GB2313443A - Analysing a surface-treated layer of a cementitious body - Google Patents

Abstract

Apparatus for analysing a surface-treated layer 2 of a cementitious body 4 by impedance spectroscopy comprises electrodes 6,8 for applying an alternating signal to the layer and body and an analyser for analysing response to the signal to determine variations in the impedance of the layer with frequency. The apparatus may be used for the in situ testing of reinforced concrete structures (figures 5,6).

Description

ANALYSING A SURFACE-TREATED LAYER OF A CEMENTITIOUS BODY This invention relates to analysis of cementitious bodies and, more particularly, to the analysis of surface-treated layers of cementitious bodies.

Cementitious materials, predominantly concrete, but possibly other materials such as mortar, are commonly used in the building of roads, bridges and other structures. These structures, roads, bridges etc. are open to the environment and, as a consequence, are prone to degradation as a result of the impact of various environmental factors thereon. For example, roads and bridges are commonly sprayed with chloride-based de-icing salts that permeate the concrete structure and cause a deterioration of the embedded reinforcing steel employed in their construction. Other ambient chemicals, such as carbon dioxide, can cause similar deterioration in the structure.

In an effort to alleviate this deterioration, it is common for surface treatments to be applied to concrete structures. The surface-treated layer that results may be a coating that sits on the surface, an impregnant that soaks into the outer layer or a sealer that both soaks in and leaves a surface coating. These treatments provide limited resistance to the penetration of aggressive substances, such as carbon dioxide and chlorides that cause the deterioration. For example, the United Kingdom Highways Agency require that its concrete bridges and other highway structures are protected by being regularly surface-treated with a silane which is absorbed into the pores of the concrete rendering it hydrophobic. By rendering the concrete hydrophobic, it is hoped that the transport of these aggressive chemicals in aqueous solution is reduced.

It is difficult to ascertain by visual inspection whether a particular surface treatment has been adequately applied. For example, silane is a colourless liquid, evaporates very quickly during application and does not change the appearance of the concrete. Similarly, it is difficult to ascertain when a given surface treatment is in need of replenishing.

Accordingly, a number of test methods have been developed to aid the assessment of surface treated layers. To reduce testing costs and delay, the majority of these tests are employable in situ. However, these traditional in situ test methods suffer from a number of disadvantages including being impractically sensitive to environmental conditions and not being able to differentiate between a surface-treated layer and the underlying cementitious material. Hence, the only reliable test method is not employable in situ and involves cutting a core from the structure, conditioning the core to a certain moisture state and then testing it in the laboratory before and after grinding off the surface-treated layer. This method is destructive, expensive and very time consuming.

In accordance with the present invention, there is provided a method of analysing a surface-treated layer of a cementitious body, said method comprising impedance spectroscopy.

The invention provides a sensitive means for measuring the amount and quality of a surface treatment. The invention also provides discrete signals for the surfacetreated layer and the underlying cementitious material such that the signals from each may be separately considered and a surface-treated layer may be analysed without having to first know the properties of the underlying material. The technique enables the non-destructive in situ assessment of a surface treatment without having to cut a core or to remove the surface-treated layer. The technique also reduces the effect of ambient environmental factors on the sensitivity of the technique and the accuracy of results.

Whilst the present invention is applicable to the analysing of any cementitious material - ie. mortar based materials - it is particularly well suited for the analysis of reinforced concrete. However, it should be remembered that any cementitious material may be analysed. Furthermore, this technique is not limited to the investigation and analysis of any particular surface treatment.

In accordance with a further aspect of the present invention, there is provided an apparatus for analysing a surface-treated layer of a cementitious body by impedance spectroscopy, the apparatus comprising: a signal generator for generating an AC signal with a varying frequency; electrodes for applying said AC signal to said surface-treated layer and said cementitious body; and an analyser for analysing response to said AC signal to determine variations in impedance of said surface-treated layer and cementitious body with frequency.

Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a Nyquist plot of the spectrum of a typical RC circuit (also shown); Figure 2 is a Bode plot of the spectrum of the typical RC circuit; Figure 3 is a measured impedance spectrum and equivalent circuit showing contributions from electrodes, a substrate and a surface treatment; Figure 4 is an example of laboratory test apparatus (exploded view); Figure 5 is an example of an in situ test apparatus (indirect mode); Figure 6 is an example of an in situ test apparatus (direct mode); Figure 7 is a plot illustrating the effect of silane dosage on resistance; Figure 8 is a plot illustrating the degradation of the component coats of a polyurethane coating with time during exposure to a sodium chloride solution; Figure 9 is a plot illustrating the resistance of a given surface treated layer vs.

chloride ion diffusion coefficient for a range of surface treatments; Figure 10 shows a set of impedance spectra plots for a range of surface treatments; and Figure 11 illustrates the effect of preconditioning relative humidity on the effectiveness of a silane surface treatment.

