| One of the most important defect in the long-term service concrete bridge deck is the delamination developed by cracking of the concrete due to the expansion of the steel bars’ rust.The Impact-Echo method is considered to be the most effective method for delaminations detection in concrete bridge deck.However,the Impact-Echo method faces the following challenges and critical needs in actual inspection: It is difficult to detect deep delamination caused by the corrosion of steel bars on the bottom surface;Making a fast and automated inspection platform has become the trend of development For this reason,this article has carried out the following work and get the following conclusions:(1)The current actual inspections usually excite the thickness mode of deep delaminations.However,deep delaminations are usually difficult to excite high-energy resonance of thickness mode;on the contrary,literatures have found that the flexural mode of deep delaminations is easy to be excited,which may become the key to solving the difficulty of deep delaminations detection.However,the current theoretical models and interpretation for the flexural mode vibration of the delamination are only applicable to the shallow delamination defects caused by the corrosion and expansion of the top steel bars.This article proposes a theoretical model suitable for flexural mode of shallow and deep delaminations based on the theory of mediumthick plates.The theoretical model provides a theoretical basis and interpretation for the flexural vibration phenomenon of delaminations,which can be used to guide actual detection.A practical calculation solution for calculating the flexural vibration frequency of delaminations with an aspect ratio greater than 2 and any aspect ratio is proposed.The flexural mode frequency of delaminations calculated by practical calculation solution is compared with the flexural mode frequency of delaminations measured by Impact-Echo testing from literatures,where dacks have been embedded with manual delaminations in laboratory.The comparison result of vibration frequency shows that the practical calculation solution has good accuracy.(2)The decisive factor of the Impact-Echo method to realize detection depends on whether the Impact source can excite the high-energy resonance mode of delaminations.The design of the impact source characteristics is very important.The current design method of Impact source characteristic is aimed to excite thickness mode,only gives an empirical design range and requires the inspector to have rich experience in the common conditions of local delaminations.There has no study on how the impact source characteristics influence on the flexural mode resonance of delaminations.This article studies the influence of impact source characteristics on the flexural mode response of delaminations,and proposes the ‘maximum resonance amplitude’design criterion of impact source characteristics,that is suitable for most common delamination conditions without relying on the experience of the inspector.the ‘maximum resonance amplitude’design criterion establishes a quantitative relationship between impact source characteristics and spectral amplitude of flexural mode response of delaminations.(3)The impact source is the key point to realize a fast and automated detection platform.This article review the current impact sources as well detection platforms and makes a comparative analysis.Aiming to break through the limitation that detection accuracy and detection speed of the current impact source cannot be achieved at the same time,this article develops a fast detection platform composed of a rotating-reciprocating impact source,a air coupled sensor and a signal processing method.The rotating-reciprocating impact source can move forward quickly while generating intensive high-energy impact and improve the detection speed while improving the detection accuracy.The rotating-reciprocating impact source adapts to harsh road conditions such as a road has gravel and potholes.A concrete bridge dack with artificial simulated delaminations has been built in the laboratory.Test has been executed on the concrete bridge dack using the fast detection platform.The test results show that the rotatingreciprocating impact source is faster than the hammer used by Sounding test,and has better accuracy than the handled steel ball used by Impact-Echo test. |
