14.1 Concrete Materials, Field Testing, Formwork & Reinforcing Steel

Concrete Materials & Mix Design Fundamentals

Concrete is a composite material formed by the chemical reaction—known as hydration—between hydraulic cement, water, fine aggregate (sand), and coarse aggregate (gravel or crushed stone). In commercial and industrial construction across Nevada, concrete specifications frequently mandate supplementary cementitious materials (SCMs) and specialized chemical admixtures to withstand harsh desert soils, intense ambient heat, and extreme freeze-thaw cycles.

                         CONCRETE VOLUMETRIC PROPORTIONS
  ┌────────────────────────────────────────────────────────────────────────┐
  │ Coarse Aggregate (Gravel/Stone)       Fine Aggregate (Sand)            │
  │ 38% – 48% Volumetric Fraction         25% – 35% Volumetric Fraction    │
  ├───────────────────────────────┬───────────────────────────────┬────────┤
  │ Portland Cement + SCMs        │ Mixing Water                  │ Air    │
  │ 7% – 15% Volumetric Fraction  │ 14% – 21% Volumetric Fraction │ 4%–8%  │
  └───────────────────────────────┴───────────────────────────────┴────────┘

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ASTM C150 Portland Cement Classifications

Cement selection dictates the rate of strength gain, heat evolution, and durability against chemical attack. ASTM C150 (Standard Specification for Portland Cement) defines five fundamental types:

• Type I (Normal / General Purpose): Used for general construction where special properties are not required (pavements, reinforced concrete buildings, bridges). It is susceptible to sulfate attack and high heat generation in mass placements.

• Type II (Moderate Sulfate Resistance & Moderate Heat): Formulated with a maximum tricalcium aluminate ($C_3A$) content of 8%. Used in drainage structures, piers, and heavy retaining walls exposed to moderate sulfate concentrations in soil or groundwater (ACI 318 Exposure Class S1: 0.10% to 0.20% water-soluble sulfate in soil, or 150 to 1,500 ppm in water).

• Type III (High Early Strength): Ground to a substantially finer particle size, accelerating hydration. Gains strength much faster than Type I; it commonly reaches in about 3 days the strength Type I concrete reaches in 7 days. Ideal for rapid formwork stripping, precast concrete fabrication, and cold weather placement where early freezing must be prevented.

• Type IV (Low Heat of Hydration): Limits $C_3A$ and tricalcium silicate ($C_3S$) to minimize the rate and total magnitude of heat generated. Engineered strictly for massive structural elements (dams, large bridge piers, mat foundations exceeding 5 to 6 feet in thickness) where internal thermal dissipation is slow and thermal gradient cracking would otherwise destroy structural integrity.

• Type V (High Sulfate Resistance): Formulated with a strict maximum $C_3A$ limit of 5%. Why it matters in Nevada: Many Southern Nevada soils contain gypsum and water-soluble sulfates. When sulfates reach the hydrated calcium aluminate in cement paste, they form expansive ettringite and gypsum. These products crack, soften, and spall the concrete. The project's geotechnical report tests the soil and groundwater, and the structural engineer assigns a sulfate exposure class under ACI 318-14 Table 19.3.1.1:

  • S1: 0.10% to under 0.20% water-soluble sulfate in soil (150 to under 1,500 ppm in water);
  • S2: 0.20% to 2.00% (1,500 to 10,000 ppm);
  • S3: over 2.00% (over 10,000 ppm).

  For S2, ACI 318 Table 19.3.2.1 calls for a maximum $w/cm$ of $0.45$, a minimum $f'_c$ of $4,500\ \text{psi}$, and Type V cement or an equivalent blended or performance cement. S3 is stricter: Type V plus pozzolan or slag cement at $0.45$, or Type V at $0.40$ and $5,000\ \text{psi}$. Type V is therefore common on Las Vegas Valley foundations, but the requirement comes from the tested exposure class, not from the county.

