CN85100680B - Electronic method and apparatus for nondetructive determination of pavement thickness - Google Patents

Abstract

The present invention relates to an electric non-destructive detecting method for the thickness of concrete layers, which is used for detecting the thickness concrete layers and even thin concrete (with steel bars or not) layers just like sprayed concrete layers. The present invention is characterized in that apparent resistivities ps of different depth can be measured by altering polar distances (AO) of electrodes supplying electricity to concrete surfaces, and the thickness of concrete layers is determined by breaking points of a ps-AO curve; in order to accurately measure the ps-AO curve, an electrode system capable of fixing telescopic electrodes to a frame is adopted and is supported and fixed by a supporting frame, and a plurality of groups of electrodes are completely installed in one time and can contact rugged and curved concrete surfaces forming various included angles with a horizontal plane.

Description

The electric measuring method of Non-Destructive Testing coagulate layer thickness

The present invention is a kind of method and apparatus of Non-Destructive Testing coagulate layer thickness, and this concrete layer is meant plain concrete layer that is covered on certain medium or the concrete layer that contains reinforcing bar.The concrete layer that this measuring method is suitable for measuring the thick plain concrete layer of 2～150cm or contains reinforcing bar is particularly suitable for detecting surface irregularity or is the thickness of shotcrete layer in the underground chamber of the dielectric surface injustice that curved surface or concrete were covered.

For the concrete layer that is overlayed on scar, the soil layer, the way that its thickness of Non-Destructive Testing is normally measured with boring.For sprayed concrete, also can be before spraying the standard of control jet thickness when fixedly survey pin or other flag things are as construction on scar.At home with the regulation of external relevant design and construction in, all stipulated in concrete brill inspection hole or the method and the quantity of burying survey pin.For example " anchor shoring shotcrete design and construction regulation ", the State Planning Commission of State Infrastructure Commission's promulgation in 1979 were about to " State Standard of the People's Republic of China-anchor shoring shotcrete design and construction and the acceptance specification " of promulgation in 1985, " New Austrian Tunneling Method construction guide " (drafts) that units such as Japan state-owned railway construction office carried out March nineteen eighty-three etc. all are to stipulate like this.According to railway tunnel spray anchor technology rule request, 1 kilometer single-track tunnel shotcrete bolt lining need detect 450 points at least, and double track tunnel then will detect 1300 points.

The method that adopts boring directly to measure has been destroyed xoncrete structure itself, and is time-consuming, finishes man-hour in engineering, and it is inconvenient that acceptance is then more felt.Especially when lining or lining cutting have thing such as splash guard behind, no matter hole or plug in pricker, do not allow to use.

Utilize sound wave or ultrasonic reflections to measure the method for concrete thickness, though also belong to Non-Destructive Testing, but it is smooth that it is only applicable to surface measurements at present, basic unit-the rock of its lining or soil layer-also more smooth above thick concrete layer of 30cm, for thin layer, shotcrete layer particularly then can not satisfy the needs of engineering.(seeing that it is " the non-damaged research report that detects of Geng Cun Tunnel Engineering concrete quality " of writing that ore deposit, Jiaozhuo institute mines).

In addition, also the someone attempts to measure with the difference of concrete layer and underlying bed resistivity the thickness of highway pavement and bridge deck concrete layer.They have directly quoted the mathematical model that the infinity plane bottom is arranged under even, isotropy conductive layer, the formula that utilizes this mathematical model to derive is measured the thickness of superstratum, and the condition of measurement is bigger highway pavement and the bridge floor of the also more smooth thickness of surfacing, substrate.

