Chapter2 Main Physical and Mechanical Properties of Concrete and Reinforcing Steel
Chapter2 Main Physical and Mechanical Properties of Concrete and Reinforcing Steel
2.1 Properties of hardened concrete The mechanical properties of hardened concrete Short term properties Long term properties Strength in compression, √Creep,:Shrinkage tension and shear Stiffness measured by modulus of elasticity The compressive strength of concrete is the most common measure for judging the quality of concrete
2.1 Properties of hardened concrete The mechanical properties of hardened concrete Short term properties Long term properties üStrength in compression, tension and shear üStiffness measured by modulus of elasticity üCreep; Shrinkage The compressive strength of concrete is the most common measure for judging the quality of concrete
2.1.1 Uniaxial compressive strength and Stress-strain curve in compression Failure mechanism of plain concrete under compression
2.1.1 Uniaxial compressive strength and Stress-strain curve in compression
Flaws in concrete 要想弄清楚Failure mechanism,首先必须了解混凝土中的Flaws
Flaws in concrete
Flaws in concrete Flaws are attributable to a number of causes. the main ones being: >Discontinuities in the cement paste(水泥胶体) owing to its complex morphology(形态) >Voids caused by shrinkage(徐变)，thermal movements and incomplete compaction(搅拌) Discontinuities at the boundary between aggregate(骨料)particles and the paste or nortar(砂浆)
Flaws are attributable to a number of causes, the main ones being: Ø Discontinuities in the cement paste （水泥胶体） owing to its complex morphology（形态） Ø Voids caused by shrinkage（徐变） ， thermal movements and incomplete compaction (搅拌） Ø Discontinuities at the boundary between aggregate （骨料）particles and the paste or mortar （砂浆）
Stress concentration caused by flaws in concrete Please judge the direction of cracks void o compression And why? tension d Tensile strength Flaws are the source of cracks 1/10 compressive strength
Tensile strength 1/10 compressive strength Please judge the direction of cracks And why？
问题：混凝土中的裂缝方向 总是沿着A方向的。 A主压应力 B主拉应力 试验照片演示
问题：混凝土中的裂缝方向 总是沿着 方向的。 A 主压应力 B 主拉应力 A 试验照片演示
lllustration倒证)of the theory -----Strength and W/C ratio >To hydrate(水化)，cement(水泥)need about38%water by weight of cement theoretically In practical,complete hydration of cement does not take place and is not aimed at.Only about 23%water by weight of cement is combined(结合) >Non-evaporable water is 23%by weight of cement >Spaces left after the loss of the evaporable water are collectively known as the voids The present of which is a great source of weakness The relation of water/cement ratio and concrete strength is intimately(密t切地)connected with the voids
Ø To hydrate（水化）, cement(水泥） need about 38% water by weight of cement theoretically Ø In practical, complete hydration of cement does not take place and is not aimed at. Only about 23% water by weight of cement is combined （结合） Ø Non-evaporable water is 23% by weight of cement Ø Spaces left after the loss of the evaporable water are collectively known as the voids Ø The present of which is a great source of weakness Ø The relation of water/cement ratio and concrete strength is intimately（密切地） connected with the voids
Failure process 8v=8a+2 al fe 10 Minimum-volume level Transverse strain (Et) Volumetric strain (E) 05 Axial strain (Fa) f=cylinder strength -4 -3 -2 -1 2 3 4 Strain:mm/m The rapid expansion at the minimum volume level,where the tensile strain at right angles (to the direction of maximum compressive stress soon exceeds the magnitude of the compressive strain is a failure feature of both uniaxial(单轴)and more general triaxial(三向)compressive behavior Concrete in the "critical'zones of compression always failed by “splitting”(劈裂)--never by“crashing”( 压碎)
（压碎） The rapid expansion at the minimum volume level, where the tensile strain at right angles（正交） to the direction of maximum compressive stress soon exceeds the magnitude of the compressive strain is a failure feature of both uniaxial （单轴）and more general triaxial （三向）compressive behavior v a t 2
lllustration of the theory -----Transverse confine (侧向约束) can greatly increase axial compressive strength >Spiral stirrups ,(螺旋箍筋) f=fc+40 Steel tube concrete (钢管混凝土)
------Transverse confine （侧向约束）can greatly increase axial compressive strength ØSpiral stirrups（螺旋箍筋） f=fc+4σr ØSteel tube concrete （钢管混凝土）