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The Water-Cementitious Materials Ratio ($w/cm$)

The water-cementitious materials ratio ($w/cm$) is the single most critical factor determining the ultimate compressive strength, density, and permeability of hardened concrete. First quantified by Duff Abrams in 1918 (Abrams' Law):

$w/cm = \frac{\text{Weight of Total Mixing Water}}{\text{Weight of Total Cementitious Materials (Cement + Fly Ash + Slag + Silica Fume)}}$

• Hydration Demand vs. Workability: Chemically complete hydration requires only about $0.23$ to $0.25\ \text{lb}$ of water per pound of Portland cement. However, unadmixed fresh concrete requires a $w/cm$ of approximately $0.45$ to $0.55$ to achieve adequate fluidity and workability for consolidation around reinforcing bars.

• Capillary Porosity: Any water added beyond the hydration requirement forms microscopic water channels. As this free water evaporates during curing, it leaves an interconnected network of capillary pores. Higher $w/cm$ ratios exponentially increase capillary porosity, drastically lowering compressive strength and facilitating the ingress of water, chlorides, and aggressive soil sulfates.

• Rule of thumb: Adding about 1 gallon of water per cubic yard raises slump by roughly 1 inch, but it can lower 28-day strength by roughly $200\ \text{psi}$ and raise shrinkage potential by about $10\%$. On a 10-cubic-yard truck, "one gallon per yard" means 10 gallons. Water may be added at the site only within the limits of the approved mix design and the ready-mix specification (ASTM C94), followed by additional drum mixing.

• Higher $w/cm$ Ratio: $\longrightarrow$ Lower Compressive Strength ($f'_c$), Higher Permeability, Shrinkage & Cracking.

• Lower $w/cm$ Ratio: $\longrightarrow$ Higher Compressive Strength ($f'_c$), Lower Permeability & Sulfate Penetration.

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Aggregates: Gradation and Sizing Rules

Aggregates constitute 60% to 75% of the total concrete volume and provide structural dimensional stability. Aggregates must meet ASTM C33 specifications for sound, clean, non-reactive materials.

• Gradation: Concrete requires a continuous, dense gradation ranging from coarse gravel down through fine sand. Proper gradation minimizes the void space between particles, reducing the volume of cement paste required to coat aggregates and bind the matrix.
• Maximum Aggregate Size Rules (ACI 318 § 26.4.2.1): To prevent honeycombing, voids, and aggregate bridging across narrow forms or reinforcement cages, the nominal maximum size of coarse aggregate must not exceed the smallest of:
  1. $1/5$ the narrowest dimension between sides of forms;
  2. $1/3$ the total depth of slabs or footings; OR
  3. $3/4$ the minimum clear spacing between individual reinforcing bars, bundles of bars, or prestressing tendons.

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Supplementary Cementitious Materials (SCMs)

SCMs replace a portion of Portland cement in the mix design, enhancing long-term durability and reducing material carbon footprints:

• Fly Ash (ASTM C618): A pozzolanic byproduct of pulverized coal combustion in power plants.
  • Class F Fly Ash: Low calcium content ($< 18\%$). Significantly mitigates Alkali-Silica Reaction (ASR), reduces early heat of hydration, and substantially improves sulfate resistance. Widely specified across Nevada civil works.
  • Class C Fly Ash: High calcium content ($> 18\%$). Provides early strength gain and cementitious properties, but offers inferior sulfate and ASR resistance compared to Class F.
  • Workability Effect: Spherical fly ash particles act as miniature ball bearings, increasing fresh concrete slump and pumpability while reducing required mixing water.
• Ground Granulated Blast-Furnace Slag (GGBFS / Slag Cement, ASTM C989): A glassy, granular byproduct of iron blast furnaces. Replaces 20% to 50% of Portland cement, generating higher ultimate 56-day and 90-day strengths, lower heat of hydration, and superior chemical resistance to chlorides and sulfates.
• Silica Fume (Microsilica, ASTM C1240): An ultrafine pozzolanic byproduct of silicon metal production, with particles roughly 100 times smaller than average cement grains. Densifies the microscopic interfacial transition zone between aggregate and paste. Yields extreme high-strength concrete ($f'_c \ge 10,000\ \text{psi}$) with near-zero permeability for parking structures, post-tensioned decks, and industrial floors subject to chemical wear.

