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1、2.2 Properties of Concrete,4.1 Compressive Strength,Cube Compressive Strength 立方体抗压强度 The cube compressive strength of concrete is given in terms of the characteristic compressive strength of 150mm size cubes tested at 28 days,The characteristic strength (fcu) is the value on the x-axis below which
2、5% of the total area under the curve falls. The value of fcu is lower fcu,m than by 1.65, where is the standard deviation of the normal distribution.,Frequency distribution of compressive strength test results follows normal distribution curve,characteristic strength,mean strength,Strength Grade of
3、Concrete,Concrete is graded on the basis of its characteristic compressive strength of 150mm size cube at 28 days and expressed in,Concrete Strength Grade,Numbers indicate the characteristic compressive strength,Uni-axial Stress versus Strain Behavior in Compression,Ec,0,cu,0.45fc,fc,12”,6”,Cylindri
4、cal Concrete Specimens,Concrete strain at max. compressive stress for normal strength concrete 0=0.002,Maximum useable strain, cu =0.0033 Used for flexural and axial compression,4.2 Tensile Strength 轴心抗拉强度,Axial tensile strength ft Splitting tensile strength Specimen: prism5 prizEm, cylinder 5silind
5、E,4.2. Tensile Strength,nearly uniform tensile stress exists at the right angle to the plain of loading application,P is the applied compressive load at failure,4.3 The modulus of elasticity Ec 混凝土的弹性模量,弹性模量的测定方法 Estimation of Elastic modulus,The modulus of elasticity is determined from a low cycle
6、loading test of prismatic specimen. The loading is limited to a maximum value of 0.5fc. The loading-unloading cycle is repeated 5 to 10 times.,4.4 Shrinkage of Concrete,Concrete, like all other materials, will slightly change in volume when it dries out. Shrinkage: Due to water loss to atmosphere (v
7、olume loss). Plastic shrinkage occurs while concrete is still “wet” Drying shrinkage occurs after concrete has set Most shrinkage occurs in first few months (80% within one year). Cycles of shrinking and swelling may occur as environment changes.,Temperature,fresh concrete,aggregates,paste,For worka
8、bility, twice as much water is required to hydrate the cement. (workable concrete mix contains more water than is needed for hydration , If the concrete is exposed to air, the large part of this free water evaporates in time ),4.4 Shrinkage of Concrete,the rate and completeness of drying depending o
9、n ambient temperature and humidity conditions.,Humidity,4.4 Shrinkage of Concrete,As the concrete dries, it shrinks in volume, probably due to the capillary tension that develops in the water remaining in the concrete. Conversely, if dry concrete is immersed in water, it expands, regaining much of t
10、he volume loss from prior shrinkage.,Drying Shrinkage Cracking,Shrinkage, which continues at a decreasing rate for several months, depending on the configuration of the member, is a detrimental property of concrete in several respects,The magnitude of the ultimate shrinkage strain depends on Cement
11、and water content Shrinkage is a function of W/C ratio Less water in the mixture, less water to escape, less shrinkage higher cement content increases the shrinkage of concrete Composition and fineness of cement. Shrinkage is a function of type of cement (Finess, Low-heat and portland-pozzolan ),4.4
12、 Shrinkage of Concrete,The magnitude of the ultimate shrinkage strain also depends on Type and Gradation of Aggregate (Modulus of Elasticity, Volume/Surface Ratio). Aggregates influence shrinkage by offering restraint to the paste High coarse aggregate content Lesser paste content Lesser water conte
13、nt Admixtures admixtures that increase the water requirement of concrete increase shrinkage admixtures that decrease the water requirement decrease shrinkage,4.4 Shrinkage of Concrete,4.5 Creep of concrete Creep - the slow deformation of a material over considerable length of time at constant stress
14、 or load. Like shrinkage, creep is not completely reversible. creep recovery Creep strain develops over time Removal of water from and rearrangement of bonds between crystallites causes creep,1,Creep of concrete,Consequences of creep,possibility of excessive deflection Instability of the structure stressing of non load bearing members Loss in pre-stress,
