9.1 Construction Joints, Contraction Joints & Surface Preparation (Roughening)
Key Takeaways
Distinguish an interface between placements from a weakened plane for shrinkage cracking and a separation that permits independent movement.
Under ACI 318-19 Section 26.5.6.2, check the nonprestressed floor and roof middle-third rule and the separate girder offset measured from the intersecting beam face, subject to approved alternatives.
For the roughened hardened-concrete condition in ACI 318-19 Table 22.9.4.2, verify a clean, laitance-free surface roughened to a full amplitude of approximately 1/4 inch.
Immediately before new concrete is placed at a construction joint, prewet the prepared interface and remove standing water.
Apply the specified saw-cut depth, layout and timing. ACI 301-20 Section 5.3.5.1 gives a default depth of one-fourth the slab thickness but not less than 1 inch.
Construction Joints, Contraction Joints & Surface Preparation
Joint inspection starts by identifying the joint’s function and the detail that applies at the measured location. A joint may transfer force, accommodate movement, control cracking or perform more than one of those functions. The joint plan establishes the line. A cross section and schedule establish what should be installed there.
| Joint | Function | Field observations |
|---|---|---|
| Construction joint | Interface intentionally created between successive concrete placements | Approved location, bulkhead, reinforcement and load-transfer details, surface preparation before next placement |
| Contraction joint | Weakened plane intended to control the location of cracking from dimensional changes | Line, saw-cut or groove depth, timing and specified reinforcement or load-transfer devices |
| Isolation joint | Separation allowing relative movement in three directions | Separation through the slab and interruption of bonded reinforcement, with the approved filler where shown |
| Expansion joint | Separation providing space for dimensional change | Gap, filler and any detailed devices that transfer load while accommodating movement |
1. Check construction joint location and the placement sequence
ACI 318-19 Section 26.5.6.2 requires the licensed design professional to review unshown or different locations and details. The nonprestressed floor and roof default is the middle third of slab, beam and girder spans. The girder offset from an intersecting beam is at least two widths of the intersecting beam measured from its face. Both location provisions allow alternatives approved by the licensed design professional. Check the proposed and approved dimensions on the same datum. If placement stopped unexpectedly, record the actual stop location and concrete condition. Obtain the disposition before treating the interface as a construction joint.
Where ACI 301-20 Section 2.2.2.5(a) is incorporated without a different project requirement, it gives defaults for construction joints in walls and columns at the undersides of slabs, beams or girders and at tops of footings or slabs. ACI 318-19 Section 26.5.7.2(a) separately requires cast-in-place vertical support concrete to be no longer plastic before placement of supported beams, girders or slabs. Section 26.5.7.2(b) calls for beams, girders, haunches, drop panels, shear caps and capitals to be placed monolithically as part of a slab system unless the construction documents show otherwise. Verify that sequence against the approved sections.
2. Check surface preparation before covering the interface
ACI 318-19 Section 26.5.6.2(d) through (f) requires cleaning and removing laitance, roughening where specified, and prewetting immediately before placement with standing water removed. Check the surface after form removal and again before it is concealed. Record debris, loose material, standing water or a surface profile that differs from the approved detail. For a keyed joint, verify the keyway shape and dimensions shown and confirm it is free of removable form material and debris.
For shear-friction design, the reinforcement crossing the interface and the contact surface condition both affect the calculation. ACI 318-19 Table 22.9.4.2 assigns the following coefficients under the conditions listed.
| Contact condition | Coefficient |
|---|---|
| Monolithically placed concrete | |
| Clean, laitance-free hardened concrete intentionally roughened to a full amplitude of approximately 1/4 inch | |
| Clean, laitance-free hardened concrete without intentional roughening | |
| Clean, paint-free as-rolled structural steel where the specified headed studs or welded deformed bars or wires transfer shear across the interface |
For normalweight concrete, . For lightweight concrete, use Section 19.2.4 and the table’s limit. The difference in coefficients does not establish the strength of an installed joint. The required crossing reinforcement and the design strength limits in Section 22.9.4 also apply.
Where the drawing requires the roughened condition, examine the profile as well as the cleanliness. For example, washing may remove loose material from a smooth bulkhead face while leaving it smooth. ACI 301-20 Section 5.3.2.6, where specified, gives interface preparation options where bond is required, including using an acceptable surface retarder or exposing coarse aggregate without leaving loosened particles or damaged concrete. Verify the resulting profile against the approved design criterion. The choice of equipment or a visible patch of aggregate alone cannot establish compliance.
3. Check contraction joint geometry and timing
Read the slab joint plan for panel boundaries, intersections and re-entrant corners. Use the detailed spacing and bar treatment. Where smooth dowels are detailed for load transfer while permitting joint opening, check their position and alignment and the specified debonding or sleeve. Follow the design for reinforcement continuity and any permitted cuts. Under ACI 318-19 Section 26.5.7.2(d), cutting or creating joints in a slab-on-ground that acts as a structural diaphragm or part of the seismic-force-resisting system requires licensed design professional approval.
When ACI 301-20 Section 5.3.5 applies, follow the specified saw-cut locations and start once the concrete can be cut without dislodging coarse aggregate. Start soon enough to control random cracking. Setting conditions, slab mixture and saw type affect the working window. Inspect and record the actual time and condition. Raveled edges can indicate that the cut began too early, while a random crack ahead of the saw shows that cracking has already occurred. Verify the cut against the specified depth. Guidance for early-entry sawing may permit a shallower cut where the detail or specification calls for it.
Under the unmodified ACI 301-20 Section 5.3.5.1 default, an 8-inch slab requires inches of cut depth, which exceeds the 1-inch floor. A 6-inch slab requires inches. Use a depth gauge at accessible representative points and record the applicable depth, locations and deviations. For example, a measured 1-1/4-inch cut in the 6-inch slab misses that default by 1/4 inch. Document it and obtain the disposition rather than prescribing a corrective cut.
Which measurement does ACI 318-19 Section 26.5.6.2(c) use for the construction joint offset in a girder with no approved alternative?
Two girder widths measured from the girder centerline
One intersecting-beam width measured from the beam centerline
At least two intersecting-beam widths measured from the beam face
At least two intersecting-beam depths measured from the beam face
Which hardened-concrete surface condition matches the coefficient in ACI 318-19 Table 22.9.4.2?
A clean screeded surface without intentional roughening
A clean, laitance-free surface intentionally roughened to a full amplitude of approximately 1/4 inch
A surface with light scoring and a retained laitance layer
Any interface with parallel grooves spaced 12 inches apart
A 6-inch slab-on-ground is specified to receive saw-cut joints under ACI 301-20 Section 5.3.5.1 with no different project depth. What depth does that provision require?
3/4 inch
1 inch
1-1/4 inches
1-1/2 inches
Sections you finish are checked off in the contents.