- 2066 Jestha (old course) · 2+3 marks
Chemical admixtures per ASTM; recommendations
Short answer
ASTM chemical admixtures include Types A–G and air-entrainng. For hot weather use a retarding admixture (B, D, or G) to delay setting and maaintain workability. For cold weather use an accelerating admixture (C or E) for faaster strength gain and frost protection. For freezing-thawing exposure use an air-entrainng admixture with a low w/c to allow expansion relief.
List different types of chemical admixtures used in concrete as per ASTM standard. What type of admixture would you recommend for concreting in (i) hot weather, (ii) cold weather, (iii) frequent freezing and thawing environment? Explain with reason.
Chemical admixtures per ASTM C494 (and ASTM C260, C1017)
| Type | Admixture |
|---|---|
| A | Water-reducing |
| B | Retarding |
| C | Accelerating |
| D | Water-reducing and retarding |
| E | Water-reducing and accelerating |
| F | High-range water-reducing (superplasticiser) |
| G | High-range water-reducing and retarding |
| (ASTM C260) | Air-entraining admixture |
| (ASTM C1017) | Flowing-concrete (plasticising) admixture |
Recommendations
(i) Hot weather - Type B or D (retarder / water-reducing retarder), or Type G. High temperature speeds up hydration, so the slump is lost quickly and the concrete sets early, which can cause cold joints. A retarder delays setting and holds the workability for transport and placing, and the water reducer cuts the water needed for the slump, reducing heat and cracking.
(ii) Cold weather - Type C or E (accelerator / water-reducing accelerator), non-chloride for RCC (e.g. calcium nitrate). Low temperature slows hydration, so setting and strength gain are delayed and the young concrete may freeze. An accelerator gives faster setting and strength gain, which protects against frost damage and allows earlier formwork removal. Air entrainment is also helpful.
(iii) Freezing and thawing exposure - Air-entraining admixture (ASTM C260; 4-7 per cent air), preferably with a water reducer to keep the w/c at about 0.45 or lower. The tiny air bubbles act as relief chambers for the 9 per cent expansion of freezing water, preventing cracking and scaling of the paste.
✓ Answer checked by Claude Opus 5.5 (high effort)
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