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Chemical Admixtures

Admixtures are added immediately before or during mixing to modify the chemical or physical behavior of fresh or hardened concrete:

• Air-Entraining Agents (ASTM C260): Introduce billions of microscopic, discrete, closely spaced air voids (typically 4% to 7% of total volume, void spacing factor $< 0.008\ \text{inches}$). As free water in hardened concrete freezes, it expands by approximately 9%; these entrained voids provide pressure relief reservoirs, preventing internal hydraulic bursting. Mandatory in Northern Nevada (Reno, Sparks, Carson City, Lake Tahoe, Elko) where freezing temperatures and deicing salts are common. Note: Air entrainment is typically omitted from interior industrial floors receiving a hard steel-troweled finish, as troweling can cause delamination and blistering.
• Water-Reducing Admixtures (ASTM C494 Type A & Type F/G): Disperse cement agglomerates through electrostatic repulsion.
  • Normal Water Reducers (Type A): Reduce water demand by 5% to 10%.
  • High-Range Water Reducers (HRWR / Superplasticizers, Type F/G): Reduce water demand by 12% to 30%, or dramatically increase slump from 2 inches to 8–10 inches without adding water, allowing self-consolidating placement in congested rebar cages.
• Retarding Admixtures (ASTM C494 Type B & Type D): Slow the initial rate of hydration, extending setting time. Crucial for hot weather concreting in Southern Nevada: Under ambient temperatures exceeding $100^\circ\text{F}$ ($38^\circ\text{C}$) in Clark County, unretarded concrete can experience flash setting or false setting before transit, pumping, and consolidation can be executed.
• Accelerating Admixtures (ASTM C494 Type C & Type E): Accelerate initial set and early strength development in cold weather. Non-chloride accelerators (calcium nitrite, calcium nitrate) must be specified for reinforced concrete to prevent corrosion of embedded steel rebar.

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Field Quality Control & Testing Protocols

Quality assurance on commercial projects requires standardized field testing of fresh concrete at the point of discharge. General contractors must ensure testing technicians hold current ACI Concrete Field Testing Technician Grade I certifications.

                    STANDARD FIELD CONCRETE TESTING PROTOCOL
  ┌─────────────────────────┬─────────────────────────┬─────────────────────────┐
  │     ASTM C143 SLUMP     │ ASTM C231/C173 AIR CONT.│  ASTM C1064 TEMPERATURE │
  │ Measures workability and│ Pressure (C231) dense   │ Measures fresh mix heat;│
  │ consistency; 12" cone in│ aggregate; Volumetric   │ sensor immersed ≥ 3";   │
  │ 3 layers, 25 rods each. │ (C173) lightweight.     │ stable in 2–5 minutes.  │
  └────────────┬────────────┴────────────┬────────────┴────────────┬────────────┘
               │                         │                         │
               └─────────────────────────┼─────────────────────────┘
                                         │
                                         ▼
                       ┌───────────────────────────────────┐
                       │      ASTM C31 / C39 CYLINDERS     │
                       │ Cast 6x12" or 4x8" test cylinders;│
                       │ Initial cure 60°F–80°F for ≤ 48h; │
                       │ Moist cure at lab; break at 7/28d.│
                       └───────────────────────────────────┘

1. ASTM C143: Slump Test

Measures the consistency and workability of fresh concrete.

