14.2 Concrete Placement, Finishing, Joints, Curing & Post-Tensioning

Concrete Placement, Consolidation & Finishing

Conveying and Placement Standards

• Free Fall and Segregation: Concrete must not segregate (coarse aggregate separating from the mortar) as it is placed. A free fall through clear space is not harmful by itself. Segregation happens when the stream hits reinforcement, form ties, or the form face, or when it is dropped at an angle. Many project specifications therefore limit unconfined drops. Placements in deep, congested wall forms, columns, or drilled shafts use tremies, drop chutes, or flexible elephant trunks.
• Horizontal Flow: Concrete must be deposited directly at or near its final position. Rakes, shovels, or vibrators must never be used to move concrete horizontally across forms, as this leaves coarse aggregate behind, producing honeycombing, rock pockets, and structural voids.
• Lift Thickness: Vertical walls and columns must be placed in uniform horizontal layers (lifts) not exceeding 12 to 20 inches in depth.

Consolidation with Internal Mechanical Vibrators

Vibration fluidizes the concrete paste by overcoming internal aggregate friction, allowing trapped air to escape and paste to flow completely around reinforcing bars and form corners.

                  PROPER INTERNAL VIBRATION TECHNIQUE
   INCORRECT: Dragging Horizontally      CORRECT: Vertical Insertions
   ┌─────────────────────────────┐       ┌─────────────────────────────┐
   │      ~~~~~~~~~~~~~~~        │       │   ↓   ↓   ↓   ↓   ↓   ↓     │
   │     /               \       │       │ ┌───┐   ┌───┐   ┌───┐       │
   │    /  (Segregation)  \      │       │ │   │   │   │   │   │       │
   │   /                   \     │       │ │   │   │   │   │   │       │
   │  ●════════════════════►     │       │ └───┘   └───┘   └───┘       │
   │  (Dragging creates voids)   │       │ Penetrate 4"–6" into lower  │
   └─────────────────────────────┘       └─────────────────────────────┘

• Insertion Spacing: Insert vibrator vertically at uniform intervals spaced approximately $1.5 \times$ the vibrator radius of action (typically 12 to 18 inches on center). The visual fields of action must overlap.
• Lift Penetration: The vibrator head must rapidly penetrate through the current lift and extend 4 to 6 inches into the previously placed underlying lift to knit the layers together and eliminate cold joints.
• Duration: Hold vibrator in place for 5 to 15 seconds until the concrete surface becomes level, acquires a uniform sheen, and large air bubbles cease rising. Rapid insertion and slow withdrawal (approximately 3 inches per second) is mandatory to allow the hole left by the vibrator head to close completely.
• Reinforcement and Forms: Do not hold the vibrator against reinforcement or form faces. Contact with the forms can damage them and mark the finished surface. Do not use the vibrator to move concrete sideways.

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Concrete Finishing Sequence

Flatwork finishing requires a precise sequence of mechanical operations coordinated with the chemical set of the paste:

1. Screeding (Strike-off): Immediately following placement, a straightedge or motorized screed is drawn across screed pipes or edge forms to level concrete to design elevation.
2. Bull Floating / Darbying: Performed immediately after screeding while the mix is plastic. A wide bull float (wood or magnesium) is pushed forward with the front edge slightly raised and pulled backward flat to smooth ridges, fill minor depressions, and embed large coarse aggregate below the surface.
3. Bleed Water Evaporation (THE CRITICAL FINISHING PAUSE): Bleeding is the upward migration of free mixing water to the surface. Absolute Rule: Finishing operations must halt while bleed water is present. Working concrete with bleed water on the surface works water back into the top paste, resulting in a weak, high $w/cm$ surface layer that causes crazing, dusting, blistering, and delamination.
4. Edging and Grooving (Jointing): As the surface sheen disappears and the slab can support foot pressure leaving an indentation of no more than $1/4\ \text{inch}$, an edging tool is run along perimeter forms to round corners (preventing chipping). Groovers cut control joints if not saw-cut later.
5. Floating: Performed using hand wood/magnesium floats or motorized power trowels equipped with float blades/shoes. Compacts the surface, removes minor imperfections, and brings a thin layer of mortar to the top.
6. Troweling: Hand steel trowels or power trowels with finish blades are run flat, then tilted slightly at higher speeds across multiple passes. Compacts and densifies the surface to produce a hard, smooth, wear-resistant interior finish. Exterior flatwork receives a textured broom finish after floating to provide non-slip traction.

