International Symposium (NDT-CE 2003)Non-Destructive Testing in Civil Engineering 2003
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THE ROLE OF NON-DESTRUCTIVE TESTS FOR EVALUATION OF A CONCRETE STRUCTURE IN THE PERSIAN GULF REGION - CASE STUDYTayebeh Parhizkar - Building and Housing Research Center - Tehran, Iran
Ali Akbar Ramezanianpour - Amir Kabir University of Technology - Tehran, Iran
Bernd Hillemeier - TU Technical University - Berlin, Germany
Nargess Mozafari - Building and Housing Research Center - Tehran, Iran
Concrete structures on the coasts of the Persian Gulf are rapidly deteriorating, due to the severe climatic and geomorphic conditions of the region (high temperature and humidity regimes) and the presence of corrosive salts containing sulfate and chloride ions, necessitating repair operations. Before choosing the appropriate materials and suitable repair operations needed to ensure a successful repair, a study of the structure's condition, in order to evaluate the causes and extent of damage is of special importance. In this respect, non-destructive testing of the concrete structure has a significant role.
In this study, the concrete port of Shahid Rajaie complex, in the south of Iran has been evaluated, using non-destructive and semi-destructive tests. The tests that were carried out include concrete visual inspection, determining crack width using a field microscope, concrete uniformity test using the Schmidt hammer, compressive strength - CAPO test , concrete cover depth over reinforcement, potential of corrosion and chloride penetration depth determination .
The test results indicate that the main cause of deterioration in this structure is penetration of chloride into the concrete. The investigations also show that the concrete cover depth over reinforcement is both inadequate and of unsuitable quality. This structure had been repaired locally, but the deterioration re-occurred. The electrical potential before and after repair operations, indicate severe corrosion and deterioration due to the ingress of chloride ions. Test results indicate that the distribution of chloride ions in the concrete was modified after the repair work, increasing the risk of corrosion due to back - diffusion of chloride from the concrete substrate.
Concrete structures on the southern coasts and ports of the Persian Gulf region are continually exposed to highly aggressive environmental conditions, namely a humid, hot atmosphere charged with corrosive ions (such as chloride and sulfate ions). These factors all affect the durability of concrete structures in this region, and cause significant concrete deterioration, namely corrosion of reinforcement, chloride and sulfate attack, salt weathering and non-structural environmental cracking, in a relatively short period of time [1,2].
The premature deterioration of concrete structures in these severe environmental conditions of the region increases the need for suitable maintenance and repair systems. Correct evaluation of the cause and extent of damage to the structures is necessary to specify appropriate repair operations, as quickly and effectively as possible. In this context, non-destructive and semi-destructive tests for inspection of concrete play an important role in evaluating the structures conditions [3,4].
There are two main groups of non-destructive test methods used for evaluation of concrete structures. The first consists of techniques that indirectly indicate concrete strength and uniformity. The second group of non-destructive test methods is used to indicate properties other than strength, such as tests for reinforcement location and size and depth of concrete cover [5,6].
In this research study an inspection was carried out on a damaged concrete port of Shahid Rajaie complex, in the south of Iran by using non-destructive and semi-destructive testing methods.
2. Testing Program
The aim of this research study consisted mainly of determining the cause and extent of damage inflicted on the concrete port situated in the Shahid Rajaie Jetty Complex. This marine complex is situated in the south of Iran, adjacent to the city/port of Bandar Abbas and was built in 1979. As there was on-going repair operations being carried out in the port complex, we were able to conduct a comprehensive research program.
In this study the deterioration of the port deck was evaluated before and after repair operations taking place there. The tests used in this evaluation include concrete visual inspection, determining crack width using a field microscope, concrete uniformity test using the Schmidt hammer, compressive strength using the CAPO test, concrete cover depth over reinforcement, potential of corrosion and chloride depth determination (profile of chloride).
2.1. Test Procedures
The "Schmidt" rebound hammer test indicates concrete strength indirectly and is also used to measure the surface hardness and check the uniformity of the concrete.
The pull-out test also known as the CAPO test can also be used to determine concrete strength. In this test a chunk of concrete is pulled out forming a cone of failure. The pulling force is directly proportional to the concrete strength.
Magnetic methods are used to locate the steel reinforcement and to indicate the size and grade of the reinforcing bars. The depth of concrete cover over the steel reinforcement was measured by using electromagnetic cover-meters.
The corrosion state of the reinforcement was obtained by measuring electrical potentials by means of standard copper/copper sulfate half-cell.
The depth of chloride penetration into the concrete was determined by obtaining the relevant chloride profiles. Samples were taken at the required depth from the surface of the structure by dry drilling.
3. Test Results and Discussion
Visual inspections of the concrete surface of the port revealed a network of shrinkage cracks caused by the accelerated drying of the concrete in the hot environment and also some parallel cracks along the length of the port slab, indicating corrosion of reinforcement. There was some discoloration from the presence of rust in some of the cracks, also a sign of rebar corrosion.
The crack widths were measured using a field microscope. The results indicate that the crack widths range between 0.2 to 1.2 mm, the most common amounts for the crack widths recorded were around 0.6-0.7 mm. A diagram of the distribution of the crack widths is shown in Fig. 1.
