Stilling Basin Raft Pour: Water Added at the Hopper and a Cement Blame Claim
A 60 m³ raft pour for the stilling basin of a barrage is under way. The approved M25 mix design specifies 340 kg/m³ of OPC 43 grade cement at a water–cement ratio of 0.45, with a retarder dosed at the batching plant. The pour window is four hours, the batching plant output is 15 m³/h, and each transit mixer carries 6 m³ with an 80-minute round trip. At the plant the slump is 75 mm; by the time the third mixer reaches the placement point after 40 minutes of transit the slump has fallen to 30 mm, and the site foreman orders 15 litres of water per cubic metre added at the hopper to restore workability. Later, two of the nine cube samples from the raft fall below 25 N/mm², and the contractor writes that the shortfall is caused by the cement supplied by the department. The site engineer must sign the pour card and answer the letter.
1. Quantify the effect of the water added at the hopper and state what should have been done instead.
Read the worked answer
Original mix water: 340 × 0.45 = 153 kg/m³.
After the addition: 153 + 15 = 168 kg/m³, giving an effective w/c of 168/340 = 0.494.
The mix is still nominally inside the IS 456 limit of 0.50 for M25 in moderate exposure, which is exactly why the foreman thought it was harmless. But the design margin is gone: as a rule of thumb a 0.05 rise in w/c beyond the design value costs roughly 8–10% of compressive strength, so the raft concrete is now a ~0.49 w/c mix with no reserve against batching variation.
Two further points matter. First, water 'ordered' at the hopper is not water measured - the actual quantity depends on the operator's judgement, so the effective w/c is unknown and unauditable on the pour card. Second, the addition is uncontrolled on a pour whose cubes are the acceptance evidence.
The correct action at the plant end: increase slump by admixture (a retarding mid-range or high-range water reducer compatible with the existing retarder), re-check slump at the placement point, and if the mix still does not place, reject the load. Never sign a pour card that records water added after batching.
2. Check whether the mixer fleet can sustain the pour within the four-hour window and identify the schedule risk.
Read the worked answer
Required delivery rate: 60 m³ / 4 h = 15 m³/h, which exactly matches the plant output - there is zero allowance for any stoppage.
Productivity of one mixer: 6 m³ in an 80-minute (1.333 h) cycle = 4.5 m³/h.
Fleet required: 15 / 4.5 = 3.33, so 4 mixers in the rotation, plus 1 standby.
The schedule risk is the matching of plant output to the window: batching must run for the full four hours with no interruption, and any plant breakdown, aggregate shortage or a single late mixer immediately converts into a cold joint in a raft that cannot practically be re-poured. A surge of trucks queuing at the hopper is equally bad, because it adds waiting time to a retarded mix.
Recommendation: start batching 30 minutes before the window, keep the fifth mixer on standby, define a designated cold-joint location and construction-joint detail in advance, and nominate one person at the placement point empowered to reject a load. Record batching time and placement time for every trip on the pour card.
3. How should the failing cube results be handled, and is the contractor's blame on departmental cement tenable?
Read the worked answer
First check the acceptance rule, not just '25 N/mm²'. Under IS 456 the criteria are: (a) each individual result ≥ fck − 3 N/mm², i.e. ≥ 22 N/mm² for M25; and (b) the mean of any four consecutive results ≥ fck + 0.825σ from the established standard deviation. Two results marginally below 25 do not by themselves mean rejection.
Second, look at causation. A w/c of 0.494 instead of 0.45 is a site-controlled deviation and is sufficient on its own to explain a shortfall. Cement is only a credible cause if the same lot fails its own tests under IS 4031/IS 269 - soundness, fineness, and 28-day mortar cube strength - and if other pours made from that lot with correct w/c are also low.
Third, act on the structure. Core testing as per IS 516 Part 5 (with adequate diameter and length-to-diameter ratio), supported by ultrasonic pulse velocity or rebound hammer for uniformity mapping, then a structural review by the designer before accepting or repairing the raft.
On the letter: even with departmental cement, the mix design, batching control and workmanship are the contractor's responsibility. Reply with the pour card showing unauthorised water addition, the cement test certificates, and a non-conformance report. The claim, as worded, is not tenable.