• Apparatus: Hollow frustum of a cone: 12 inches high, 8-inch base diameter, 4-inch top diameter. Tamping rod: 5/8-inch diameter, 24 inches long with rounded hemispherical tip.
• Procedure: Dampen cone and place on flat, rigid, non-absorbent surface. Fill in three equal layers by volume (depths of approximately $2\text{-}5/8\ \text{inches}$, $6\text{-}1/8\ \text{inches}$, and $12\ \text{inches}$). Rod each layer 25 times throughout its depth, with strokes distributed evenly over the cross section. For the second and third layers, penetrate approximately 1 inch into the underlying layer. Strike off excess concrete, clean perimeter, and lift cone vertically in $5 \pm 2$ seconds without lateral or twisting motion.
• Measurement: Invert cone beside concrete and measure the vertical subsidence from the top of the mold to the displaced original center of the top surface to the nearest $1/4\ \text{inch}$.
• Failure Modes: If a distinct portion of the concrete shears off laterally (shear slump) or collapses completely, discard the test and retest on a fresh portion of the sample.

2. ASTM C231 & ASTM C173: Air Content Testing

• ASTM C231 (Pressure Method, Type B Meter): Standard method for concrete made with relatively dense aggregates. Uses Boyle's Law to correlate pressure reduction in a sealed chamber to the volume of air voids compressed in the concrete sample.
• ASTM C173 (Volumetric Method / Roll-A-Meter): Mandatory for lightweight aggregate concrete (or porous slag). Because porous aggregates contain internal cellular air that would be compressed under pressure testing—yielding falsely elevated air readings—the volumetric method washes air out of the concrete using water and isopropyl alcohol.

3. ASTM C1064: Fresh Concrete Temperature

• Sensor Requirements: Calibrated temperature-measuring device accurate to $\pm 1^\circ\text{F}$ ($\pm 0.5^\circ\text{C}$) over a range of $30^\circ\text{F}$ to $120^\circ\text{F}$.
• Procedure: Submerge temperature sensor at least 3 inches into the fresh concrete sample. Close the void around the stem and leave it immersed for at least 2 minutes, or until the reading stabilizes. Complete the measurement within 5 minutes of obtaining the sample. Record temperature to the nearest $1^\circ\text{F}$ ($0.5^\circ\text{C}$).
• Specification Limits: ACI 305.1 sets a default maximum concrete temperature of $95^\circ\text{F}$ ($35^\circ\text{C}$) at discharge unless the project specification states another limit. Some specifications set a lower limit, such as $90^\circ\text{F}$, or allow a higher one only for a mix with a proven hot-weather record. Read the project specification before the first summer pour.

4. ASTM C31 & ASTM C39: Compressive Strength Testing

• Molding Cylinders (ASTM C31):
  • Standard $6 \times 12$ inch cylinders: Filled in 3 equal layers, rodded 25 times per layer, tapped 10 to 15 times with a rubber mallet per layer to close rodding voids.
  • Standard $4 \times 8$ inch cylinders: Permitted when the specification allows them; the cylinder diameter must be at least three times the nominal maximum aggregate size. Filled in 2 equal layers, rodded 25 times per layer, and tapped with mallet.
• Initial Field Curing: Maintain cylinders in a moisture-tight environment between $60^\circ\text{F}$ and $80^\circ\text{F}$ ($16^\circ\text{C}$ to $27^\circ\text{C}$) for up to 48 hours on site. (For high-strength concrete $\ge 6,000\ \text{psi}$, initial cure range is tightened to $68^\circ\text{F}$ to $78^\circ\text{F}$).
• Laboratory Moist Curing & Breaking (ASTM C39): Transported to testing laboratory within 48 hours; cured in moist room at $73.5 \pm 3.5^\circ\text{F}$ ($23.0 \pm 2.0^\circ\text{C}$) with free moisture at all times. Standard breaks are conducted at 7 days (typically yielding 65% to 75% of design strength) and 28 days (the definitive contractual design strength, $f'_c$).
• ACI 318 Acceptance Criteria (§ 26.12.3): The strength level of an individual concrete class is legally acceptable if:
  1. Every arithmetic average of any three consecutive strength tests equals or exceeds specified design strength ($f'_c$); AND
  2. No individual strength test (the average strength of at least two $6 \times 12$ or three $4 \times 8$ cylinders made from the same sample and tested at 28 days or the specified test age) falls below $f'_c$ by more than $500\ \text{psi}$ (when $f'_c \le 5,000\ \text{psi}$), or by more than $0.10 f'_c$ (when $f'_c > 5,000\ \text{psi}$).

