Performance testing of concrete protective coatings

0 Introduction:
reinforced concrete is the most used building material in engineering and the most widely used structural form in the world. China spends more than 100 billion yuan per year on concrete structures, and the direct economic losses caused by the failure of reinforced concrete structures amount to 600 billion yuan/a, and the durability of concrete is becoming more and more serious. Concrete protective coating technology, as a method that can effectively improve the durability of concrete structures, has gradually received people's attention
china
. However, the current test and evaluation methods on the performance and protective effect of concrete protective coating materials are not yet mature, which seriously restricts the healthy development and application of such materials.
1 Performance testing methods for coated concrete
With the development of concrete protective coating materials and their wide application in engineering, the performance testing and evaluation methods of concrete protective coatings have become an increasing concern for manufacturers and engineering units. Domestic performance indicators for coating materials are also limited to the material itself, but there are few performance and protective effect indicators for coating materials in use, especially in a particular environment. The following are some test methods for coating concrete to provide reference for evaluating coating materials.
1.1 Coating concrete salt mist cycle aging test
In the marine environment, salt spray corrosion is an important factor leading to the destruction of concrete structure. Concrete protective coating has gradually become an important measure of offshore concrete protection, the effect is very obvious, but there is little research on the salt spray corrosion test of coated concrete. Using scanning electroscopes (SEMs) and Fourier Transform Infrared Spectroscopy (FTIR), LU Ping studied the aging behavior of polyaspartic acid (PAE) polyester coating under accelerated climate aging conditions by examining the surface shape and structural form of the coating after aging through the salt spray cycle.
FTIR graph
before and after the aging of the polysaccharine coating by salt mist is the SEM diagram after the coating has been aged by salt spray 70 d and 200 d. After exposure of 70 d in the salt mist tank (Figure 1a), the coating surface is dense and continuous, indicating that the salt spray corrosion at this stage has no effect on the coating;
that the PAE polycarp coating is well resistant to salt spray erosion. Figure 2 is the ATR-FTIR spectra before the aging of the PAE polysaccharin coating and after the aging of the salt spray cycle by 200 d, with little change, i.e. no significant changes in the surface appearance and corrosion resistance of the coating before and after aging. It is stated that the PAE polycarp coating has excellent corrosion resistance to salt mist, which is consistent with the previous conclusions. At the same time, the effect of salt spray aging on the antifreeze resistance of coated concrete is also studied. By measuring the relative elastic mould of concrete to determine its antifreeze resistance, after aging of 300 d salt mist, the antifreeze effect on coated concrete is small, indicating that THE polysaccharid coating improves the antifreeze resistance of concrete.
, the aging performance of coated concrete can be analyzed by examining whether it has been damaged and its adhesion after salt spray aging test. With the extension of salt spray aging time, the concrete adhesion of different protective coatings is reduced to varying degrees, but the adhesion effect on PAE polysaccharin-coated concrete is very small, which indicates that the aging performance of PAE coating is better than that of conventional anti-corrosion coating.
of the 1.2 coating concrete adhesion test
one of the key properties of the coating material is the adhesion or adhesion to the substrate. When the coating is not bonded closely to the substrate, it will fall off, lose its protective effect, and damage the substrate. By measuring the adhesion between the coating and concrete, the advantages and disadvantages of the coating can be evaluated well. The concrete working environment has a great influence on the adhesion of the coating, such as the concrete in the marine environment will withstand freezing and melting cycle, dry and wet alternating, temperature alternating or corrosion media erosion, etc. , to measure the adhesion at this time, we can evaluate the attachment of the coating and concrete under different circumstances, thus providing a reference for the choice of coating.
Wang Weiying used the peeling bonding test to study the effect of substrate strength, maintenance age, primer, temperature and humidity on peeling strength, and used regression analysis method to establish the relationship between peeling strength and time in the above-mentioned study. Through the analysis of the destruction model, the type of bonding damage between polyyurea and substrate is obtained.
The PosiTest Adhesion Tester is a newly developed adhesion test instrument, as shown in Figure 3. It shows the adhesion of coating and concrete directly and accurately, which is simpler, easier and more suitable for practical engineering than the peeling bonding test. The specific operation is to lightly polish the coated concrete with sandpaper, then wipe clean, glue the 50 mm pull-off unit to the coating, dry at room temperature 1 d or put in the oven at 50 degrees C for a few hours, and then dry and cool. Secure the locator to the pull-off unit and apply a pull-off to the hydraulic pump until the pull-off is made, and the display displays an adhesion reading.
1.3 coating concrete AC impedance spectrum
the method of AC impedance spectrum has been widely paid attention to in recent years, and has become a powerful tool for studying material structure, performance and reaction structure. The AC impedance method is applied to the study of cement slurry and concrete properties, based on the view of cement slurry and concrete as an electrochemical system. Hardened cement slurry and concrete are porous media with a porous solution as an electrolyte, so an inert metal electrode (stainless steel electrode) can be placed on each end face of hardened cement slurry and concrete, which can be used as an electrochemical system.
the change of the composition structure of cement-based material causes the change of electrochemical behavior, the information of its composition and structure can be analyzed from the impedance spectrum of cement-based material. Applying concrete protective coatings to cement mortar and concrete surfaces can change their interfaces, so it is predictable that the interfaces of painted and unpainted protective coatings have different AC impedance responses. Therefore, it is theoretically feasible to describe and express the protective properties of concrete protective coatings with different responses of AC impedance. Ge Haiyan used the AC impedance instrument (Figure 4) to study the effect of concrete protective coating on the ac impedance spectrum characteristics of the sample.the test results of
AC impedance device schematic
show that the protective coating has good impermeability to concrete, and according to the AC impedance spectrograph, it can be judged the capacity of various coatings in improving the compaction of concrete structure, that is, the capacity of anti-chloride ion permeability.
