Effect of corrosion on concrete reinforcement mechanical and physical 2
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    Effect of corrosion on concrete reinforcement mechanical and physical 2 Effect of corrosion on concrete reinforcement mechanical and physical 2 Document Transcript

    • International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 4, Issue 3, May - June (2013), © IAEME 176 EFFECT OF CORROSION ON CONCRETE REINFORCEMENT MECHANICAL AND PHYSICAL PROPERTIES Sanad A.M.1 and Hassan H.A.2 (Construction & Building Department, College of Engineering & Technology, Arab Academy for Science, Technology & Maritime Transport, Cairo, Egypt) ABSTRACT Corrosion of concrete reinforcement is a major factor affecting the deterioration of RC structures. During corrosion, steel undergoes several phases of chemical reactions with consequent variation in steel section geometry and mechanical properties. At ultimate corrosion stage, the effective cross section area of steel is reduced with equivalent decrease in load carrying capacity leading to unsafe structures. During initial phase of corrosion, chemical reactions generate new products which irregularly increase bars’ diameters. The resulted products induce additional stresses on the structural member, causing cracking and spalling of the concrete cover. Corrosion cracking increases further corrosion rate by loss of protective cover and direct exposure to corrosive environment. A comprehensive experimental program was conducted to evaluate the effect of several degrees of corrosion on the mechanical and physical properties of concrete reinforcement bars. Three types of carbon steel bars were used: plain, deformed, and epoxy-coated deformed bars. The results showed clearly the effect of corrosion on increasing the rate of deterioration of RC members’ strength. Keywords: Steel corrosion, Reinforced Concrete Deterioration, Mechanical Properties. 1. INTRODUCTION Recently the aspects of concrete durability and performance have become a major subject of discussion especially when the concrete is subjected to severe environment. Corrosion of steel bars is the main factor influencing both the concrete durability and strength [1]. The corrosion products of the steel reinforcement can expand four to five times its original volume, developing high pressures within the concrete, which cause cracking and INTERNATIONAL JOURNAL OF CIVIL ENGINEERING AND TECHNOLOGY (IJCIET) ISSN 0976 – 6308 (Print) ISSN 0976 – 6316(Online) Volume 4, Issue 3, May - June (2013), pp. 176-184 © IAEME: www.iaeme.com/ijciet.asp Journal Impact Factor (2013): 5.3277 (Calculated by GISI) www.jifactor.com IJCIET © IAEME
    • International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 4, Issue 3, May - June (2013), © IAEME 177 spalling of the concrete cover and expose the rebar to further corrosion activity. The potential consequences of the corrosion problem can be summed up in the continuous reduction in strength, stiffness, durability and designed life time of concrete structural elements reinforced with conventional steel. According to the ASTM [2], corrosion is defined as “the chemical or electrochemical reaction between a material, usually a metal, and its environment that produces a deterioration of the material and its properties”. Corrosion reduces the ribs height of the deformed bar which causes reduction in the contact area between the ribs and the concrete leading to reduction in the bond strength. Corrosion may also affect the rib face angle in the advanced stages; moreover, ribs of deformed bars are eventually lost at high level of corrosion. Corrosion of reinforced bars is usually associated with the increase of the crack width [3]. The increase of the corrosion products around the bar leads to increase of bursting force and tension cracking of the surrounding concrete, as the corrosion increases, the crack width becomes wider and the bond strength decreases [4]. Rapid deterioration of reinforced concrete buildings in Alexandria, Egypt has become a major problem for sea front buildings’ dwellers, where the disaster exceeds the value of money and extends to human lives. In the last decade many reinforced concrete buildings collapsed with a majority located in coastal cities. Over eighty percent of these collapses were at Alexandria and Damietta which are located at the north coast of Egypt facing the salt attack of the Mediterranean Sea. This high percentage highlights the importance of investigating the common factors that lead to the collapse of those buildings [1]. 2. RESEARCH SCOPE AND OBJECTIVES The main objective of this paper is to study the rate of reinforcement corrosion for three different types of steel embedded in four types of concrete. The effect of corrosion on the mechanical & physical properties of the embedded reinforcement is also investigated and the study is conducted at four phases of corrosion; un-corroded bars, pre-cracking, cracking and severely corroded bars. A comprehensive experimental program was implemented to identify the effect of four parameters; the water/cement ratio, concrete strength, type of steel reinforcement and coating material. The tested mechanical properties included the loss of the tensile strength and steel bars’ ductility; where, the measurement of physical properties included the mass and rib height loss of the bars. 3. EXPERIMENTAL PROGRAM The effect of steel corrosion on the durability of seafront reinforced concrete structures was investigated experimentally at AASTMT Labs where the four types of concretes considered had variable water/cement ratios and variable ultimate strengths as shown in Table 1. The effect of corrosion in regular carbon steel bars was evaluated for Plain bars, un-coated deformed bars and epoxy coated deformed bars. The bars diameter was 16mm and used to reinforce a 100mm diameter by 200mm height concrete cylinder. The specimen was reinforced with a single bar located in the center as shown in Fig. 1.