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Concrete Formwork Systems & Engineering

Formwork is a temporary structural mold engineered to support fresh concrete, reinforcing steel, construction equipment, and workers until the concrete develops sufficient self-supporting strength.

Lateral Pressure of Fresh Concrete (ACI 347R)

Freshly placed concrete acts as a dense, semi-fluid slurry exerting intense lateral hydrostatic pressure against vertical wall and column form faces. Under ACI 347R, maximum lateral pressure ($P_{\max}$, in $\text{lb/ft}^2$ or $\text{psf}$) is governed by:

1. Rate of vertical placement ($R$, in $\text{feet per hour}$);
2. Concrete mix temperature ($T$, in $^\circ\text{F}$);
3. Unit weight of concrete ($w$, standard $150\ \text{pcf}$);
4. Slump and admixture chemistry (retarders extend liquid state);
5. Depth of mechanical vibration.

$\text{Maximum Hydrostatic Limit: } P_{\max} = w \times h = 150 \times h$

For walls with placement rate $R \le 7\ \text{ft/hr}$ and placement height $\le 14\ \text{ft}$:

$P_{\max} = C_w C_c \left[ 150 + \frac{7200 R}{T} \right]$

For columns:

$P_{\max} = C_w C_c \left[ 150 + \frac{9000 R}{T} \right]$

Walls placed faster than $7\ \text{ft/hr}$, or taller than $14\ \text{ft}$ at up to $7\ \text{ft/hr}$, use a separate ACI 347 wall equation. In every case, the design pressure is at least $600 C_w\ \text{psf}$ and never more than the full liquid head $wh$. $C_w$ is the unit weight coefficient ($1.0$ for $150\ \text{pcf}$ concrete). $C_c$ is the chemistry coefficient: $1.0$ for Type I, II or III cement without a retarder, rising to $1.2$–$1.4$ for retarders and high slag or fly ash blends.

Worked example (wall): $R = 5\ \text{ft/hr}$, $T = 70^\circ\text{F}$, $C_w = C_c = 1.0$, wall height $10\ \text{ft}$.

$P_{\max} = 150 + \frac{7200 \times 5}{70} = 150 + 514 = 664\ \text{psf}$

The liquid head is $150 \times 10 = 1,500\ \text{psf}$, so the formula value of about $664\ \text{psf}$ governs.

Worked example (column): $R = 10\ \text{ft/hr}$, $T = 50^\circ\text{F}$, column height $12\ \text{ft}$. The formula gives $150 + 9000 \times 10 / 50 = 1,950\ \text{psf}$. The liquid head is only $150 \times 12 = 1,800\ \text{psf}$, so the form is designed for $1,800\ \text{psf}$.

Exam Key Concept: Cold weather and rapid vertical pumping rates dramatically increase form pressure because concrete takes longer to hydrate and stiffen. Conversely, hot weather accelerates setting, shortening the liquid head and reducing maximum lateral form pressure.

                          WALL FORMWORK ELEVATION
     ┌────────────────────────────────────────────────────────┐
     │ [Double 2x4 Top Waler]                                 │
     │  ║                                                  ║  │
     │  ║  ┌──────────┐                     ┌──────────┐   ║  │
     │  ║  │ Plywood  │  ● Snap Tie         │ Plywood  │   ║  │
     │  ║  │ Sheathing│  │                  │ Sheathing│   ║  │
     │  ║  └──────────┘  │                  └──────────┘   ║  │
     │  ║                ▼                                 ║  │
     │══╬══════════════════════════════════════════════════╬══│ <── [Double 2x4 Waler]
     │  ║         ◄────────── Form Tie ──────────►         ║  │
     │  ║                                                  ║  │
     │  ║ 2x4 Vertical Studs @ 12" or 16" o.c.             ║  │
     │  ║                                                  ║  │
     │══╬══════════════════════════════════════════════════╬══│ <── [Bottom Waler]
     │  ║                                                  ║  │
     │  ▼                                                  ▼  │
     │ [Concrete Footing / Keyway]                            │
     └────────────────────────────────────────────────────────┘

