|M.Sc Student||Wang Fengzhe|
|Subject||Assessment of Strength, Hardness and Elastic Properties of|
Concrete Using Nondestructive Methods
|Department||Department of Civil and Environmental Engineering||Supervisor||Professor Konstantin Kovler|
|Full Thesis text|
The needs of concrete assessments and safety monitoring were growing along with the increasing of global concrete consumption and the development of the science of concrete materials. Nondestructive testing (NDT) methods can offer strong infer to the properties of concrete, e.g. concrete hardness, strength and elastic properties, while traditional inspection and test cannot.
The objectives of present study were:
1) to analyze the relationships between compressive strength of concrete and several NDT values (e.g. Schmidt hardness value, Leeb hardness value, and ultrasonic pulse velocity (UPV) of concrete.)
2) to obtain a better understanding of dynamic tests (based on vibration and wave propagation) used to estimate the dynamic Young’s Modulus (Ed) and to establish the correlation between the static Young’s modulus (Es) and Ed in concrete
Numerous proposals and suggested empirical formulae have been suggested to described the correlations between NDT values and compressive strength or elastic modulus of concrete. However, the relationships varied from each other and limited the application of NDT methods. In this study, the selected NDT methods were applied on concrete prism specimens with varied water cement ratio (0.33, 0.4 and 0.5).
Among the nondestructive testing methods estimating the compressive strength of concrete, it was observed that Leeb rebound value was correlated with compressive strength in nearly linear shape, which showed a highest confidence level compared to Schmidt hammer and UPV tests. The correlations between Schmidt value and uniaxial compressive strength (UCS) show different patterns for normal strength concretes (w/c=0.4, 0.5) and high strength concretes (w/c=0.33). Leeb rebound hammer test shows a more consistent behavior of correlation between the Leeb rebound number and compressive strength for concretes with various w/c ratios.
The classic Schmidt rebound hammer (N-type) is not recommended to be used on concrete specimen at early age (less than 3 days) or concrete with expected UCS less than 14 MPa, due to the semi-destructive character of Schmidt hammer method. Schmidt number was insensitive to the change of uniaxial compressive strength in lower strength concrete, and it is not recommended for testing concrete with UCS less, than 20 MPa. Such constraints do not necessarily apply to the lower impact Leeb rebound devices. At the same time, Leeb hammer was sensitive to moisture condition of specimen.
Two dynamic test methods, the UPV and resonance frequency methods, were conducted on concrete specimens to assess the dynamic modulus of elasticity. In concrete, dynamic modulus of elasticity (Ed) was always greater than static elastic modulus (Es). The ratio of the static to dynamic modulus of elasticity (Es/Ed) ranged between 64% to 75% when Ed was obtained by ultrasonic pulse velocity method. The ratio Es/Ed ranged between 89% to 98% when estimated Ed using resonance frequency method. The ratio of the static modulus of elasticity to the dynamic modulus of elasticity of the concrete grew with the increased UCS of concrete. The static modulus of elasticity was reliably estimated by the dynamic modulus of elasticity, which can help avoiding the experimental problems in measuring the static elastic modulus of concrete.