Plain & Reinforced Concrete-1 CE-313Lecture # 16th Feb 2008Introduction

Plain & Reinforced Concrete-1

Plain ConcreteConstituent 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.

Plain & Reinforced Concrete-1

Mechanics of Reinforced ConcreteBasics 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 methodSimply 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.

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 applicationsPracticalPhysical 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.

Plain & Reinforced Concrete-1Text BooksDesign of Concrete Structures (13th Edition) by Arthur H. Nilson, David Darwin & Charles W. DolanConcrete Structures by Prof. Dr. Zahid Ahmed SiddiquiReferencesReinforced Concrete (5th Edition) by Edward G. NawyBuilding Code Requirements for Structural Concrete (ACI 318-08)

Plain & Reinforced Concrete-1ConcreteConcrete 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)

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

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

Plain & Reinforced Concrete-1Merits of Concrete ConstructionGood Control over cross sectional dimensions and Shape One 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 Materials All the constituent materials are earthen materials (cement, sand, crush) and easily available in abundance.Economic Structures All the materials are easily available so structures are economical.Good Insulation Concrete is a good insulator of Noise & heat and does not allow them to transmit completely.

Plain & Reinforced Concrete-1Merits of Concrete Construction (contd)Good Binding Between Steel and Concrete there is a very good development of bond between steel and concrete.Stable Structure Concrete 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 Buckling Concrete members are not slim like steel members so chances of buckling are much less. Aesthetics concrete structures are aesthetically good and cladding is not required

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

Plain & Reinforced Concrete-1Demerits of Concrete ConstructionUnpredictable Behavior If same conditions are provided for mixing, placing and curing even then properties can differ for the concrete prepared at two different times.Inelastic Behavior concrete is an inelastic material, its stress-strains curve is not straight so its behavior is more difficult to understand.Shrinkage and Creep Shrinkage 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 Behavior Most of the time concrete is cast-in-situ so it has limited industrial behavior.

Plain & Reinforced Concrete-1Specification & Codes These are rules given by various organizations in order to guide the designers for safe and economical design of structuresVarious Codes of Practices areACI 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.

Plain & Reinforced Concrete-1Specification & 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

Plain & Reinforced Concrete-1Design LoadsDead 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 Load It is the load of structural member due to its own weight.Superimposed Load It 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.

Plain & Reinforced Concrete-1Design 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.

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

Occupancy / UseLive Load(Kg/m2)Residential/House/Class Room200Offices250-500Library Reading Room300Library Stack Room750Warehouse/Heavy storage1250

Plain & Reinforced Concrete-1Basic Design EquationApplied Action x F.O.S = Max. Internal ResistanceFactor of SafetyF.O.S. = Max. Failure load/Max. Service Load Following points are relevant to F.O.SIt is used to cover uncertainties due toApplied loadsMaterial strengthPoor workmanshipUnexpected behavior of structureThermal stressesFabricationResidual stressesIf F.O.S is provided then at service loads deflection and cracks are within limits.It covers the natural disasters.

Plain & Reinforced Concrete-1Ultimate Strength Design (USD)/LRFD MethodStrength 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}

Plain & Reinforced Concrete-1Ultimate Strength Design (USD)/LRFD Method (contd...) Sn UWhereSn = Nominal StrengthSn = Design Strength = Strength Reduction FactorU = Required Strength, calculated by applying load factorsFor a member subjected to moment, shear and axial load: Mn Mu Vn Vu Pn Pu

Plain & Reinforced Concrete-1Allowable 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 LoadsIn both Allowable strength design and Ultimate strength design analysis carried out in elastic range. fcfc/2ConcreteSteelfyfy/2fuStrainStrainStress Stress

Plain & Reinforced Concrete-1Plastic 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 an warning before the failure of structures

Plain & Reinforced Concrete-1Capacity AnalysisIn 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 StructureIn design of structure load, span and material properties are known and cross sectional dimensions and amount of reinforcement are to be determined.

