Corrosion depth of concrete can reflect the damage state of the load-carrying capacity and durability of the concrete structures servicing in severe environment. Ultrasonic technology was studied to evaluate the corrosion depth quantitatively. Three acidic environments with the pH level of 3.5, 2.5, and 1.5 were simulated by the mixture of sulfate and nitric acid solutions in the laboratory. 354 prism specimens with the dimension of 150 mm × 150 mm × 300 mm were prepared. The prepared specimens were first immersed in the acidic mixture for certain periods, followed by physical, mechanical, computerized tomography (CT) and ultrasonic test. Damage depths of the concrete specimen under different corrosion states were obtained from both CT and ultrasonic test. Based on the ultrasonic test, a bilinear regression model is proposed to estimate the corrosion depth. It is shown that the results achieved by ultrasonic and CT test are in good agreement with each other. Relation between the corrosion depth of concrete specimen and the mechanical indices such as mass loss, compressive strength, and elastic modulus is discussed in detail. It can be drawn that the ultrasonic test is a reliable nondestructive way to measure the damage depth of concrete exposed to acidic environment.
Since 1940s, pollution of acid rain has become to be a significant environmental problem confronting the environmentalists. In the past two decades, it is reported that acid rain falls have become more and more serious worldwide [
Presently, nondestructive techniques are available for concrete evaluation, which include radar, pulse velocity, acoustic emission, radiography, infrared thermograph, and many others. The ultrasonic wave reflection technique was first applied to the areas of cementations materials in 1981 [
Since physical and chemical reactions that take place on concrete structure exposed to acidic environment are complex, the concrete cover increases their porosity and microcracks density compared with sound concrete. As the corrosion continues, the degraded layer is expanded to the heart of the structure, and larger cracks occurred; thus concrete strength decreased significantly. The corrosion depth is an important factor to describe the damage state of the concrete, and it takes a significant effect on the mechanical property of the damaged concrete. A rational evaluation of the concrete corrosion depth will give a crucial accordance for the health monitoring and safety evaluation of the servicing concrete structures.
In this study, the application of ultrasonic nondestructive testing technique to monitor the corrosion depth of concrete at different corrosion states was explored. 354 prism specimens (150 mm × 150 mm × 300 mm) were prepared. Three pH levels sulfate and nitric acid solutions are mixed to simulate the acidic environment with different acidity, respectively, (pH 3.5, pH 2.5 and pH 1.5). After being immersed in the simulated acid rain for different periods, ultrasonic nondestructive test was conducted on the concrete specimens under different corrosion states. Corrosion depths of the concrete specimens under different corrosion states were obtained. A bilinear model was then built up to predict the corrosion depth of concrete under different corrosion periods. To verify the validity of the proposed model, computerized tomography (CT) test was performed, and the corrosion depth is estimated. It is shown that ultrasonic NDT technology is a reliable nondestructive method to measure the damage depth of concrete. Based on compressive test results executed on the damaged concrete specimen, Fan et al. [
A kind of nondestructive technology, NM-4 nonmetal ultrasonic tester, which is shown in Figure
NM-4 nonmetal ultrasonic tester.
Layout of the transducer.
Time-distance graph.
If the concrete specimen is corroded, the surface will become loose, and the sound velocity
When the distance between the transducers is short, the ultrasonic wave will transmit in the damaged layer. Once the distance exceeds a threshold value which is denoted as
Wave velocities that penetrate through the specimens under different corrosion states can be achieved, and the corrosion depths of concrete specimens after different immersion periods can be calculated by the expression (
Concrete specimens exposed to the mixed acid solution.
To reduce the differences caused in the production process, commercial concrete is used in this study. The details of concrete mixtures are shown in Table
Mix proportion of the concrete mixtures.