    • International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 (Print), ISSN 0976 – 6316(Online) Volume 4, Issue 3, May Table 1: Mixture Type Concrete Density (kN/m3 ) 30MPa, w/c=0.32 23.1 44MPa, w/c=0.32 23.3 60MPa, w/c=0.32 23.3 44MPa, w/c=0.52 24.2 Figure 1. Different dosages of super plasticizers were added to the mixtures having low water cement ratios (w/c=0.32) to obtain mixture with high water cement ratio (w/c=0.52). This technique was used to investigate separately the effect of concrete strength and the effect of water cement ratio without any other variable that might affect the mechanical properties of the concrete. The tested mechanical and physical properties of the used steel bars before corrosion initiation are shown in Table 2. nternational Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 6316(Online) Volume 4, Issue 3, May - June (2013), © IAEME 178 Table 1: Test results for concrete specimens Slump (mm) Test Type Average Stress of three samples (after 56 days) (MPa) 29 Compressive Strength 33.74 Splitting Tensile Strength 30 Compressive Strength 48.58 Splitting Tensile Strength 33 Compressive Strength 62.90 Splitting Tensile Strength 32 Compressive Strength 49.88 Splitting Tensile Strength Figure 1. cylinderical concrete specimens Different dosages of super plasticizers were added to the mixtures having low water cement ratios (w/c=0.32) to obtain approximately the same slump range as the 44MPa mixture with high water cement ratio (w/c=0.52). This technique was used to investigate separately the effect of concrete strength and the effect of water cement ratio without any fect the mechanical properties of the concrete. The tested mechanical and physical properties of the used steel bars before corrosion initiation are nternational Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 June (2013), © IAEME Average Stress of three samples (after 56 days) (MPa) 33.74 3.69 48.58 4.60 62.90 4.40 49.88 3.98 Different dosages of super plasticizers were added to the mixtures having low water approximately the same slump range as the 44MPa mixture with high water cement ratio (w/c=0.52). This technique was used to investigate separately the effect of concrete strength and the effect of water cement ratio without any fect the mechanical properties of the concrete. The tested mechanical and physical properties of the used steel bars before corrosion initiation are
    • International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 (Print), ISSN 0976 – 6316(Online) Volume 4, Issue 3, May Table 2: Tested mechanical and physical properties of steel bars 16mm un coated plain steel bars Steel Grade Code PL Effective diameter Yield stress (MPa) Tensile strength (MPa) Elongation percentage 4. ACCELERATED CORROSION Accelerated corrosion tests are used to obtain qualitative information on corrosion behavior in a relative short period compared to the field corrosion test. Accelerated corrosion tests have been used successfully to determine the susceptibility of the reinforcing and other forms of structural steel to localized attacks such as pitting corrosion, stress corrosion and other forms of corrosion [5]. Before testing, all concrete samples were months and specimens tested at end of this period were defined as zero corrosion samples and served as the control specimens. The rest of specimens were placed in fiber tank with dimension 165x85 cm containing an electrolytic solution weight of water]. A steel mesh was placed in the bottom of tank to carry the specimens and connected to 12V power supply through a single steel bar as shown schematically in Fig. 2. The direction of electrical current was ar while the specimens’ bars served as anodes. Based on the crack width; three corrosion phases were defined; Pre considered when the electrical current measurement started to increase but was visible. Cracking stage considered when the first crack appeared on the concrete specimen regardless its width, and severe corrosion stage considered when any crack width reached 4mm. The accelerated corrosion test was terminated when al took place for all steel types. Figure 2: schematic diagram showing the accelerated corrosion set nternational Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 6316(Online) Volume 4, Issue 3, May - June (2013), © IAEME 179 Tested mechanical and physical properties of steel bars 16mm un coated plain steel bars 16mm un coated deformed steel bars 16mm epoxy coated deformed