Plain & Reinforced Concrete-1 CE-313

1. Plain & Reinforced Concrete-1CE-313 Lecture # 1 31th Jan 2012 Introduction

2. Plain & Reinforced Concrete-1 Plain Concrete • Constituent material of concrete and their properties. • Hydration of cement. • Properties of fresh and hardened concrete and factors effecting them. • Curing of concrete and its significance. • Testing of concrete for various properties including physical tests, strength tests. • Crushing or ultimate strain. • Modulus of elasticity of concrete, types, tests. Determination and significance. • Design of normal concrete mixes, factors affecting the workability of the fresh concrete and strength & durability of the hardened concrete. • Alkali aggregate reaction, carbonation and sulfate attack. • Additives and admixtures for concrete. • Cracks in concrete.

3. Plain & Reinforced Concrete-1 Mechanics of Reinforced Concrete • Basics of composite action of steel and concrete. • Stress-strains curves of steel and concrete. • Actual, simplified and equivalent stress blocks. • Behavior of reinforced concrete members including columns, beams and slabs at working and ultimate loads. • Specifications, codes of practice and design loads. • Analysis, design and detailing of • Simply supported rectangular and T-beam by ultimate strength design method • Simply supported and continuous one way and two way slabs. • Reinforced concrete members for axial compression and tension. • Tied and spiral columns. • ACI code provisions for design of columns.

4. Plain & Reinforced Concrete-1 Mechanics of Reinforced Concrete (contd…) • Shear and diagonal tension in concrete, design and detailing of flexural members for shear. • Corner reinforcement in slabs. • Assessment of crack width in flexural members. • Introduction to alternate method of design with applications Practical • Physical testing of constituent material for concrete. • Acceptance test for cement. • Test on fresh and reinforced concrete for workability, compressive strength, tensile strength, modulus of rupture and modulus of elasticity. • Casting of different types of beams and columns and testing to study the effects of various factors. • Detailing of designed elements.

5. Plain & Reinforced Concrete-1 • Text Book • Design of Concrete Structures (13th Edition) by Arthur H. Nilson, David Darwin & Charles W. Dolan • Concrete Structures (Part-I) by Zahid Ahmad Siddiqi • References • Reinforced Concrete (5th Edition) by Edward G. Nawy • Building Code Requirements for Structural Concrete (ACI 318-08)

6. Plain & Reinforced Concrete-1 Concrete Concrete is a mixture of cement, fine and coarse aggregate. Concrete mainly consists of a binding material and filler material. If filler material size is < 5mm it is fine aggregate and > 5mm is coarse aggregate. Plain Cement Concrete (PCC) Mixture of cement , sand and coarse aggregate without any reinforcement is known as PCC. PCC is strong in compression and week in tension. Its tensile strength is so small that it can be neglected in design. Reinforced Cement Concrete (RCC) Mixture of cement , sand and coarse aggregate with reinforcement is known as RCC. (Tensile strength is improved)

7. Plain & Reinforced Concrete-1 Reinforced Cement Concrete (RCC) contd.. Mix Proportion Cement : Sand : Crush 1 : 1.5 : 3 1 : 2 : 4 1 : 4 : 8 Water Cement Ratio (W/C) W/C = 0.5 – 0.6 For a mix proportion of 1:2:4 and W/C = 0.5, if cement is 50 kg Sand = 2 x 50 = 100 Kg Crush = 4 x 50 = 200 Kg Batching By Weight Water = 50 x 0.5 = 25 Kg

8. Plain & Reinforced Concrete-1 Mechanism of Load Transfer Function of structure is to transfer all the loads safely to ground. A particular structural member transfers load to other structural member.

9. Plain & Reinforced Concrete-1 Merits of Concrete Construction • Good Control over cross sectional dimensions and ShapeOne of the major advantage of concrete structures is the full control over the dimensions and structural shape. Any size and shape can be obtained by preparing the formwork accordingly. • Availability of MaterialsAll the constituent materials are earthen materials (cement, sand, crush) and easily available in abundance. • Economic StructuresAll the materials are easily available so structures are economical. • Good InsulationConcrete is a good insulator of Noise & heat and does not allow them to transmit completely.

