Pump Line Blockage Mid-Pour on a Raft
You are placing a 600 mm thick raft slab, 18 m by 12 m, in a single continuous pour. The mix is a 40 MPa, 160 mm slump, 20 mm aggregate pump mix with a retarder. Halfway through, the pump line blocks. The crew clears it, but by the time pumping resumes the concrete already placed has been sitting for about 50 minutes in warm conditions. The pump operator wants to add water at the hopper to get the mix flowing again. The steel fixer is worried about a cold joint forming where the new concrete meets the old.
1. Evaluate the proposal to add water at the hopper, and explain what it would do to the concrete already placed and to the strength of the raft.
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Adding water at the hopper is not acceptable.
- It changes the mix design. The water/cement ratio is the single biggest control on strength and durability. Adding water on site raises the w/c ratio, reducing strength and increasing permeability and shrinkage - exactly the wrong outcome for a 40 MPa raft.
- It is uncontrolled. The amount added at the hopper is not measured against the batch, so the actual w/c ratio of that portion is unknown. The delivered mix already has a designed slump of 160 mm; if it has stiffened, that is a time/temperature effect, not a reason to add water.
- It affects concrete already placed. The water-rich concrete will be weaker and more permeable where it lands, and it will not bond as well to the older, stiffer concrete. It can also cause bleeding and segregation at the interface.
- It may breach the specification. Most specifications prohibit adding water on site except within strict limits with approval, and the batch records would no longer match what was placed.
What to do instead:
- If the mix has stiffened, the correct remedy is a approved retarder or superplasticiser dosed to the batch under the supplier's direction - not water.
- Check the batch time and temperature; if the concrete is outside its working life, it should not be placed.
- The pump blockage itself should be investigated (aggregate size vs line diameter, line length, elbow count, priming) so it does not recur.
Bottom line: water at the hopper trades a short-term flow problem for a permanent strength and durability problem in a structural raft. It should be refused.
2. The pour must continue. Explain how you would manage the 50-minute-old concrete and the risk of a cold joint, and what you would record.
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Managing the interface:
- Assess the placed concrete. With a retarder and warm conditions, 50 minutes may still be within the initial set window, so the surface may still be workable. Check by probing: if the concrete is still plastic and can be vibrated, a cold joint may be avoided.
- If still plastic: recommence placing as soon as possible, vibrate the new concrete into the old layer (penetrating the vibrator into the previous layer) so the two are integrated. Keep the surface from drying out in the meantime.
- If it has started to set: treat it as a cold joint. Prepare the surface - remove laitance and loose material, roughen, clean, and use a bonding agent or a rich mortar as specified - before placing new concrete. Do not simply dump new concrete on a set surface.
- Keep the pour moving. Once pumping resumes, maintain a steady rate and avoid a second stoppage; have a standby pump or a second line if the risk is high.
- Consider a construction joint. If the delay is long enough that a proper cold joint cannot be avoided, form a deliberate construction joint at a designed location (with a key and continuity of reinforcement) rather than leaving a random, unprepared cold joint.
Records:
- Time of stoppage and resumption, and the temperature of the placed concrete.
- Whether the interface was still plastic or had set, and the preparation applied.
- The batch/docket details and any admixture added (with approval).
- A non-conformance or inspection record for the joint, and any testing (e.g. cores) the engineer requires.
Bottom line: the priority is to keep the concrete alive and integrate it if possible; if not, make a proper, prepared construction joint and document it. Adding water is never the answer.