The measurement and analysis of the variation of impedance with frequency of a system is called Impedance Spectroscopy (IS) or Electrochemical Impedance Spectroscopy in the fields of dielectric and electrochemical research respectively.

The variation of impedance with frequency is known as an impedance spectrum. IS is an accurate and powerful tool for characterizing materials, being rapid and nondestructive. IS may be conveniently employed and the resulting data can be analysed with commercially available software or in some cases by manual calculation. The electrical parameters of the resulting data may be related to various fundamental properties of the material under test.

Impedance is defined as Z(j#)= E(j#) I(j#) where: j is the complex number (1)o.5; # is angular frequency 2nf; f is frequency (Hz); E(jw) is the AC voltage signal applied to the system under test (V); and I(jw) is the current response (A).

E(jw) and I(jw) can be written as: E(j#)= Emsin(#t) I(j#)=Imsin(#t+#) where: Em is the voltage amplitude applied; Im is the amplitude of the response current; and ç is the phase shift.

The impedance can be expressed by a complex number: Z(i)= z'+jZ" where: Z'=|Z|cos# Z"= |Z|sin# Then:

where: |Z| is the Impedance modulus.

Impedance spectrum data is usually presented as either a Nyquist plot or a Bode plot. Plotting the imaginary part of impedance data, Z", against the real part of impedance, Z', for every frequency point produces a Nyquist plot. For example, a typical RC circuit and a Nyquist plot thereof is shown in Figure 1. The IS gives a semi-circle, the diameter of which is the resistance of a parallel resistor (rip) and the distance between the Y axis and the intersection of the IS with the X axis is the resistance of a series resistor (R3. The value of the capacitor can then be obtained from: co C T PRp where: (ST is the frequency of the top point of the semi-circle (Hz).

In this way, for some simple systems, the electrical parameters of a material under test can be easily calculated or estimated from the Nyquist plot.

As an alternative to a Nyquist plot, a Bode plot may be employed. Bode plots express impedance data by plotting the impedance modulus and phase shift against frequency on a logarithmic scale. An example of a typical Bode plot is shown in Figure 2. As with the Nyquist plot of Figure 1, the electrical parameters of a material under test can be obtained from the Bode plot.

IS employs the principle that by giving a certain stimulation to a material which is initially considered as an unknown, the response of the material can be analysed to determine its electrical composition. As a first step, a meaningful equivalent circuit for the material under test may be constructed. The validity of the equivalent circuit is judged by curve fitting to assess whether measured data from the material is identical or close to calculated data from the equivalent circuit and whether the equivalent circuit makes any sense in real physical or chemical terms.

Selection of a reasonable equivalent circuit for a coating material is not problematic to persons skilled in the art. An example of such an equivalent circuit for surface-treated concrete is shown at the top of Figure 3. However, concrete is a dielectric and hence equivalent circuit analysis has to take the concrete into account.

An equivalent circuit for a non-dielectric material consists of pure resistors and capacitors or inductors. However, for an electrolyte or for a dielectric material, the relaxation time has a certain distribution. Relaxation time cannot be modelled by an equivalent circuit consisting of ideal discrete elements alone. To adequately model a dielectric material, the equivalent circuit may employ distributed circuit elements.

The most commonly used distributed element for electrolytes and dielectric materials is a Constant Phase Element (CPE). The definition of a CPE is Z=R0(J#)-n where: R0 is a constant; and n is the index which determines the constant phase angle a = n(X/2).

When n=1, the CPE is a pure capacitor and when n=O the CPE is a pure resistor.

Advantageously, surface treatments, impregnated concrete and untreated concrete all behave as dielectric materials. The spectra of surface treatments, untreated concrete, impregnated concrete and test electrodes are generally located in discrete frequency bands. For example, the spectra of a substrate (untreated concrete), surface treatment and test electrodes are generally located in very high (lOMHz-lOkHz), medium (100KHz-I Hz) and low (1KHz-lOmHz) frequency bands respectively. Figure 3 shows just such a distribution with the constituent elements being easily resolvable.

As mentioned above, both the substrate and surface treatment can be represented by an equivalent circuit containing a constant phase element. The resistance and other dielectric parameters of a surface treatment and a substrate can be extracted by IS analysis.

The protective qualities of a surface treatment may be usefully characterised by its resistance per unit area of treated surface.