10. Plain & Reinforced Concrete-1 Merits of Concrete Construction (contd…) • Good Binding Between Steel and Concretethere is a very good development of bond between steel and concrete. • Stable StructureConcrete is strong in compression but week in tension and steel as strong in tension so their combination give a strong stable structure. • Less Chances of BucklingConcrete members are not slim like steel members so chances of buckling are much less. • Aestheticsconcrete structures are aesthetically good and cladding is not required

11. Plain & Reinforced Concrete-1 Merits of Concrete Construction (contd…) • Lesser Chances of Rustingsteel reinforcement is enclosed in concrete so chances of rusting are reduced. Demerits of Concrete Construction • Week in tensionConcrete is week in tension so large amount of steel is required. • Increased Self WeightConcrete structures have more self weight compared with steel structures so large cross-section is required only to resist self weight, making structure costly. • CrackingUnlike steel structures concrete structures can have cracks. More cracks with smaller width are better than one crack of larger width.

12. Plain & Reinforced Concrete-1 Demerits of Concrete Construction • Unpredictable BehaviorIf same conditions are provided for mixing, placing and curing even then properties can differ for the concrete prepared at two different times. • Inelastic Behaviorconcrete is an inelastic material, its stress-strains curve is not straight so its behavior is more difficult to understand. • Shrinkage and CreepShrinkage is reduction in volume. It takes place due to loss of water even when no load is acting over it. Creep is reduction in volume due to sustained loading when it acts for long duration. This problem is not in steel structures. • Limited Industrial BehaviorMost of the time concrete is cast-in-situ so it has limited industrial behavior.

13. Plain & Reinforced Concrete-1 Specification & Codes These are rules given by various organizations in order to guide the designers for safe and economical design of structures Various Codes of Practices are • ACI 318-05 By American Concrete Institute. For general concrete constructions (buildings) • AASHTO Specifications for Concrete Bridges. By American Association of State Highway and Transportation Officials. • ASTM (American Standards for Testing and Materials) for testing of materials.

14. Plain & Reinforced Concrete-1 Specification & Codes (contd…) No code or design specification can be construed as substitute for sound engineering judgment in the design of concrete structures. In the structural practice, special circumstances are frequently encountered where code provisions can only serve as a guide, and engineer must rely upon a firm understanding of the basic principles of structural mechanics applied to reinforced or pre-stressed concrete, and the intimate knowledge of nature of materials

15. Plain & Reinforced Concrete-1 Design Loads • Dead Load“The loads which do not change their magnitude and position w.r.t. time within the life of structure”Dead load mainly consist of superimposed loads and self load of structure. • Self LoadIt is the load of structural member due to its own weight. • Superimposed LoadIt is the load supported by a structural member. For instance self weight of column is self load and load of beam and slab over it is superimposed load.

16. Plain & Reinforced Concrete-1 Design Loads (contd…) • Live Load“Live loads consist chiefly of occupancy loads in buildings and traffic loads on bridges” • They may be either fully or partially in place or not present at all, and may also change in location. • Their magnitude and distribution at any given time are uncertain, and even their maximum intensities throughout the life time of the structure are not known with precision. • The minimum live loads for which the floor and roof of a building should be designed are usually specified in the building codes that governs at the site construction.

17. Plain & Reinforced Concrete-1 Densities of Important Materials Intensities of Live Loads (Table 1.1, Design of concrete structures by Nilson)

18. Plain & Reinforced Concrete-1 Basic Design Equation Applied Action x F.O.S = Max. Internal Resistance Factor of Safety F.O.S. = Max. Failure load/Max. Service Load Following points are relevant to F.O.S • It is used to cover uncertainties due to • Applied loads • Material strength • Poor workmanship • Unexpected behavior of structure • Thermal stresses • Fabrication • Residual stresses • If F.O.S is provided then at service loads deflection and cracks are within limits. • It covers the natural disasters.

19. Plain & Reinforced Concrete-1 Ultimate Strength Design (USD)/LRFD Method Strength design method is based on the philosophy of dividing F.O.S. in such a way that Bigger part is applied on loads and smaller part is applied on material strength. Material Strength ≥ Applied Load x F.O.S.1 x F.O.S.2 {1 / F.O.S.2} Material Strength ≥ Applied Load x F.O.S.1 F.O.S.1 = Overload factor or Load Factor {greater than 1} 1/F.O.S.2 = Strength Reduction factor or Resistance Factor {less than 1}

20. Plain & Reinforced Concrete-1 Ultimate Strength Design (USD)/LRFD Method (contd...) ΦSn ≥ U Where Sn = Nominal Strength ΦSn = Design Strength Φ = Strength Reduction Factor U = Required Strength, calculated by applying load factors For a member subjected to moment, shear and axial load: ΦMn ≥ Mu ΦVn ≥ Vu ΦPn ≥ Pu

21. Plain & Reinforced Concrete-1 Allowable Strength Design (ASD) In allowable strength design the whole F.O.S. is applied on material strength and service loads (un-factored) are taken as it is. Material Strength / F.O.S. ≥ Service Loads In both Allowable strength design and Ultimate strength design analysis carried out in elastic range. fc’ fu Stress fy fc’/2 Stress Concrete Steel fy/2 Strain Strain

