Short Course 101-1111-00L:Fundamentals and Applications of Acoustic EmissionNovember 1, 2007 5-1 Concrete – Diagnosis 5-2 Concrete – Damage and Fracture Mechanics 5-3 Superstructure – Building and Bridge
5-1 Concrete - DiagnosisNovember 1, 2007 (1/3) Introduction Damage Assessment Corrosion Monitoring in Reinforced Concrete
Damage Assessment In concrete engineering, two types of failure modes in the reinforced concrete beam are known. One is a bending failure, as bending cracks are nucleated in the bending span and the beam results in final failure. The other is a shear (diagonal-shear) failure, as diagonal-shear cracks are suddenly generated in the shear span at the final moment.
AE behaviors of the two modes in Reinforced Concrete Beams In the case of a under-reinforced beam, sliding between reinforcement and concrete was observed due to yielding of the reinforcement. As a result, AE count rate increases exponentially. In the case that the reinforcement withstood in an over-reinforced specimen, AE events are observed at constant rate until the final failure. Right before the final stage, diagonal cracks were suddenly observed without any precursors.
Observation fact • The results demonstrate a potential for the prediction of failure mode of reinforced concrete beams by AE observation. • It is confirmed that AE activities are sensitive to local instability of the structure.
Recommended practice for in situ monitoring of concrete structures by AE [NDIS-2421 2000] In order to assess the damage levels of reinforced concrete beams, one criterion to qualify the damage levels is proposed on the basis of two ratios associated with the Kaiser effect.
Damage Qualification of RC Beams (a) Ratio ofload at the onset of AE activity to previous load: Load ratio = load at the onset of AE activity in the subsequent loading / the previous load. (b) Ratio of cumulative AE activity during the unloading process to that of the last maximum loading cycle: Calm ratio = the number of cumulative AE activity during the unloading process/ total AE activity during the last whole loading cycle.
Experiment Two reinforced concrete beams of 3.2 m length were tested. AE sensor:150 kHz resonance frequency Frequency range :10 kHz to 1 MHz Total amplification : 80 dB gain. Crack-mouth opening displacement (CMOD) recorded
AE observation under salt attack onshore Mostly observed AE events were generated due to rainfall. After three year exposure, in one beam, AE activities following raindrops were observed.
Corrosion Process in Reinforced Concrete Continuous monitoring of AE events were conducted in an accelerated corrosion test and a cyclic wet-dry test. An RC slab of dimensions 10 cm x 25 cm x 40 cm In a cyclic wet-dry test, the slab was soaked in the same tank without electric charge for a week and then taken out to get dry for a week. This cycle was repeated. AE sensors : 50 kHz resonance (RA5) Amplification : 40 dB gain in total Frequency range : from10 kHz to 1 MHz
Corrosion Monitoring by AE Activities ：Half-cell potential ：AE hits The second stage The first stage Corrosion standard, -350mV AE hits Half-cell potential (mV) Time, day
Phenomenological model for corrosion loss At the 1st phase, corrosion initiates in reinforcement. The rate of the corrosion loss decreases at the 2nd phase under aerobic conditions. At the 3rd phase of anaerobic corrosion, expansion of rebar due to corrosion products nucleates concrete cracking.
AE parameter analysis RA = the rise time / the maximum amplitude Average Frequency (Fa) = AE ringdown-count / the duration time Modified JCMS-III B5706
Phenomenological AE observation 1st stage: small other-type AE events →Onset of corrosion in rebar 2nd stage: fairly large tensile-type(SEM) AE events → Nucleation of corrosion-induced cracking in concrete
Deterioration process due to salt attack, prescribed in the codes The presence of two stages:
5-2 Concrete – Damage Mechanics and Fracture MechanicsNovember 1, 2007 (2/3) Introduction Damage Evaluation Application to Fracture Mechanics Remarks
Introduction Relations between any AE parameters and other physical parameters are quantitatively analyzed and then applied to the diagnosis of concrete structures. This is because the change of AE activity could be related with the rate of the deterioration process. Since AE techniques directly deal with the occurrence of micro-cracks, applications to damage mechanics and fracture mechanics are straightforward.
Damage Evaluation The rate process analysis was introduced to evaluate quantitatively the change of the activity [Ohtsu & Watanabe 2001]. When concrete contains a number of critical micro-cracks, active AE occurrence is expected under loading due to crack propagation from existing defects or micro-cracks. In contrast, AE activity in sound concrete is known to be stable and low up to final failure.
