--- title: "Chemical admixtures per ASTM; recommendations" subject: "Concrete Technology and Masonry Structure" chapter: 1 key: "semester_5/concrete_technology_and_masonry_structure:admixture-recommendation-weather" source: https://www.bctengineer.com/q/semester_5/concrete_technology_and_masonry_structure/admixture-recommendation-weather site: BCT Engineering --- # Chemical admixtures per ASTM; recommendations Concrete Technology and Masonry Structure · Chapter 1: Introduction to concrete and concrete materials - Asked in: 2066 Jestha (old course), 2+3 marks - Video: [CH7 - Part 3: Chemical Admixtures for Concrete](https://www.youtube.com/watch?v=Apn9dValdME) — Nathan Robles - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/concrete_technology_and_masonry_structure/admixture-recommendation-weather - Whole chapter: https://www.bctengineer.com/md/semester_5/concrete_technology_and_masonry_structure/chapter-1 ## Question 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. ## 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. ## Answer #### 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. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which admixture suits hot-weather concreting, and why? A. Accelerator (Type C), to gain strength faster B. Retarder or water-reducing retarder (Type B/D), to delay setting and hold workability C. Air-entraining agent, to relieve expansion D. Chloride-based accelerator, to cut heat Answer: B. High temperature speeds hydration, causing slump loss and flash set. A retarder delays setting and keeps the mix workable. (Key point: Hot weather admixture) 2. For cold-weather RCC, which admixture is recommended? A. Type B retarder B. Non-chloride accelerator (Type C/E) such as calcium nitrate C. Chloride accelerator only D. Type F superplasticiser only Answer: B. Low temperature slows hydration, so an accelerator speeds setting and strength gain. It should be non-chloride to protect steel. (Key point: Cold weather admixture) 3. What protects concrete from frequent freezing and thawing? A. Retarder B. Accelerator C. Air-entraining admixture giving about 4-7 per cent air D. Higher w/c ratio for more voids Answer: C. Tiny air bubbles act as relief chambers for the expansion of freezing water, preventing cracking and scaling. (Key point: Freeze-thaw admixture) 4. In ASTM C494, which type is a high-range water reducer (superplasticiser)? A. Type A B. Type C C. Type D D. Type F Answer: D. Type F is high-range water-reducing; A is water-reducing, C accelerating, D water-reducing and retarding. 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