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L03- Concrete Properties.pdf

11 Pages

Civil Engineering
Course Code
Ghani Razaqpur

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Properties of Fresh Concrete Concrete in its fresh state is a fluid and it should have the property of being easily: 1. Transported 2. Placed 3. Compacted 4. Finished Good concrete has good workability. Workability is the ease with which concrete can be placed and finished, without segregation, bleeding, honeycombing, etc. Bleeding is the accumulation of the mix water on the free surface of the concrete. Over- vibration, over trowling and lean mixes (mixes with small quantities of cement) increase the possibility of bleeding. Segregation is the separation of the coarse aggregates from the paste. This happens when the mix is very wet and lacking adequate fine particles. Air entrainment reduces the tendency for segregation. Honeycombing: The presence of large and distributed voids in hardened concrete Consistency is a measure of concrete fluidity, and measured by a slump test. Typical Recommended Slump Values Max (mm) Min (mm) Footing and other substructures: 75 25 Beams, walls, columns: 100 25 Pavement slabs: 75 25 Mass concrete: 50 25 Factors affecting Consistency and Workability of Concrete 1. w/c ratio (increase with higher ratio) 2. fineness of cement (increases with fineness) 3. admixtures (superplasticizers and air-entraining agents increase consistency) 4. Surface area of the aggregates (consistency decreases with increased surface area) 5. Higher fineness modulus (increases workability) 6. High temperature and rapid loss of water through evaporation decreases workability. 1 Curing of Concrete Curing refers to the maintenance of enough moisture and proper temperature during the early stages of hydration until concrete has attained adequate strength, with experiencing excessive shrinkage or loss of internal moisture Types of curing 1. In air 2. Moist 3. Steam 4. Wrapped and sealed Effects of curing on hardened concrete properties Proper curing increases concrete strength, durability, and water tightness. Moist curing increases early strength. For adequate curing, the humidity should be maintained above 80 %. Properties of Hardened Concrete The properties of interest are: 1. Strength 2. Modulus of elasticity 3. Durability 4. Creep 5. Shrinkage 6. Water tightness Compressive strength: It is the most important type of concrete strength, which is a good indicator of its strength under other types of stresses, such tension and shear. The compressive strength, designated as f’ cs measured by axially loading cylinders of 150 mm diameter and 300 mm height to destruction. For high strength concrete smaller size cylinders may be used (100 x 200 mm). If the axial compressive load causing failure is P, the compressive strength is calculated by dividing the load P by the cross-section area of the cylinder. 4P fc' 2 D where D is the cylinder diameter. In SI system, unit of length is metre and that of force is Newton (N). Stress is measured in Pascal (Pa) = N/m or kPa = 1000 Pa or Megapascal 6 (MPa) = 10 Pa . Concrete compressive strength can vary between 15 to 120 MPa. 2 Stress-Strain Curve of Concrete in Uniaxial Compression The stress-strain curve of concrete under axial compression follows approximately a parabolic ascending path up to the maximum stress, or compressive strength, thereafter it follows a descending path. The descending path is closer to a straight line, but it depends on the concrete strength. Higher strength concrete shows a more steep descent than lower strength concrete. Concrete with compressive strength higher than 70 MPa is considered high strength while that with strength less than 25 MPa may be considered low strength. Concrete in the range of 35-45 Mpa is commonly used in today’s practice. The parabolic stress-strain curve for concrete under uniaxial compression most commonly used in research works is called the Hognestad’s parabola and it gives a reasonable approximation of the behaviour up to the peak point. It is given by 2 c c fc fc'' for c o o o " For c of c fc 1 100 c o f c f “ c ε ε ε c o cmax . . c c 3 Modulus of Elasticity of Concrete (E ): Thecmodulus of elasticity is a measure of the stiffness of a material, i.e. how much stress is needed to cause a unit deformation or strain in the material. For concrete its modulus of elasticity can be determined in accordance with ASTM Standard ASTMC469: Test Method for Static Modulus of Elasticity and Poisson’s Ratio. The standard specifies that from the test data, E , can ce calculated as E S 2 S 1 c 0.00005 2 Where S is2the compressive stress corresponding to 40% of the maximum load and ε is 2 its corresponding strain, S is1the strain corresponding to a stress of 50 microstrain =0.000050. These values are read off the stress-strain curve of concrete obtained from the test. The above modulus is called the secant modulus and can be used to relate concrete compressive stress and strain up to 40% of its strength. Beyond this limit, the modulus varies more rapidly with stress because concrete exhibits much more nonlinearity, but increments of stress and strain can be related by the tangent modulus at the specified stress level. The tangent modulus is simply the slope of the stress-strain curve. In practice, designers use an approximate value for the concrete secant modulus, referred to simply as the elastic modulus. This approximate value is also limited to 40% of concrete strength. The CSA Standard A23.3, which governs concrete design in Canada, gives two expressions for E as follows: c For normal density concrete in units of MPa it gives: ' Ec 4500 fc (MPa) 3 More generally, for concrete of any density γ (kg/mc), 1.5 ' c E c (3300 fc 6900) 2300 (MPa) 3 3 where γ ic the concrete density in kg/m . For normal density concrete γ ≈ 2300 kgcm Tensile strength: The tensile strength of concrete is relatively low and that is the reason for its cracking under moderate levels of load or in other tensile stress inducing situations, such as restrained thermal contraction or shrinkage. The tensile strength can be determined by one of the following three methods: (a) Flexural testing of unreinforced concrete beam (prism) specimens (b) Splitting or Brazilian cylinder test (c) Direct tension of coupon by an axial load 4 T YPES OF T ENSION TESTS 1. Direct tension: Often this test is not used because it is difficult to carry out in the lab. The holding devices often introduce high stress
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