Rebound Control on a Wet-Mix Tunnel Lining
A crew is placing a 100 mm wet-mix lining on the crown of a tunnel using a robotic arm. The mix has a water/cement ratio of 0.45, 10 mm aggregate, and a target slump of 90 mm. The nozzleman holds the nozzle about 1.5 m from the surface at roughly 45 degrees to the receiving plane. The first 30 m² pass shows heavy rebound pooling at the springline, and a test panel taken from the same batch shows 28-day compressive strength of 32 MPa versus a 40 MPa design requirement. The aggregate gradation certificate shows the fine fraction is at the low end of the allowable band. Ambient temperature is 8 °C and the substrate is damp but free of standing water.
1. Diagnose the most likely causes of the excessive rebound and low panel strength, and rank them.
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Two interacting causes stand out.
- Nozzle distance and angle. At 1.5 m the stream has lost velocity and coherence, so material ricochets rather than embedding. The 45-degree angle also drives material off the vertical face. Correct practice is roughly perpendicular (within about 15 degrees of normal) at about 0.6–1.0 m for wet-mix. This alone explains rebound pooling at the springline, where the angle to the surface changes fastest.
- Deficient fine fraction. A gradation at the low end of the fine band reduces paste volume and cohesion, so the mix is 'bony' and rebounds more. That also lowers strength because the paste cannot fully coat and bind the coarse particles.
Lower priority: the 8 °C ambient slows early strength gain but does not by itself explain a 32 vs 40 MPa gap at 28 days if curing was adequate. The 0.45 w/c is reasonable for wet mix; do not blame it first. The damp substrate is acceptable.
Ranking: nozzle technique first (it is the dominant, immediately correctable variable and matches the springline symptom), gradation second (it explains both the rebound and the strength shortfall), temperature a distant third.
2. What corrective actions would you direct before the next pass, and what would you verify?
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Immediate: stop the pass, reposition the nozzle to about 0.6–1.0 m and near-perpendicular to the surface, and slow the robotic arm traverse so each pass builds a consistent thickness. Re-check that the gunning rate matches the arm speed.
Mix: request a revised gradation with the fine fraction moved toward the middle/upper part of the band, or adjust the mix (e.g., add fines or a supplementary cementitious/paste component) so cohesion improves. Do not simply add water, which would raise w/c and further cut strength.
Verify:
- Shoot a new test panel from the revised mix and revised technique; test at 28 days against the 40 MPa requirement, and take early-age cores or beams if a decision is needed sooner.
- Measure rebound volume on the next pass as an objective check.
- Core the placed lining to confirm in-place thickness and strength where rebound was heavy, since rebound-rich zones are typically weaker and may be below the 100 mm thickness.
- Confirm curing protection at 8 °C (insulation or heated enclosure) so early strength develops.
3. Rebound material is being collected and a worker proposes reusing it in the next batch. Evaluate.
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Reject it. Rebound has lost most of its cement paste and fines, so its gradation is coarser and its effective w/c and binder content are unpredictable. Reusing it changes the mix design without documentation, typically reduces strength and durability, and can introduce contamination from the substrate. It also violates the principle that the approved mix design is what gets placed. Rebound should be removed and disposed of per the project's waste plan. If the contractor wants to reduce waste, the correct lever is improving nozzle technique and mix cohesion, not recycling rebound.