For example, deliver in U.S. PB in 1972 report (PB-210245) by Russell, in the test report " Nondestructive Determination of Pavement Thickness using Electrical Resistivity Techniques " (" coming non-destructive determination highway pavement thickness ") of propositions such as James E., introduced relevant test method with resistivity technique.It is to utilize at a big wooden frame (60 " * 25 ⁹The wire electrode that is provided with in/16 ") (12 " and 24 " long) be placed on the concrete surface of firm pouring; electrode is all contacted; traveling electrode position one by one in wooden frame with the road surface, measure the resistivity that the electrode of several different distance obtains after, with the electricalresistivity of it and concrete layer ₁, medium that concrete was covered the electricalresistivity ₂The input robot calculator, utilization is in electrically even, isotropy, infinity, the conducting medium of the uniform thickness down formula that mathematical model derived of different another layer of infinitely great planar interface, the resistivity of lining calculates concrete thickness, and the test of quoting in the report is thick 8 " the road surface on carry out.Report is pointed out, their test method is applicable to that substrate can regard infinity plane as, overlayer be electrically evenly, the situation of isotropy, absolute conductive layer, so just can measure accurate result, and be not suitable for the concrete road surface of having solidified to the road surface of firm pouring.For guaranteeing that near last cap rock be the testing precondition of electrical homogeneous isotropism, so adopt wire electrode.But the kind electrode heaviness, requiring has 1～2 foot contacting with the survey concrete surface, so only be applicable on the flat road surface of surfacing.Will constantly change the position of electrode in wooden frame during measurement, use calculation by computer then, this method wastes time and energy, and institute's Thickness Measurement by Microwave can not be thinner than 5cm.After 1973, do not see the report that further research and use are arranged.Also exclude this method in the handbook of relevant measurement concrete component, lining cutting, lining, the standard.

Why difficult with physical method measurement concrete thickness, also because concrete itself is aggregate and the fixed potpourri that forms of hydrated cementitious that is not waited by particle size, reinforcing bar is often arranged also in the xoncrete structure, physical property is extremely inhomogeneous, when bed thickness was thin, this unevenness was more obvious.In addition, the basal plane of its covering and sprayed concrete surface, also often rough and uneven in surface or be curved surface, this brings many difficulties to thickness measuring.

Task of the present invention is the method that a kind of harmless measurement coagulate layer thickness will be provided, and no matter this concrete is that newly lay or of the remote past, and the still non-reinforced of reinforcing bar arranged, the still thick-layer of thin layer, the surface is smooth or uneven.

Task of the present invention realizes as follows, its principle of first accompanying drawings:

Fig. 1-measurement apparent resistivity method synoptic diagram

The section survey apparent resistivity synoptic diagram of Fig. 2-two-layer dielectric is formed

Fig. 3-electrode synoptic diagram

1-insulator cap 2-contact tube 3-conducting rod wherein

4-conductive rubber cap 5-spring

Fig. 4-three-pole device electrode system synoptic diagram

1-insulation fixed mount wherein

2-spring retractable electrode

3-bull socket, each contact conductor all causes socket

Fig. 5-four utmost point device electrode is a synoptic diagram

1-insulation fixed mount wherein

2-spring retractable electrode

3-bull socket, each contact conductor all causes socket

Fig. 6-support and be synoptic diagram with bracket leg top fixed electorde

1-electrode system wherein

2-bull plug

The 3-polycore cable

4～6 supports

The rack leg of the extending shortening of 4-

The Zhi Dingtou of 5-support

6-can make bracket head change the hinge joint joint of a drift angle degree

The ρ that Fig. 7-south mountain tunnel measuring point 5 records _S-AO curve is the plain concrete layer, and electrical measurement thickness 10cm chisels and measures thickness 9～9.5cm

The ρ that Fig. 8-Wu Zhuan tunnel measuring point 8 records _S-AO curve, for containing the sprayed concrete of bar-mat reinforcement, electrical measurement thickness 20cm chisels and measures thickness 19～19.5cm

At least lay two pairs of electrodes on the surface of intending the survey coagulate layer thickness, a pair of is A, B, uses for feeding electric current; Another is used for measuring the potential difference (PD) △ V between MN to being M, N _MN(see figure 1), after the energising, recording strength of current is I, then the apparent resistivity ρ of medium _SFor

