Abstract: This article introduces the latest developments in the testing technology of the surface permeability of concrete in foreign countries. It also explains the on-site testing of the surface permeability of concrete in combination with domestic engineering examples, and proposes several possible development directions in the future.
Keywords: concrete, surface permeability, on-site testing
1 Introduction The durability of concrete structures is closely related to the permeability of concrete materials, especially surface concrete, which is the first line of defense against the erosion of harmful media such as water and CO2. Since the concrete structure has been built, there are a lot of unconnected micro-cracks. Under the influence of environmental factors and stress, these micro-cracks continue to expand until they are connected. At this time, the permeability of the concrete determines the speed of water or harmful media entering the interior, which determines the Deteriorating the speed of development. Crack propagation (controlled by concrete fracture energy) accelerates the penetration rate of water and other media, and also accelerates the degradation process. The interaction model is shown in Figure 1.
Figure 1 Interaction model of concrete performance deterioration [1]
It can be seen from Figure 1 that the deterioration of concrete is related to its microstructure and the transmission of aggressive media. The permeability of concrete depends on its own microstructure and saturation, which is the key factor that determines the deterioration of concrete performance. Therefore, the durability of concrete can be evaluated by testing its permeability.
Since the beginning of the 1960s and 1970s, foreign countries have started to study methods for detecting concrete permeability on site. They are roughly divided into three categories: water absorption test, permeability test (water or air penetration) and ion diffusion test. The more famous such as ISAT test (initial surface water absorption test) has been listed as the British standard in 1970: BS 1881: part 5; the drilling method developed by JW Figg in 1973 can detect the water absorption and permeability indicators of surface concrete; Later, there were air permeability tests developed by RK Dhir, Schonlin, etc. The German company Germanns also developed a water and gas permeability tester for on-site evaluation in the early 1990s. The degree of automation of the above test methods is not high.
At present, the more advanced in the world is the Autoclam automatic permeability tester [2] developed by Queen ’s University in Belfast, UK, which can automatically detect three indicators of water absorption, water permeability and gas permeability at the same time. This article mainly focuses on the principle of this instrument And applications.
2 The principle and application of automatic penetration meter
2.1 Principle
The main components of the Autoclam system are shown in Figure 2.
Figure 2 Automatic Penetrometer System
The first step of the three tests of automatic detection of water absorption, water permeability and air permeability is to fix or glue a steel base ring to the concrete surface, so that air and water can be sealed. The inner diameter of the base ring is usually 50mm, the actual "AUTOCLAM" test device is fixed on the base ring, as shown in Figure 3-a.
The main body of the instrument includes a pressure transducer to detect the pressure in the test area; a cylinder barrel for the piston to run in; a main valve to introduce water or air (or other gases); a release valve to release water Or gas. By measuring the distance the piston moves in the column barrel, the water absorption of the concrete surface can be calculated. The design considers different column barrels and piston sizes to suit different concrete permeability. Because the moisture content of the surface concrete affects permeability, it is generally required that the concrete surface tested is dry.
For the water absorption test, a pressure of 2Kpa was used, which is equivalent to the 200mm water column used by the ISAT method. The whole test is automatic, and the test time is about 15 minutes. Plotting the curve of the cumulative water absorption versus the square root of time gives a linear relationship, the slope of which is the water absorption index in m3 / √min.
The principle of automated water permeability test is the same as that of automated water absorption test, except that the test pressure is higher. As shown in Figure 3-b, for the automated water permeability test, the pressure is maintained at 150Kpa, and the water permeability index is calculated using the same method as the water absorption index.
Automated air permeability test is shown in Figure 3-c. After the pressure in the test device increases to about 150Kpa, close the on-off valve and record the pressure decay curve with time. It can be found that the natural logarithm of the pressure is linear with time. Take the slope of the linear regression line as the air permeability index, the unit is ln (unit of pressure) / min.
In fact, the penetration of air and water has not reached a steady state, so these tests can only get relative indicators related to permeability. Both tests are simple and fast, suitable for laboratory and field applications, but the water permeability is less affected by the moisture content of the concrete.
Figure 3 Schematic diagram of automated test (a) The foundation ring fixed on the concrete surface;
(B) Test method for water permeability;
(C) Air permeability method
2.2 Relationship between permeability index and durability index
2.2.1 Air permeability index and durability The test examined the relationship between carbonation depth, salt erosion depth, freeze-thaw cycle erosion and Autoclam air permeability coefficient. The results are shown in Figure 4 (ac). Because Autoclam's water seepage coefficient has a better relationship with freeze-thaw cycle erosion, in Figure 6 Autoclam's water seepage coefficient is the abscissa. It can be seen from the figure that the durability has a good correlation with the Autoclam index.
