10.2 Conveying, Depositing & Layer Placement (Segregation & Cold Joint Prevention)
Key Takeaways
Under ACI 301-20, use conveying and deposition methods that avoid segregation or loss of constituents and deposit the concrete near its final position.
Check the discharge path for obstructions and apply any project-specific drop limit. ACI 304R-00 permits direct vertical discharge where the forms are sufficiently clear.
Verify where the contractor diverts pump primer and washwater. ACI 301-20 prohibits placing concrete contaminated with materials that have not been specified or accepted.
Coordinate lift thickness and delivery rate with available consolidation capacity and the formwork placement limits.
Record an interruption and obtain the disposition if the next layer cannot be integrated into the earlier layer. Apply approved construction-joint requirements before placement resumes at that interface.
10.2 Conveying, Depositing & Layer Placement
Model IBC Table 1705.3 lists continuous special inspection of concrete placement for proper application techniques. Verify the approved placement procedure and watch how the concrete travels from discharge to its final position. ACI 301-20 Section 5.3.2, where specified, addresses equipment capacity, segregation, deposition and consolidation. ACI 304R-00 provides guidance for arranging the equipment and the discharge path.
1. Check each transfer and discharge point
Segregation can occur when the stream splits around reinforcing bars or when the coarse aggregate separates from mortar during transfer. Look for a steady, cohesive stream directed near its final position. Note places where concrete runs laterally a long distance or where obstructions divert part of the stream. A rock pocket or concentrated mortar at one location may indicate segregation. Follow the applicable disposition if the concrete cannot be placed or consolidated by the available method.
| Conveyance method | What to observe |
|---|---|
| Pump | Steady discharge without segregation or unapproved mixture changes. Follow the approved startup plan to divert unaccepted primer and washwater before structural concrete enters the forms. |
| Bucket | Control the gate and discharge near the final position. Check that the stream has an unobstructed route through the cage. |
| Chute | Under ACI 301-20 Section 5.3.2.3(b), the standard arrangement uses a rounded metal or metal-lined chute sloped between 1:2 and 1:3. A chute longer than 20 ft or outside that slope range may be used if it discharges to a hopper before distribution into the forms. |
| Belt conveyor | Under ACI 301-20 Section 5.3.2.3(a), check the discharge baffle or hopper and that the belt slope does not cause segregation. |
| Tremie in a wet shaft | Check the watertight pipe, the approved start procedure that separates the initial charge from water or slurry, and that the discharge end remains embedded in concrete to the specified depth. |
ACI 304.2R-17 describes the grout or commercial primer used to coat a pump line. This operation prepares the line for the approved concrete mixture. ACI 301-20 Section 5.3.2.4(c) prohibits placing concrete contaminated with unaccepted or unspecified materials. If the primer is unapproved for incorporation, verify the first discharge is diverted until the specified concrete reaches the outlet. Where the project calls for comparing air content before and after pumping, use the specified sampling points and procedures and record where each result was obtained.
2. Direct concrete into position
ACI 304R-00 Section 5.4.1 favors an uninterrupted vertical path. Direct vertical discharge is an option when the forms are open and clear enough for concrete to fall without disruption. There is no universal 3-to-5-ft limit in that provision. Where reinforcement or an embed intercepts and separates the stream, a properly directed hose, drop chute or other approved placement method can bypass the obstruction. Check any drop height specified for the project.
A 47P practice exhibit for wall W2 shows two access openings and a placement note on detail 4/S5. The note directs discharge near the final position without striking the cage, limits the free fall to 6 ft for this wall and directs diversion of pump primer to a separate container. Opening A gives a 3-ft drop, but the stream strikes two bars and a large embed plate. Opening B gives a 5-ft unobstructed drop near the final position. Trace the discharge path on the section and compare both heights with the specified 6-ft limit. Opening B meets both placement conditions. Verify that the crew diverts the primer as detailed before using B.
3. Plan layers and watch for interruptions
ACI 301-20 Section 5.3.2.4 permits single or multiple layers. Where a deep member is placed in successive layers with internal vibration, coordinate the lift thickness with the head’s ability to reach through the new concrete into the preceding plastic layer. ACI 309R-05 Section 7.2 recommends limiting layers to 20 inches in large mats and heavy pedestals. For a congested wall, check the layer depth in the approved procedure and whether the available vibrator can reach through it. Avoid mounding material and using vibration to drag it across the form. Monitor the placement rate against the formwork plan. If discharge outruns consolidation, the next layer can conceal a portion of the current layer before it has been consolidated.
For a continuous layered placement, ACI 309R-05 Section 7.2 recommends that the vibrator reach the bottom of the new layer and at least 6 inches into the preceding plastic layer. Use the approved sequence so that layer thickness, access and consolidation keep pace. For a supported beam, girder or slab on a cast-in-place vertical member, ACI 318-19 Section 26.5.7.2(a) separately requires the supporting concrete to be no longer plastic before the supported member is placed.
An interruption can cause the leading edge or previous layer to stiffen so the new concrete no longer integrates. Identify the affected station, time and concrete condition. ACI 301-20 Sections 5.3.2.4(b) and 5.3.2.6 require meeting the construction-joint provisions where placement against the earlier concrete would otherwise result in a cold joint. Do not infer setting time from a universal truck interval. Bring the condition to the contractor for correction. If the proposed joint is unshown or different from the approved detail, obtain the applicable design review under ACI 318-19 Section 26.5.6.2(a). Verify the disposition and applicable surface preparation before the next placement.
The pump line is primed with a product that is not accepted for incorporation into the concrete. What should the inspector verify at startup?
Mix the first discharge into the footing if its slump is acceptable.
Place the primer against the wall face and extend consolidation.
Use the primer to correct the concrete’s slump in the pump hopper.
Divert the startup discharge until the specified concrete reaches the outlet.
Detail 4/S5 limits free fall to 6 ft for wall W2 and requires an unobstructed discharge path near the final position. The detail also calls for diverting pump primer. Opening A drops 3 ft across the cage. Opening B drops 5 ft with no obstruction near the final position. Which placement meets the detail?
Use Opening B after diverting pump primer as required.
Use Opening A because it gives the shorter fall.
Use either opening because both heights are below 6 ft.
Use Opening A and move the concrete to its final position with a vibrator.
Why should a deep wall placement use a layer thickness suited to the available internal vibrator?
So the layer stays unconsolidated until the next delivery.
So the head can reach through the new layer and into the preceding layer while it remains plastic.
So the head needs to operate only at the top surface.
So the layer can harden before the adjacent layer is placed.
Sections you finish are checked off in the contents.