ρ _S＝ＫΔＶ _ＭＮ/Ｉ……（1）

Wherein

$\mathrm{K=}\frac{\pi }{（\frac{I}{A_M}-\frac{I}{A_N}\mathrm{）-（}\frac{I}{B_M}-\frac{I}{B_N}）}$

If A, B, M, N are point-blank, and are symmetrical in MN mid point O, and AB＞MN, then be called four utmost point devices, at this moment

Ｋ＝πＡ _Ｍ·Ａ _Ｎ/ＭＮ

This device uses when concrete layer is thin, and measuring accuracy is higher.

If wherein B point places distant place (BO＞10MN), then be called three-pole device, K=2 π at this moment, K=2 π A _MA _N/ MN

Three-pole device is used for concrete layer when thicker, and its measuring accuracy is lower than with four utmost point device persons.

When medium was switched on, the distance of the distance of transmitting electrode or transmitting electrode and MN was big more at dielectric surface, and the degree of depth of then measured apparent resistivity reflection is big more.Change polar distance of supplying power ascendingly and record corresponding apparent resistivity, just can reflect the electric conductivity of different depth in the medium from shallow to deep.

Because the concrete resistivity and the rock of its lining or the resistivity of soil have tangible difference, so its separatrix can be at " ρ _S-AO " reflect on the curve.

In fact, can change the AO size at the scene one by one, record corresponding △ V _MNAnd electric current I, try to achieve corresponding ρ with formula (1) calculating _S, and draw " ρ _S-AO " curve, curve draws concrete thickness h thus.

For avoiding continuous mobile A, the B electrode, accelerate measuring speed, guarantee accuracy of observation, the present invention has adopted the electrode system of being made up of a plurality of electrodes, electrode used therein (Fig. 3) wherein, be that an end of circular conducting rod (3) is put into the contact tube (2) that inside keeps spring (5) and connects power supply, this contact tube (2) internal diameter is greater than conducting rod external diameter 0.1～0.2mm and be fixed on the insulator cap (1), the other end that conducting rod (3) contacts with concrete is installed conductive rubber cap (4), one end of spring (5) and insulator cap (1) offset, the other end then joins with an end of conducting rod (3), so conducting rod (3) can freely stretch in contact tube (2), become retractable electrode (Fig. 3), so that electrode and concrete layer are closely connected.

Fig. 4 and shown in Figure 5 be electrode system figure, they are with a plurality of retractable electrodes (2), according to the required whole spread of electrodes of actual measurement, be fixed on the insulating frame (1) with four utmost point devices and three-pole device form, Figure 5 shows that four utmost point device electrodes system, this be in each electrode spread point-blank, corresponding a pair of A, B electrode are symmetrical in the mid point O of M, N electrode; Figure 4 shows that the three-pole device electrode system, in this electrode system each A electrode and M, N electrode spread point-blank, shared B electrode during measurement, this electrode places near the measuring point, make BO greater than 10 times AO maximal value, and with conducting medium that the testing concrete layer links to each other in.Each electrode is drawn with polycore cable by bull socket (3), by switch, needs and can connect respectively according to test.Fig. 6 is by a length-adjustable rack leg (4), by changing the hinge joint joint (6) of direction, connects a support that the Zhi Dingtou (5) of top electrode system (1) is constituted, and electrode system (1) is fixed on the concrete surface.Make electrode grounding resistance surpass the instrument allowed band as if the concrete surface too dry, can spray water in concrete surface.

For avoiding the transmitting electrode polarization, between AB, adopt low frequency ac.

Calculate ρ for asking _S, available various types of transistorized millivoltmeters are measured the potential difference (PD) △ V between supply current intensity I and MN _MNSupporting with the present invention, manufactured and designed direct-reading ρ _SLittle sounder, can shorten Measuring Time.If instrument is equipped with microprocessor, directly print ρ _S-AO curve will make measurement more convenient.