Procedure of damage evaluation 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 modulus E* of elasticity in concrete from the database, relative damages, Eo/E* of concrete in existing structures are successfully estimated.
Compression test of core-drilled samples In usual cases, cylindrical samples of 10 cm in diameter and 20cm in height are tested. AE sensor of wide-band type (UT-1000,PAC): resonance frequency: approx. 1MHz) Frequency range : from 60kHz to 1MHz.
Rate Process Analysis AE occurrence from stress level V(%) to V+dV (%) is represented as a function of the incremental number of AE events, dN, f(V) dV = dN/N, where N is the accumulated number of AE events up to stress level V(%), which is normalized by the compressive strength.
Approximation of probability fn. f(V) To discriminate AE activities at low stress level whether high or low, the function f(V) is approximated as the following hyperbolic function, f(V) = a/V +b where a and b are empirical constants.
Histograms on the value f(V) directly estimated as dN/(N･dV) at each increment dV in concrete
Damaged Core Samples Core samples were taken from an outlet of cooling water in a nuclear power plant
Results of the pore volumes over 0. 5 mm radius, the rate a, and the strength
AE events versus stress From two equations; f(V) dV = dN/N and f(V) = a/V +b, a relationship between the number of total AE events N and stress level V(%) is obtained, N = C V a exp (bV). Here, C is the integration constant.
A relationship between the number of total AE events N and stress level V(%) : N = C Va exp (bV)
Damage mechanics According to the continuum damage mechanics, the state of damage is represented by the scalar damage parameter W from the modulus of elasticity, E, of a damaged material is expressed as, E = E*(1 - W), where the modulus E* is that of an intact material.
Estimation of intact modulus: E* Wc - Wo = (Eo - Ec)/E*. A linear correlation between loge(Eo－Ec) and the rate ‘a ‘ value is proposed as, loge(Eo－Ec) = Da + c. Then, it is assumed that Eo = E* when a = 0. This allows us to estimate Young's modulus of intact concrete E* from, E* = Ec + exp(c).
Database Relative damage is estimated as Eo/E*
Application to Fracture Mechanics The fracture process zone is created ahead of a notch (crack) in concrete, without revealing the notch sensitivity. Nucleation of micro-cracks in the fracture process zone is clarified, which was ideally introduced in order to explain the tension-softening behavior.
Relation between the area of the zone and the size of aggregate With the increase of the size of aggregate, the fracture process zone grows broadly.
In the expansion test, which simulates crack propagation due to corrosion of reinforcing steel-bar, the moment tensor analysis was performed.
Remarks AE techniques have been applied to concrete engineering for more than four decades. A variety of practical applications are achieved and further going to be standardized. Based on these research activities, RILEM technical committee : TC212-ACD ( Acoustic Emission and Related NDE Techniques for Crack Detection and Damage Evaluation in Concrete) was established in 2004 to 2009. Recommendation practices are under preparation. Thus, the world-wide standards are to be established in concrete engineering.
5-3 Superstructure – Building and BridgeNovember 1, 2007 (3/3) Introduction Building Concrete Bridge Steel Bridge Steel-Concrete Composite Slab Remarks
Introduction • An enormous number of infrastructures, in particular, buildings and bridges have been constructed. However, many of them are currently known to have been aged and deteriorated after long-year service. • In concrete structures, damages due to deteriorated or aged materials including corrosion of reinforcement and poor-workmanship responsible for initial cracking are often reported. • Steel structures are deteriorated mostly by fatigue, resulting fro increasing span length and overloading of traffic vehicles.
Standard Specifications for Concrete Structures [JSCE 2001] step 1: Inspection, step 2: Evaluation of inspected results, step 3: Prediction by deterioration model, and step 4: Counter-measures for maintenance and management. Thus, inspection procedures for the maintenance and management of structures are of fundamental importance prior to making a prediction model for deterioration and deciding counter-measures for repair and retrofit.
Inspection methods for superstructures as bridges and buildings. • Visual inspection is most extensively employed. • Since defects, deterioration, and damage normally grow inside structure, a safety assessment cannot be based solely on the visual observation of cracks and signs of damages in structural elements. • The technique should enable to provide definitive and quantitative evaluation in a short time. • To inspect concrete and steel of superstructures, applications of AE techniques are in progress.
AE Applications The AE method is expected to become a very useful technique for evaluating the soundness or for detecting damages of the superstructure. This is because the measurement can be carried out without stopping traffic in a bridge or without evacuation of residents in a building. Some successful results on the applications to buildings and bridges are stated.