21. What is the relationship between the maximum size of aggregates a.pdf
- 1. 21. What is the relationship between the maximum size of aggregates and concrete strengths for the same water-cement ratio 22. Certain harmful materias in aggregates, such as such as some shales, will cause popouts. What are the mechanisms of the popouts? 23. What are the minerals that are considered low shrinkage aggregates? 24. List three negative effects of organic impurities in concrete Solution 21) Relationship between maximum size of aggregates and concrete compreesive strength: Compressive strength of concrete depends on the aggreagte strength not necessarily on the aggragate size. However, with an increase in size of the aggreagte to the maximum the consumption of water-cement ratio decreases, thereby the density of concrete increases. With increase in density, the compressive strength increases. The surface texture of aggregate can either be smooth or rough. A smooth surface can improve the workability, yet a rougher surface generates a stronger bond between the paste and the agggregates creating a higher strength. Angular and elongated aggregates have higher surface to volume ratio, better bond characteristics but require more cement paste to produce a workable mixture. So for the same water-cement ratio, smooth aggregates are preferred to produce higher compressive strength. 22) Mechanisms of popouts: Shales have been found to be montmorillonitic. Popouts could be attributed to the pressures generated by the absorption of water by the monmorillonite. The reaction between cement alkalis and the reactive shale particles form an alkali-silica gel at the surface of the reacted particle. The quantity of gel produced from a single reactive particle, and the rate at which it forms, will be the greatest if the particles are exposed to high-alkali concentrations. Once formed, the alkali silica gel absorbs water and start to swell. The swollen gel occupies the available pore space in the concrete creating pressures, which often causes popouts. 23) Minerals that are considered low shrinkage aggregates: Metakalinite: It is a highly reactive material which is formed as a result of roasting of natural kaolinite at temperatures of 700oC to 900oC. Zeolite: The source of zeolite formation is due to settlement of volcanic ash and then reaction with lake salts. Fly ash: It is a by-product obtained during combustion of coal in thermal power plants. Silica fumes: It is a by-product of the production of elemental silicon or alloys containing silicon, also known as condensed silica fume or microsilica.
- 2. Slag: Blast furnace slag is a by product of extraction of iron ore. Amongst the mineral admixtures, slag possesses the highest specific gravity( around 2.8 to 3.0). Rice hush ash: It is a highly reactive pozzolan obtained by controlled calcination of rice husks. 24) Negative effects of organic impurities: - Organic impurities derived from decaying vegetation delay the setting time and hardening of concrete. - Silt, clay and dust may form a coating on the aggragate particles, resulting in weakened bond between aggregates and the cement paste. - High content of chlorides in water tends to show efflorescence, dampness and makes the steel reinforcement prone to corrosion.