Experimental study on AE characteristics of
three-point-bending concrete beams
Bing Chen a,*,Juanyu Liu b
a
School of Civil Engineering and Mechanics,Shanghai Jiaotong University,Shanghai 200240,PR China
b
Department of Civil Engineering,Texas A&M University,College Station,TX 77843,USA
Received 18March 2003;accepted 18August 2003
罗定文塔Abstract
In this rearch,acoustic emission (AE)characteristics of three-point-bending concrete beams were investigated during the entire loading period.It was found that the relative notch depth significantly influenced AE characteristics.The occurrence of AE events decread greatly with an increa of the re
lative notch depth.The influences of different fibers in concrete on AE characteristics were investigated as well.The experimental results indicated that the Weibull function can be ud to describe quantitatively the influences of the relative notch depth and fibers on AE characteristics,fracture characteristics,and brittleness of concrete.The two parameters,h and m ,of the Weibull function depended on the geometry of the concrete specimens and the brittleness of concrete,respectively.D 2004Elvier Ltd.All rights rerved.
Keywords:Acoustic emission,(AE);Concrete;Microcracking;Brittleness
1.Introduction
Acoustic emission (AE)is a kind of microismic wave generated from dislocations,microcracking,and other irreversible changes in a stresd material.The monitoring of stress waves is accomplished by piezoelectric trans-ducers,which convert the mechanical waves to electrical signals.
乒乓球球拍
The AE technique is a widely ud tool for nondes-tructive evaluation of metallic and nonmetallic materials and engineering structures [1–4].It differs from most other nondestructive methods in two significant aspects [5].First,the energy detected is relead from the interior of the tested object rath
er than from some external sources.Second,AE techniques are capable of detecting the dynamic process associated with the degradation of structural integrity.Recently,the AE technique has also been applied to studies of concrete mechanics,with a focus
on the properties of crack extension during the fracture process [6–8].
In the rearch on fracture toughness of concrete [9–12],a three-point-bending concrete beam with a single notch is a typical specimen ud model concrete fracture mechanics who relative notch depth has significant effects on the brittleness and fracture properties of the specimen [13].
In order to investigate the sources of brittleness of concrete,AE characteristics of three-point-bending concrete beams with and without different fibers were investigated in this study,and the effects of the relative notch depth and intrinsic properties of concrete specimens on the brittleness of concrete were studied as well.
2.Experimental details 2.1.Sample preparation
Two ries of concrete beams were prepared for this study:(1)normal concrete beams with different notch depths and the same mix proportions;and (2)concrete beams containing different fibers at the notch depth of 50.0mm.The details of the mix proportions are listed in
李扯火
0008-8846/$–e front matter D 2004Elvier Ltd.All rights rerved.doi:10.res.2003.08.021
*Corresponding author.Tel.:+86-21-5474-4095;fax:+86-21-5474-4255.
二十四节气立冬
E-mail address:hntchen@sjtu.edu (B.Chen).Cement and Concrete Rearch 34(2004)391
索取–397
Table 1.The specimens were demolded 1day after casting and then were cured in a curing chamber at 17–23j C and a relative humidity greater than 90%.Two days before testing,they were taken out for single edge notch cutting according to the requirements of the experiments.2.2.Testing equipments
Three-point-bending tests were performed using an Ins-tron 8501Digital Servo hydraulic testing system.The specimen dimensions were 100Â100Â515mm,with sin-gle edge notch at depths of 20.0,50.0,or 80.0mm,as shown in Fig.1.Six PACR15transducers with a resonant frequency of approximately 150kHz were glued onto the two opposite surfaces of the specimens to monitor the AE activities.The deflections at the midspan of beams were also measured.
A SPARTAN-AT2000AE system was ud for AE data acquisition.The AE signals were amplified with a gain of 40d
B in a preamplifier and a gain of 20dB in the system.A threshold of 40dB was lected to ensure a high signal-to-noi ratio.
