Question 8 of 8: Concrete Curing and Steel Hardenability/Jominy Correlation

Question 8: Concrete Curing and Steel Hardenability/Jominy Correlation (20 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Given. (a) — general concrete-technology question. (b) 2.5 in. diameter bar, hardness traverse when still-oil quenched: surface 50 HRC, centre ≈ 36 HRC (both surfaces symmetric); four candidate hardenability curves A, B, C, D (Fig., "Hardenability Curves for the Four Steel Lots"); four position–diameter–quench correlation charts (surface, ¾-radius, mid-radius, centre), each carrying curves 1 (still water), 2 (mildly agitated oil), 3 (still oil), 4 (mildly agitated molten salt).

Find. (a) Concrete constituents, curing mechanism, effect of water/cement ratio. (b) Which lot (A/B/C/D) matches the unknown bar; the predicted hardness traverse if the bar were reheated and water-quenched instead.

(a) Concrete's main constituents are Portland cement, fine aggregate (sand), coarse aggregate (gravel or crushed stone), and water (often with admixtures and/or entrained air). Curing is the hydration reaction between the cement's calcium-silicate/aluminate compounds and water, producing calcium-silicate-hydrate (C–S–H) gel and calcium hydroxide; this exothermic reaction binds the aggregate into a hardened, load-bearing matrix, and it continues to consume water and develop strength over an extended curing period (commonly quoted as reaching design strength around 28 days) provided sufficient moisture and favourable temperature are maintained — premature drying halts hydration and permanently limits the final strength. The water/cement ratio strongly affects properties: a lower w/c ratio leaves less capillary porosity once hydration is complete, giving higher strength, lower permeability and better durability, but poorer workability and greater risk of incomplete compaction; a higher w/c ratio improves workability/placeability but leaves more capillary pores, lowering strength and durability and increasing shrinkage, permeability, and vulnerability to freeze–thaw damage and reinforcement corrosion.

Approach (b)

For each of the four bar positions (surface, ¾-radius, mid-radius, centre), read the equivalent Jominy (end-quench) distance for a 2.5 in. bar quenched in still oil (curve 3) from the corresponding correlation chart, then read the predicted hardness at that Jominy distance from each of the four candidate hardenability curves (A–D); the lot whose predicted surface–to–centre profile best matches the given traverse (50, …, 36, …, 50 HRC) is the unknown bar. Repeating the same four position readings using curve 1 (still water, a more severe quench) instead of curve 3, then reading hardness from the identified lot's own curve, gives the predicted water-quenched traverse.

1. Equivalent Jominy distances, still-oil quench (curve 3), $d=2.5$ in. Reading each of the four correlation charts at $d=2.5$ in.: surface ≈ 0.62 in, ¾-radius ≈ 0.82 in, mid-radius ≈ 0.89 in, centre ≈ 0.98 in (equivalent Jominy distance increases toward the centre, since the bar's interior always cools more slowly than its surface).
2. Predicted hardness at those distances, each lot. Reading the hardenability chart at 0.62 / 0.82 / 0.89 / 0.98 in for each curve gives approximately: Lot A ≈ 55/50/48/46; Lot B ≈ 50/42/39/37; Lot C ≈ 36/29/28/26; Lot D ≈ 23/21/20/20 HRC.
3. Compare with the observed traverse. The observed profile (surface 50, ¾R ≈ 46, mid-R ≈ 41, centre 36 HRC) tracks Lot B (50/42/39/37) to within about 4 HRC at every position, while Lot A is 4–10 HRC too hard throughout and Lots C and D are 10–27 HRC too soft — the unknown bar is Lot B.
4. Equivalent Jominy distances, still-water quench (curve 1), $d=2.5$ in. Reading curve 1 on the same four charts (water quenches faster, so every equivalent distance is smaller than for oil): surface ≈ 0.19 in, ¾-radius ≈ 0.41 in, mid-radius ≈ 0.57 in, centre ≈ 0.64 in.
5. Predicted water-quench traverse. Reading Lot B's own hardenability curve at these smaller distances gives the reheated-and-water-quenched traverse: $$\text{surface}\approx57,\ \tfrac34\text{R}\approx56,\ \text{mid-R}\approx52,\ \text{centre}\approx\boxed{49\ \text{HRC}}$$ — higher and noticeably flatter than the original still-oil traverse, but still showing a shallow dip at the centre because even a water quench cannot fully harden the core of a 2.5 in. bar all the way through.