lecture3混凝土结构设计原理-英文课件.ppt

1、Lecture 3-FundamentalsJune 6,2003CVEN 444Lecture GoalsConcrete PropertiesSteel PropertiesConcrete Mixing and ProportioningQualityWorkabilityEconomicalIn the design of concrete mixes,three principal requirements for concrete are of importance:Concrete Mixing and ProportioningQuality of concrete is me

2、asured by its strength and durability.The principal factors affecting the strength of concrete,assuming a sound aggregates,W/C ratio,and the extent to which hydration has progressed.Durability of concrete is the ability of the concrete to resist disintegration due to freezing and thawing and chemica

3、l attack.Concrete Mixing and ProportioningWorkability of concrete may be defined as a composite characteristic indicative of the ease with which the mass of plastic material may deposited in its final place without segregation during placement,and its ability to conform to fine forming detail.Concre

4、te Mixing and ProportioningEconomical takes into account effective use of materials,effective operation,and ease of handling.The cost of producing good quality concrete is an important consideration in the overall cost of the construction project.Concrete Mixing and ProportioningThe influence of ing

5、redients on properties of concrete.Concrete Mixing and Proportioning3.WorkabilitynWorkability measured by slump test 1.Layer 1:Fill 1/3 full.25 stokes2.Layer 2:Fill 2/3 full.25 stokes3.Layer 3:Fill full.25 stokes4.Lift cone and measure slump(typically 2-6 in.)1 2 3 412”slumpConcrete Mixing and Propo

6、rtioningSlump test-The measurement of the consistency of the mix is done with the slump-cone test.The recommend consistency for various classes of concrete structures.Concrete Mixing and Proportioning4.AdmixturesnApplications:wImprove workabilitywAccelerate or retard setting and hardeningwAid in cur

7、ingwImprove durabilityConcrete Mixing and Proportioning4.AdmixturesnAir-Entrainment:Add air voids with bubbleswHelp with freeze/thaw cycles,workability,etc.wDecreases density:reduces strength,but also decreases W/CnSuperplasticizers:increase workability by chemically releasing water from fine aggreg

8、ates.Concrete Mixing and Proportioning5.Types of CementnType I:General PurposenType II:Lower heat of hydration than Type InType III:High Early StrengthwHigher heat of hydration quicker strength(7 days vs.28 days for Type I)Concrete Mixing and Proportioning5.Types of CementnType IV:Low Heat of Hydrat

9、ionwGradually heats up,less distortion(massive structures).nType V:Sulfate ResistingwFor footings,basements,sewers,etc.exposed to soils with sulfates.Concrete Mixing and ProportioningFailure Mechanism of ConcreteShrinkage Microcracks are the initial shrinkage cracks due to carbonation shrinkage,hydr

10、ation shrinkage,and drying shrinkage.Concrete Mixing and ProportioningFailure Mechanism of ConcreteBond Microcracks are extensions of shrinkage microcracks,as the compression stress field increases,the shrinkage microcracks widen but do not propagates into the matrix.Occur at 15-20%ultimate strength

11、 of concrete.Concrete Mixing and ProportioningFailure Mechanism of ConcreteMatrix Microcracks-are microcracks that occur in the matrix.The propagate from 20%fc.Occur up to 30-45%ultimate strength of concrete.Matrix microcracks start bridge one another at 75%.Aggregate microcracks occur just before f

12、ailure(90%).Concrete Properties1.Uniaxial Stress versus Strain Behavior in CompressioncEcou0.45fcfc fc12”6”Concrete PropertiesThe standard strength test generally uses a cylindrical sample.It is tested after 28 days to test for strength,fc.The concrete will continue to harden with time and for a nor

13、mal Portland cement will increase with time as follows:Concrete PropertiesnCompressive Strength,fcwNormally use 28-day strength for design strengthnPoissons Ratio,nw n 0.15 to 0.20wUsually use n=0.17cEcou0.45fcfc fcConcrete PropertiesnModulus of Elasticity,EcwCorresponds to secant modulus at 0.45 fc

14、wACI 318-02(Sec.8.5.1):where w=unit weight(pcf)90 pcf wc 155 pcfFor normal weight concrete(wc 145 pcf)Concrete PropertiesCompute Ec for fc=4500 psi for normal weight(145 pcf)concrete using both ACI equations:Concrete PropertiesnConcrete strain at pressive stress,owFor typical curves in compressionwo

15、 varies between 0.0015-0.003wFor normal strength concrete,o 0.002 Ecou0.45fcfc fcConcrete PropertiesnMaximum useable strain,u wACI Code:u=0.003wUsed for flexural and axial compressionEcou0.45fcfc fcConcrete PropertiesTypical Concrete Stress-Strain Curves in CompressionConcrete PropertiesTypes of com

16、pression failureThere are three modes of failure.a Under axial compression concrete fails in shear.b the separation of the specimen into columnar pieces by what is known as splitting or columnar fracture.c Combination of shear and splitting failure.Concrete Properties2.Tensile StrengthnTensile strength 8%to 15%of fcnModulus of Rupture,frwFor deflection calculations,use:nTest:ACI Eq.9-10PfrMmax=P/2*aunreinforced concrete beamConcrete Properties2.Tensile Strength(cont.)nSplitting Tensile Strength,