Measure the ρ of different transmitting electrode distances _SAfter, with ρ _SWith AO be coordinate, draw ρ _S-AO curve is the decidable coagulate layer thickness then.According to known concrete resistivity and its lining body-rock or soil-resistivity, can find the concrete leading portion curve of reflection easily, its ρ _SApproximate concrete resistivity, and reflection is by the back segment curve of concrete overlay rock mass, its ρ _SApproximate the resistivity of rock mass.With the curve leading portion to the corresponding AO numerical value of back segment turning point, promptly equal the thickness of concrete layer.If contain bar-mat reinforcement in the concrete, then must make ρ _SThe stage casing ρ of-AO curve _SValue reduces, at this moment by the curve stage casing to the pairing AO numerical value of the turning point of back segment, be the thickness that contains the bar-mat reinforcement concrete layer.

The invention has the advantages that:

1. directly by the ρ that surveys _S-AO curve break interpretation coagulate layer thickness, and need not utilize the concrete layer electricalresistivity ₁The resistivity of media ρ that is covered with it ₂Exact value calculate its thickness, even thereby the electric conductivity of concrete layer is inhomogeneous, anisotropy, concrete resistivity do not detect situations such as accurate numerical value and dielectric surface injustice that concrete was covered along degree of depth gradual change or concrete resistivity, can not influence the employing of this method.

2. even variation has taken place in the resistivity of same measuring point concrete layer, so that the ρ of two different times gained _S-AO curve is all or part of not to be overlapped, sentence read result that can influence curve yet.

3. the retractable electrode that has spring that the present invention is designed, make this method not be only applicable to the smooth concrete of test surfaces, and under the situation of or surface curvature rough and uneven in surface at concrete surface, can be closely connected with it because of electrode, still can measure correct relevant data.

4. the electrode system that uses of the present invention is arranged electrodes according to whole spread of electrodes that testing is required, and this electrode system is once installed and finished and select the electrode of required usefulness by switch, can record a series of data rapidly like this when measuring.Avoid in rotating detection process traveling electrode repeatedly, thereby accelerate measuring speed, reduce the error of arranging electrode.Simultaneously, allow the testing instrument not near testing point, even put not intervisibility or the also normally testing of conversation difficulty with testing.

5. electrode system used herein is comparatively light, and cooperation can be regulated the support of a length and a conversion drift angle degree, all can install horizontal by the place of various angles at concrete surface, is particularly useful for testing each different parts in the underground chamber.

6. the present invention both can measure the coagulate layer thickness of new perfusion, also can measure the thickness of fixed concrete layer, both can be used for plain concrete, also can be used for reinforced concrete, big can the arriving more than the 1m of test thickness, little can be thinned to 2cm, and especially measuring thin-lift concrete is its speciality.

Illustrate below in conjunction with accompanying drawing:

＜example 1〉south mountain tunnel inducer adopts concrete spraying support, has detected some places jetting thickness in this Duan Zhongyong the present invention.Wherein, recognize that according to this section geologic map geology belongs to ampelitic limestone and limestone, on exposed scar, measure its resistivity and be higher than 100 Ω m that the resistivity of spray-up then is lower than 80 Ω m as the NO5 measuring point.

The NO5 measuring point is at the tunnel side wall place, estimates that jetting thickness in 20cm, adopts four utmost point devices, and the employing pole span is MN=1cm, and AO is respectively 1.5,3.5,4.5,6,8,12,14,20cm.According to the spray-up surface undulation.With the electrode system horizontal positioned, and make all electrodes all can be closely connected with the spray-up surface, fixing with the bracket leg top.Power to spray-up by A, B with 50 all industrial-frequency alternating currents, measure △ V with the DA-16 transistorized millivoltmeter _MNAnd I, calculate ρ then _SMeasure the ρ of different AO _SAfter, draw ρ _S-AO curve (Fig. 7).From the curve as can be seen, curve leading portion resistivity is that a section of 30～40 Ω m has reflected spray-up resistivity, one section resistivity that reflects rock of curve back segment 120 Ω m, with the corresponding AO numerical value of turning point that changes to back segment by the curve leading portion be 10cm, determine that thus jetting thickness is 10cm.