Figure 2 a. Relationship between air permeability index and concrete carbonation depth
2.2.2 On-site water absorption index and durability Figure 5 shows the relationship between on-site water absorption index and different durability indicators. The correlation between carbonation depth and water absorption is not as good as the correlation between the water permeability index and the water permeability index in Figure 4. There is a similar phenomenon in the depth of salt erosion. The reduction of these correlations is because the water absorption is directly related to the porosity of the concrete near the surface layer, but the correlation with the pore structure of deeper concrete is not strong.
However, as can be seen from Figures 5-c and d, the amount of chloride ion intruding into the protective layer has a good correlation with the water absorption index. The E diagram further shows that the onset time of corrosion is highly correlated with the water absorption of the concrete surface, and the role of the protective layer is also very obvious.
The above test is part of the Autoclam field test index and durability test. It can be seen that these indexes have a good correlation with multiple durability indicators of concrete.
3 Domestic status
3.1 Research in the laboratory What the national standard of China stipulates is the impermeability grade method to determine the permeability of concrete [3]. Recently, with the in-depth study of the durability of concrete, many researchers have carried out the improvement of the hydraulic method, and some have also used the penetration of CH2Cl2 for evaluation [4-5]. Another hot spot is to study the diffusion coefficient of chloride ions in concrete. A large number of tests have been conducted in China in combination with the proposals of the United States and Europe, mainly including the electricity method, the natural diffusion method and the Nel method. These works are of great significance for studying the durability of concrete structures. However, they all have a common feature, and they need to be sampled and operated in the laboratory, and the existing structure cannot be tested on site.
3.2 On-site testing In China, the permeability testing of existing structures is generally carried out by sampling to the laboratory, and field testing is rarely carried out. This unit has introduced a series of concrete durability non-destructive testing equipment from Denmark since the mid-1990s. Among them are gas permeability testers and water penetration testers, which can be tested on site.
For example, in November 2004, an existing tunnel of a subway was shut down for overhaul, and the concrete permeability needs to be evaluated in the durability evaluation of the concrete structure. Our center used GWT (Germanns Water Permeability) instrument to test and evaluate the concrete of the tunnel side wall and received good results [7].
4 Significance of on-site testing and evaluation of concrete permeability
4.1 Controlling the construction quality At present, the momentum of large-scale construction in China will continue for several decades. Many key civil infrastructures and landmark buildings have put forward requirements for a service life of 100 years, such as the Hangzhou Bay Bridge and the main stadium of the Beijing 2008 Olympic Games. For concrete structures, the quality of on-site construction largely determines the future durability of concrete structures, and the quality of surface concrete (generally refers to the concrete protective layer of about 40mm) should be carefully controlled.
In the past, it was mainly evaluated by on-site visual inspection, and there was no scientific method to check its compactness. In May 2004, the China Construction Standards Association Standard "Construction and Design Guidelines for Durability of Concrete Structures" Article 6.3.2 clearly requires that for important projects, the gas permeability of the surface concrete on the site should be evaluated by portable instruments on the site Or water permeability.
Relevant work is currently underway. The permeability of the on-site surface concrete will be set according to the characteristics of China's concrete to set different indexes for quality control in engineering.
4.2 Classification of permeability of existing structures When testing durability of existing important structures such as bridges and tunnels, the permeability index of surface concrete is also very important. Over the past 10 years, foreign studies have shown that water absorption, water permeability, and gas permeability indicators are closely related to many durability indicators [8-10]. If the first-hand data is obtained on site, the carbonation rate, salt corrosion resistance, freeze-thaw resistance, and steel corrosion start time can be predicted based on the relationship between the two obtained in the laboratory, and then the deterioration model of the concrete structure can be established It is of great significance to formulate a constructed maintenance and repair strategy.
4.3 Testing the effect of protective measures Research at home and abroad has shown that impregnation on the surface of concrete or protection with film-forming paint can greatly extend the service life of the structure. In the past, it was only possible to test the effect after painting on small laboratory samples, which could not be evaluated on site. Using a permeability tester, such as Autoclam, can detect the change of the permeability index of the concrete surface before and after painting, and directly calculate the protection effect, which is more intuitive [11-12]. Further, the aging rate of the concrete structure can be predicted on the basis of the permeability index after protection.
5 Suggestions In view of the above-mentioned significance of on-site testing of surface concrete permeability, the author believes that the following aspects are the future development direction:
5.1 Carry out laboratory tests to find out the relationship between the surface concrete permeability and conventional permeability tests;
5.2 Continue on-site testing and accumulate data to prepare for the preparation of standards for assessing the quality of surface concrete on site construction;
5.3 Through a comparative study of laboratory and field tests, it lays the foundation for establishing a concrete deterioration model based on surface permeability.
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