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Concrete Joint Design: Types, Sizing & Saw-Cutting

Concrete naturally shrinks as excess mixing water evaporates (drying shrinkage ranges from $1/8\ \text{inch}$ to $1/4\ \text{inch}$ per 100 feet). Restrained shrinkage creates tensile stress greater than the young concrete's tensile capacity, so the slab cracks at random. General contractors use three primary joint types:

                         THE THREE CONCRETE JOINT TYPES

  1. CONTRACTION (CONTROL) JOINT:  Pre-planned plane of weakness;
     Minimum Depth = D / 4         guides shrinkage cracks.
     ┌──────────────────┬──────────────────┐
     │                  ▼ Saw-Cut (D/4)    │
     │              │                      │
     │              : Crack occurs below   │
     └──────────────┴──────────────────────┘

  2. ISOLATION (EXPANSION) JOINT:  Complete structural separation;
     Full Depth Separation         allows independent movement.
     ┌──────────────┐ █ Joint Filler ┌──────────────┐
     │   Column /   │ █ (1/2" Foam/  │  Floor Slab  │
     │  Foundation  │ █ Bituminous)  │              │
     └──────────────┘ █              └──────────────┘

  3. CONSTRUCTION JOINT:           Stopping point between pours;
     Keyed or Dowelled             transfers vertical shear.
     ┌──────────────────┬──────────────────┐
     │ Initial Pour     │ ═══ Dowel ═══    │ Future Pour
     └──────────────────┴──────────────────┘

1. Contraction Joints (Control Joints)

Create a planned plane of weakness that forces shrinkage cracking to occur neatly at the bottom of the cut rather than randomly across the slab.

• Joint Spacing Guidance: For unreinforced slabs on ground, ACI 302.1R and ACI 360R place contraction joints at about 24 to 36 times the slab thickness. In feet, that is 2 to 3 times the thickness in inches. For example:

  • 4-inch slab: $8$ to $12\ \text{ft}$.
  • 5-inch slab: $10$ to $15\ \text{ft}$.
  • 6-inch slab: $12$ to $18\ \text{ft}$.

  Use the shorter end of the range for high-shrinkage conditions, such as hot, dry, windy placements or mixes with high water content, and always follow the joint layout on the drawings.

• Panel Geometry: Joint layouts must form panels as close to square as possible. Avoid rectangular panels where the long dimension exceeds $1.25$ to $1.5$ times the short dimension, and eliminate all re-entrant L-shaped corners by placing joints at column corners.

• Saw-Cut Depth: Minimum depth must be one-fourth ($1/4$) the slab thickness ($D/4$), or a minimum of 1 inch for a 4-inch slab, and $1.5\ \text{inches}$ for a 6-inch slab.

• Saw-Cut Timing: Timing is critical. Conventional wet saw-cutting must occur as soon as the concrete is hard enough to resist raveling or dislodging aggregate along the cut, typically 4 to 12 hours after finishing (and always before random shrinkage cracking begins). Early-entry dry-cut saws ("soft-cut") can cut within about 1 to 4 hours after finishing, to a shallower depth (typically about 1 to 1-1/4 inches) set by the saw manufacturer.

2. Isolation Joints (Expansion Joints)

Provide complete, full-depth structural separation between slabs and rigid vertical elements (columns, foundation walls, equipment pads, and adjacent pavements). Uses a compressible premolded joint filler (1/2-inch thick asphalt-saturated fiber or closed-cell polyethylene foam) extending through the full depth of the slab to prevent cracking from differential thermal expansion or foundation settlement.

