimeter forms). Grade beam supports perimeter against freeze/thaw movement.
- Elevated Slabs: Require shoring and decking. Three construction methods:
- Monolithic design (beams and slab poured together)
- Precast concrete or structural steel beam support
- Steel beams with corrugated decking
Formwork must be constructed using safe construction procedures such as those established by the American Concrete Institute (ACI). Shoring and bracing costs are significant for elevated slab construction.
Formwork Quantity Takeoff
Calculating panels, snap ties, studs, walers, and ICFs
Key Measurement: SFCA
Formwork is measured in Square Feet of Contact Area (SFCA) — the area where the form face actually touches the concrete. For walls, this is calculated for both sides.
Contact Area (one side) = Height × Length
Total Contact Area = Height × Length × 2
Wall: 8′ high × 22′ long
Contact area = 8 × 22 = 176 sq ft
Panels one side = 176 ÷ 32 = 5.5 → use 6 panels
Panels both sides = 6 × 2 = 12 panels
Snap ties at 24″ OC both ways on 8″ thick wall:
Each 4’×8′ panel = 32 sq ft → requires 16 ties (1 per 2 sq ft)
176 sq ft contact area ÷ 2 sq ft per tie = 88 ties
Total sq ft ÷ 4 = number of snap ties (total SFCA ÷ 4 sq ft/tie)
Studs one side = [(22 ÷ 1.33′) × 2] + 2 = 35 studs
Studs both sides = 35 × 2 = 70 studs
Rows = 8 ÷ 2 = 4 rows
Walers per row = 4 × 2 = 8 walers
LF one side = 8 × 22 = 176′
LF both sides = 176 × 2 = 352 LF
Total surface = 8 × 26 = 208 sq ft
Total ICFs = 208 ÷ 5.33 = 39
Corner ICFs = (96″ ÷ 16″) × 1 = 6
Wall ICFs = 39 − 6 = 33; + 5% (≈2) = 35 wall ICFs
PROBLEM 6.2-A
Determine wall form panel count for an elevator pit- Calculate total contact area (one side) for all four walls
- Divide by 32 sq ft (area of one 4’×8′ panel)
- Round up to the nearest whole panel
Cast-in-Place Concrete Materials
Concrete composition, cement types, aggregate, admixtures, and reinforcement
Concrete = Portland Cement + Fine Aggregate + Coarse Aggregate + Water ± Admixtures
Aggregate represents approximately 70% of concrete’s total volume.
Overall strength is stated as psi (pounds per square inch) at 28 days.
Portland Cement Types
Admixtures
Substances added to concrete to alter properties such as air content, setting time, freeze resistance, and workability. Key types:
- Air Entrainment: Creates microscopic air pockets that increase workability and freeze-thaw resistance
- Water Reducers (Plasticizers): Reduce water needed while maintaining workability
- Accelerators: Speed up setting time for cold weather or fast-track work
- Retarders: Slow setting time for hot weather or long transit distances
Water-Cement Ratio
The weight of water divided by the weight of cement in one cubic yard of concrete. Lower w/c ratio = stronger concrete. The proper amount of water is needed to fully hydrate the cement; excess water weakens the mixture. A slump test measures the consistency (slump) of concrete — concrete slump can indicate water content.
Reinforcement
| Bar # | Diameter (in) | Weight (lb/ft) | Area (sq in) |
|---|---|---|---|
| #3 | 0.375 | 0.376 | 0.11 |
| #4 | 0.500 | 0.668 | 0.20 |
| #5 | 0.625 | 1.043 | 0.31 |
| #6 | 0.750 | 1.502 | 0.44 |
| #7 | 0.875 | 2.044 | 0.60 |
| #8 | 1.000 | 2.670 | 0.79 |
| #9 | 1.128 | 3.400 | 1.00 |
| #10 | 1.270 | 4.303 | 1.27 |
| #11 | 1.410 | 5.313 | 1.56 |
Bar # × ⅛” = diameter. e.g. #4 bar = 4 × ⅛” = ½” diameter
| Grade | Min Yield (psi) | Common Use |
|---|---|---|
| Grade 40 | 40,000 | Light residential, footings |
| Grade 50 | 50,000 | General structural |
| Grade 60 | 60,000 | Most common commercial/structural |
| Grade 75 | 75,000 | High-strength applications |
Epoxy-coated rebar is used in exterior applications (roads, bridges) to minimize corrosion effects.
