Research On The Strength Test Of Underwater Concrete Based On Rebound Method

It is well known that underwater concrete structures normally serve under ambient water environment for a long period.Moisture will penetrate into concrete through capillaries and pores,and its strength will change due to the influence of humidity.In engineering practice,when testing the strength of concrete structures in an underwater environment,it is difficult to obtain strength data due to factors such as low visibility in water and high operation difficulties factor,and cannot provide data support for maintenance decisions.In this thesis,the underwater rebound test is carried out on concrete cube test blocks(150mm×150mm×150mm)from multiple strength grades under different detection conditions by the underwater rebound method.Through data regression analysis,the strength measurement curve of underwater rebound value against strength has been constructed along with the correlation between them.Furthermore,a simple and efficient method for nondestructive testing for underwater concrete strength was proposed.The main research content of this thesis includes following aspects:(1)At present,the concrete rebound hammers widely used in the industry are lack of waterproofing.Working underwater,therefore,causes severe damage to the equipment which in return affects the test results.Under such an industrial gap,considering the material and sealing of the rebound hammer,the test proposed in this thesis upgrades the ordinary rebound hammers to increase its service ability under water.Hence,the upgraded equipment overcomes the incapability of the traditional rebound hammer which is not able to operate in the water environment.(2)In order to study the influence of moisture on the mechanical properties of concrete materials in a quantitative manner,in this thesis,firstly,author has put cured concrete blocks of five strength grades such as C10,C20,C30,C40,C50 in a drying box for drying.After the test blocks are completely dry,those blocks have been placed in ambient temperature water tank for immersion and measuring to record the underwater rebound value and moisture content of the concrete test block at regular intervals.A detailed analyses has then carried to observe the change law of the two in different immersion times.Finally,the relationship between the underwater rebound value and the water content has established based on recordings.(3)Due to the influence of equipment improvement and underwater testing environment,the strength measurement and evaluation methods given in the current national standard "Technical Specification for Inspecting of Concrete Compressive Strength by Rebound Method"(JGJ/T23-2011)are only applicable to the testing of concrete strength in conventional environment,so it is necessary to establish corresponding specification for rebound testing in water environment.This thesis aims to improve the corresponding inspection specification for underwater rebound method.In order to apply all kinds of test conditions,in this thesis,the correction values of the pouring side of the concrete block and the horizontal plane are ±30°,±45°,±60°,±90°,the pouring top and bottom surface relative to the pouring side,and different water depths relative to 0m water depth,are used for the underwater environmental rebound test research to obtain the rebound correction value under different detection conditions.After the rebound test,all the test blocks were loaded to obtain their compressive strengths.With those strength,the rebound-strength curve of concrete in water environment was constructed through statistical and regression analysis.(4)The one-dimensional water absorption test was carried out on concrete test blocks with five strength grades such as C10,C20,C30,C40 and C50 with recording of the curve of water absorption quality and water absorption height in a time domain.According to the parameters obtained from the test,the internal humidity field of the concrete test block at different immersion time was simulated using the finite element software.To obtain closest simulation,the time-varying load was applied to the test block after water saturation in the FEA modelling.Its elastic modulus was further obtained and compared with the test data for validation.Based on the above research,a set of non-destructive testing methods for underwater concrete strength is established,which provides data support for the strength testing of underwater concrete engineering.The key advantages of proposed workflow,e.g.high accuracy,low operation difficulty coefficient and low price of testing equipment are for certain address the limitations of traditional methos.The verification can further prove that the proposed methods can be widely used in underwater concrete strength testing projects.