MICROWAVE NON-DESTRUCTIVE TESTING OF FIBRE CONCRETE USING FREE SPACE MICROWAVE MEASUREMENTS

Abstract

Microwave nondestructive testing (MNDT) techniques such as reflection, transmission and dielectric measurements have advantages over other NDT methods regarding low cost, good penetration in nonmetallic materials, good resolution and contactless feature of microwave sensor (antenna). In this paper, reflection coefficients, transmission coefficients, dielectric properties (ε’ = dielectric constant and ε” = loss Factor) were measured to detect fibre distribution and concentration in fibre-reinforced concrete (FRC). For FRC specimens, reflection (S11) and transmission (S21) coefficients were measured in the frequency range 8 – 12.5 GHz by using free-space microwave measurement (FSMM) system. FSMM system consists of a pair of spot-focusing horn lens antenna, mode transitions, coaxial cables and vector network analyzer, ε’ and ε” values can be evaluated from the measured S11 and S21. FRC specimens were manufactured with polypropylene (PP) fibre (Fibermesh® fibres, length = 19 mm) and steel fibres (Convotex fibres, length = 30 mm). FRC specimens were cast with fibre concentration of 0.0, 0.6, 0.9 and 1.2 kg /m3 for PP fibres and 0, 10, 20, and 30 kg/m3 for steel fibres. The results show decreasing reflection coefficients, transmission coefficients, and dielectric properties with increasing curing age. These properties also show the potential to be used to determine the percentage of fibre content in concrete. INTRODUCTION DIELECTRIC CONSTANT AND LOSS FACTOR MEASUREMENT SYSTEM SETUP MATERIALS AND SPECIMENS EXPERIMENTAL RESULTS CONCLUSIONS REFERENCES