3. Construction Joints

Stopping points established where concrete placement ends for the day. Can be keyed or reinforced with smooth steel dowels (aligned parallel to each other and lubricated on one side) to permit horizontal thermal contraction while transferring vertical shear loads across the joint.

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Hot Weather Concreting (ACI 305R) & Curing Regimes

Hot weather concreting is defined by any combination of high ambient temperature, low relative humidity, high wind speed, and solar radiation that accelerates hydration and evaporation.

The Plastic Shrinkage Hazard Nomograph

When surface water evaporation exceeds the rate at which internal bleed water rises to the surface, the concrete surface shrinks while the underlying matrix remains plastic, producing plastic shrinkage cracks (wide, irregular fissures appearing before final set).

                      PLASTIC SHRINKAGE CRACKING RISK

  EVAPORATION RATE (lb/ft²/hr)             RISK LEVEL & MANDATORY ACTIONS
  ─────────────────────────────────────────────────────────────────────────────
  E < 0.10 lb/ft²/hr          Low Risk: Standard placement and finishing.
  0.10 ≤ E < 0.20 lb/ft²/hr   Moderate Risk: Precautions advised (windbreaks).
  E ≥ 0.20 lb/ft²/hr          CRITICAL RISK: Plastic shrinkage cracking imminent!
                              Mandatory fogging, windbreaks, evaporative
                              retarders, and immediate curing measures.

• ACI 305R Evaporation Threshold: When the evaporation rate exceeds $0.20\ \text{lb/ft}^2/\text{hr}$ ($1.0\ \text{kg/m}^2/\text{hr}$)—or $0.10\ \text{lb/ft}^2/\text{hr}$ for mixes with silica fume or low bleed rates—mandatory mitigation is required.
• Field Mitigation Actions in Nevada:
  1. Schedule Night Pours: Place concrete between midnight and 6:00 AM when ambient temperatures and solar radiation are lowest.
  2. Pre-wet Subgrade and Forms: Spray subgrade and forms with cool water prior to placement to prevent moisture absorption from the mix (leaving no standing puddles).
  3. Cooling Mix Components: Replace part of the batch water with shaved or crushed ice; inject liquid nitrogen into truck drums at the batch plant; store aggregates in shaded bins.
  4. Erect Windbreaks and Sunshades: Reduce surface air velocity and direct solar heating.
  5. Apply Evaporative Retarders: Spray monomolecular film retarders across the surface immediately following bull floating to reduce moisture loss.

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Concrete Curing Methods

Curing maintains adequate moisture and temperature inside the fresh concrete matrix for a sufficient duration to ensure continuous cement hydration and microstructural strength gain:

• Continuous Water Curing: Ponding, continuous water fogging, or saturated burlap/cotton mats covered with 4-mil polyethylene sheeting. Represents the highest quality curing regime, eliminating drying shrinkage during the critical hydration phase.
• Liquid Membrane-Forming Curing Compounds (ASTM C309): Spray-applied liquid resins or waxes forming an impervious surface seal that traps internal moisture.
  • Type 1: Clear or translucent.
  • Type 2: White-pigmented. Preferred for exterior flatwork in hot, sunny climates: the white pigment reflects solar heat and keeps the slab cooler.
  • Application Rate: Must be applied uniformly at manufacturer rates (typically $150$ to $200\ \text{sq ft/gallon}$) immediately following surface finishing as the water sheen disappears.
• Minimum Curing Duration (ACI 301 / ACI 308.1): Unless the specification says otherwise, keep curing for at least 7 days, or until tests on field-cured cylinders show at least 70% of $f'_c$. High-early-strength concrete needs at least 3 days.