1.4 Coating concrete anti-chloride ion permeability test
chloride ions is one of the important reasons for the destruction of the structure of offshore reinforced concrete, so anti-chlorine ion permeability is an important indicator of concrete durability in marine environment. The test methods mainly include fast chloride ion penetration test method (RCM method), natural immersion method and electro-pass method.
Yanglin used natural immersion and rapid chloride ion penetration (RCM) to study the performance of concrete protective coating against chloride ion penetration. Considering that both methods can effectively evaluate the ability of concrete protective coating to resist chlorine ion penetration, the ostodiffic coefficient of concrete in the actual engineering can be approximated to the DRCM (chloride ion diffusion coefficient) value when it is exposed to chlorine salts at an early stage.
When the coating concrete impermeability test was carried out by rapid chloride ion permeation (RCM), the chlorine ion diffusion coefficient of the coated concrete was significantly reduced compared with the comparison sample, indicating that the protective coating improved the chlorine ion impermeability of the concrete, and with the increase of the coating thickness, the permeability of the anti-chloride ion increased. And it is proved that the two methods have good linear correlation.
1.5 Water absorption and impermeability testing of coated concrete
For coated concrete structures, the protective coating is painted to prevent seawater from invading the concrete interior, i.e. water absorption is also an important performance indicator of the coating. The change of water absorption rate of the test piece after the treatment of concrete protective coating is an intuitive and effective evaluation method. Wang Weiying uses water absorption rate to evaluate the protective effect of concrete protective coating and the protective effect of protective coating under the influence of different media. The test results show that the water absorption rate of concrete is greatly reduced, whether it is impregnated concrete protection liquid or sprayed polypropyl elastomer (SPUA) coating.
addition to the SPUSA coating, other coatings with the longer immersion time, the absorption rate has a certain increase, indicating that the SPAA coating has excellent water resistance. In order to better evaluate the performance of concrete protective coating, it is necessary to measure the change of water absorption rate in the corrosive medium environment. The ability of concrete protective coating to resist corrosion media erosion is evaluated by measuring the change of water absorption rate of the test piece in acid, alkali and other corrosive environments, and it provides a reference for the selection of protective coating in special environment.
concrete impermeability refers to the ability of concrete to resist the penetration of pressurized medium, and is one of the important comprehensive indicators to evaluate the durability of concrete. At present, the main research methods are water seepage, breathability, surface water absorption, chloride ion penetration, power and so on. The relevant literatures put forward some methods to improve the impermeability of concrete, mainly in view of the factors affecting the impermeability of concrete, to take various measures to improve the hole structure of concrete interior, refine the aperture, reduce or prevent the penetration channel of concrete internal connectivity, in order to improve the impermeability of concrete, and pointed out that the surface coating method is one of the important measures to improve the impermeability of concrete.
Yanglin tested the impermeability of concrete by using the national standard water seepage method, and examined the impermeability of concrete by applying different water pressures that were raised step by step to the coated concrete. The results show that the impermeability of concrete is greatly improved after coating, and the impermeability of different coatings is different.
1.6 Coating concrete freeze-thaw cycle test (fast-frozen method)
in accordance with GBJ 82-85 "normal concrete long-term performance and durability test method" in the antifreeze performance test "fast-frozen method." During freezing and melting, the relative dynamic elastic mould and mass loss of concrete are measured, and the relative dynamic elastic mould reflects the development of micro-cracks inside concrete, and the mass loss is caused by surface peeling, which in severe cases leads to bone exposure.
the literature measures these two indicators to reflect the extent of damage to the interior and surface of concrete, and the test results show that the quality and dynamic elastic modules of uncoated concrete have changed dramatically during the freezing and melting cycle, while the coated concrete has not changed significantly. It is stated that the coating can significantly improve the antifreeze performance of concrete.
1.7 electrochemical methods
in the study of reinforced concrete rust, electrochemical test methods are used in many ways. The electrochemical detection method of steel corrosion in concrete mainly includes natural position method and polarization measurement technology. Constant power method, electrochemical noise method, concrete resistance method, harmonic method, etc. are also in development, but for field detection is rare.
Liu Yujun used electrochemical methods to measure the corrosion of concrete by reference to ASTM C 876-91. The test results show that after applying concrete protective coating in the same environment, it can delay the time when the rebar passivation film begins to be active, reduce the activity degree of the passivation film, and significantly reduce the corrosion speed of the rebar.
2 Conclusions and Prospects
The above-mentioned research will provide the necessary theoretical preparation and technical support for the development of the test and evaluation method of concrete protective coating performance and protective effect, and will also promote the development and engineering application of concrete protective coating materials. The application of AC impedance spectrum method in the study of concrete protective coating needs further study, and try to find a mathematical model that describes impedance parameters and coating type, dosing and processing time, with a view to quantitatively evaluating the performance of concrete protective coating by USing AC impedance method.
in actual engineering, concrete structure is not simply eroded by a corrosive medium, but by the combination of several corrosive media, but may also be affected by load, the corrosion in this environment will be more serious. In this paper, the effect of salt spray aging on the properties of coated concrete antifreeze and adhesion is studied, but there are few test methods in this area. Therefore, the corrosion behavior of concrete protective coating in multi-factor environment needs to be further studied to provide reference for the selection of concrete protective coating in engineering practice.