Building construction/Unit 2 /Basic civil engineering
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The document provides information on concrete foundations, including the key ingredients of concrete and their functions. It discusses different grades of concrete based on their compressive strength. Formwork, mixing, placing, compaction and curing of concrete are explained. Different types of foundations are described, including shallow foundations like spread footings, strap footings and mat foundations. Deep foundations such as pile foundations and pier foundations are also summarized. Load bearing and framed structures are compared in terms of their suitability for different types of buildings and soils.
Building construction/Unit 2 /Basic civil engineering
- 1. UNIT – 2 BUILDING CONSTRUCTION CONCRETE FOUNDATION
- 2. TOPICS TO BE COVERED : • CONCRETE - Ingredients of Cement Concrete, Grades of Concrete, proportions for Nominal mix concrete, Workability & Compressive Strength of Concrete, Curing of Concrete. • FOUNDATION - Necessity of foundations, Definitions of Safe bearing capacity, Ultimate bearing capacity and factor of safety, Difference between Load Bearing & Framed Construction.
- 3. CONCRETE • Plain concrete, commonly known as concrete, is an intimate mixture of binding material, fine aggregate, coarse aggregate and water. • This can be moulded into desired shape and size before it losses plasticity and hardens. • Plain concrete is strong in compression but very week in tension.
- 4. INGREDIENTS OF CEMENT CONCRETE : • Major ingredients of concrete are: 1. Binding material (like cement, lime, polymer) 2. Fine aggregate (sand) 3. Coarse aggregates (crushed stone, jelly) 4. Water. • A small quantity of admixtures like air entraining agents, water proofing agents, workability agents etc. may also be added to impart special properties to the plain concrete mixture. Depending upon the proportion of ingredient, strength of concrete varies. It is possible to determine the proportion of the ingredients for a particular strength by mix design procedure. In the absence of mix design the ingredients are proportioned as 1:1:2, 1:1.5:3, 1:2:4, 1:3:6 and 1:4:8, which is the ratio of weights of cement to sand to coarse aggregate.
- 5. PROPORTION OF CEMENT, SAND AND COARSE AGGREGATES IN CONCRETE • In proportioning of concrete it is kept in mind that voids in coarse aggregates are filled with sand and the voids in sand are filled with cement paste. Proportion of ingredients usually adopted for various works are shown in Table below.
- 6. GRADES OF CONCRETE Grade of concrete Strength in N/mm2 Proportion M5 5 1:5:10 M7.5 7.5 1:4:8 M10 10 1:3:6 M15 15 1:2:4 M20 20 1:1.5:3 M25 (Design Mix) 25 1:1:2 Grade of concrete is defined as the minimum strength the concrete must posses after 28 days of construction with proper quality control. Grade of concrete is denoted by prefixing M to the desired strength in MPa. ... For example, for M20 concrete, mix proportion will be 1:1.5:3 for cement : sand: coarse aggregates.
- 8. • A minimum w/c ratio (water-to-cement ratio) of about 0.3 by weight is necessary to ensure that the water comes into contact with all cement particles (thus assuring complete hydration). • Typical values are in the 0.4 to 0.6 Advantages of low water/cement ratio: •Increased strength •Lower permeability •Increased resistance to weathering •Better bond between concrete and reinforcement •Reduced drying shrinkage and cracking •Less volume change from wetting and drying
- 9. FUNCTIONS OF VARIOUS INGREDIENTS • Cement is the binding material. After addition of water it hydrates and binds aggregates and the surrounding surfaces like stone and bricks. Generally richer mix (with more cement) gives more strength. • Setting time starts after 30 minutes and ends after 10 hours. Hence concrete should be laid in its mould before 30 minutes of mixing of water and should not be subjected to any external forces till final setting takes place.
- 10. • Coarse aggregate consists of crushed stones. It should be well graded and the stones should be of igneous origin. They should be clean, sharp, angular and hard. They give mass to the concrete and prevent shrinkage of cement. • Fine aggregate consists of river sand. It prevents shrinkage of cement. • When surrounded by cement it gains mobility enters the voids in coarse aggregates and binding of ingredients takes place. It adds density to concrete, since it fills the voids. Denser the concrete higher is its strength.
- 11. • Water used for making concrete should be clean. It activates the hydration of cement and forms plastic mass. As it sets completely concrete becomes hard mass. • Water gives workability to concrete which means water makes it possible to mix the concrete with ease and place it in final position. More the water better is the workability. However excess water reduces the strength of concrete. Figure shows the variation of strength of concrete with water cement ratio. • To achieve required workability and at the same time good strength a water cement ratio of 0.4 to 0.45 is used, in case of machine mixing and water cement ratio of 0.5 to 0.6 is used for hand mixing.
- 12. COMPRESSIVE STRENGTH OF CONCRETE • Compressive Strength - is defined as the measured maximum resistance of a concrete specimen to an axial load, usually expressed in N/mm2 at an age of 28 days
- 13. In practical terms, about 90% of its strength is gained in the first 28 days.