Formwork Components & Release Agents

• Form Ties: Tension rods (snap ties, loop ties, coil ties, through-bolts) that span across opposing form faces to resist lateral burst pressures. Snap ties feature break-backs (typically 1 to 1.5 inches deep) allowing the protruding tie end to be snapped off flush below the concrete surface; the resulting hole is grouted or plugged to prevent water intrusion and corrosion.
• Form Release Agents: Applied to form faces prior to placing rebar to prevent mechanical bonding between cement paste and forms.
  • Barrier Coatings: Form oils, waxes, and silicones that physically separate paste from form surfaces.
  • Chemically Reactive Agents: Contain fatty acids that chemically react with free lime ($\text{Ca(OH)}_2$) in the fresh cement paste to form a water-insoluble calcium soap film. These provide cleaner releases, eliminate surface staining, and leave no residue that could impair adhesion of future stucco, waterproofing, or drywall compounds.
• Form Stripping and Reshoring Schedules: Formwork must remain in place until concrete achieves specified stripping strength (verified by field-cured cylinder breaks). Wall and column forms (vertical elements supporting only lateral pressure) can typically be stripped in 12 to 24 hours. Supported floor slabs, beams, and soffits stay shored until the concrete reaches the stripping strength set by the engineer, often a stated percentage of $f'_c$. Reshores go in as the original shores come out whenever the slab must carry more load than it can yet support. OSHA 1926.703(e) bars removing forms and shores (other than for slabs on grade and slip forms) until the employer determines that the concrete can carry its own weight and the superimposed loads. That determination must rest on the removal conditions in the plans and specifications or on ASTM strength tests. Reshores stay in place until the concrete they support has enough strength for all loads on it.

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Reinforcing Steel (Rebar) Technology

Plain unreinforced concrete possesses immense compressive strength but very low tensile strength (tensile strength is approximately 8% to 12% of compressive strength). Steel reinforcement is cast inside concrete to carry tensile, flexural, and shear stresses.

Material Specifications & Sizing

• ASTM A615 (Grade 60): The standard deformed carbon-steel billet rebar used in building construction. Yield strength $F_y = 60,000\ \text{psi}$ ($60\ \text{ksi}$). Tensile strength $F_u = 90,000\ \text{psi}$.
• ASTM A706 (Grade 60 / Low-Alloy): Tightly controlled chemistry (carbon equivalent $\le 0.55\%$) gives dependable ductility and weldability. ACI 318-14 §20.2.2.5 requires A706 for longitudinal bars that resist earthquake forces in special moment frames and special structural walls. A615 Grade 60 is allowed there only when mill tests show an actual yield strength no more than $18,000\ \text{psi}$ above the specified value and a tensile-to-yield ratio of at least $1.25$. Welding A615 bars needs a procedure under AWS D1.4 based on the bar's carbon equivalent, often with preheat.
• Rebar Numbering System: For sizes #3 through #8, the bar number represents the nominal diameter in eighths of an inch ($1/8\ \text{inch}$):

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Minimum Concrete Cover Requirements (ACI 318-14 Table 20.6.1.3.1)

Concrete cover is the physical thickness of concrete between the outer surface of reinforcing steel (including ties and stirrups) and the nearest exterior surface of the concrete member. Cover provides passivation against corrosion, structural fireproofing, and ensures adequate mechanical bond:

                         ACI 318 MINIMUM CONCRETE COVER
  ┌─────────────────────────────────────────────────────────────┬──────────┐
  │ EXPOSURE CONDITION                                          │ MIN COVER│
  ├─────────────────────────────────────────────────────────────┼──────────┤
  │ Concrete cast against and permanently in contact with earth │ 3.0 in   │
  │ (e.g., bottom and unformed sides of footings, caissons)     │ (75 mm)  │
  ├─────────────────────────────────────────────────────────────┼──────────┤
  │ Concrete exposed to earth or weather after form removal:    │          │
  │   • #6 through #18 bars                                     │ 2.0 in   │
  │   • #5 bar, W31/D31 wire, and smaller                       │ 1.5 in   │
  ├─────────────────────────────────────────────────────────────┼──────────┤
  │ Concrete not exposed to weather or in contact with ground:  │          │
  │   • Slabs, joists, and walls (#11 and smaller)              │ 0.75 in  │
  │   • Beams and columns (primary rebar, ties, stirrups)       │ 1.5 in   │
  └─────────────────────────────────────────────────────────────┴──────────┘

