Fine Aggregate FM Shift After a Supplier Source Change
Over three weeks, the fineness modulus of the plant's fine aggregate has risen from 2.82 to 3.14 after the supplier switched to a new pit. The mix is a 4,000 psi precast box culvert mix with 564 lb/yd³ of Type I/II cement, 0.45 w/cm, 1,250 lb/yd³ of 1-in nominal coarse aggregate, and a Type A water reducer at 4 oz/cwt. The sand is now harsher, and the plant observes 1.5 in of bleed water on a 12-in-deep test block, segregation at the end of a 30-ft chute, and slump loss from 5 in to 2.5 in in 25 minutes at 88°F. The cement lot also changed; its C3A is 8.5% versus 6.0% previously. Batch water is adjusted by probe. The producer wants to add water at the truck to restore slump. The QC technician must decide what to test, whether to accept the sand, and how to adjust the mix without sacrificing strength or durability.
1. What tests and observations should the QC technician require before accepting the new sand, and what do the bleeding and segregation suggest?
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A fineness modulus alone is not an acceptance test. The technician should require a full gradation against ASTM C33 limits, including the percent passing the #50 and #100 sieves, because an FM rise from 2.82 to 3.14 usually means a loss of fine material. Also test absorption, bulk specific gravity, material finer than the #200 sieve, clay lumps and friable particles, organic impurities, and soundness if the source is new. Run a mortar or concrete trial at the actual mix proportions and measure bleed water, slump loss, air content, setting time, and compressive strength. The 1.5 in of bleed water and chute segregation point to a cohesion deficiency: the sand is coarser and has fewer fines to hold the paste, so the mix is more prone to settling and aggregate separation. The higher C3A cement can increase water demand and early stiffening, so the slump loss may be a combined effect rather than sand alone. The technician should not attribute all symptoms to one variable without trial data.
2. Propose a numerical adjustment path and explain the order of adjustments.
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Do not add water at the truck as the first move; that raises w/cm above 0.45 and directly reduces strength and durability. First, restore grading and cohesion by increasing the fine aggregate content or blending in a finer sand. For example, if the sand is 1,250 lb/yd³, adding 50 to 80 lb/yd³ of finer sand or blending 30% of a 2.70 FM sand may restore the combined FM toward 2.90. Second, increase the water reducer from 4 to 6 oz/cwt and consider a retarder for the 88°F placement; this addresses slump loss without adding water. Third, rebalance coarse aggregate downward only if the sand increase overfills the void system. Maintain cement at 564 lb/yd³ and hold w/cm at or below 0.45. If slump still cannot be maintained, adjust the mix design in a trial batch and verify 7- and 28-day strength. Adding water at the truck treats the symptom and makes the batch noncompliant with the approved w/cm.
3. Should the plant accept the sand? Draft the supplier communication.
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Acceptance should be conditional, not blanket. The plant can accept the sand for trial use if each stockpile is segregated and tested, but it should not release production concrete until the trial batch meets slump, air, bleed, and strength requirements. The supplier communication should state the observed FM shift from 2.82 to 3.14, the resulting bleeding and segregation, and the specific C33 gradation limits that are in question. Request the pit's gradation and absorption data, the source change notice, and a plan to blend or screen the sand to restore a target FM of 2.80 to 3.00. Set temporary acceptance testing at every 500 tons or each stockpile, whichever is more frequent, plus a weekly gradation check. If the supplier cannot meet the limits, the plant should reject the stockpile and qualify an alternate source. Document the decision and the basis so the project engineer can review it.