Aggregate Choice: Charnockite Versus Khondalite
Two quarries near Thrissur supply aggregate for an M25 concrete pavement and a road base. Quarry A is a charnockite with coarse pyroxene and quartz; petrography shows 2% strained quartz and no clay minerals. Quarry B is a khondalite with garnet, sillimanite, graphite, and 8% weathered feldspar. Test results: A has UCS 180 MPa, water absorption 0.3%, flakiness index 18%, and ASR expansion 0.08% at 14 days; B has UCS 95 MPa, absorption 0.9%, flakiness 32%, and ASR expansion 0.18% at 14 days. The contractor wants to use B for both because it is cheaper by about 150 per tonne. The concrete will be exposed to coastal humidity and cyclic wetting. No petrographic examination of the fine aggregate is available. The specification requires ASR expansion below 0.10% at 14 days and absorption below 0.5% for coarse aggregate. The quarry B face contains discontinuous graphite-rich bands and sheared zones.
1. Evaluate A and B against the specification and recommend a use for each. Justify with the test data and petrography.
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Quarry A meets the stated limits: UCS 180 MPa, absorption 0.3% is below 0.5%, flakiness 18% is moderate, and ASR expansion 0.08% is below 0.10%. Its petrography shows only 2% strained quartz and no clay, so it is the safer choice for M25 concrete. A should be accepted for concrete, subject to checking the fine aggregate and doing a trial mix.
Quarry B fails two specification limits: absorption 0.9% is above 0.5%, and ASR expansion 0.18% is above 0.10%. The flakiness index of 32% is also high, and the rock contains 8% weathered feldspar plus graphite-rich sheared bands. Weathered feldspar increases absorption and reduces durability; graphite is soft and can create weak, flaky particles. B is not acceptable for the concrete pavement as tested. It might be considered for road base only after Los Angeles abrasion, CBR, and soundness testing, and even then its high flakiness and absorption make it marginal. The lower price does not compensate for non-compliance.
2. Explain why the strained quartz and graphite-bearing shear zones matter, and what additional tests you would order before accepting any aggregate.
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Strained quartz can react with alkalis from cement to form an alkali-silica gel. The gel absorbs water and expands, cracking concrete, especially under cyclic wetting and coastal humidity. The 2% strained quartz in A is low, but the fine aggregate could still be reactive, so it must be tested. Graphite is soft and acts as a lubricant; graphite-rich shear zones are planes of weakness that can produce flaky, low-strength particles and increase water absorption. Sheared zones also vary laterally, so a single bulk sample may hide worse material.
Additional tests should include: petrographic examination of both coarse and fine aggregate; mortar-bar or concrete-prism ASR testing using the actual cement; Los Angeles abrasion; sodium sulphate soundness; water absorption on separate size fractions; sulphate and chloride content; and trial mixes. For road base, add CBR and repeated-load tests. Sampling should be designed to include the graphite-rich bands, not just the average rock.
3. The contractor proposes blending 30% B with 70% A to reduce cost. Would you accept the blend on the basis of the arithmetic? Show the calculation and the non-arithmetic risks.
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A simple arithmetic blend gives ASR expansion = 0.70 x 0.08 + 0.30 x 0.18 = 0.056 + 0.054 = 0.110%, which is still above the 0.10% limit. Absorption = 0.70 x 0.3 + 0.30 x 0.9 = 0.21 + 0.27 = 0.48%, which is below 0.5%, but passing one limit does not cancel failing the other.
The arithmetic also hides variability. A graphite-rich batch of B could have much higher absorption and lower strength than the tested average, and blending does not remove the weak particles. The fine aggregate is still untested. I would not accept the blend on arithmetic alone. Acceptance would require petrography of the blended product, ASR testing of the actual blend and cement, durability tests, and a trial mix; otherwise B should be kept out of the concrete.