Properties of Hardened Concrete

Transcription

1 Outline Properties of Hardened Concrete Dr. Kimberly Kurtis School of Civil Engineering Georgia Institute of Technology Atlanta, Georgia Compressive strength E Tensile strength Drying Shrinkage Creep Compression Testing Uniaxial compressive strength of concrete is easy to measure It has become the standard gauge of concrete quality (for better or worse) Some notes about failure : With most materials, failure is associated with the appearance of cracks Concrete intrinsically contains many cracks, which will propagate under loading However, cracks may or may not be visible at the surface when concrete fails Compression Testing Compressive strength is determined according to ASTM C469, where a 6x12 or 4x8 cylinder, cured for 28 days, is tested at a load rate of psi/sec. Can also be performed 1d, 3d, 7d, 28d, 90d. Typical 28-day strengths are Normal strength 3-6 ksi High strength 6-9 ksi Ultrahigh strength ksi Time Curing conditions W/C or W/CM Number/size voids Cement content

2 Cement Type (composition) Cement fineness Use of chemical admixtures Use of SCMs Aggregate strength Aggregate MSA Aggregate/paste bond strength Test Parameters Specimen size Specimen shape Load rate Stress-Strain Behavior Stress-Strain Behavior Why is concrete less brittle than the aggregate and cement paste it is composed of? 4800psi 3500psi

3 Elastic Modulus Elastic Modulus: Estimations Can also be estimated from compressive strength: E c = 33 w c 1.5 f c 0.5 (ACI 318) * E c = elastic modulus of concrete, psi W = unit weight, pcf f c =28d compressive strength of standard cylinders, psi Valid to strengths of at least 6000 psi (perhaps to as high as 9000 psi) The unit weight is used to account for the presence and density of the aggregate E agg is rarely known and this is a useful way to include its effect in E * E c = w c 1.5 f c 0.5 for E c in MPa, where w is in kg/m 3 and f c is in MPa Elastic Modulus: Estimations For normal weight concrete (145pcf),the ACI 318 equation reduces to E c = f 0.5 c for E c in psi E c = 4.73 f 0.5 c for E c in GPa where f c is in MPa Typical values for E c are 2-6x10 6 psi for normal weight, normal strength concrete Parallel Model E c = E p V p + E a V a Assumes ε is same in aggregate and paste Elastic Modulus: Models E c =E concrete E p =E cement paste E a =E agg Assume V p +V a =1 V p =vol paste V a =vol agg Series Model 1/E c =V p /E p +V a /E a Assumes σ is same in aggregate and paste For lightweight concrete, there is a correction for aggregate density E c = 0.043ρ 1.5 f c 0.5 for E c in GPa where f c is in MPa Elastic Modulus: Models Factors Influencing E c Parallel model overestimates E c Series model overestimates E c Combination models (like Hirsch or Counto, see Ch. 9) do a pretty good job Deviations from actual behavior are believed to be due to ITZ effects E c Aggregate volume E agg Aggregate porosity MSA Aggregate shape Influence microcracking Aggregate surface texture in the ITZ Aggregate mineralogy Porosity of the paste ITZ Testing parameters (speed, moisture state)

4 Splitting Tension f t ~ 8-12% of f c ASTM C496 or the Brazilian Test is performed on 6x12 cylinders f t = 2P/πDL Can be estimated by: f t =6.7(f c ) 0.5 for normal strength concrete where units are psi Splitting tension test introduces some compressive stress at top and bottom of (6x12 ) cylinder Measured strength is 10-15% higher than nominal strength Splitting Tension Deformation in Concrete EARLY AGE CONCRETE Plastic shrinkage shrinkage strain associated with early moisture loss Thermal shrinkage shrinkage strain associated with cooling LATER AGE CONCRETE Drying shrinkage -shrinkage strain associated with moisture loss in the hardened material Deformations occur under loading - Elastic - Viscoelastic (including creep) Drying Shrinkage and Creep Both result from movement of water in the hydrated cement paste, which results in new bonds forming in the C-S-H; the driving force differs. For drying shrinkage, environmental conditions (e.g., low external RH) are the driving force For creep, stress is the driving force. Drying Shrinkage Inadequate allowance for drying shrinkage can lead to cracking and warping or curling Must provide adequately spaced joints in slabs and pavements Joints define where the crack will form, rather than allowing for random crack formation Can then seal joints to prevent moisture ingress Creep Creep can be both beneficial and problematic. Creep of concrete in prestressed members Creep in concrete can reduce the pre-stress and possibly lead to cracking Prestressing steel strand embedded in concrete Induced compressive P stress balances tensile stresses expected during service

5 Creep Creep can be both beneficial and problematic. Creep and Shrinkage Stress relaxation, the complement to creep, can reduce stress in the concrete at early ages and reduce the likelihood for early age cracking. Drying Shrinkage and Creep Parameters Affecting Drying Shrinkage and Creep Influence of Aggregate Aggregate volume fraction is an important parameter ε c = ε paste (1-V agg ) n where n~1.8 Influence of Aggregate E agg is another important factor

6 Influence of Paste Properties Prolonged hydration or hydration at elevated temperatures increase chemical bonding, reducing creep and shrinkage Lower w/c concrete creep and shrink less But, generally, these relationships are complex and require testing to confirm anticipated behavior