BMS_ASSIGNMENT_SHRUTIGARG (1).pdf
- 1. BUILDING MATERIAL SCIENCE-3 ASSIGNMENT -2 AGGREGATES AND ADMIXTURES AGGREGATES ● WHAT ARE AGGREGATES? ● Aggregate, in building and construction, material used for mixing with cement, bitumen, lime, gypsum, or other adhesive to form concrete or mortar.
- 2. ● The aggregate gives volume, stability, resistance to wear or erosion, and other desired physical properties to the finished product. ORIGIN OF AGGREGATES Fine aggregates are obtained from a variety of sources. The sources of aggregate are invariably close to their demand locality; It is difficult to transport the large quantity of aggregate (in tonnes) and there will be high cost of transportation. They can be sourced from pits, riverbanks and beds, the seabed, gravelly or sandy terraces, beaches and dunes. The other deposits that provide granular materials can be processed with minimal extra effort or cost. Sand and gravel, which are unconsolidated sedimentary materials, are important sources of natural aggregate. The occurrence of high quality natural sands and gravels with in economic distance of major urban areas may be critical for viable concrete construction in those areas. 2
- 3. MANUFACTURING PROCESS ● The first stage of aggregate processing involves quarrying where a large deposit of desirable aggregate is identified and extracted from the ground. Hardened substances are either blasted or cut into smaller manageable-sized rubble and transported by truck or conveyor belts to the processing area. ● At the processing site, the aggregates are fed through a crusher. In most cases, a plant will have a primary crusher, which will reduce the size of the quarried rubble down to chunks about six-inches in circumference. ● The product from the primary crusher is then screened for larger usable products. The remaining material is transported by a conveyor belt to a second 3
- 4. Vibratory feeder for secondary crushing and screening which is reducing and separating the aggregate into some more usable products. ● Finally, after tertiary crushing, the Aggregate Screen — with single, double, and triple deck configurations — allows for the quick identification and separation of variously sized aggregate for use in concrete and asphalt. CLASSIFICATION OF AGGREGATES ❖ Classification of aggregates based on: Grain Size ❖ Classification of aggregates based on: Density ❖ Classification of aggregates based on: Geographical Origin ❖ Classification of aggregates based on: Shape ● Classification of aggregates based on: Grain Size 4
- 5. If you separate aggregates by size, there are two overriding categories: ● Fine ● Coarse The size of fine aggregates is defined as 4.75mm or smaller. That is, aggregates which can be passed through a number 4 sieve, with a mesh size of 4.75mm. Fine aggregates include things such as sand, silt and clay. Crushed stone and crushed gravel might also fall under this category. Typically, fine aggregates are used to improve workability of a concrete mix. Coarse aggregates measure above the 4.75mm limit. These are more likely to be natural stone or gravel that has not been crushed or processed. These aggregates will reduce the amount of water needed for a concrete mix, which may also reduce workability but improve its innate strength. ● Classification of aggregates based on: Density There are three weight-based variations of aggregates: ● Lightweight ● Standard ● High density Different density aggregates will have much different applications. Lightweight and ultra lightweight aggregates are more porous than their heavier counterparts, so 5
- 6. they can be put to great use in green roof construction, for example. They are also used in mixes for concrete blocks and pavements, as well as insulation and fireproofing. High density aggregates are used to form heavyweight concrete. They are used for when high strength, durable concrete structures are required – building foundations or pipework ballasting, for example. ● Classification of aggregates based on: Geographical Origin Another way to classify aggregates is by their origin. You can do this with two groups: ● Natural ● Processed Aggregates taken from natural sources, such as riverbeds, quarries and mines. Sand, gravel, stone and rock are the most common, and these can be fine or coarse. Artificial aggregates or ‘by-product’ aggregates, they are commonly taken from industrial or engineering waste, then treated to form construction aggregates for high quality concrete. Common processed aggregates include industrial slag, as well as burnt clay. Processed aggregates are used for both lightweight and high-density concrete mixes. ● Classification of aggregates based on: Shape 6
- 7. Shape is one of the most effective ways of differentiating aggregates. The shape of your chosen aggregates will have a significant effect on the workability of your concrete. Aggregates purchased in batches from a reputable supplier can be consistent in shape, if required, but you can also mix aggregate shapes if you need to. The different shapes of aggregates are: ● Rounded – Natural aggregates smoothed by weathering, erosion and attrition. Rocks, stone, sand and gravel found in riverbeds are your most common rounded aggregates. Rounded aggregates are the main factor behind workability. ● Irregular – These are also shaped by attrition, but are not fully rounded. These consist of small stones and gravel, and offer reduced workability to rounded aggregates. ● Angular – Used for higher strength concrete, angular aggregates come in the form of crushed rock and stone. Workability is low, but this can be offset by filling voids with rounded or smaller aggregates. ● Flaky – Defined as aggregates that are thin in comparison to length and width. Increases surface area in a concrete mix. ● Elongated – Also adds more surface area to a mix – meaning more cement paste is needed. Elongated aggregates are longer than they are thick or wide. ● Flaky and elongated – A mix of the previous two – and the least efficient form of aggregate with regards to workability. 7