3.Experimental results
3.1.Effect of relative notch depth on AE characteristic The correlations between AE events and the P –d curves of concrete beams with different relative notch depths are shown in Fig.2.It was found that the occurrence of AE had a good correlation with the load versus the deflection curve.In the ascending branch of the P –d curves,the AE charac-teristics of concrete beams with different relative notch depths were similar:(1)During the initial loading stage,there was little occurrence of AE activities;(2)As the load reached about 80%of the failure load,the AE activity became more inten;(3)When the external load exceeded the ultimate strength,the AE activity incread rapidly.In the descending branch of the P –d curves,the relative notch depth had a great effect on AE characteristics:(1)The position of the peak of occurring AE events changed with the relative notch depth.The peak appeared in the region of 85–95%of ultimate strength on the descending branch of the P –d curves when the relative notch depth was 0.2,while
it appeared in the region of 60–80%P max in the descending branch of the P –d curves when the rel
ative notch depth was 0.5.However,there did not exist an AE peak during the entire loading process when the relative notch depth was 0.8.(2)The width of the occurring AE events peak incread with an increa of the relative notch depth.Therefore,the position of the peak shifted to the latter part of the descending branch of the P –d curves,the width of the peak incread,and the magnitude of the peak decread with an increa of the relative notch depth.As the relative notch depth became larger,no peak occurred,which indi-cated that the occurrence of AE decread with an increa of the relative notch depth;this is the brittleness of the specimen decread.
3.2.Effect of fibers on AE characteristic
The effects of fibers on AE characteristics during the entire loading process are shown in Fig.3.From the P –d curves,it can be en that there existed a plateau at the maximum load for the fiber reinforced concrete,which indicated that fibers decread the brittleness of the concrete beams.Accordingly,AE characteristics showed the similar trends:the distribution of occurring AE events for normal concrete peaked in the region of 80–90%of the maximum load in the descending branch of the P –d curve,while that for fiber reinforced concrete was scattered,with no obvious occurring AE event peak.
3.3.Correlation between AE events and fracture process A typical experimental record for the P –d and dN /dt –d curves is shown in Fig.
4.It was found that the relationship between dN /dt and d could be divided into three stages:(1)dN /dt =,there was no AE);(2)the relationship between dN /dt and d was discontinuous;and
Table 1
劳务用工Mix proportion Series
Water,kg/m 3Cement,kg/m 3Slag,kg/m 3Sand,kg/m 3Gravel,kg/m 3Fiber,kg/m 3Normal concrete 1682671157401110/Steel
fiber concrete 168267115740111070PP
fiber concrete
168
267
115
740
1110
2
Fig.1.Geometry of specimen and position of transducers.
B.Chen,J.Liu /Cement and Concrete Rearch 34(2004)391–397attend
392
(3)the relationship between dN /dt and d was continuous.From further analysis of dN /dt –d curves,it can be considered that the fracture process of three-point-bending beams consisted of three stages:stable microcracking,coalescence of microflaws and growth of the microcracks,and connection of the main cracks.(1)Stable microcracking:when the load was less than the
maximum load,microflaws in the interior of the concrete could not extend and they were in a stable state.
(2)Coalescence of microflaws and growth of microcracks:
when the load reached the maximum load
(which
Fig.2.AE characteristic for concrete beams with different relative notch depth;1—load,2—AE event rate,3—cumulative AE event number.
B.Chen,J.Liu /Cement and Concrete Rearch 34(2004)391–397393
means the deflections beyond the deflection at peak load),the microcracks around the tip of the main cracks began to grow while the main cracks themlves did not extend.Hence,the relationship between dN /dt and d was discontinuous.
(3)Growth of main cracks:When the external load reached
to the critical load,the main cracks began to grow.Thus,the relationship between dN /dt and d was continuous.
Therefore,the trend of occurring AE events corresponds to the microfracture process of concrete in three-point-bending.
4.Analysis and discussion
According to the statistical theory of damage of quasi-brittle materials,concrete can be considered to be
compod
Fig.3.AE characteristic for concrete beams with different fibers;1—load,2—AE event rate,3—cumulative AE event number.