Welded Wire Reinforcement (WWR)
Rolls or mats of welded wire stretched out and set in place to reinforce slabs (sidewalks, driveways, floor slabs). Designated by W number (wire spacing and cross-sectional area). Yield strength: 60,000–70,000 psi. Calculated in square feet of slab area.
| New Designation | Old Designation | Spacing | Weight (lb/100 sq ft) |
|---|---|---|---|
| 6×6–W1.4×W1.4 | 6×6–10×10 | 6″×6″ | 21 |
| 6×6–W2.0×W2.0 | 6×6–8×8 | 6″×6″ | 29 |
| 4×4–W2.0×W2.0 | 4×4–8×8 | 4″×4″ | 43 |
| 4×4–W4.0×W4.0 | 4×4–4×4 | 4″×4″ | 85 |
Cast-in-Place Concrete Methods
Placement, finishing, and labor cost factors
Concrete Delivery Methods
- Transit-mix (Ready-mix) truck: Mixes and delivers concrete to job site. Most economical when truck has direct access.
- Concrete pump: Used when forms are far below/above grade or cannot be accessed by transit-mix truck. Prevents long drops that cause separation of cement paste and aggregate.
- Crane and bucket: For remote placements where pumps cannot reach.
- Motorized buggy / wheelbarrow: For slabs on grade where access is limited.
For most foundation walls, concrete can be discharged from the transit-mix truck directly into wall forms using a chute. Where walls are far below or above grade, a concrete pump is used.
Wall Finishes
- Smooth: Surface rubbed and stoned smooth after form removal; voids patched.
- Rough-textured: Produced by inclusion of decorative aggregate and/or sandblasting.
- Architectural: Wood, metal, or plastic form liners inside the form create patterns (grooves, brick pattern, etc.).
Slab Finishes
- Broom finish: Common for sidewalks and roadways.
- Trowel finish: Smooth, used for most interior floor slabs.
- Exposed aggregate: Decorative aggregate included; cement paste washed off.
- Superflat: For industrial floors; laser screeds used during finishing.
- Patterned: Rubber or metal patterns to create brick/stone look. Can cost 25–100% more than standard slab.
Forming Labor Costs Include:
- Transporting and distributing concrete formwork to the proper location on the job site
- Erection of the forms
- Blockout installation
- Application of form-release agents
- Bracing and securing the forms
- Form removal, cleaning, and transportation
Concrete Placement Labor Includes:
- Transportation of concrete to forms
- Spreading and screeding to proper height and thickness
- Consolidation through vibration
- Initial surface finishing
- Additional surface treatments (rubbing, sandblasting, sawing control joints)
- Weather protection installation/removal (if required)
Cast-in-Place Concrete Quantity Takeoff
Calculating concrete volume for walls, slabs, columns, and stairs
All concrete volume is expressed in cubic yards (CY). Since standard measurements are in feet and inches, divide cubic feet by 27 to convert (1 CY = 27 CF). Add 2–5% waste for spillage; up to 10% waste for footings in contact with earth.
V = (0.67′ × 7.5′ × 32′) ÷ 27 = 160.8 ÷ 27 = 5.96 CY
V = (10 × 20 × 0.5) ÷ 27 = 100 ÷ 27 = 3.7 CY
V = (2 × 2 × 8.5) ÷ 27 = 34 ÷ 27 = 1.26 CY
V ≈ 10 × 3.5 = 35 CF → 35 ÷ 27 = 1.3 CY
PROBLEM 6.5-A
Foundation wall concrete volume- Convert 10″ thickness to feet
- Multiply thickness × height × length to get cubic feet
- Divide by 27 for cubic yards
- Apply 3% waste factor
PROBLEM 6.5-B
Slab on grade volumePROBLEM 6.5-C
Column volumePrecast Concrete Materials
Beams, slabs, walls — types, prestressing, and identification
What is Precast Concrete?
Precast concrete members are cast off-site at a plant (or on-site before erection) and then lifted into position. Used for beams, walls, slabs, piles, and other structural applications. They require special reinforcement for transportation stresses in addition to structural loads.
Pretensioning
- Steel cables (tendons) are tensioned before concrete is placed
- After concrete sets, tension is released, compressing the beam
- Tendons are in tension before concrete is placed
- Common in plant precast (standardized)
Post-tensioning
- Cables are tensioned after concrete has hardened
- Cable ducts cast into the concrete; cables added after set
- Used for both precast and cast-in-place applications
- Allows on-site adjustability
Precast Beam Profiles
- Rectangular: Most common; easy to form and cast. Used for general applications.
