Investigation of severe corrosion observed at intersection points of steel rebar mesh in reinforced concrete construction

Construction and Building Materials 37 (2012) 67–81

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Investigation of severe corrosion observed at intersection points of steel rebar mesh in reinforced concrete construction Abdulrahman Alhozaimy a, Raja Rizwan Hussain b,⇑, Rajeh Al-Zaid a, Abdulaziz Al Negheimish a a b

Center of Excellence for Concrete Research and Testing, Civil Engineering Department, King Saud University, Saudi Arabia Center of Excellence for Concrete Research and Testing, Civil Engineering Department, College of Engineering, King Saud University, Saudi Arabia

h i g h l i g h t s " High corrosion observed at intersections of steel rebars in the mesh of RC construction. " Effect of variation in rebar spacing, connection type as well as the binding wire material. " Improved steel rebar fixing and placement methods in RC construction. " Protection of steel reinforced concrete construction against corrosion.

a r t i c l e

i n f o

Article history: Received 31 December 2011 Received in revised form 28 June 2012 Accepted 13 July 2012 Available online 24 August 2012 Keywords: Corrosion Reinforced concrete construction Steel rebar intersections Mesh spacing Connection type Binding wire material Microstructure

a b s t r a c t This paper investigates the phenomenon of high corrosion observed at many intersections of steel rebars in the wall footing of an existing reinforced concrete structure. This phenomenon is rather new and unusual and has not been reported in the past. Therefore, an experimental program was carried out to confirm the field observations and clarify the mechanism involved. Chloride-contaminated reinforced concrete panels were cast; laboratory measurements were conducted to determine the half-cell potential, corrosion current and concrete resistivity; and scanning electron microscopy and mercury intrusion porosimetry were performed. The experimental measurements at the intersection of steel rebars were found to be mostly higher than the areas between them. The high corrosion rate observed at steel intersection points appeared to be due to the coupled effects of the corrosive binding wire material, the electrical connectivity, the reduced center-to-center (c/c) steel bar spacing and poor concrete microstructure at the rebar intersection. Further detailed investigation is required for a better understanding of the effects of these factors individually. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction The corrosion of reinforced concrete structures has always been an issue of great concern for professional civil engineers and researchers all over the world. Reinforced concrete (RC) structures are corroded due to the effect of various environmental factors, such as chloride, carbonation and temperature. The rapid deterioration of RC structures due to corrosion is becoming a very serious threat for the world in terms of the enormous economic and safety implications. It requires necessary countermeasures for the protection and maintenance of corroding infrastructures while also considering the alleviation of the problem in future construction.

⇑ Corresponding author. Address: Center of Excellence for Concrete Research and Testing, Civil Engineering Department, College of Engineering, King Saud University, PO Box 800, Riyadh 11421, Saudi Arabia. Tel.: +966 562556969. E-mail address: [email protected] (R.R. Hussain). 0950-0618/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.conbuildmat.2012.07.011

According to ACI committee 201 [1], the durability of concrete is defined as its ability to withstand the environmental conditions to which it is exposed. Thus, durable concrete should resist rebar corrosion, weathering actions, chemical attack, abrasion, and other forms of deterioration. The fast deterioration of reinforced concrete is a very serious issue with huge economical drawbacks and serviceability related problems. Therefore, engineers are facing maintenance problems involving the aging and deterioration of RC structures due to a number of mechanisms. One of the most severe problems is the corrosion of the reinforcing steel. Reinforcement corrosion is the most urgent durability problem for RC structures throughout the world. Extensive research work has been performed regarding the corrosion of RC structures in the past. These studies cover many aspects related to the corrosion of RC structures, including the effect of chloride on corrosion [2]; the combined effects of chloride, humidity and temperature [3]; the factors affecting the threshold chloride values for corrosion [4]; and the effectiveness of corrosion inhibitors [5,6]. In addition, the corrosion

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(a) RC structure under severe corrosion

(b) Severe corrosion at intersection points of steel bars in concrete Fig. 1. Unique Corrosion Pattern Found in the Foundations of a Typical RC Complex in Riyadh, Saudi Arabia

of RC structures in the harsh environment of the Gulf region has been established [7–11]. Reviews on the corrosion in concrete structures, their monitoring and service life prediction have been conducted [12,13]. Structural damage propagation modelling for chloride-induced corrosion in RC structures has been proposed [14]. Procedures for the in situ measurement of the corrosion rate of rebars have been developed [15]. Methods such as nuclear techniques have been used for the non-destructive analysis of corroding concrete [16]. The effect of oxygen diffusion on the corrosion rate in concrete has been investigated [17]. Corrosion uncertainty modelling and the assessment of stainless steel reinforcement for the rehabilitation of concrete structures has also been carried out in the past [18,19]. In the Middle Eastern areas, such as Saudi Arabia, the need to emphasize the durability of corroding RC structures is paramount given the widespread use of concrete in all types of construction. The corrosion of reinforced concrete in Saudi Arabia has received increasing attention, especially in the Eastern Province, where concrete deterioration due to rebar corrosion has become fatal. The new Saudi Building Code [20] is expected to alleviate this problem in new construction by implementing strict durability standards for RC structures. Proper design, the selection of good material, good construction practices, and periodic monitoring, maintenance, and repair should be considered together to more effectively extend the service life of reinforced concrete structures.

