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- Concrete Technology - CT
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**Visvesvaraya Technological University Regional Center - VTU**- Civil Engineering
- B.Tech
- 5 Topics
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Here the following three main types of deformations in hardened concrete subjected to external load and environment are discussed. Elastic strains These are the instantaneous deformations that occur when an external stress is first applied Shrinkage strains These deformations occur either on loss of moisture from the concrete on cooling of concrete Creep It is the time-dependent deformation that occurs on the prolonged application of stress Deformation Effect Any one or combinations of the above types of deformations in a hardened concrete leads to cracking. 1. Elastic Strains Elastic strain in concrete, as defined above, depends on the externally applied stress and the modulus of elasticity of concrete: Elastic strain = Externally applied stress/Modulus of elasticity of concrete Modulus of Elasticity of Concrete Typical Stress-Strain Plot of Concrete (1) At stress below 30% of ultimate strength, the transition zone cracks remain stable. The stress-strain plot remains linear. (2) At stress between 30% and 50% of ultimate strength, the transition zone microcracks begin to increase in length, width and numbers. The stress-strain plot becomes non-linear. (3) At 50 to 60% of the ultimate stress, cracks begin to form in the matrix. With further increase to about 75% of the ultimate stress, the cracks in the transition become unstable, and crack propagation in the matrix will increase. The stress-strain curve bends towards the horizontal. (4) At 75 to 80% of the ultimate stress, the stress reaches a critical stress level for spontaneous crack growth under a sustained stress. Cracks propagate rapidly in both the matrix and the transition zone. Failure occurs when the cracks join together and become continuous. 2

• Concrete is not a truly elastic material, as evident from the nonlinear stress-strain curve for concrete, shown in the following Fig.: fo Stress ff ε0 εu Strain • Since the stress-strain curve for concrete is nonlinear, following methods for computing the modulus of elasticity of concrete are used yielding various types of modulus of elasticity for concrete: 1. The “initial tangent modulus” It is given by the slope of a line drawn tangent to the stress-strain curve at the origin 2. The “tangent modulus” It is given by the slope of a line drawn tangent to the stress-strain curve at any point on the curve 3. The “secant modulus” It is given by the slope of a line drawn from the origin to a point on the curve corresponding to a 40% stress of the failure stress 4. The “chord modulus” It is given by the slope of a line drawn between two points on the stress-strain curve Calculation of the above four types of moduli of elasticity for concrete has been explained below using a typical stress-strain curve, as shown in the following Fig.: Initial tangent fo Tangent IT ff Stress • Chord Secant ε0 3 Strain εu

• Modulus of elasticity for concrete determined from an experimental stress-strain relation curve, as described above, is generally termed as static modulus of elasticity (Ec) whereas the modulus of elasticity determined through the longitudinal vibration test is termed as dynamic modulus of elasticity (Ed) Static modulus of elasticity (Ec)for concrete • Static modulus of elasticity of concrete has been related to its compressive strength by the various Standards Relationship between modulus of elasticity of concrete and compressive strength • BS 8110:Part 2:1985 has recommended the following expression for 28-day Ec in terms of 28-day cube compressive strength (fcu), for normal weight concrete (i.e. concrete with density, ρ ≈ 2400 kg/m3): Ec28 = 20 + 0.2 fcu28 (where Ec28 is in GPa and fcu28 is in MPa) Note: For lightweight concrete the above values of Ec28 should be multiplied by the factors (ρ/2400)2 and (ρ/150)2 respectively. • ACI Building Code 318-89 recommends the following expression for (Ec) in terms of cylinder compressive strength (fcyl), for normal weight concrete (i.e. concrete with density, ρ ≈ 2400 kg/m3): Ec = 4.7 (fcyl)0.5 (where Ec is in GPa and fcyl is in MPa) Ec, 28 = 9.1fcu0.33 - for normal weight concrete of density = 2400 kg/m3, and Ec, 28= 1.7ρ2 fcu0.33 x10-6 for lightweight concrete - (ρ) =1400–2400 kg/m3 • CEB - FIP Model Code (Euro-International) E = 2.15 X 104 (fcm/10)1/3, E in MPa and fcm in MPa. Static modulus of elasticity (Ed) for concrete • Experimental stress-strain relation curve, as described above, is generally termed as static modulus of elasticity (Ec) and is short term modulus. • If creep effect is considered at a given load, the modulus determined is referred to as long term modulus of elasticity. ELong = EShort/(1+θ), 4

Where θ is creep coefficient and Creep coefficient is the ratio of creep strain to elastic strain ELong < Eshort Dynamic modulus of elasticity (Ed) for concrete Modulus of elasticity determined through the longitudinal vibration test by velocity of sound or frequency of sound is termed as dynamic modulus of elasticity (Ed) Dynamic modulus of elasticity of concrete (Ed) is approximately taken as equal to the initial tangent modulus of elasticity of concrete. Ed is more as creep effect is not considered. Dynamic modulus of elasticity for normal and light weight concrete in GN/m2 (GPa) is given by Ec = 1.25 Ed – 19 - for Normal weight concrete and Ec = 1.04 Ed – 4.1 - for light weight concrete, GN/m2 If M20 NWC is used, Ec = 22.4 GPa, Ed = 33.12 GPa, 48% more Conduct NDT on concrete prism Subject beam to longitudinal vibration at its natural frequency and measure the resonant frequency (n, Hz) or the UPV (km/s) through it. Ed = Kn2L2ρ ; If L in mm, ρ in kg/m3, then Ed = 4x10-15n2L2ρ, in GPa Appx. Ranges of Resonant Frequencies of Concrete beam 100 x 100 x 500 mm Transverse 900–1600 Hz, Longitudinal 2500–4500 Hz. If n= 4000 Hz, Ed = 38.4 GPa Conduct NDT on concrete prism and measure the UPV (km/s) through it. UPV = Path length/Transit time 5

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