Tentative Guidelines on Cement-Fly Ash Concrete for Rigid Pavement Construction
1976 Edition

Per IRC 68 (1976), fly ash incorporated in cement-fly ash concrete should comply with IS: 3812-1966 (Part I) standards. Key requirements include lime reactivity of at least 40 kg/cm², a Blaine surface area minimum of 3200 cm²/g, and a carbon content not exceeding 12%. For lignite fly ash, additional limits specify magnesium oxide content below 5% and sulphur trioxide less than 3%. These criteria ensure the fly ash exhibits adequate pozzolanic properties and compatibility with cement, promoting strength and durability in pavements.

To design an equivalent cement-fly ash concrete mix following IRC 68, begin with the original plain cement concrete proportions: water (W), cement (C), sand (S), and coarse aggregate (A). Determine replacement factors for cement (p_c, generally ~0.85) and sand (p_s, typically ~0.90). Calculate the reduced cement and sand quantities as C' = p_c × C and S' = p_s × S. Compute the fly ash amount (F) based on equal volume substitution, using the formula F = ((C - C') / S_c) + ((S - S') / S_f), where S_c and S_f are specific gravities of cement and fly ash respectively, converting volumes back to weights. Adjust water content to maintain the water-to-effective binder ratio: W' = W × (C' + F) / C. Lastly, modify coarse aggregate quantity to compensate for volumetric changes: A' = A + (W - W') × S_a, where S_a is aggregate specific gravity. Trial mixes are recommended to fine-tune replacement ratios for desired workability and strength.

The recommended batching and mixing approach for cement-fly ash concrete per IRC 68 includes batching fly ash, cement, sand, and aggregates by weight, while water may be measured by calibrated volume. Fly ash moisture content should be checked and weights adjusted accordingly. For small quantities, fly ash can be weighed on a pan balance and added directly. The mixing sequence is critical: water is added first to the mixer, followed by fly ash to create a slurry, then cement, and finally aggregates. This ensures uniform dispersion of fly ash, preventing clumping and maintaining consistency in the concrete mix.

Fly ash, due to its fine particle size and propensity to become airborne, should be stored in covered environments such as bagged forms or specially excavated rectangular trapezoidal pits. The tops of these storage pits must be protected with tarpaulins or equivalent covers to guard against moisture ingress and dust emission. During transportation, fly ash should be either bagged or wetted at the top to minimize dust. For batching, fly ash must be weighed by weight with moisture content regularly monitored; adjustments should be made for wet fly ash. Following this, fly ash is added after water during mixing to form a slurry, ensuring even distribution and preserving its pozzolanic effectiveness.

Key quality control procedures for cement-fly ash concrete involve verifying fly ash compliance with IS: 3812 (Part I)-1966 upon receipt and for every new consignment. Moisture content of fly ash should be checked once or twice daily to maintain dosing accuracy. The mixing process must follow the sequence of adding water first, then fly ash slurry, followed by cement and aggregates, ensuring thorough and uniform mixing. Batching should be done by weight with moisture corrections as needed. Additionally, material storage must prevent dust dispersion, typically by covered pits or bagging. Regular testing and inspections following IRC SP: 11-1973 guidelines are necessary to confirm the concrete’s workability and strength meet design requirements.