- 14. • During the first week to 10 days of curing it is important that the concrete not be permitted to freeze or dry out • Concrete compressive strength depends upon many factors: • Quality and proportions of the ingredients • The curing environment
- 15. • Workability - that property of freshly mixed concrete that determines its working characteristics, i.e. the ease with which it can be mixed, placed, compacted and finished. • Factors effecting workability: • Method and duration of transportation • Quantity and characteristics of cementing materials • Concrete consistency (slump) • Aggregate grading, shape & surface texture • % entrained air • Water content • Concrete & ambient air temperature • Admixtures WORKABILITY OF CONCRETE
- 16. SLUMP CONE TEST • Concrete slump test or slump cone test is to determine the workability or consistency of concrete mix prepared at the laboratory or the construction site during the progress of the work. • Concrete slump test is carried out from batch to batch to check the uniform quality of concrete during construction. • A good indication of the water content of a mix and thus the workability can be DETERMINED from a standard slump test.
- 19. SLUMP CONE • The slump cone consists of a metallic mould in the form of a frustum of a cone having internal dimensions as • Bottom diameter : 20 cm • Top diameter : 10 cm • Height : 30 cm • Thickness : ≤ 16 mm
- 21. CURING OF CONCRETE • Curing - maintenance of a satisfactory moisture content and temperature in concrete for a suitable period of time immediately following placing & finishing so that the desired properties may develop. • Factors that effect curing: • Time • Temperature • Moisture
- 22. • Curing of Concrete is a method by which the concrete is protected against loss of moisture required for hydration and kept within the recommended temperature range. Curing will increase the strength and decrease the permeability of hardened concrete. Curing of Concrete : - • Shading concrete work • Covering concrete surfaces with hessian or gunny bags • Sprinkling of water • Ponding method • Membrane curing • Steam curing
- 23. FOUNDATIONS • FOUNDATION : In engineering, a foundation is the element of a structure which connects it to the ground, and transfers loads from the structure to the ground. • Foundations are generally considered either shallow or deep. • Foundation engineering is the application of soil mechanics and rock mechanics (Geotechnical engineering) in the design of foundation elements of structures.
- 26. It should be able to bear live and dead load It should be taken sufficiently deep so as to reach hard strata. Base of the foundation should be rigid so that differential settlements are minimized. Foundation should transfer the load of the super structure to the base soil in such a way that there is no settlement. The load acting on the sub soil per unit area should always be less than the safe bearing capacity (SBC) of the soil . It must distribute the load of the structure on sufficiently large area. Area of the footing should be such large that the intensity of the load at its base doesn't exceed the SBC of the sub soil. They should be taken sufficiently deep to guard the building against damage or distress caused by swelling or shrinkage of the sub soil.
- 28. Bearing capacity of the soil ≈ supporting power of the soil
- 30. Load bearing Structure Framed structure A load bearing masonry structure has load bearing walls which receive the load and transmit the same to the ground through their foundations A framed structure has columns erected which in turn are braced together by beams and slab. These load bearing walls supports the entire load including their self weight. Space between column and beams are filled by panel walls. Foundations for load bearing walls may be of two types: i) Simple strip footing ii) Strip footing with masonry offsets (Stepped footing). Foundations for framed structures are of two types: i) Isolated footing spread footing slopped footing stepped footing ii) Pad footing simple pad footing stepped pad footing Suitable up to three floors Suitable for any number of floors Cost is less Cost is more Less space More space Not suitable for all types of soil Suitable for any type of soil.
- 31. Advantages of framed structure They are meant for multistoried buildings Framed structures provide greater floor area. The walls and partition walls which are thin resulting increase of floor area. Additions and alterations can be more easily done in the case of framed structures. Construction time is less in framed constructions They can resist earth quake shocks better than a load bearing structure.
- 33. Types of Foundation Broadly Classified to two types: 1) Shallow foundation : Depth ≤ Breadth 2) Deep foundation : Depth > Breadth Shallow foundations located just below the lowest part of the superstructure they support; deep foundations extend considerably deeper in to earth.
- 35. Spread footing It is the type of footing which supports either one wall or one column spread the superimposed load of wall or column over a large area. Spread footing for single column: It contains only one column. It is of three types Single footing Stepped footing Slopped footing
- 37. Spread footing for walls
- 41. STRAP Footing or CANTILEVER Footing Independent footings connected by a beam Provided when the distance between the columns are large
- 46. Mat foundation: It is a combined footing that covers the entire area beneath a structure and supports all the walls and columns. It is suitable when the load coming on the soil is practically uniform or soil is of yielding nature or where soil tends to cause differential settlement or spread footing would cover more than half the area of the foundation.
- 47. Pile Foundation: It is the type of foundation in which load is transmitted to the lower level by means of vertical members known as piles. Concrete piles are made up of concrete. 30 to 50 cm in Φ and 20 m length Made of seasoned timber. 20 to 50 cm in Φ and length 20 times Φ Bottom is sharpened and provided with iron shoe. It can be of I or hollow pipe section. Composition of wood and concrete. Wooden portion will be at the bottom portion and concrete at the upper.
- 50. Pier Foundation Pier foundation is generally shallower in depth that the pile foundation It is preferred when there exists hard strata at considerable depths