Concrete | Cement (kg/m3) | Sand (kg/m3) | Coarse aggregate (kg/m3) | Water (kg/m3) | Ratio (w/c) | Reducing water agent/kg | Undisturbed fly ash/kg |
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C40 | 450 | 678 | 1040 | 159 | 0.353 | 12.8 | 60.0 |
Since the in-service concrete structures are subjected to a variety of exposure conditions, such as chemical attack, drying and wetting cycles, temperatures, superficial carbonation, and drying shrinkage, all the conditions will affect the damage process of concrete. Therefore, it is difficult to duplicate the field attack condition in the laboratory exactly. Although the site corrosion test can better reflect the actual corrosion process, it always took much time to fulfill the corrosion. To clarify the effect of acid rain on concrete, long-term exposure tests were performed [
Viewing the various acidities of acidic environment worldwide, three acid solutions with pH level of 3.5, 2.5, and 1.5 were deposed to simulate the acidic environments with different acidities. Since the mechanical property of concrete always changes with age, aging effect is taken into consideration herein. To well discuss the deterioration character of mechanical property of concrete specimens exposed to acid solution, corresponding specimen exposed to water is used as reference. Exposure conditions were listed in detail in Table
Experimental conditions.
Designation | Exposing condition | Solution acidity | Specimen grouping | Immersion time (days) |
---|---|---|---|---|
Series 1 | Pure water | pH 7.0 | 6 specimens in a group, 18 groups | 5, 10, 15, 20, 25, 30, 35, 40, 45, 55, 65, 75, 100, 105, 130, 180 |
Series 2 | H2SO4 + HNO3 (molar ratio is 9: 1) | pH 3.5 | 6 specimens in a group, 15 groups | 5, 10, 15, 20, 25, 30, 35, 40, 45, 55, 65, 75, 100, 130, 180 |
Series 3 | H2SO4 + HNO3 (molar ratio is 9: 1) | pH 2.5 | 6 specimens in a group, 15 groups | 5, 10, 15, 20, 25, 30, 35, 40, 45, 55, 65, 75, 100, 130, 180 |
Series 4 | H2SO4 + HNO3 (molar ratio is 9: 1) | pH 1.5 | 6 specimens in a group, 11 groups | 5, 10, 25, 35, 45, 55, 65, 80, 90, 105, 130 |
Immersing and spraying methods are the two main ways to simulate corrosion in building materials by acid rain in the laboratory. Based on the laboratory test, it has been verified that both methods are comparable and the results are reliable, and it is also discovered that the immersing method is more suitable to perform a fast test on corrosion of cement concrete [
PB-10 Sartorius Acidometer.
After being immersed in the acid solutions for different periods, six specimens will be taken out for the compressive test as a testing group periodically. Then, the corroded specimens were dried for about two to three days. Weighting, ultrasonic nondestructive test, and compressive test will be executed on the corroded concrete specimens successively. Before starting the experiment, the surfaces of the prisms were cleaned to remove the loose material, and the upper and bottom surfaces were ground to be plane. Nondestructive test, such as ultrasonic wave test, CT test, and weight, was executed. The corrosion depth and mass loss of the damaged concrete are achieved. Then, mechanical properties such as compressive strength and elastic modulus of the corroded specimens are achieved by the destruction tests.
In the acidic environment, the chemical reactions occur between acid medium and the concrete constituent, and the hydration products (e.g., C-S-H, calcium aluminate hydrate, calcium alumina-ferrite hydrate, etc.) will lose calcium and finally break down into amorphous silica hydrogel. PH values in the concrete decrease gradually, which will result in the mass loss and strength reduction. In the RC structure, the chemical reaction will result in the depassivation of steel and subsequently in corrosion. Phenolphthalein test is the regular method to obtain the neutralized depth of concrete, which has a disadvantage of the integrity of concrete specimen. To obtain the corrosion depth of the corroded concrete but not damaging the integrity of concrete, ultrasonic nondestructive test was performed on concrete specimens under different corrosion states, and NM-4 nonmetal ultrasonic tester shown in Figure
Corrosion depth of concrete specimens immersed in acid solution.
From Figure
Regression analysis.
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pH 3.5 | 0.53456 | 0.01021 | 0.97 | 1.04109 | 0.00150 | 0.84 | 60 |
pH 2.5 | 0.72544 | 0.01211 | 0.90 | 1.32113 | 0.00102 | 0.91 | 55 |
pH 1.5 | 1.02944 | 0.00730 | 0.98 | 0.78430 | 0.03569 | 1.00 | 10 |
Since the X-rays from computerized tomography (CT) are absorbed by the concrete specimen according to composition and density of the material, different features can be detected. Objects with a higher density absorb less X-rays, resulting in bright areas. Consequently, the lower the density, the more X-rays are absorbed, creating darker regions. To verify the efficiency of the bilinear model proposed previously, another nondestructive method, CT test, was applied to discover the corrosion of concrete on mesoscale. For the measurements, a Siemens Somatom Sensation 16-slice spiral computed tomography scanner made in Germany (shown in Figure
Comparison of CT images of specimen exposed to acid solution for different periods.
