Damage evaluation and corrosion detection in concrete by acoustic emission

Acoustic emission (AE) techniques have been extensively studied and applied to nondestructive testings (NDT) of concrete and concrete structures. For damage evaluation, AE behavior of concrete under compression could be analyzed, applying the rate process theory. Based on Loland's model in damage mechanics, a relation between AE rate and the damage parameter is correlated. By quantifying intact moduli of elasticity of concrete from the database on the relationship, relative damages of concrete in existing structures are successfully estimated by the compression test of concrete samples. The technique is applied to estimate concrete samples of recycled aggregate. The deteriorated degree of recycled concrete is reasonably estimated. For corrosion detection, continuous monitoring of AE signals is useful for earlier warning of corrosion in reinforcement. It is demonstrated that the onset of corrosion in reinforcement and the nucleation of corrosion cracking in concrete could be clearly identified by AE parameter analysis. which is normally defined as the initiation time of corrosion. The latter is the transition from the initiation stage (phase II) to the accelerated stage (phase III), and is critically important to assess the durability of reinforced concrete structures. For nondestructive evaluation (NDE) of corrosion, electrochemical techniques of half-cell potential and polarization resistance have been widely employed. Yet, it is known that these techniques can not provide precise information on the two transition periods. As a result, these periods are normally defined by chloride concentration levels at cover thickness in concrete. Here, continuous AE measurement is applied to identify the transition periods at the onset of corrosion and at the nucleation cracking. 2 ANALYSIS OF AE SIGNALS 2.1 AE rate-process analysis AE behavior of a concrete sample under compression is associated with generation of micro-cracks. These micro-cracks are gradually accumulated until final failure. The number of AE events, which correspond to the generation of these cracks, increases acceleratedly along with the accumulation of micro-cracks. It appears that the process is dependent on the number of cracks at a certain stress level as to be subjected to a stochastic process. Therefore, the rate process theory is introduced to quantify AE behavior under compression. The following equation of the rate process is introduced to formulate the number of AE hits dN due to the increment of stress from V to V+dV, N dN dV V f = ) ( (1) where N is the total number of AE events and ƒ(V) is the probability function of AE at stress level V(%). For ƒ (V) in Equation1, the following hyperbolic function is assumed, b V a V f + = ) ( (2) where a and b are empirical constants. Here-inafter, the value ‘a’ is called the rate, which reflects AE activity at a designated stress level. At a low stress level the probability varies, depending on whether the rate ‘a’ is positive or negative. In the case that the rate ‘a’ is positive, the probability of AE activity is high at a low stress level, suggesting that concrete is damaged. In the case of the negative rate, the probability is low at a low stress level, revealing that the structure could be in stable condition. Substituting Equation 2 into Equation 1, a relationship between total number of AE events N and stress level V is obtained as the following equation, ) exp(bV CV N a = (3) where C is the integration constant. A damage parameter Ω in damage mechanics can be defined from a relative ratio of the moduli of elasticity (Loland, 1980),