Chisel the spray-up check subsequently, can be observed its back rock is ampelitic limestone, measures the thick 9～9.5cm of being of spray-up with steel ruler, very close with the electrical measurement result.

＜example 2〉Wu Zhuan tunnel outlet section adopts sprayed concrete as preliminary bracing, has detected some somes jetting thicknesses with the present invention here, as measuring point 8, learns that according to geologic information this place passes sand shale alternating layers section, contains reinforcing bar in the sprayed concrete.Measure ρ by example 1 described program _SAnd draw ρ _S-AO curve (Fig. 8) is because of not clear thickness, so adopt three-pole device.Adopt that MN=1cm, AO are respectively 1.5,2.5,3.5,4.5,6,8,10,14,20,30,40cm.As seen from the figure, curve leading portion ρ _SBe about 30 Ω m, reflected the pneumatically placed concrete layer, from AO greater than 2.5cm after, ρ _SSharply reduce, reflected the influence of reinforcing bar, curve back segment ρ _SRise to again more than the 10 Ω m, reflected shale, the corresponding AO of turning point that changes to back segment with the curve stage casing is 20cm, determines that thus jetting thickness is 20cm.

Chisel the spray-up check subsequently, run into reinforcing bar at the 2.0cm place, run into aubergine shale through measuring at 19～19.5cm place with steel ruler, the result is consistent with electrical measurement.

With the present invention the result of 6 tunnel survey jetting thicknesses with chisel thickness that the pneumatically placed concrete layer measures subsequently to such as shown in the subordinate list.In 90 measuring points, 42 of the persons that has the bar-mat reinforcement in the concrete, 48 of no-reinforcing-bar net persons wherein have 1～2 day person in the back of injection for concrete, have after the injection still less than support the survivor in 14 days, and existing 1～2 year person in the back of injection is arranged.Its rock behind has eight kinds.Through check results, have only 2 points to declare mistake, account for 2.7%, one actual concrete thickness of point and surpass and to require the thickness measured, all the other 87 some electrical measurement thickness h with chisel concrete and measure h ' contrast as a result, use formula

$\mathrm{\sigma ＝\pm }\sqrt{\frac{{\mathrm{\sum （ｈ－ｈ\prime ）}}^{２}}{\mathrm{２ｎ}}}$

Calculate meansquaredeviation, then σ=± 0.87cm

Harmless electrical measurement sprayed concrete layer thickness with the actual measurement thickness table of comparisons

In proper order

Thickness range

The place

Survey period

Bar-mat reinforcement

Electrical measurement thickness cm

Chisel spray-up and measure thick cm

Difference cm

Remarks

１

＜３ｃｍ

Wu Zhuan

③

Do not have

１.５

１

０

The canescence sandstone

２

South mountain tunnel

⑦

Do not have

２.５～３.５

１.５～３.５

０

Limestone, yeath alternating layers

３

⑩

Do not have

２.５

３

０.５

The same

４

Do not have

１.５

０.５

０

Yeath

５

Do not have

１.５

１

０

Limestone

６

Do not have

１.５

１.５～２.５

１

７

Draw the Luan water-conveyance tunnel

①

Do not have

２.５～３.５

２

０.５～１.５

Gneiss

８

⑥

Do not have

２.５

２～３

０

９

Collapse 1

Do not have

２.５

２～３

０

１０

Collapse 2

Do not have

２.５

２

０.５

１１

Collapse 3

Do not have

２.５

２

０.５

１２

３～１０cm

Preceding tunnel, mountain range

①

Do not have

１.５

３.５～４

０.５～１

Strong yellow soil

１３

②

Do not have

８

８～９

０～１

１４

③

Do not have

８

８

０

１５

④

Do not have

１０

８～９

１～２

１６

⑤

Do not have

４.５

４.５

０

１７

⑥

Do not have

４.５

４.５

０

１８

The Wu Zhuan tunnel

①

Do not have

６

６.５

０.５

The canescence sandstone

１９

④

Do not have

４.５

３.５

１

２０

⑤

Do not have

６

５

１

２１

South mountain tunnel

Do not have

３.５

３.５

０

Gravelly soil (the aubergine weathered shale presss from both sides grey sillar stone)

