South Carolina Concrete

6 questions | 12% of exam | Reference: 29 CFR 1926 (OSHA Construction)

Key Concepts

Personal Protective Equipment (PPE)

• Per §1926.20(f)(1), PPE requirements impose a separate compliance duty for each employee. Failing to provide PPE to even one worker is a separate violation.
• Concrete work requires: hard hats, safety glasses/goggles, gloves (cement is caustic — pH ~12–13), rubber boots, and respirators where silica dust is present.
• Silica (crystalline): Action Level = 25 µg/m³ TWA; PEL = 50 µg/m³ TWA (8-hour) per §1926.1153. Employers must implement a written exposure control plan when workers are exposed at or above the action level.

Fall Protection – Formwork & Reinforcing Steel

• Fall protection required at 6 feet above a lower level for construction workers (§1926.502).
• Acceptable systems: guardrail systems, safety net systems, or personal fall arrest systems (PFAS).
• Reinforcing steel (rebar): Employees working more than 6 feet above ground on rebar must be protected. Impalement protection required — cap or bend exposed rebar ends.

Formwork and Shoring Safety (§1926.703)

• Formwork must be designed by a qualified designer and able to support all anticipated loads.
• Shoring must be inspected by a competent person before, during, and after concrete placement.
• Do not remove shoring until concrete has achieved adequate strength to support its own weight and imposed loads — as specified by the engineer of record.
• Reshoring plans must be developed when stripping forms early.

7 questions | 14% of exam

Key Concepts

Soil Types and Bearing Capacity

• Bearing capacity = soil's ability to support load without excessive settlement. Depends on: composition, confinement, and moisture content.
• Strongest soils: compacted gravel mixed with sand or clay (hardpan) — excellent bearing for most foundations.
• Compacted loam (sand + silt) at considerable depth = good bearing.
• Clay is unreliable: firm when dry, expands and weakens when wet. Expansive clays require geotechnical evaluation.
• Dry sand or wet sand confined = generally acceptable; porous sandy soils wash away easily.
• Marshy/quick soils = typically require pile foundations.
• Solid rock = best bearing surface, but partial bearing on rock risks uneven settlement.

Geotechnical Investigations and Soil Testing

• Open test pits = reliable for shallow exploration; allow visual inspection of soil layers and field testing.
• Power augers and drilling rigs = used for deep samples and to locate the water table elevation.
• Laboratory soil testing required for heavy construction and deep excavations.
• Geotechnical engineer's report must be reviewed before construction begins — compare actual site conditions to report descriptions.
• If site is on fill: determine quality of material and degree of compaction before proceeding.

Key Concepts

Mixture Design & Proportioning

• Water-cementitious material (w/cm) ratio is the single most important factor controlling concrete strength and durability. Lower w/cm = higher strength. Typical structural concrete: 0.40–0.50 w/cm.
• Slump measures workability. Standard slump for most structural concrete: 3–4 inches. Pumped concrete may run 5+ inches. Increasing water to boost slump without adjusting cement destroys the w/cm ratio.
• Air entrainment is required for freeze-thaw exposure. Total air content typically 4–7% depending on aggregate size and exposure class. Air entrainment reduces strength ~5% per 1% air added.
• Admixtures: Water reducers maintain workability without adding water. Accelerators speed set in cold weather. Retarders slow set in hot weather. Fly ash, slag, and silica fume are supplementary cementitious materials (SCMs) that replace a portion of portland cement.
• Strength testing: Standard cylinders are 4×8 in. or 6×12 in., cured 28 days. A strength test = average of two cylinders from the same sample. Per ACI 318, a strength test fails if: (1) any individual test falls more than 500 psi below f'c, or (2) the average of any three consecutive tests falls below f'c.

Reading Concrete Specifications

• Prescriptive specs dictate ingredients and proportions (e.g., cement content, max w/cm). Performance specs specify the end result (f'c, air content) and let the contractor choose the mix.
• Tolerances are mandatory — not guidelines. Know allowable tolerances for slump, air content, and placement dimensions per the project specification and ACI 117.
• Acceptance testing must be done at point of delivery, not at the pump or end of chute.

Key Concepts

Types of Reinforcement

• Deformed bars (rebar): Most common; deformations (ribs) improve mechanical bond with concrete. Used in beams, columns, slabs, walls.
• Welded Wire Reinforcement (WWR): Grid of wires; crosswires provide anchorage (hooks not required). Used in slabs and pavements.
• Fiber-Reinforced Concrete: Fibers (steel, synthetic, glass) dispersed throughout mix; controls plastic shrinkage cracking, does NOT replace structural rebar.
• Prestressing Tendons: Wires, strands, or bars placed under tension. Pretensioning = tendons tensioned before concrete placement. Post-tensioning = tendons tensioned after concrete hardens.

Reinforcing Bar Grades, Sizes, and Identification Markings

• Sizes: Bar number = approximate diameter in eighths of an inch (e.g., #4 bar = 4/8 = 1/2 in. diameter).
• Grades (yield strength): Grade 40 (40,000 psi), Grade 60 (60,000 psi — most common in structural work), Grade 75, Grade 80.
• Identification markings on bar: (1) Producer's mill mark, (2) Bar size number, (3) Steel type (S = billet, A = axle, R = rail, W = low-alloy), (4) Grade mark — Grade 60 has two lines or the number "4" (since 60 = 4 × 15); Grade 40/50 has one line or no grade mark.

Key Concepts

Formwork Definition (OSHA §1926.700): Formwork = the total system, including forms, shores, reshores, hardware, and braces — not just the form skin itself.

Formwork Loads: - Vertical dead load: Weight of concrete (150 pcf normal weight) + form weight - Vertical live load: Personnel, equipment, material storage (minimum 50 psf for design, or 75 psf if motorized buggies are used) - Lateral load: Wind, seismic, or minimum 100 lb/ft applied at top of form as a design floor - Load combinations: Dead + Live govern vertical; lateral pressure governs wall/column form design

ACI 347R Lateral Concrete Pressure — Key Factors: - Rate of placement (ft/hr) — faster placement = higher pressure - Concrete temperature — lower temp = slower set = higher pressure - Concrete unit weight - Admixtures (retarders increase pressure; accelerators decrease it) - Vibration depth (internal vibrators within 6 ft of bottom increase fluid head)

ACI 347R Pressure Equations (Wall Forms):

For walls, when rate of placement R ≤ 7 ft/hr:

p = C~w~ C~c~ [150 + 9000R/T] (max = w·h, min = 600 C~w~)

For rates between 7–15 ft/hr:

p = C~w~ C~c~ [150 + 43,400/T + 2800R/T]

Where: p = lateral pressure (psf), R = rate of placement (ft/hr), T = concrete temp (°F), C~w~ = unit weight coefficient, C~c~ = chemistry coefficient

For columns: Same form as the low-rate wall equation; maximum = full hydrostatic (wh).