Exam Key Rule: For footings cast directly against earth without forms, the minimum rebar clearance from the dirt subgrade is 3.0 inches. Reinforcement must be supported on solid concrete dobies (mortar blocks matching the design compressive strength), plastic chairs, or composite standoffs. Never use broken bricks, wood blocks, or scrap stone.

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Rebar Lap Splices & Welded Wire Reinforcement (WWR)

• Lap Splices: When rebar cannot be placed in a single continuous length, bars are lapped alongside each other so that tension or compression is transferred through the surrounding concrete paste via shear bond.
  • Tension Lap Splices (ACI 318 § 25.5.2): A Class A splice is $1.0 l_d$, where $l_d$ is the calculated development length. It is allowed only where the steel provided is at least twice the steel required and no more than half the bars are spliced within the lap length. Every other tension splice is Class B, $1.3 l_d$.
  • Read the splice schedule: Development length depends on bar size, concrete strength, cover, spacing, bar position, and epoxy coating. Take lap lengths from the structural drawings or splice schedule, never from a rule of thumb.
  • Large bars: ACI 318 does not permit lap splices of bars larger than No. 11, except in limited compression and footing-dowel cases. No. 14 and No. 18 bars are joined with mechanical couplers or welded splices.
• Welded Wire Reinforcement (WWR / Welded Wire Fabric, ASTM A1064): Cold-drawn steel wire welded in a square or rectangular grid. Designated by spacing and cross-sectional wire area (e.g., $6 \times 6 - \text{W}1.4/\text{W}1.4$ indicates wires spaced 6 inches on center in both directions with a wire area of $0.014\ \text{in}^2$). WWR provides crack-control (temperature and shrinkage reinforcement) in slabs-on-grade. Support it on chairs or concrete blocks at the depth shown on the drawings. Industry guidance (ACI 302.1R) discourages "hooking," which means pulling the mesh up with a rake during placement, because the wire usually ends up near the bottom of the slab where it does little to control cracks.

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OSHA Subpart Q: Formwork, Shoring & Rebar Safety

The trade exam may test the federal concrete rules in 29 CFR 1926 Subpart Q, which Nevada OSHA enforces on construction sites:

• Formwork drawings on site (1926.703(a)(2)): Drawings or plans for the jack layout, formwork (including shoring), working decks, and scaffolds, with all revisions, must be available at the jobsite.
• Shoring inspections (1926.703(b)): Inspect shoring equipment before erection. Inspect erected shoring immediately before, during, and immediately after the concrete is placed. Damaged shoring that falls below the required strength may not be used, and shoring found weakened after erection must be reinforced immediately. Do not adjust single-post shores to raise formwork after concrete is placed. Tiered single-post shores need a qualified designer, and an engineer qualified in structural design must inspect them once erected.
• Construction loads (1926.701(a)): Do not place construction loads on a concrete structure unless the employer has determined, from information provided by a person qualified in structural design, that the structure can support them.
• Impalement (1926.701(b)): All protruding reinforcing steel that employees could fall onto or into must be guarded to eliminate the impalement hazard.
• Vertical rebar and mesh (1926.703(d)): Support wall, pier, and column reinforcing so it cannot overturn or collapse. Keep unrolled wire mesh from recoiling, for example by securing each end or turning the roll over.

Exam tip: Questions often pair a technical rule with its safety twin. Form pressure (ACI 347) tells you how strong the forms must be; Subpart Q tells you who checks the shoring and when the forms may come off.