２２

①

Do not have

６～８

６～７

０～１

The flesh pink sandstone

２３

②

Do not have

８～１０

８～１０

０

２４

③

Do not have

４.５～６

４～６

０

The limestone slabstone

２５

④

Do not have

４.５

４～５

０

Yeath

２６

⑤

Do not have

１０

９～９.５

０.５～１

２７

⑥

Do not have

８～１０

７～１０

０～１

２８

⑧

Do not have

３.５

４～４.５

０.５～１

Yeath, limestone alternating layers

Harmless electrical measurement sprayed concrete layer thickness with the actual measurement thickness table of comparisons

In proper order	Thickness range	The place	Survey period	Bar-mat reinforcement	Electrical measurement thickness cm	Chisel spray-up and measure thick cm	Difference cm	Remarks
									29	3～10cm	South mountain tunnel	⑨	Do not have	4.5	3.5	1	Limestone
									30				Do not have	6	5～5.5	0.5～1
31		Do not have	8	8	0
						32		Do not have	3.5			4	0.5
33		Do not have	10	7	3
						34		Do not have	3.5			3.5～4.5	0～1
35		Do not have	＜1.5	4～5	Mistake
						36	Do not have	4.5	5			0.5	The flesh pink sandstone
37		Do not have	10	9～10	0～1
						38	The tunnel, Dayao Mountain	①	Have		10	8	2	The tawny slate
39	⑧	Have	10	10	0
						40			Have		8	10	2
41		Have	8	7.5～9	0.5～1									The grey slate
						42			Have		8	8	0		The tawny decayed rock
43	Have	8	8.5	0.5
44		Have	10	10		0
								45			Have	8	8～9	0～1
46		Have	12	9	3	The tawny decayed rock
								47			Have	4.5	4.5	0	The tawny slate
48	The water-conveyance tunnel of diverting Luanhe water to Tianjin	②	Do not have	2～4.5	2～4.5	0	Gneiss
49		③	Do not have	6～8	6～8	0
								50	④		Do not have	4.5	4	0.5
51		⑤	Do not have	8	6	2
								52	⑦		Do not have	3.5	3.5	0
53		Barracks ground	Do not have	3.5	7	0.5	It below the sand-cement slurry common brick
								54	Have		10	9	1	Gneiss
55			Have	10	9～10	1～0
							56		Have		10	9～10	1～0	Mylonite

Harmless electrical measurement sprayed concrete layer thickness with the actual measurement thickness table of comparisons

In proper order	Thickness range	The place	Survey period	Bar-mat reinforcement	Electrical measurement thickness cm	Chisel spray-up and measure thick cm	Difference cm	Remarks
									57	＞	Preceding tunnel, mountain range	⑦	Have	14	14	0	Strong yellow soil
									58			⑧	Have	14	15	1
59	Tunnel, waist Xian river	①	Do not have	14	14～14.5	0～0.5
60							②	Do not have	16.5		17	0.5
	61	The Wu Zhuan tunnel	②	Do not have	14	12.5～13							1～1.5	The canescence sandstone
62	⑥						Have	14	13.5		0.5	Aubergine shale
63	⑦		Have	20	18～18.5	1.5～2
							64	⑧	Have		20		19～19.5	0.5～1
65	South mountain tunnel	Do not have	10	11	1	The flesh pink sandstone
66			Do not have	14	16		2
	67					The tunnel, Dayao Mountain		②	Have		10	11	1	The tawny slate
68	③	Have	10	13	3
							69	④	Have		14	12	2
70	⑤	Have	8	＞20	Mistake
							71	⑥	Have		12	14	2	The grey slate
72	⑦	Have	＞20	＞20	0										The tawny slate
73	⑨	Have	20	20	0
							74	⑩	Have		14	14	0
75		Have	10	11	1
							76	Have	20		20	0	The tawny wind slap
77		Have	10	12	2
							78		Have		10	12		2
79	The water-conveyance tunnel of diverting Luanhe water to Tianjin	⑧	Have	14	14	0							Gneiss
80							⑨	Have	？		＞29			Curve can't the interpretation gneiss
		81	⑩	Have	14	15～16							1～2		Gneiss
82								Have	10		10～11	0～1		Mylonite
		83	Have	10	10～11	0～1							Gneiss
84			Have	25	22	3