The test results regarding concrete uniformity by using the Schmidt rebound hammer is shown in Fig.2, taken from the port deck surface. The hammer readings are between 20 to 45 on the hammer scale, with an average of about 33 and a deviation of ± 6.9 which shows that the concrete quality on the deck surface is very non-uniform.
The CAPO test was used to determine the concrete strength of the concrete cover of the port deck, to check the stability of the structure. The results of this test are shown in Fig. 3. The average strength recorded is about 20.6 ± 1.99 N/mm2, which is comparatively low. A comparison was made with the compressive strength recorded at the time of the port construction (obtained from site records) which was stated to be 38-42 N/mm2 for cube samples. The difference in strengths is due to several factors such as unsuitable concrete practice at the time of construction, concrete segregation, use of raw materials with unsuitable quality as well as the effect of environmental conditions on the concrete strength.
The depth of concrete cover over the steel reinforcement was measured by using an electromagnetic cover-meter. The areas having the most cracks were used as a guide to where the readings were made. The concrete depth was found to vary between 15-55 mm with a deviation of ±12.8, which indicates non-uniformity in the concrete cover over the rebars. The distribution of the results of this test is shown in Fig. 4.
The corrosion of the reinforcement was assessed with the half-cell electrical potential test, by using a copper/copper sulfate electrode. The area evaluated was in accordance with the visual inspections; areas having the most deterioration. The area tested with this method was sub-divided into 20x20 cm grids. The test results are represented by equipotential contours and shown in Fig. 5.
The test results indicate that there is active reinforcement corrosion in many areas of the port deck tested, the highest electrical potential recorded being - 400 mv CSE according to the ASTM C 876 standard, resulting in a corrosion probability of 90%. The highest amount of corrosion seemed to have occurred in the areas closest to the Persian Gulf. At these points, we were able to uncover the rebars (due to the on-going repair operations) and visually inspect the reinforcement, which showed a large degree of corrosion. A comparison was made of the two amounts of corrosion recorded, using visual inspection and the electrical potential method, the results from the latter method was about 30% higher.
In a comparison of the concrete cover depth results with the electrical half-cell potentials recorded, it can be noted that the more negative figures were in areas where the concrete cover was less than 40 mm. It can be said that the permeability of the concrete has an inverse relationship with the concrete cover depth and is directly related to the corrosion of the reinforcement.
Chloride profiles were obtained by testing samples taken from the various required depths of the concrete structure by dry drilling up to a depth of 100 mm. The chloride ions were determined by using the BRE test method for determining the chloride content of hardened concrete . A total of seven locations of the port deck were evaluated and a typical chloride profile of the testing locations of the structure is shown in Fig. 6.
It is apparent that the concentration of the chloride ions is reduced as the depth of concrete increases and the maximum amount of chloride is present at the surface of the concrete. As can be seen from Fig. 6, even at the depth of 100 mm the amount of chloride is still more than the critical amount (0.1 percent of cement weight). By comparing these results with the concrete cover depths at each testing location, it is apparent that the chloride ions have reached the rebars and caused corrosion of the reinforcement. The extent of chloride penetration into the concrete depends on several factors. One of the main factors is porosity of concrete; inferior concrete cover quality and high permeability results in an increase in the concentration of chloride ions in the concrete. As the graph shows, the slope of the curve represents an indirect relationship between the decrease in chloride ions with the depth of concrete, which indicates that most of the chloride ions present in the concrete has entered from the atmosphere. The inspection of the reinforcements showed that the corrosion of the steel was uniform, indicating that the rebars were uniformly exposed to chloride ions and that the corrosion was induced by chloride attack.
After a two-year interval and after the completion of the repair operations, the half-cell electrical potential of the deck was obtained again. The half-cell test results are shown in Fig. 7.
The potential half-cell readings taken after the repair operations, shown as contours in Fig. 7, indicate that the corrosion had increased, as the readings were more negative than before. Inspections of the reinforcing bars at this time showed pit corrosion, with upto 60 % reduction in rebar diameter. The increase in corrosion rate seen after the repair of the structure is attributed to the fact that a patch repair system was used; only 10-20% of the port deck was repaired and due to the fact that the contaminated concrete was not removed completely and that the repair operations disrupted the distribution of chloride ions in the concrete, back - diffusion of chloride ions occured from the concrete substrate into the repaired area. This process results in an increase in reinforcement corrosion; the repaired area is converted into an active cathode and the surrounding concrete acts as an anode, thus the anode and cathodic areas are reversed and the corrosion rate in the surrounding areas increase greatly. The most negative electrical potential readings were -450 mv CSE in comparison with -400 mv CSE recorded before the repairs, despite the fact that epoxy coatings had been used to protect the rebars in the repair operations .
Inspection of the structure and the non-destructive and semi-destructive tests carried out in this study confirm that the main cause of deterioration in this concrete port was due to chloride penetration and chloride induced corrosion of reinforcement, accelerated by the severe environmental conditions of the Persian Gulf region and also by the improper repair strategy applied.
This study shows that non-destructive tests (NDT) are effective methods for assessing deterioration in existing concrete structures.