Abstract
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.
1. Introduction
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 [1–6]. It is well known that concrete has become and will continue to be the most widely used construction material in civil engineering. However, when concrete is subject to acid rain, physical and chemical reactions occurred. The complex reactions will alter the internal structure of concrete, which will result in the change of its material property. Many famous buildings, such as Emei Mountain, Leshan Grand Buddha, Acropolis monument in Greece, and the Statue of Liberty in the United States, have been strongly damaged by acid rain in the previous decades. The damage effect of acid rain on concrete structures has attracted more and more attention of civil engineers gradually. A better understanding of the material properties of concrete exposed to acidic environment can help to discover the damage process of the in-service structures. Some experimental studies have been performed to discover the corrosion mechanism of acid rain attacking concrete, and simulation test of the acid rain attacking concrete was analyzed [7–12]. However, it is still difficult to evaluate the damage state of the concrete in acidic environment in practical engineering.
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 [13]. Since ultrasonic wave technology has an obvious advantage of not affecting the integrity of concrete, it has been applied to predict concrete strength, and detect the internal defects of concrete such as cracks, delaminations, and/or honeycombs [14]. Ultrasonic nondestructive testing (NDT) technology has been applied extensively in a wide variety of civil engineering practices. Based on the theoretical studies, researches have applied the ultrasonic technology to estimate properties of concrete, mortar, and cement pastes [15–21].
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. [12] discussed in detail the relations between corrosion depth and other parameters, such as mass loss, axial compressive strength, and elastic modulus of corroded concrete specimens.
2. Theoretical Background
A kind of nondestructive technology, NM-4 nonmetal ultrasonic tester, which is shown in Figure 1, was used to measure the corrosion depth of the concrete specimen under different corrosion states. In the experimental procedure, low frequency transducer whose frequency is below 50 kHz was selected. Considering that the dimension of the specimen, 40 mm, was determined as the measuring space, five measuring points B1, B2, B3, B4, and B5 (shown in Figure 2) were found. One transducer A (shown in Figure 2) is put on a permanent position, and the other transducer is placed on the measuring points B1, B2, B3, B4, and B5 in turn. In the test, sound velocity on each measuring point was recorded, and the distance between the two transducers was obtained. Time-distance graph shown in Figure 3 can be achieved.
If the concrete specimen is corroded, the surface will become loose, and the sound velocity is less than the sound velocity measured by undamaged concrete (denoted by ). and can be calculated by (1), respectively:
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 , some of the ultrasonic wave will penetrate through the damaged layer and transmit along the undamaged surface rapidly and will reach the receiver transducer prior to the wave transmit through the damaged layer. Therefore, a step change will occur.
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 (2). Relations between the corrosion depth and immersion time under different acid solutions are shown in Figure 4: where is the corrosion depth, is the threshold distance value between the two measured points, which can be achieved by the time-distance diagram, is the sound velocity for undamaged concrete, and is the sound velocity for damaged concrete.
3. Experimental Procedure
3.1. Materials and Specimen Preparation
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 1. To discover the deterioration process of concrete specimens under different corrosion state, 354 prism specimens with the dimension of 150 mm × 150 mm × 300 mm were prepared.
3.2. Techniques and Procedures
3.2.1. Simulated Acidic Environment
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 [22]. Accelerated corrosion test is applied in most of the present researches as well. Considering that in most of Chinese area, the acid rain is due to sulfuric acid [23], sulfuric acid type acid rain is simulated herein. Sulfate and nitric acid solutions are mixed to simulate the acidic environment in this paper.
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 2. Depending on the exposing conditions, the prism specimens were divided into 4 groups denoted as series 1, series 2, series 3, and series 4.
3.2.2. Test Procedure
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 [10]. Therefore, immersing test is a reasonable choice of accelerate method in this paper to fulfill the accelerated corrosion process. A corrosion-resisting rectangular tank was used to contain acid solution and the concrete specimens. After curing in water for 28 days, the specimens were divided into four groups. One group of specimens was continuously cured in water, while the other three groups of specimens were completly immersed in the three tanks with pH 3.5, pH 2.5, and pH 1.5 acid solutions, respectively, (shown in Figure 4). Since pH level of the mixed solution will change with the exposure period, nitric acid was added to the solution and stirred constantly to keep the pH level of the mixed solution constant. Solution acidity will be detected by PB-10 Sartorius pH-meter (shown in Figure 5) periodically.
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.
4. Results
4.1. Corrosion Depth
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 1 was used. Relation between the immersion time and corrosion depth is plotted in Figure 6.
From Figure 6, it can be seen that the corrosion depth achieved by ultrasonic nondestructive test increases quickly at the initial stage of corrosion, and then slows down gradually. It was also confirmed that the eroded depth of the specimen has a good bilinear relation to the immersion time under simulated acid rain with various acidities. The lower pH of the simulated acid solution is, the thicker of the corrosion depth will be. The results are coincident with those obtained from the long exposure test [22]. Hence, it can be drawn that the 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. Based on the test results, regression analysis is performed, and a simple bilinear model is put forward. The regression results in Table 3 show significant correlation between neutralized depth and immersion time, which can be expressed as where y is the corrosion depth and t is exposure duration. Both and are regression constants. and are correlation coefficients and is the critical exposure moment of the segment lines. All the constants and correlation coefficients are listed in Table 3.
4.2. Verification of Ultrasonic Test Result
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 7(a)) was used. Samples were scanned with a fixed X-ray source, at 140 kV, 200 mA, and 22.60 mGy CTD. Each scan was 1 mm thick, and the total number of the scans was about 300. Based on the images obtained from CT test, the corrosion depth of concrete was evaluated as well.
(a) 16-slice Spiral computed tomography scanner
(b) 0 day
(c) 120 days
(d) 3D reconstruction specimen
(e) Cross-sections
(f) Scanning layer of cross-section 1-1
(g) Scanning layer of cross-section 2-2
(h) 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 7(b)-7(c). Scanning images at the three cross-sections (Figure 7(e)) of concrete exposed to the simulated acid solution for 0 day and 120 days are given in this paper (Figures 7(f)–7(h)).
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 (3), the damage depth of concrete is 1.498 cm, which is in good correlation with the results obtained by CT test. Therefore, it is verified that ultrasonic wave technology is an efficient method to determine the corrosion depth of concrete in aggressive environment.
5. Analysis and Discussions
Besides the NDT of corroded concrete, compressive mechanical parameters such as compressive strength and elastic modulus are achieved [12]. Relation between corrosion depth and other mechanical properties of concrete is discussed.
5.1. Relation between Corrosion Depth and Mass Loss
To compare the deterioration regularity of the concrete corroded by the simulated acid environment, relative mass variation ratio is defined as where is the relative mass loss ratio of corroded concrete specimen, is the mass of specimen immersed in the acid solution for t days, and is the mass of sound concrete specimen. The relation between and immersion time is given in Figure 8, and the relation between and the corrosion depth is plotted in Figure 9.
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 8). Although the same corrosion depth is detected, the mass loss is different for specimen damaged by different acid solutions. The higher the acidity of the solution is, the higher the mass loss of concrete is.
5.2. Relation between Corrosion Depth and Compressive Strength
To compare the strength deterioration trend of the concrete specimen immersed in the acid solution, relation between concrete strength and the corrosion depth is achieved, which is plotted in Figure 10.
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.
5.3. Relation between Corrosion Depth and Elastic Modulus
According to the testing results, relation between elastic modulus and corrosion depth of concrete specimens is shown in Figure 11.
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.
6. Conclusions
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.
Acknowledgments
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.