- L-shaped: Used as perimeter/spandrel beams around openings and for spandrels.
- Inverted-T (IT): Used for spanning between column caps, beams, or girders. Can be doubled side-by-side for bridge beams and road surfaces.
Precast Slabs
- Hollow Core (HC): Designation uses HC prefix (e.g., 4HC8 = 4′ wide, 8″ thick hollow core). Void cores reduce weight. Used for above-grade floor and roof panels.
- Solid Core (FS — Flat Slab): Designation uses FS prefix (e.g., FS6 = solid flat slab, 6″ thick). Heavier but simpler to fabricate.
- Types and sizes noted on floor plans, elevations, and panel schedules.
Three Primary Types of Precast Wall Panels
- Structural: Most commonly cast on the job site and tilted up into place (tilt-up construction).
- Finish: Cast off-site with various concrete colors, aggregate sizes/types, and surface finishes. Typically a separate panel schedule is provided.
- Curtain Wall: A non-load-bearing prefabricated panel suspended on or fastened to structural members. Often cast off-site with various finishes.
Precast Concrete Construction Methods
Plant precast vs. site precast, tilt-up, and lift-slab construction
Precasting at Plant
- Concrete pipe, curtain walls, beams, pavers cast off-site
- Transported to job site and placed by crane
- Estimators determine from prints if standard-size or special casting is required
- Costs stated as cost per unit from casting plant (include transportation)
Precasting On-Site
- Members too large to transport are cast on-site
- Minimizes formwork and concrete movement
- Many panels cast directly on the floor slab of the building
- Methods include Tilt-Up and Lift-Slab construction
Tilt-Up Construction
Concrete members are cast horizontally on or near their final position, then tilted vertically and placed after forms are stripped. Common for low-rise concrete structures of 1–3 stories. Steps:
- First floor slab set → low forms built on slab for tilt-up panel perimeter
- Bond breaker applied to slab → reinforcement, lift anchors, and blockouts set in place
- Concrete placed and finished
- After concrete reaches sufficient strength, crane with spreader bar lifts panel into final position
- Panels braced → floors and roofs tie panels together
- Panels fastened using flush-cast pilasters, cast-in-place columns, precast columns, steel columns, or flush steel plates
Lift-Slab Construction
Concrete slabs are cast on top of each other, then lifted using jacks to the proper elevation and fastened to columns and beams. Minimizes shoring costs compared to conventional elevated-slab construction. Steps:
- Forms and support for lowest slab set on grade → bond breaker applied
- Series of slabs cast on top of each other with blockouts for columns
- After concrete sets, slabs jacked to proper elevation and fastened to supporting columns and beams
- Reinforcing steel and lifting anchors detailed on the detail drawings
Forming costs are generally lower for tilt-up panels than cast-in-place walls (high walls can be cast with minimal forming expense). However, lifting costs include crane ownership/rental, operator expenses, bracing and attachment costs. For large projects, panels of the same design can be scheduled to minimize duplication in the takeoff.
Precast Concrete Quantity Takeoff
Plant-precast costs, site-precast material quantities, tilt-up panels, and lift slabs
Plant-Precast Members
- Consult casting plant for costs per unit
- Costs include: concrete, forming materials, labor
- Stated for the estimator as a cost per unit
- May include transportation to job site
- Placement costs (lifting, setting, joining) added separately
Site-Precast Members
- Concrete volume, reinforcement, and surface finish calculated same as CIP
- Differences vs. CIP: forming costs, lifting, bracing, and joining costs
- Tilt-up panel concrete = same as CIP slab (finishing costs similar)
- Lift slab: calculate concrete volume, reinforcement, and finishing same as slab on grade; add lifting equipment costs
Tilt-Up Panel Additional Expenses
- Lifting costs: crane ownership or rental, operator expenses
- Temporary bracing costs until support columns are set and roof is in place
- For large projects, create a panel schedule to track identical panels and minimize takeoff duplication
Lift Slab Additional Expenses
- Additional reinforcement for lifting the slab
- Jacking and lifting-equipment operations
- Shoring during placing and setting of support columns
PROBLEM 6.8-A
Tilt-up panel concrete volumeChapter 6 Quiz
Test your knowledge across all sections — 20 questions
Glossary of Terms
All key vocabulary from Chapter 6 — Concrete
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