B.Chen,J.Liu /Cement and Concrete Rearch 34(2004)391–397
国企和央企的区别394
of many microelements.Under the external load,the rupture probability of each microelement is different but normally complies with a Weibull distribution.For instance,some rearch has shown that under uniform uniaxial tensile strain,the rupture probability density function of the micro-element could be described as follows:f ðe Þ¼h m ðe Àe 0Þm À1exp ½Àh ðe Àe 0Þm
ð1Þ
where h and m are parameters related to the size and deformation properties of the specimen,respectively.e 0is a constant determined by experiments.
When the strain reaches a certain value,the rupture probability of the microelement occurring in the material becomes:
F ðe Þ¼Z e
f ðe Þ¼1Àexp ½Àh ðe Àe 0Þm ð2Þ
Although the rupture condition of three-point-bend beams is much more complicated than that of the speci-mens under uniaxial tensile loading and the distribution of strain is not uniform,it is still reasonable to consider that each AE event is the result of the rupture of a microelement.Therefore,the rupture probability density of the microelements in concrete at a time (corresponding to a certain deflection)can be prented by the ratio of the AE hit ratio (dN /d d )to the total number of AE hits (N tot ),i.e.,
f ðd Þ¼dN d
d d N tot
ð3ÞWhile the rupture probability of the microelement at that time is:F ðd Þ¼N d =N tot
ð4Þ
From the fit of the experimental data,it was found that the Weibull function was perfectly suitable for describing
the statistical distribution characteristics of AE events and the rupture probability of the microelement for three-point-bending concrete materials,with a correlation (average of nine specimens)of 99.5%.A
s an example,Fig.5shows the coincidences between the Weibull function and AE hit ratio curves of three-point bending beams with three different relative notch depths.In Fig.5,the points reprent the experimental data,while the straight line was the result of bilogarithmic fitting of AE cumulation obtained from the experiments.The correlation between Weibull function integration curves and the measured AE hit ratio–deflection curves is shown at the top left of the figure,while the correlation between Weibull function integration curves and tested AE cumulation curves is shown at the bottom right of the figure.Therefore,the rupture probability density and the rupture probability ratio of the microelement for three-point-bending concrete beams can be described by the Weibull function,namely,
f ðd Þ¼ðdN d =d d Þ=N tot ¼h m ðd Àd 0Þm À1exp ½Àh ðd Àd 0Þm
ð5ÞF ðd Þ¼N d =N tot ¼1Àexp ½Àh ðd Àd 0Þm
ð6Þ
The values of m ,h ,and fracture energy G f for concrete beams with different relative notch depths are shown in Table 2.For the concrete beams,the components of concrete were the same while the dimensions of the speci-mens were different.It was found that the value of m did not change with
a change of the relative notch depth,while the values of h and G f decread greatly with increasing relative notch depths.Table 3gives the values of m ,h ,and fracture energy G f for concrete beams with different fibers.For the concrete beams,the dimensions of the specimens were the same and the components different.In this ca,the values of m and G f changed greatly for concrete beams containing different fibers,while the value of h was almost unchanged.In addition,the values of m and G f were the least for normal concretes and the largest for concretes with steel fibers.
The fracture properties and brittleness of concrete come from two aspects:one is from the intrinsic brittleness of the material,the other is from the geometric properties of the specimen.According to the experimental results,the two parameters,h and m ,in the Weibull function depend on the geometry of the concrete specimens and the brittleness of the concrete,respectively.Therefore,the brittleness of concrete beams can be determined quantitatively by AE techniques during the entire loading process.
5.Conclusions
(1)The failure process of three-point-bending concrete
beams with different relative notch depths and same specimen dimensions have different AE
characteristics.
Fig.4.Typical P –d curve and dN /dt –d curve from experiment.
B.Chen,J.Liu /Cement and Concrete Rearch 34(2004)391–397395