- 8. PROPERTIES OF AGGREGATES But there are properties or characteristics of aggregate which influence the properties of resulting concrete mix. These are as follows- 1. Composition 2. Size & Shape 3. Surface Texture 4. Specific Gravity 5. Bulk Density 6. Voids 7. Porosity & Absorption 8. Bulking of Sand 9. Fineness Modulus of Aggregate 10.Surface Index of Aggregate 11.Deleterious Material 8
- 9. 12.Crushing Value of Aggregate 13.Impact Value of Aggregate 14.Abrasion Value of Aggregate Maximum size of aggregate should be less than- ● One-fourth of the minimum dimension of the concrete member. ● One-fifth of the minimum dimension of the reinforced concrete member. ● The minimum clear spacing between reinforced bars or 5 mm less than the minimum cover between the reinforced bars and form, whichever is smaller for heavily reinforced concrete members such as the ribs of the main bars. Depending upon the amount of moisture content in aggregates, it can exist in any of the 4 conditions- ● Very dry aggregate ( having no moisture) ● Dry aggregate (contain some moisture in its pores) ● Saturated surface dry aggregate (pores completely filled with moisture but no moisture on surface) ● Moist or wet aggregates (pores are filled with moisture and also having moisture on surface) Aggregates should not contain any harmful material in such a quantity so as to affect the strength and durability of the concrete. Such harmful materials are called deleterious materials. Deleterious materials may cause one of the following effects- ● To interfere hydration of cement 9
- 10. ● To prevent development of proper bond ● To reduce strength and durability ● To modify setting times Deleterious materials generally found in aggregates, may be grouped as under ● Organic impurities ● Clay , silt & dust ● Salt contamination APPLICATIONS OF AGGREGATES ● They are added to concrete mixes to keep cement costs down and to reduce the shrinkage and cracking that is often found in concrete structures. 10
- 11. ● They are used to line railroad tracks, helping carry the load of trains and providing better drainage of water away from the rails. ● They fill in voids around the foundations of homes and to protect underground pipes that are located beneath homes and other structures. ● They are used in the manufacture of some brands of cat litter. ● They are included in roofing shingles to make them durable enough to stand up to the weather. ● They are used to build roads, with about 30,000 tons of aggregates needed to build one mile of a four-lane highway. ● They are incorporated in landscaping designs to create drainage for plant life and to boost the aesthetic appeal of homes and businesses. ● They are used when creating bricks for use in construction, helping to make them stronger. ● Crushed limestone is used at power plants to reduce the high levels of sulfur dioxide emissions. ● They are used to provide the minerals that are essential for creating cosmetics, soaps, fertilizers, glues and many other everyday products. ADMIXTURES 11
- 12. ● Admixtures are artificial or natural materials added to the concrete besides cement, water and aggregate to improve certain properties of concrete during casting or setting or service stage. ● To improve the properties of the concrete required. It can be divided in 2 groups that is: 1. Chemical 2. Mineral ● Major functions and advantages of using admixtures: ● To improve workability of fresh concrete ● To improve durability by entrainment of air ● To reduce the water required ● To accelerate setting & hardening & thus to produce high early strength ● To aid curing ● To impart water repellent / waterproofing property ● To cause dispersion of the cement particles when mixed with water ● To retard setting ● To improve wear resistance (hardness) ● To offset / reduce shrinkage during setting & hardening ● To cause expansion of concrete and automatic prestressing of steel. ● To aerate mortar / concrete to produce a light-weight product ● To impart colour to concrete ● To offset or reduce some chemical reaction ● To reduce bleeding 12
- 13. ● To reduce the evolution of heat ● TYPES OF CHEMICAL ADMIXTURES Additives are used in concrete mixes to enhance its properties. Depending on the specific function or action on the concrete property, it can be classified into following: ● Accelerator ● Water reducing Admixture ● Superplasticizer ● Air Entraining ● Retarding ● Corrosion Inhibitors ● Alkali-Aggregate Reaction Inhibiting ● Shrinkage Reducing Accelerator ● Added to increase the rate of hydration of concrete mix which then lead to the increase in the rate of development of strength and ● greater heat evolution. ● Shorten the setting time. ● More rapid gain of strength & rapid setting. ● Disadvantages is possible cracking due to heat evolution & possibility of corrosion of embedded reinforcement. ● Water reducing Admixture/Plasticizers 13
- 14. ● Used to reduce the amount of water necessary to produce a concrete of a given consistency ● To increase the slump for a given water content ● To obtain specified strength at lower cement content ● Increases workability with faster gain of strength ● Increase the slump, reduce water content, save cement ● Disadvantages, it has the risk of corrosion. Superplastizers ● By adding to a hydraulic binder, gives very high workability and allows a large decrease in water content for a given workability ● Allows the particles to be more workable where it enable working with low w/c ratio ● Enhances hydration process, increases strength ● Eliminate concrete segregation & allow good dispersion of cement particles in water,accelerating the rate of hydration ● Uniform distribution of cement particles is partly responsible for the highly early strength in concrete made with superplasticizer ● Advantages of using Superplasticizers are a) Decreased time to place and finish b) Accelerated curing period c) Early removal of formwork ● Excessive dosage may render concrete too fluid, causing severe segregation. 14
- 15. Secondary case study Admixtures- Crushed stone, for construction. rs 675/ton ECA aggregates rs 600/litre 15
- 16. Expanded clay aggregate natural brown aggregates hydroclay. rs690/ bag Whitish limestone chips, packaging types. rs5.3/ kilogram Admixtures- Perma concrete admixtures, packaging 16
- 17. rs78/kilogram 17