| Abstract |
| The article researched the
method of testing and evaluation to crack's depth on concrete road
surface by means of nonmetal ultrasonic flaw detector.In this article,it
compared the measuring accuracy about crack's depth using the Tc-To,BS
and T methods,regard as these methods must synthesized for
measurement,because of the crack's depth is an unknown quantity at
beginning.First use the T-method to measure,then use the Tc-To and BS
method to prove in order to obtain well accuracy. |
| Introduction |
| At present,the measuring
method mainly is three kinds about crack's depth in concrete structure: |
| 1.Tc-To method,as shown in
Fig.1 |
| 2.BS method,as shown in
Fig.2 |
| 3.T method,as shown in Fig.3 |
|
|
|
|
Fig.1 |
Fig.2 |
|
In the
first method (Fig.1),is need that the distance of the probe to
the crack should approximately equal to the crack's depth so
that obtaining well accuracy.
In the second method (Fig.2),if the distance of the probe to the
crack is not suitable,then it will cause larger error.
But the crack's depth is don't known in practical testing,so the
Tc-To and BS method is difficult to carry.
In the third method (Fig.3),it is take the receiver step by step
approach to the crack (in this time,the receiver is to receive
through-wave) until cross the crack.Measuring out the
propagation time of each step,then drawing the coordinate-curve
from the relation between propagation time and the opposite
distance of both probes,so that able |
|
|
Fig.3 |
|
| to obtain the crack's depth
from the mutation part on the curve.The advantage of the T-method is
need'nt know the requirement of probe's place. |
| Testing process |
| Using instrument is a type
of CTS-25 nonmetal ultrasonic flaw detector,the probe is 100KP40F
(100KHz,40mm dia. piezocrystal) and 50KP28 (50KHz,40mm dia.
piezocrystal),manufactured by Chinese SANTOU city.The couplant is
liquid-glass (Sodium silicate). |
| The sample is concrete block
with artifical crack (air slit,insertion of thin steel-plate or glued
wood-plate).The block's thickness is 250mm.The concrete's aggregates
were oval-stone and river-sand,the cement's mark number is 525#.Their
mixture is according to the proportion of 1:1:1. |
| Partial measuring results
following: |
| 1.Tc-To method |
| Real crack's
depth(mm) |
a(mm) |
Measuring
value(mm) |
Error(%) |
| Hs=152(insert thin
steel-plate with thickness 0.5mm) |
119 |
144(50KHz) |
-5.3 |
| Hs=56 |
69 |
61(50KHz) |
+8.9 |
| Ha=80(air slit) |
128 |
83(100KHz) |
+3.8 |
| Ha=175 |
219 |
174(50KHz) |
-0.6 |
| Ha=175 |
169 |
175(50KHz) |
0 |
|
| In this method,the key is
identification of the first-wave and accurately read-out the propagating
sound-time of the first-wave.In addition,it should measuring the
different distance between both probe and their corresponding
propagating time of the first-wave on the sample without crack before
measurement for the crack's depth,i.e,to measure out the To. |
| From above results,if the
distance between probe and crack is approximately equal to the crack's
depth and the place of both probes on two side of the crack is
symmetric,then the accurater result is obtained. |
| 2.BS method |
| Take two probes to place on
two side of crack separately,and the placement is symmetric.First take
the distance between both probes is L1 and measuring the
sound-time T1 of first-wave,then take the distance up to L2
and measuring the soun-time T2 of first-wave.The crack's
depth is: |
|
h=(L1/2)[(4T12-T22)/(T22-T12)]1/2 |
| In the measurement,it is
found that the error is more large using the formula for computing.If
change above formula to: |
|
h=L1'¡¤[(4T12-T22)/(T22-T12)]1/2 |
| there L1' is the
distance between both probes on their inner-side,that can obtaining
accurater results.For example: |
| Real crack's
depth(mm) |
L'(mm) |
T(¦Ìs) |
result(mm) |
error(%) |
| Hs=137 |
L1'=100
L2'=200 |
T1=41.4
T2=58.8 |
139.6(50KHz) |
1.9 |
| Hs=137 |
L1'=100
L2'=200 |
T1=38.7
T2=57.3 |
123.1(100KHz) |
-10.1 |
| Hs=100 |
L1'=100
L2'=200 |
T1=37.9
T2=62.4 |
86.8(50KHz) |
-13.2 |
| Ha=120 |
L1'=100
L2'=200 |
T1=74.3
T2=107 |
133.9(100KHz) |
11.6 |
| Ha=100 |
L1'=100
L2'=200 |
T1=67.6
T2=111.6 |
85.9(50KHz) |
-14.1 |
| Ha=175 |
L1'=200
L2'=400 |
T1=99.6
T2=170.4 |
149.2(100KHz) |
-14.7 |
|
| From above results,it found
that the error is larger if the distance between both probes is chose
not fit,in addition,the testing frequency is concerned too. |
| For the Tc-To and BS method
they are request certain arrangement of distance between probe and crack
for obtaining accurater and reliabler results.But the crack's depth is
an unknown parameter in advance,so these methods difficult to carry out
in the practical application. |
| 3.T-method |
It take
the transmiting probe (T-probe) to place at a side of crack and
distance is keeping changeless.The receiving probe (R-probe) is
placed at same side with the T-probe,firstly,then move the
R-probe toward the crack up to cross the crack and continue to
outside until certain distance.This movement is step by step
taking certain interval from near to far.In this case,it is
measure the first-wave's sound-time of each distance one by
one.Then drawn out a set of "time-distance" curve,as
shown in Fig.4.
At crack that has a time's jump (uncontinuous time),then to
compute the crack's depth according to following formula:
h=T¡¤ctg¦Á(T¡¤ctg¦Á+2a)/2(T¡¤ctg¦Á+a) |
|
|
Fig.4 |
|
| there: ¦Á--inclined angle of
the "time-distance" curve; T--uncontinuous time at the crack;
a--distance between crack and center of T-probe. |
| As
shown in Fig.4,its key is measure the time change
("T") when the R-probe is close to the placement on two side
of crack.For example: |
| Real crack's
depth(mm) |
T(¦Ìs) |
a(mm) |
¦Á(¡ã) |
result(mm) |
error(%) |
| Ha=100 |
24.5 |
200 |
13.885 |
82.72(50KHz) |
-17.28 |
| Ha=60 |
19.5 |
225 |
12.78 |
74.09(50KHz) |
+23.5 |
|
| The T-method has larger
error yet,but it is able to know the probable depth of crack and easily
carry out in practice.If we can use the T-method as basis,then go a step
further to prove using Tc-To and BS method according to measuring result
of T-method,we will obtain the more accurate result.In the T-method,it
should take the R-probe close to two side of crack as full as possible
in order to decrease the error of uncontinuous time measured. |
| Conclusion |
| On the basis of
T-method,then go a step further to prove using the Tc-To and BS method
that is a synthesis method.This synthesis method can to obtain higher
accuracy for measurement of crack's depth on the road-surface made of
concrete. |