16-slice Spiral computed tomography scanner
0 day
120 days
3D reconstruction specimen
Cross-sections
Scanning layer of cross-section 1-1
Scanning layer of cross-section 2-2
Scanning layer of cross-section 3-3
After being cured in water for 28 days, the specimen was taken out and scanned to describe the mesolevel structure based on 300 along the axis of specimen. The specimens were then conditioned in the acid solution with a pH value of 1.5 for another 120 days and scanned again. Surface appearances of the specimen exposed to the acidic solution for 0 day and 120 days are shown in Figures
From the scanning images of the specimen, it is estimated that the damage depth is about 1.5 cm. Based on the proposed bilinear regression model (
Besides the NDT of corroded concrete, compressive mechanical parameters such as compressive strength and elastic modulus are achieved [
To compare the deterioration regularity of the concrete corroded by the simulated acid environment, relative mass variation ratio is defined as
Relation between mass loss ratio and corrosion time.
Relation between corrosion depth and mass loss.
After being immersed in acid solutions with pH 3.5 and 2.5 up to 180 days, relative mass loss ratio of the specimens will decrease 0.40 percent and 1.58 percent of the mass of sound specimen, respectively. For the specimens under pH 1.5 acid solutions, relative mass loss ratio of the specimens after the 105 days exposure will decrease 7.5 percent of that of sound specimen. Therefore, mass loss of concrete specimen is very sensitive to acidity of corrosion environment (Figure
To compare the strength deterioration trend of the concrete specimen immersed in the acid solution, relation between concrete strength
Relation between corrosion depth and compressive strength of corroded concrete.
Regardless of the acidity of the immersion solution, strength variations of concrete specimens exposed to acid solutions show the same trend. At the initial immersion period, the strength of corroded specimens will have a slight increase, and then the strength will decrease gradually. It can also be concluded that concrete compressive strength is related to the acidity of corrosion environment. Although the same corrosion depth is detected, the compressive strength is different for specimen damaged by various acid solutions. The higher the acidity of the solution is, the higher the degradation of concrete compressive strength occurs. Once the corrosion depth exceeds 11 to 13 mm, the concrete strength will decrease rapidly.
According to the testing results, relation between elastic modulus and corrosion depth of concrete specimens is shown in Figure
Relation between corrosion depth and Elastic modulus.
The same tendency was achieved on elastic modulus for the specimens exposed to all the three immersion solutions. At the initial immersion period, elastic modulus is not stable. It can also be concluded that concrete compressive strength is related to the acidity of corrosion environment. Although the same corrosion depth is detected, the elastic modulus is different for specimen damaged by various acid solutions. After the detected corrosion depth exceeds eleven to 13 mm, the variation tendency becomes to be evident.
In the present study, corrosion depth of concrete exposed to acidic environment was measured by ultrasonic wave technology in the laboratory. Acid solutions mixed by sulfuric acid and nitric acid solutions were deposed to simulate the acidic environment with three pH levels of 3.5, 2.5, and 1.5. To fulfill the accelerated corrosion in the laboratory, the prism concrete specimens were immersed in the deposed acid solutions. After being exposed to the three kinds of acid solutions for some periods, corrosion depth of the corroded specimens was tested. Based on the experimental results, a bilinear regression model that has a good correlation was proposed to predict the corrosion depth of concrete. To verify the validity of the proposed model, CT test was executed on the concrete specimens as well. It is shown that ultrasonic nondestructive test, which has an obvious advantage of not affecting the integrity of concrete, is a reliable method to measure the damage depth of concrete under acid environment. It is verified that the suggested bilinear model proposed by ultrasonic wave technology is an efficient method to determine the corrosion depth of concrete. Based on the other compressive parameters achieved in another experiment, relations between corrosion depth and the mechanical property indices (such as mass loss, compressive strength, and elastic modulus.) are discussed in detail.
This research was financially supported by the National Natural Science Foundation of China (Grant no. 51178069), National Natural Science Foundation of China (Grant no. 50708010), Liaoning Provincial Funded project (Grant no. 20092149), and the Fundamental Research Funds for the Central Universities.