Advantageously, the present technique may be equally well employed in situ or in the laboratory. An IS analysis apparatus essentially comprises electrodes for applying an impedance signal to a surface treated layer and substrate and an analyser for analysing the impedance signal to determine variations in impedance of a surface treated layer with frequency.

An impedance spectrum is produced by measuring impedance at different frequencies using an impedance analyser. A suitable impedance analyser is the Schlumberger 1260 Frequency Response Analyser which gives a frequency range of 32MHz to lmHz, enveloping the frequencies in which the impedance spectra of concrete and surface treatments are located.

The electrode material preferably presents a low impedance. Alternatively or additionally the frequency range where the corresponding impedance spectrum of the electrode is located should be well away from the impedance spectrum of a material under test. Furthermore, the electrode material should be corrosion resistant to avoid contamination of the specimen under test with corrosion products. A variety of electrode materials including stainless steel, copper mesh and graphite plate have been tested. Graphite has been found to have suitable impedance and corrosion resistance properties. Other electrode materials and configurations are also employable.

Electrical contact between the material/structure under test and the electrodes may be made by pushing the electrode, which could be in the form of a conductive sponge, against the concrete surface, painting conductive paint onto the concrete, applying conductive gel or using a conductive solution seeping through the porous base of a wet cell. Choice of electrical contact will depend primarily on whether the method is to be applied in situ or in the laboratory and on the purpose of the test.

Electrical conduction through concrete and through most surface treatments is electrolytic, i.e. by the migration of ions within the pore water. The moisture state of the material, which inevitably varies with environmental exposure, therefore has a major effect on its impedance. Variations in the impedance of the concrete due to moisture content variations are reduced as the IS method allows the effect of the concrete substrate to be quantified and isolated. Ideally the surface-treated layer should be tested at a known moisture state. In the laboratory this may involve storing specimens at a known temperature and relative humidity for a certain period, or until a constant weight, prior to testing. On site a more practical approach is to use a wet cell and to monitor the impedance of the surface-treated layer until it reaches a constant value. It should be noted that this takes far less time than the time for the substrate to reach a constant moisture state which is the requirement for the effective application of all of the existing test methods. If the water uptake during the test is also monitored, the test provides additional information about the resistance of the surface treatment to water transport.

As mentioned above, the present technique may be used in the laboratory to analyse a given surface treatment. A variety of possible designs of laboratory test apparatus to measure the impedance of a surface-treated specimen may be employed.

One such laboratory apparatus is shown in Figure 4. This apparatus design involves applying a surface treatment to one substantially flat face 2 of a concrete disc 4 or other test material and then placing electrodes 6, 8 on either side of the disc 4, parallel to each of the flat faces 2, 10. An electrolytic solution 12 may be used to provide electrical contact on the surface-treated face of the specimen and a conductive coating 14 may be used to provide an electrical contact on the opposite face 10. This type of apparatus also allows the water absorption of a specimen to be monitored with time. The choice of solution should take account of the purpose and exposure environment of the surface treatment. If the primary purpose of the surface treatment is to protect the concrete from chloride penetration, a chloride solution (say one mol sodium chloride solution (lM NaCI)) would be an appropriate choice. Solutions that react with the surface treatment or concrete in a way that would not occur in practice should be avoided.

As discussed above, the present technique may also be employed in situ.

There are two approaches to in situ testing - direct testing and indirect testing. The indirect approach uses two identical wet cells placed on the surface treated layer of the test material - as shown in Figure 5. In the indirect approach, the distance between the wet cells should be small in relation to the distance from any steel embedded in the test material in order to avoid the steel providing a low resistance path between the electrodes that masks the other resistances within the path.

The direct approach, shown in Figure 6, uses a wet cell as one electrode and reinforcing steel in the concrete as the other electrode. With foresight, an electrical connection can be installed during construction. In the vast majority of situations the concrete will contain a large number of steel reinforcing rods close to the surface (e.g. 50mm beneath surface). The large area of the reinforcing steel, and hence its small impedance, causes the analysis of data to be simplified.

With reference to Figure 5, an example of an indirect in situ testing apparatus is shown. As mentioned above, the apparatus comprises a first and a second wet cell 16, 18. Each of these cells may have an identical configuration, and comprise a container having, at least, a porous base 20 suitable for allowing electrolyte 19 contained therein to seep through. An electrode 22 is placed within the container and below the surface of the electrolytic solution. An impedance signal generator/analyser (not shown) may then be connected directly between the electrodes 22. The electrodes 22 are placed on a surface 24 of a material under test and an electrical contact may be enhanced between the containers' bases 20 and the material under test by conductive gel, for example.