steel bars 24/35 40/60 40/60 PL-UC DF-UC DF 16 15.75 15.85 250 564.5 546.5 360 676 658 21.9 13 12.5 CORROSION SET-UP Accelerated corrosion tests are used to obtain qualitative information on corrosion behavior in a relative short period compared to the field corrosion test. Accelerated corrosion have been used successfully to determine the susceptibility of the reinforcing and other forms of structural steel to localized attacks such as pitting corrosion, stress corrosion and other forms of corrosion [5]. Before testing, all concrete samples were set for curing for 2 months and specimens tested at end of this period were defined as zero corrosion samples and served as the control specimens. The rest of specimens were placed in fiber tank with dimension 165x85 cm containing an electrolytic solution [5% sodium chloride NaCl by the weight of water]. A steel mesh was placed in the bottom of tank to carry the specimens and connected to 12V power supply through a single steel bar as shown schematically in Fig. 2. The direction of electrical current was arranged so that the single steel bar served as cathode, while the specimens’ bars served as anodes. Based on the crack width; three corrosion phases were defined; Pre-cracking stage considered when the electrical current measurement started to increase but before any crack was visible. Cracking stage considered when the first crack appeared on the concrete width, and severe corrosion stage considered when any crack width The accelerated corrosion test was terminated when all stages of corrosion chematic diagram showing the accelerated corrosion set-up nternational Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 June (2013), © IAEME 16mm epoxy coated deformed steel bars 40/60 DF-C 15.85 546.5 658 12.5 Accelerated corrosion tests are used to obtain qualitative information on corrosion behavior in a relative short period compared to the field corrosion test. Accelerated corrosion have been used successfully to determine the susceptibility of the reinforcing and other forms of structural steel to localized attacks such as pitting corrosion, stress corrosion and set for curing for 2 months and specimens tested at end of this period were defined as zero corrosion samples and served as the control specimens. The rest of specimens were placed in fiber tank with [5% sodium chloride NaCl by the weight of water]. A steel mesh was placed in the bottom of tank to carry the specimens and connected to 12V power supply through a single steel bar as shown schematically in Fig. 2. ranged so that the single steel bar served as cathode, cracking stage before any crack was visible. Cracking stage considered when the first crack appeared on the concrete width, and severe corrosion stage considered when any crack width l stages of corrosion
    • International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 4, Issue 3, May - June (2013), © IAEME 180 5. EXPERIMENTAL PROGRAM RESULTS The results of this research are divided into two parts; the first concerning the effect of corrosion on the mechanical properties of the reinforcing bars including the yield stress, the tensile strength and the ductility. The second part shows the effect of corrosion on the physical properties including the mass, the rib height, and the cross-sectional area of the bars. 5.1 Effect of Corrosion on the Bars’ Mechanical Properties The corroded bars were removed from the samples, after performing a pull-out test for concrete-reinforcement bond strength. Then, theywere subjected to axial tensile test to study the effect of corrosion on the tensile strength and ductility of the bars. The stress-strain curves were plotted for each concrete and steel type at different degrees of corrosion. Figs. 3 to 5 show an example of stress-strain behaviors for the bars embedded in the 44MPa, 0.52 water/cement ratio concrete cylinders.The tensile stress was calculated by dividing the tensile load by the average cross sectional area of bar, taken as the average between the area of the corroded embedded part and the un-corroded area of the protruding part. The strain was calculated by dividing the extension values taken from the machine LVDT by the bar initial length.From the figures, it can be seen that corrosion affects the steel mechanical properties negatively. At zero corrosion stage, all bars showed large ductility and high yield stress. However, these ductility regions start to disappear as the corrosion propagated from pre- cracking to severe corrosion stage and all bars failed at lower extensions due to large decrease in ductility.Nearly all steel grades showed similarreduction in ductility values. Figure 3: stress-strain curve for un-coated plain bars 0 50 100 150 200 250 300 350 400 0 0.05 0.1 0.15 0.2 0.25 Tensilestress(Mpa) Strain (mm/mm) Zero corrosion Pre-cracking Cracking Severe corrosion