Harmless electrical measurement sprayed concrete layer thickness with the actual measurement thickness table of comparisons

In proper order	Thickness range	The place	Survey period	Bar-mat reinforcement	Electrical measurement thickness cm	Chisel spray-up and measure thick cm	Difference cm	Remarks
									85	＞10cm	The input tunnel of diverting Luanhe water to Tianjin	Have	17	19	2	Gneiss
									86				Have	14	15		1
87		Have	10	10.5	0.5
						88		Have	14			14	0
89		Have	14	16	2
						90		Have	14			16～17	2～3

〈 ﹠﹠〉Fig. 1 〈 ﹠﹠〉Fig. 2 〈 ﹠﹠〉Fig. 3 〈 ﹠﹠〉Fig. 4 〈 ﹠﹠〉Fig. 5 〈 ﹠﹠〉Fig. 6 〈 ﹠﹠〉Fig. 7 〈 ﹠﹠〉Fig. 8

Claims (9)

1, a kind of harmless electrical measurement method of being covered in the plain concrete on certain medium or containing the coagulate layer thickness of reinforcing bar comprises:

(1) on the measurement straight line of concrete surface, a pair of supply terminals electrode A, B and a pair of measurement point electrode M, N are set, the O point is point among this measurement point electrode M, the N on the measurement straight line, this supply terminals electrode A, B are positioned at the outside of measurement point electrode M, N;

(2) feed low frequency ac to this supply terminals electrode A, B;

(3) change the pole span of this supply terminals electrode A, B one by one;

(4) measure and the interelectrode potential difference (PD) △ V of the measurement point of above-mentioned corresponding pole span _MNWith supply current I;

(5) utilize formula ρ _S=K Δ V _MN/ I goes out corresponding apparent resistivity;

(6) utilize the ρ of different poles distance _SValue is asked the calculation coagulate layer thickness;

The invention is characterized in: according to above-mentioned each ρ _SWith the AO value, make ρ _S-AO curve is by this ρ _SThe corresponding AO value in turning point place that the apparent resistivity curve section of measured plain concrete that-AO curve is reflected or the concrete layer that contains reinforcing bar is intersegmental with the apparent resistivity curve that is capped medium is directly read the coagulate layer thickness that is covered on this medium.

2, the measurement mechanism that uses in accordance with the method for claim 1 is by transmitting electrode, potential electrode power supply and apparent resistivity ρ _SMeasuring instrument is formed, the invention is characterized in and use a pair of measurement point electrode M, N, a pair of above supply terminals electrode A, B, each supply terminals electrode is connected power supply through a polycore cable in turn by switch, the conducting rod of all point electrodes can overcome behind the elastic force of contact tube inside spin spring rectilinear motion in contact tube, to guarantee that each point electrode contacts with the detected concrete layer, premises electrode is fixedly mounted on the insulating frame according to the required spread of electrodes of measurement and forms electrode system, this electrode system props up the support support of drift angle degree by regulating height and conversion, thereby makes on the concrete surface its plain concrete that stably is placed in differing heights and degree of tilt or that contain reinforcing bar.