Study On The Test Method For Impact Resistance Of Cement Concrete

As it is known,concrete is one of the most widely used building materials in modern architectures.However,in civil engineering,all kinds of concrete structures inevitably encounter dynamic load during the design lifetime,except in the case of static load.For example,bridges and high buildings encounter wind loading,dams suffer from hydrodynamic pressure loading,ocean flats encounter impacting of ocean waves,airport runways must withstand repeated dynamic loads due to aircraft takeoff and landing,protective structures subjected to projectile or aircraft impact,and structures sustaining impact loads during earthquakes.Dynamic loads(such as those loads)can cause the damage and collapse of buildings,which can lead to casualties and property losses.Because of their unpredictability and destructive capacity,these kinds of loading always become important factors in controlling structural design.In addition,the impact resistance is a very important parameter for evaluating the dynamic performance of concrete,thus it is very necessary to study concrete under impact load.Current understanding of the impact resistance of concrete is very limited.At the heart of the problem is the absence of a standardized test technique for testing concrete under impact.Various investigators have used different impact machines,specimen configurations,specimen sizes,and instrumentation,and they have also adopted differing analysis schemes,so,it is difficult to compare the test results.In this regard,American Concrete Institute 544 has proposed a drop-weight impact test to evaluate the impact resistance of concrete.The test is widely used since it is simple and economical.However,the results obtained from this test are often noticeab ly scattered due to nature of the test and nonhomogeneous condition of the concrete.The sources of large variations in results obtained from the ACI544 impact test can be summarized as follows:(1)The cylindrical specimens are used in the test,which allow cracks to occur anywhere and in any direction.This increases the subjectivity of the test and makes the visual identification of the first crack difficult;(2)The criteria for accepted or rejected failure mode is absence,which increases the scatter o f the results.In order to reduce the large variation in results of the impact test and predetermine the location of the crack,the impact resistance of concrete was measured using self-designed U-shape concrete specimens and a newly designed drop-weight impact test apparatus in this paper.The impact test results were subjected to comprehensive statistical analysis.The results are as follows:1.Using U-shape specimens can force cracks to occur in a predefined path,and for each specimen,just a single crack occurred,which can facilitate to observe the crack initiation and propagation.The newly designed drop-weight impact test apparatus is feasible and the operation is easy.2.For plain concrete,in order to reduce the error caused by human factors,compressive strength tests were also conducted on cubic specimens as a means of quality control.Through the discreteness control of compressive strength values to feedback the differences between specimens,avoiding the large difference due to the poor quality of concrete specimens.A series of drop-weight tests were carried out for U-shape concrete specimens with four different quality level of drop hammer and three kinds of age.The research results show that the impact life has a logarithmic linear correlation with the mass of drop hammer.3.Large variation is a common problem in impact test.The values of coefficient of dispersion were calculated for the plain concrete,basalt fiber reinforced concrete and steel fiber reinforced concrete.The values of coefficient of dispersion of impact life were also calculated from the literatures using ACI544 drop-weight impact test.The results show that the drop-weight impact test apparatus of this paper can reduce the discreteness of data.Through the frequency histogram figures,we can see that the compressive strength results are almost normally distributed and the impact resistance results are poor to follow a normal distribution.In the case of the same fiber volume content,the impact resistance of steel fiber reinforced concrete is better basalt fiber reinforced concrete.4.Applying to the model the least-squares method to analyze the impact life,The study results shows that the first-crack impact life has a positive linear relationship with the ultimate failure impact life.5.An important feature of impact is the probability characteristics,so it is important to study the impact problem using the theory of probability.The impact life N is seen as a random variable.According to the expectation estimation of mean rank method,the concepts of order statistics and rank are introduced to estimate the values of the function of cumulative failure probability F(N)and survival probability R(N).The study results shows that the impact lives follow both the Lognormal distribution and the Weibull distribution,but the Weibull distribution is more suitable.The relationship between the impact life and failure probability is established based on the Weibull distribution,and it is used to estimate the impact life under d ifferent failure probabilities.The impact resistance of fiber reinforced concrete can be tested and predicted by the Weibull distribution model.At the experimental work present,the number of specimens is determined based on experience,and the theoretical foundation is absence.So,it is subjective arbitrariness and blindness to do the experimental work.According to the statistics t-distribution,the minimum number of specimens is determined by the method of mathematical statistics in this paper.The ba sis and recommendations are also given.6.Based on the statistical damage model of weibull distruction,the damage evolution equations of concrete compressive strength are established,and the damage evolution equations of fiber reinforced concrete impact strength are also established.