22. Plain & Reinforced Concrete-1 Plastic Design In plastic design, plastic analysis is carried out in order to find the behavior of structure near collapse state. In this type of design material strength is taken from inelastic range. It is observed that whether the failure is sudden or ductile. Ductile failure is most favorable because it gives warning before the failure of structures

23. Plain & Reinforced Concrete-1 Capacity Analysis In capacity analysis size, shape, material strengths and cross sectional dimensions are known and maximum load carrying capacity of the structure is calculated. Capacity analysis is generally carried out for the existing structures. Design of Structure In design of structure load, span and material properties are known and cross sectional dimensions and amount of reinforcement are to be determined.

24. Plain & Reinforced Concrete-1 Objectives of Designer There are two main objectives • Safety • Economy Safety The structure should be safe enough to carry all the applied throughout the life. Economy Structures should be economical. Lighter structures are more economical. Economy α 1/self weight (More valid for Steel Structures) In concrete Structures overall cost of construction decides the economy, not just the self weight.

25. Plain & Reinforced Concrete-1 Load Combinations To combine various loads in such a way to get a critical situation. Load Factor = Factor by which a load is to be increased x probability of occurrence • 1.2D + 1.6L • 1.4D • 1.2D + 1.6L + 0.5Lr • 1.2D + 1.6Lr + (1.0L or 0.8W) Where D = Dead load L = Live load on intermediate floors Lr = Live load on roof W = Wind Load

26. Plain & Reinforced Concrete-1 Strength Reduction Factor / Resistance Factor, Φ

27. Plain & Reinforced Concrete-1 Shrinkage “Shrinkage is reduction in volume of concrete due to loss of water” Coefficient of shrinkage varies with time. Coefficient of shortening is: • 0.00025 at 28 days • 0.00035 at 3 months • 0.0005 at 12 months Shrinkage = Shrinkage coefficient x Length Excessive shrinkage can be avoided by proper curing during first 28 days because half of the total shrinkage takes place during this period

28. Plain & Reinforced Concrete-1 Creep “creep is the slow deformation of material over considerable lengths of time at constant stress or load” Creep deformations for a given concrete are practically proportional to the magnitude of the applied stress; at any given stress, high strength concrete show less creep than lower strength concrete.

29. Plain & Reinforced Concrete-1 Creep (contd…) How to calculate shortenings due to creep? Consider a column of 3m which is under sustained load for several years. Compressive strength, fc’ = 30 MPa Sustained stress due to load = 10 MPa Specific creep for 28 MPa fc’ = 116 x 10-6 per MPa Creep Strain = 10 x 116 x 10-6 = 116 x 10-5 Shortening due to creep = 3000 x 116 x 10-5 = 3.48 mm

30. Plain & Reinforced Concrete-1 Specified Compressive Strength Concrete, fc’ “28 days cylinder strength of concrete” • The cylinder has 150mm dia and 300mm length. • According to ASTM standards at least two cylinders should be tested and their average is to be taken. ACI 5.1.1: for concrete designed and constructed in accordance with ACI code, fc’ shall not be less than 17 Mpa (2500 psi)

31. Plain & Reinforced Concrete-1 Specified Concrete Compressive Strength, fc’ BS specifies the compressive strength in terms of cube strength. • Standard size of cube is 6”x6”x6” • BS recommends testing three cubes and taking their average as the compressive strength of concrete Cylinder Strength = (0.75 to 0.8) times Cube Strength

32. Plain & Reinforced Concrete-1 Concrete Cylinder Concrete Cube

33. Plain & Reinforced Concrete-1 Relevant ASTM Standards • “Methods of Sampling Freshly Mixed Concrete” (ASTM C 172) • Practice for Making and Curing Concrete Test Specimens in Field” (ASTM C 31) • “Test Methods for Compressive Strength of Cylindrical Concrete Specimen” (ASTM C 39)

34. Plain & Reinforced Concrete-1 Testing of Samples for Compressive Strength Cylinders should be tested in moist condition because in dry state it gives more strength. ACI 5.6.2.1: Samples for strength tests of each class of concrete placed each day shall be taken : • Not less than once a day • Not less than once for each 110m3 of concrete. • Not less than once for each 460m2 of concrete. Code allows the site engineer to ask for casting the test sample if he regards it necessary.

