6.4 Concrete Placement, Consolidation, and Finishing

Concrete Placement, Consolidation, and Finishing

Successfully executing a concrete pour requires careful logistics, appropriate equipment, and skilled labor. Once the cement meets the water, a countdown begins. The contractor must transport, place, consolidate, and finish the concrete before it begins to set. For the PE exam, understanding the limitations and best practices of these operations is crucial for ensuring the final structural integrity of the concrete.

Transport and Placement Methods

Ready-mix concrete is typically transported in transit mixers. Standard specifications (like ASTM C94) limit the time between batching (when water meets cement) and discharge to 90 minutes, or a maximum of 300 drum revolutions, to prevent the concrete from setting in the truck. In hot weather, this time limit is often reduced.

Once on site, concrete can be placed into the forms using several methods:

• Chutes: Direct discharge from the truck, limited by reach and slope.
• Pumps: Concrete boom pumps or line pumps are used to move concrete vertically or across large slabs. Pumping requires a specific mix design with sufficient fine aggregate and paste to prevent blockages (plugging).
• Crane and Buckets: Used for high-rise columns, dams, or areas inaccessible by pumps.
• Tremie: A pipe used to place concrete underwater. The bottom of the tremie pipe must remain submerged in the freshly placed concrete to prevent the cement paste from washing away into the surrounding water.

Segregation: A primary rule of placement is avoiding segregation, which is the separation of heavy coarse aggregates from the lighter cement paste. Segregation occurs if concrete is dropped from too high (free-fall limits are typically 3 to 5 feet) or if it is allowed to ricochet off formwork and rebar.

Consolidation

Freshly placed concrete contains a significant amount of entrapped air voids. Consolidation is the process of removing these large voids and ensuring the concrete flows intimately around the reinforcing steel and into the corners of the forms. This is almost exclusively achieved using internal (spud) vibrators.

Proper vibration technique is frequently tested on the exam:

• Vibrators must be inserted vertically into the concrete at regular intervals.
• They should penetrate rapidly to the bottom of the lift (and slightly into the previous, still-plastic lift) and be withdrawn slowly to allow the hole to close behind the vibrator.
• Crucially, vibrators must never be used to drag or push concrete laterally along the forms. Doing so causes the heavier aggregates to settle out, leading to severe segregation and rock pockets (honeycombing).

Finishing and Jointing

Slab finishing is a multi-step process that must be timed perfectly with the concrete's setting behavior:

1. Screeding (Strike-off): Leveling the concrete to the proper elevation immediately after placement using a straightedge.
2. Bullfloating: Smoothing the surface and pushing large aggregates down while bringing "cream" (paste) to the surface.
3. Waiting: The finishers must wait for the bleed water (water rising to the surface) to evaporate. Finishing while bleed water is present drives water back into the top surface, weakening it and causing scaling later.
4. Troweling/Brooming: Applying the final hard, smooth finish with steel trowels, or a slip-resistant broom finish.
5. Curing: Immediately retaining moisture (via wet burlap, plastic sheeting, or chemical curing compounds) to allow the hydration reaction to continue and maximize strength.

Joints

Concrete shrinks as it cures and is subject to thermal expansion and contraction. Joints manage these movements to prevent random cracking.

• Construction Joints: Intentional stopping points between different concrete placements (e.g., pouring half a slab one day and the rest the next day).
• Expansion/Isolation Joints: Full-depth joints filled with compressible material. They separate the concrete from other structures (like walls or columns) to allow independent movement and prevent stress transfer.
• Control (Contraction) Joints: Intentional planes of weakness created to dictate where shrinkage cracking occurs. For slabs on grade, they are typically saw-cut to a depth of 1/4 of the slab thickness as soon as the concrete is hard enough to resist raveling. By cutting the surface, the natural crack is forced to form in a straight line below the cut.

Extreme Weather Concrete Placement

Weather drastically affects the hydration of cement.

Hot Weather Concreting

High temperatures accelerate setting, reducing the time available for placement and finishing. It also increases the rate of evaporation, leading to severe plastic shrinkage cracking on the surface before the concrete has gained any strength.

• Mitigation: Use set-retarding admixtures, substitute ice for a portion of the mix water, shade the materials, and erect windbreaks to reduce evaporation.

Cold Weather Concreting

Low temperatures drastically slow the hydration reaction, delaying strength gain and form stripping. If the water inside the concrete freezes before the concrete reaches a compressive strength of about 500 psi, the expanding ice crystals will destroy the internal matrix, permanently ruining the concrete.

• Mitigation: Heat the mix water and aggregates at the plant. Enclose the work area and provide supplemental heat. Use insulating blankets over slabs to retain the heat generated by the exothermic hydration reaction. Use accelerating admixtures (though calcium chloride must be avoided in reinforced concrete as it corrodes steel).