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

Plain & Reinforced Concrete-1Load CombinationsTo 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 occurrence1.2D + 1.6L1.4D1.2D + 1.6L + 0.5Lr1.2D + 1.6Lr + (1.0L or 0.8W)WhereD = Dead loadL = Live load on intermediate floorsLr = Live load on roofW = Wind Load

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

Plain & Reinforced Concrete-1ShrinkageShrinkage is reduction in volume of concrete due to loss of waterCoefficient 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

Plain & Reinforced Concrete-1Creepcreep is the slow deformation of material over considerable lengths of time at constant stress or loadCreep 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.

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

Plain & Reinforced Concrete-1Specified Compressive Strength Concrete, fc28 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 17.5 Mpa (2500 psi)

Plain & Reinforced Concrete-1Specified Concrete Compressive Strength, fcBSS specifies the compressive strength in terms of cube strength. Standard size of cube is 6x6x6 BSS recommends testing three cubes and taking their average as the compressive strength of concrete Cylinder Strength = (0.75 to 0.8) times Cube Strength

Plain & Reinforced Concrete-1Concrete Cylinder Concrete Cube

Plain & Reinforced Concrete-1Relevant 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)

Plain & Reinforced Concrete-1Testing of Samples for Compressive StrengthCylinders 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 115m3 of concrete. Not less than once for each 450m2 of concrete. Code allows the site engineer to ask for casting the test sample if he regards it necessary.

Plain & Reinforced Concrete-1Acceptance Criteria for Concrete QualityACI 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

Plain & Reinforced Concrete-1Acceptance Criteria for Concrete Quality (contd)ExampleFor Required fc = 20 MPa, if following are the test results of 7 samples19, 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 MPaEvery arithmetic average of any three consecutive strength tests equals or exceeds fc.None of the test results fall below required fc by 3.5 MPa.Considering these two point the quality of concrete is acceptable

Plain & Reinforced Concrete-1Mix DesignIngredients 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

Plain & Reinforced Concrete-1Mix Design (contd)ACI-5.3.2 Required Average Compressive StrengthTable 5.3.2.1-Required Average Compressive Strength when Data are Available to Establish a Sample Standard DeviationSs = Standard deviation of compressive strength test

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

Plain & Reinforced Concrete-1Stress Strain Curve of Concretefc0.85fcStressStrainCrushing0.0028 to 0.0045, generally 0.003The 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 strain0.4 fc

Plain & Reinforced Concrete-1Modulus of ElasticityConcrete is not an elastic material therefore it does not have a fixed value of modulus of elasticityStrainStressSecant ModulusTangent ModulusInitial tangent ModulusTangent and Secant Moduli of Concrete0.4fc

Plain & Reinforced Concrete-1Modulus of Elasticity (contd) Secant modulus (Ec) is the one which is being used in design.

Ec = 0.043 wc1.5fc wc = density of concrete in kg/m3fc = specified cylinder strength in MPaFor normal weight concrete, say wc = 2300 kg/m3 Ec = 4700fc

Plain & Reinforced Concrete-1Reinforcing Steel Steel bars are:PlainDeformed (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, fyUltimate strength, fuSize and diameter of bar

Plain & Reinforced Concrete-1Steel Bars

Plain & Reinforced Concrete-1Reinforcing Steel (contd..)

Stress Strain Curve for Steelfyfy/2fuStrainStress Strain Hardeningyielding

Plain & Reinforced Concrete-1Reinforcing Steel (contd) Steel Grade Designation Grade 300, fy = 300 MPaGrade 40Grade 420, fy = 420 MPaGrade 60Grade 520, fy = 520 MPaGrade 70FPSStrainGrade 300Grade 450Grade 520StressFor hot rolled steel barsCold twisted steel bars are available in grade 420For hot rolled steel bars

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

Concluded