35. Plain & Reinforced Concrete-1 Acceptance Criteria for Concrete Quality ACI 5.6.3.3: Strength level of an individual class of concrete shall be considered satisfactory if both of the following requirements are met: • Every arithmetic average of any three consecutive strength tests equals or exceeds fc’. • No individual strength test (average of two cylinders) falls below fc’ • by more than 3.5 MPa (500 psi) when fc’ is 35 MPa (5000 psi) or less; or • by more than 0.10fc’ when fc’ is more than 35 MPa

36. Plain & Reinforced Concrete-1 Acceptance Criteria for Concrete Quality(contd…) Example For Required fc’ = 20 MPa, if following are the test results of 7 samples • 19, 20, 22, 23, 19, 18, 24 MPa Mean 1 = (19 + 20 + 22) / 3 = 20.33 MPa Mean 2 = (20 + 22 + 23) / 3 = 21.67 MPa Mean 3 = (22 + 23 + 19) / 3 = 21.33 MPa Mean 4 = (23 + 19 + 18) / 3 = 20.00 MPa Mean 5 = (19 + 18 + 24) / 3 = 20.33 MPa • Every arithmetic average of any three consecutive strength tests equals or exceeds fc’. • Non of the test results fall below required fc’ by 3.5 MPa. Considering these two point the quality of concrete is acceptable

37. Plain & Reinforced Concrete-1 Mix Design • Ingredients of concrete are mixed together in order to get a specified Required Average Strength, fcr’ . • If we use fc’ as target strength during mix design the average strength achieved may fall below fc’. • To avoid under-strength concrete fcr’ is used as target strength in-place of fc’. fcr’ > fc’

38. Plain & Reinforced Concrete-1 Mix Design(contd…) ACI-5.3.2 Required Average Compressive Strength Table 5.3.2.1-Required Average Compressive Strength when Data Are Available to Establish a Sample Standard Deviation Ss = Standard deviation of compressive strength test

39. Plain & Reinforced Concrete-1 Mix Design(contd…) Table 5.3.2.2-Required Average Compressive Strength when Data Are Not Available to Establish a Sample Standard Deviation

40. Plain & Reinforced Concrete-1 Stress Strain Curve of Concrete • The first portion of curve, to about 40% of the ultimate strength fc’, can be considered linear. • The lower the strength of concrete the greater will be the failure strain Crushing Stress fc’ 0.85fc’ 0.4 fc’ 0.0028 to 0.0045,generally 0.003 Strain

41. Plain & Reinforced Concrete-1 Modulus of Elasticity Concrete is not an elastic material therefore it does not have a fixed value of modulus of elasticity Initial tangent Modulus Tangent Modulus 0.4fc’ Stress Secant Modulus Strain Tangent and Secant Moduli of Concrete

42. Plain & Reinforced Concrete-1 Modulus of Elasticity (contd…) Secant modulus (Ec) is the one which is being used in design. Ec = 0.043 wc1.5√fc’ wc = density of concrete in kg/m3 fc’ = specified cylinder strength in MPa For normal weight concrete, say wc = 2300 kg/m3 Ec = 4700√fc’

43. Plain & Reinforced Concrete-1 Reinforcing SteelSteel bars are: • Plain • Deformed (currently in use) Deformed bars have longitudinal and transverse ribs. Ribs provide a good bond between steel and concrete. If this bond fails steel becomes in effective. The most important properties for reinforcing steel are: • Young's modulus, E (200 GPa) • Yield strength, fy • Ultimate strength, fu • Size and diameter of bar

44. Plain & Reinforced Concrete-1 Steel Bars

45. Plain & Reinforced Concrete-1 Reinforcing Steel (contd..) Stress Strain Curve for Steel fu yielding Strain Hardening fy Stress fy/2 Strain

46. Plain & Reinforced Concrete-1 Reinforcing Steel (contd…) Steel Grade Designation • Grade 300, fy = 300 MPa Grade 40 • Grade 420, fy = 420 MPa Grade 60 • Grade 520, fy = 520 MPa Grade 70 FPS For hot rolled steel bars Cold twisted steel bars are available in grade 420 Grade 520 Grade 420 Stress Grade 300 For hot rolled steel bars Strain

47. Plain & Reinforced Concrete-1 Reinforcing Steel (contd..) For simplification the stress strain diagram is consider bilinear because after yielding cracks appear and concrete becomes in effective. Bilinear Curve Stress Strain

48. Plain & Reinforced Concrete-1 Marks Distribution • Mid Term Examination = 20 Marks • Final Term Examination = 40 Marks • Sessional = 20 Marks • Quiz - I = 37.5 % • Quiz – II = 37.5 % • Assignments = 25 % • Lab Work = 20 Marks

49. Concluded