In use, an impedance generator/analyser is connected to the electrodes 22 and an impedance signal is applied to the material under test. The analyser may then be employed to measure the variation of the impedance with frequency of the applied signal.

With reference to Figure 6, an example of a direct in situ testing apparatus is shown. This apparatus is particularly useful when employed in situations where the material under test is provided with at least one electrically conductive reinforcing member 26 (although in practice there are likely to be a great many reinforcing rods present). The reinforcing member 26 may be used to provide one electrode of the test apparatus. As mentioned above, with foresight, electrical connections may be provided when reinforcing members are first embedded within the material under test.

Alternatively, electrical connectors may be provided after construction of the material under test by drilling holes through the material under test and attaching an electrical contact to at least one exposed reinforcing member.

In common with Figure 5, the apparatus of Figure 6 may employ a wet cell 28 having an identical configuration to the wet cell shown in Figure 5 as the other electrode. An impedance signal generator/analyser may then be connected between an electrode of the wet cell and an electrical connection connected to the at least one reinforcing member to provide a circuit through which an impedance signal may be generated and analysed.

IS as embodied in the present technique may be used to investigate a variety of different parameters of surface-treated concrete and other cementitious materials and is able to distinguish between signals from the surface-treated layer and signals from the underlying substrate. IS may also be used to investigate other factors affecting the performance of a given treatment and the suitability of a given surface treatment for a given purpose. For example, Figure 7 illustrates a graphical assessment of adequate surface treatment - in this case Silane - application in terms of amount applied or protection afforded. As shown in Figure 7, a decrease in resistance indicates a reduction in applied dosage and thus a decrease in quality of test material.

The present technique may also be used for monitoring of protective qualities over a period of time to determine when to retreat the concrete or other cementitious material. For example, Figure 8 shows the degradation of the component coats of a polyurethane coating with time as a result of exposure to a sodium chloride solution.

Figure 9 shows the correlation between resistance and chloride ion diffusion rate through a surface treated specimen for a range of surface treatments. This assessment of resistance to ion diffusion provides an indication of the resistance of a given surface treatment to environmental factors such as applied aqueous chlorine.

The present technique may also be employed to provide a means for developing more effective surface treatments. Figure 10 shows impedance spectra for a range of surface treatments. From measurements such as these, the effectiveness of a given treatment may be assessed.

The present technique may also be employed to assist in the investigation of factors affecting surface treatment performance, e.g. substrate composition and moisture content, application method and penetration depth. Figure 11 shows the effect of preconditioning relative humidity on the effectiveness of a silane surface treatment.

It will be understood, of course, that the present invention has been described above by way of example only and that modifications may be made to the example described herein without departing from the scope of the appended claims.

For example, whilst the present system is primarily intended for the analysis of surface-treated layers of a test material, it may also be employed simultaneously to provide useful analysis of an untreated test material.

Claims (11)

1. A method of analysing a surface-treated layer of a cementitious body, said method comprising impedance spectroscopy.

2. A method according to Claim 1, wherein said impedance spectroscopy also analyses said cementitious body beneath said surface-treated layer.

3. A method according to Claim 1 or Claim 2, wherein said cementitious body is concrete.

4. A method according to Claim 1 or Claim 2, wherein said cementitious body is mortar.

5. A method as claimed in any one of Claims 1 to 4, wherein said impedance spectroscopy measures resistance of said surface-treated layer.

6. Apparatus for analysing a surface-treated layer of a cementitious body by impedance spectroscopy, the apparatus comprising: a signal generator for generating an AC signal with a varying frequency; electrodes for applying said AC signal to said surface-treated layer and said cementitious body; and an analyser for analysing response to said AC signal to determine variations in impedance of said surface-treated layer and cementitious body with frequency.

7. An apparatus according to Claim 6 wherein said electrodes comprise a pair of wet cells placed on said surface treated layer, each being spaced from the other and comprising: a container having a porous base, such that, in use, an electrolyte within said container seeps through said porous base; and an electrode disposed within said container such that, in use, said electrode is immersed in said electrolyte.

8. An apparatus according to Claim 6 wherein said electrodes comprise: a first electrode that is a wet cell and comprises: a container having a porous base, such that, in use, an electrolyte within said container seeps through said porous base; and an electrode disposed within said container such that, in use, said electrode is immersed in said electrolyte; and a second electrode that is an electrically conductive reinforcing member embedded within said cementitious body.

9. Use of an apparatus according to any of Claims 6 to 8 in accordance with a method according to any of Claims 1 to 5.

10. A method substantially as hereinbefore described.

11. An apparatus substantially as hereinbefore described with reference to Figures 4 to 6 of the accompanying drawings.