    • International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 4, Issue 3, May - June (2013), © IAEME 181 Figure 4: stress-strain curve for un-coated deformed bars Figure 5: Stress-strain curve for epoxy-coated deformed bars A clear reduction in yield stress and ultimate stress is also observed in all specimens. As corrosion propagates, the safety factor used in the designing process is drastically reduced due to the decrease in yield stress and the over-all advantage of structural ductility also disappears, leading to sudden failure without signs of large deformation in the structure. Corrosion of steel over whelms its advantages when the effective cross section is reduced, the ultimate stress is decreased and the ability for large elongation at yield limit is lost. 0 100 200 300 400 500 600 700 800 0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 Tensilestress(Mpa) Strain (mm/mm) Zero corrosion Pre-cracking Cracking Severe corrosion 0 100 200 300 400 500 600 700 0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 Tensilestress(Mpa) Strain (mm/mm) Zero corrosion Pre-cracking Cracking Severe corrosion
    • International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 4, Issue 3, May - June (2013), © IAEME 182 5.2 Effect of Corrosion on the Bars’ Physical Properties The effect of corrosion on steel bars physical properties was studied including the mass and rib loss for each type of steel at different degrees of corrosion. The mass loss was obtained as the difference between the mass of the corroded bar, after removal of the loose corrosion products, and its mass before corrosion. The ribs height were measured after the corrosion took place, and the rib profile loss was obtained as the difference between the rib height of the corroded bar and its height before corrosion. Tables 3 and 4 show the mass and rib profile loss of steel bars at different corrosion stages respectively. Table 3: Mass loss of steel bars at different degrees of corrosion Concrete Type Steel Type Mass loss as a percentage of zero corrosion mass (%) Pre-cracking Cracking Severe Corrosion 30MPa, w/c=0.32 Plain (St37) 0.8 2.6 3.5 Deformed Uncoated (St60) 1.2 2.9 4.2 Deformed Coated (St60) 1.6 2.4 3.6 44MPa, w/c=0.32 Plain (St37) 0.4 0.6 2.9 Deformed Uncoated (St60) 1 1.2 3.6 Deformed Coated (St60) 1 1.1 3.3 60MPa, w/c=0.32 Plain (St37) 0.4 1.2 2.2 Deformed Uncoated (St60) 0.5 1.7 2.9 Deformed Coated (St60) 0.4 1.7 2.6 44MPa, w/c=0.52 Plain (St37) 0.6 2.6 3.7 Deformed Uncoated (St60) 0.4 1.6 4.2 Deformed Coated (St60) 0.5 2.9 4.1 From Table 3, the un-coated deformed bars have the greatest mass loss while the plain bars have the least mass loss in all concrete mixes. The epoxy coated bars have less mass loss compared to the uncoated deformed bars which proves the efficiency of the epoxy coating in corrosion protection. The epoxy coating is well known for its good protection for steel bars against corrosion. The epoxy coating in this research was applied to the whole length of the bar but its ends were left un-coated as practiced in the construction field. Therefore the corrosion is concentrated in the uncoated end leading to specific core rupture across the diameter. Figs. 3 and 4 show the difference in crack propagation between the un-coated and epoxy coated deformed bars respectively. The comparison between the un-coated deformed bars and plain bars, showed that the plain bars have less mass loss than the deformed ones. This can be related to the high surface area of deformed bars. The plain bars had also less mass loss than the deformed coated ones. The plain bars are steel 37, while the deformed bars are steel 60, therefore the difference in corrosion rate can be attributed to the difference in