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Cold Weather Concreting (ACI 306) in Northern Nevada

Reno, Carson City, Elko, and the higher valleys freeze regularly in winter. ACI 306 defines cold weather as a period when the air temperature has fallen to, or is expected to fall below, $40^\circ\text{F}$ during the protection period. Key controls:

• Protect from early freezing: Fresh concrete that freezes before it reaches about $500\ \text{psi}$ can lose much of its potential strength and durability. Use insulated blankets, heated enclosures, or both until that strength is reached.
• Never place on frozen ground: Remove snow and ice from forms and reinforcement, and do not place concrete on a frozen subgrade. Thawing subgrade settles unevenly and draws heat from the slab.
• Vent heaters: Combustion heaters inside enclosures must be vented. Exhaust carbon dioxide reacts with the fresh surface and causes a soft, dusting surface (carbonation), and carbon monoxide is a hazard to workers.
• Avoid thermal shock: Remove blankets and enclosures gradually so the surface does not cool much faster than the core.
• Use non-chloride accelerators in reinforced concrete, as noted in the admixture discussion.

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Post-Tensioned Slabs (PTI Level 1 Field Fundamentals)

The B trade exam lists the Post-Tensioning Institute's unbonded field-personnel course as a reference. Most post-tensioned (PT) slabs and slab-on-ground foundations in Nevada use unbonded monostrand tendons:

• Tendon: A 0.5-inch, seven-wire, low-relaxation strand (ASTM A416, $f_{pu} = 270\ \text{ksi}$, area $0.153\ \text{in}^2$), coated with corrosion-inhibiting grease and sealed inside an extruded plastic sheath. Each tendon has a fixed (dead) end anchor and a stressing (live) end anchor. Long pours may use intermediate anchors at construction joints.
• Placement and profile: Tendons are draped to the high and low points shown on the shop drawings and held by chairs and support bars. ACI 117 tolerances for the vertical position of prestressing steel are $\pm 1/4$ inch for members up to 8 inches deep, $\pm 3/8$ inch for 8 to 24 inches, and $\pm 1/2$ inch over 24 inches.
• Pre-pour inspection: Repair damaged sheathing, keep anchors square to the tendon and tight to the edge form, install pocket formers and bursting (backup) reinforcement at anchorages, and check that tendons have smooth profiles without kinks.
• Stressing: Stress only after the concrete reaches the strength the engineer specifies, usually confirmed by field-cured cylinders. ACI 318 limits jacking stress to $0.94 f_{py}$ but not more than $0.80 f_{pu}$. For a 0.5-inch strand: $0.80 \times 270 \times 0.153 = 33.0\ \text{kips}$.
• Elongation check: The crew records the elongation of every tendon and compares it with the calculated value. ACI 318 requires that the cause be found and corrected if the force indicated by elongation and the force indicated by the jack gauge differ by more than 7% for post-tensioned construction.

Worked example: Estimate the elongation of a 100-foot tendon with an average force of 30 kips after friction losses, using $E = 28,500\ \text{ksi}$.

$\Delta L = \frac{P L}{A E} = \frac{30 \times 1,200}{0.153 \times 28,500} = \frac{36,000}{4,360.5} \approx 8.3\ \text{in}$

A rule of thumb of "about 8 inches per 100 feet" is a useful field check.

• Jack safety (OSHA 1926.701(c)): No employee except those essential to the post-tensioning operation may be behind the jack during tensioning. Signs and barriers must limit access to the stressing area. A tendon that breaks or slips can shoot out of the slab edge with lethal force.
• Finishing the work: After the engineer accepts the elongations, cut the tendon tails and fill the stressing pockets with non-shrink grout to protect the anchors.
• Never cut blindly: Before coring, saw-cutting, or anchoring into an existing PT slab, locate the tendons with ground-penetrating radar or X-ray. Cutting a stressed tendon is dangerous and costly to repair.

Other Subpart Q Rules During Placement

• Concrete buckets (1926.701(d)–(e)): No employee may ride a concrete bucket. No one may work under a bucket while it is being raised or lowered, and elevated buckets should be routed so that as few employees as possible are exposed.
• Pneumatically applied concrete (1926.701(f)): Workers applying a cement, sand, and water mixture through a pneumatic hose (shotcrete or gunite) must wear protective head and face equipment.