    • International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 4, Issue 3, May - June (2013), © IAEME 183 chemical composition between the two types of steel, showing that steel 37 is more resistant to corrosion than steel 60. The uncoated bars also showed uniform corrosion along the total bar length, while the epoxy coated bars showed concentrated corrosion at un-coated end leading to localized stresses at the bottom third of the concrete cylinder. Table 4: Rib and diameter loss of steel bars at different degrees of corrosion Concrete Type Steel Type Rib profile loss as a percentage of zero corrosion rib height (%) Pre-cracking Cracking Severe Corrosion 30MPa, w/c=0.32 Plain St37(diameter loss)* 8.7 16.1 18.8 Deformed Uncoated St60 69.2 98.56 (131.2)** Deformed Coated St60 81.2 92.1 (122.1)** 44MPa, w/c=0.32 Plain ST37 (diameter loss)* 6.1 7.5 17.1 Deformed Uncoated St60 38.7 69.6 (121.9)** Deformed Coated St60 43.6 66 (116.9)** 60MPa, w/c=0.32 Plain ST37 (diameter loss)* 6 11.1 14.7 Deformed Uncoated St60 47.2 84.7 (108.4)** Deformed Coated St60 40.8 83.6 (103.9)** 44MPa, w/c=0.52 Plain ST37 (diameter loss)* 7.6 16 19.1 Deformed Uncoated St60 63.9 81.8 (132.1)** Deformed Coated St60 63.1 94.2 (130.9)** *For plain bars with no ribs, the percentages were calculated from the total cross-sectional area of the bar. **Results exceeding the 100% indicate that the corrosion causes total loss in the ribs and extends to the inner bar diameter. Figure 6 Crack propagation in concrete cylinders reinforced with uncoated defromed bars Figure 7 Crack propagation in concrete cylinders reinforced with epoxy coated defromed bars
    • International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 4, Issue 3, May - June (2013), © IAEME 184 Concerning the rib height loss, Table 4 indicates that un-coated deformed bars exhibit the maximum rib loss when compared to the plain and epoxy coated deformed bars. From the previous two tables, it can be concluded that the rate of corrosion increases in case of un- coated deformed bars. This can be attributed to the greater surface area of deformed bars compared to the plain bars and their greater conductivity when compared to the epoxy coated bars, where the epoxy acts as a barrier to chloride penetration. 6. CONCLUSION The purpose of this research is to study the effect corrosion on reinforced concrete structures especially those near the sea side and structures located in salt-laden environments. The influence of corrosion on the mechanical and physical properties of steel bars was investigated. This paper includes results from experimental program performed with four concrete mixes and three types of steel bars. All concrete mixes were reinforced by three different carbon steel types: Plain, deformed, and epoxy-coated deformed bars. The experimental results of steel bars tensile tests, showed that all bars demonstrate large ductility at yielding regions before corrosion. However, these regions starts to disappear as the corrosion propagated from pre-cracking to severe corrosion. At advanced corrosion stages; corrosion reduces the cross-sectional area of steel bars, and affect the height of ribs of deformed bars. Corrosion causes high loss of steel ductility, large reduction of yield and ultimate stresses. When combined with reduction of effective cross-sectional area of steel, corrosion leads to serious deterioration of load carrying of reinforced concrete members. REFERENCES [1] Hassan A.H., Sanad A.M., and Moussa M.A., Environment Impact on Sea Front Reinforced Concrete Structures in Egypt, Global Climate Change, Biodiversity and Sustainability, April 2013. [2] ASTM STP 1065, "Corrosion Rates of Steel in Concrete," ISBN13: 978-0-8031-1458- 6, 1990. [3] Bertolini, L., (2004), “Corrosion of Steel in Concrete,” ISBN: 3-527-30800-8. [4] Sanad A.M., Hassan A.H. and Moussa M.A., Finite Element Modeling of Steel Corrosion in Reinforced Concrete Cylinders, The 3rd International Conference on Advanced Engineering Materials and Technology, May 2013 [5] Assem Adel Abdel Aal Hassan, (2003), "Bond of Reinforcement in Concrete with Different Types of Corroded Bars" Theses and dissertations, Ryerson University. [6] Siddhant Datta , B.M. Nagabhushana and R. Harikrishna, “A New Nano-Ceria Reinforced Epoxy Polymer Composite with Improved Mechanical Properties”, International Journal of Advanced Research in Engineering & Technology (IJARET), Volume 3, Issue 2, 2012, pp. 248 - 256, ISSN Print: 0976-6480, ISSN Online: 0976-6499. [7] Dr. Abdulkader Ismail Abdulwahab Al-Hadithi, “Improving Impact and Mechanical Properties of Gap-Graded Concrete by Adding Waste Plastic Fibers”, International Journal of Civil Engineering & Technology (IJCIET), Volume 4, Issue 2, 2013, pp. 118 - 131, ISSN Print: 0976 – 6308, ISSN Online: 0976 – 6316.