Fig. 1 shows a typical residential building complex in Riyadh, Saudi Arabia, which was built approximately 30 years ago and is suffering from severe corrosion in the foundations. The interesting point is that the reinforcement bars show very severe corrosion at many of the intersection points of the horizontal and vertical steel bars in the mesh and limited corrosion between the intersection points. The huge difference observed in the magnitude of the corrosion rate at the steel rebar intersection points and areas in between them is a rather new and unusual finding. Despite the substantial amount of research work reported in the past, as mentioned in the previous section of this paper, the phenomenon of the severe corrosion rate of reinforcement steel at the intersection points of the rebar mesh compared to the other areas is yet to be explored. Therefore, the experimental scheme implemented in this research was intended to investigate, under controlled, simulated laboratory conditions, the very high corrosion rate observed at the steel rebar mesh intersection points in comparison to very low or no corrosion Table 1 Material mix proportions. w/c

Binder (kg/m3)

Water (kg/m3)

Crushed sand (kg/m3)

Silica sand (kg/m3)

20 mm Aggregate (kg/m3)

10 mm Aggregate (kg/m3)

0.55

350

193

280

420

730

320

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300 mm

10 mm Dia. deformed mild steel bars

300 mm

Lean Concrete

Concrete with

200 mm

W/C: 0.55

W/C: 0.55 and 5% admixed

200 mm

10 mm Dia. deformed mild steel bars

5% Admixed Cl -

Cl-

200 mm

500 mm

500mm

(a) Steel reinforced concrete panel 1 with copper binding wire

300 mm

300mm

200 mm

1000 mm x 1000 mm x 100 mm

(b) Steel reinforced concrete panel 2 with copper binding wire

Fig. 2. Experiment Specimens

in other areas of the mesh. The investigation will enable clarification and understanding of the mechanisms involved therein. To the best of the authors’ knowledge, there is no similar research work related to this phenomenon in the published literature. The results of this research will form the basis for evaluating the adequacy of current durability provisions and will provide the scope for future improvement of the quality and efficacy of RC structures against corrosion. 2. Experiment program 2.1. Materials and mix proportions 2.1.1. Concrete Ordinary Portland cement of Type I as per ASTM C150 [21] was used. The coarse and fine aggregates were tested to ensure that the gradation satisfies the standard specification requirements of ASTM C33 [22]. The gradation of fine aggregate was blended with natural silica sand and manufactured sand obtained from crushed limestone. Crushed sand and natural silica sand passed through sieve No. 4 (4.75mm openings) were used as the fine aggregate with a specific gravity of 2.60 and 2.66 and a water absorption of 2.48% and 0.28%, respectively. Crushed limestone with a blended size of 20 mm and 10 mm was used as the coarse aggregate with specific gravities of 2.67 and 2.66 and water absorptions of 0.68% and 0.77%, respectively. The specific gravity and absorption of the aggregates were determined in accordance with ASTM C127 [23] and C 128 [24], respectively. 2.1.2. Steel reinforcement Deformed round carbon mild steel bars 10 mm in diameter were used as the construction steel reinforcement. The surface of the steel bars was polished by No. 200 sandpaper. Finally, steel bars were degreased with acetone immediately prior to being placed in the mould. 2.1.3. Mix proportions of concrete Mix proportions were designed with a water to cement ratio (w/c) of 0.55, and the chloride content was maintained at 5% total chloride (Cl) by mass of cement to initiate a high corrosion rate and faster production of experimental results. Trial concrete mixing was first conducted to achieve a 150-mm slump. The mix proportions are shown in Table 1.

2.2. Specimen preparation and experiment scheme A specific experiment was devised and performed, in which 100 mm thick reinforced concrete panels with dimensions of 1.0 m  1.0 m  0.1 m were cast with a single layer of 10-mm diameter reinforcement bars at the middle that perpendicularly intersect each other to observe the high corrosion expected to occur at the rebar intersection points. The clear cover thickness for the upper rebar was 40 mm from the top and for the lower rebar was 40 mm from the bottom of the concrete panel. Uniform chloride content has been used as a corrosion initiator while considering the corrosion cell as a fused anode cathode system. The reinforcing bars in the mesh had a variable c/c spacing configuration to observe its effect on the high corrosion rate expected to occur at rebar intersection points. Furthermore, three different types of binding wires (copper binding wire, steel binding wire and nylon binding wire with insulating plastic spacers between the two intersecting bars) were used at the rebar intersections. The specimens were cured by sprinkling water over burlap for 28 days under room temperature (20 ± 2 °C). Fig. 2 shows a detailed schematic and actual diagrams of the specimens cast in this research. 2.3. Experiment measurements A comprehensive experimental measurement methodology was conducted to clearly understand the role of intersecting steel reinforcement bars in the acceleration of the corrosion rate. The steel rebar corrosion current (Icorr), corrosion potential (Ecorr) and concrete resistivity (K) were measured for all specimens using a GECOR device [25] and ASTM C-876 [26]. The GECOR measures the corrosion rate as reflected by the corrosion current density and the half-cell corrosion potential. A true measure of the corrosion rate is possible by the polarization resistance technique. It has been well established by Stern and Geary that the corrosion current is linearly related to the polarization resistance, which gives a direct quantitative measurement of the amount of steel turning into oxide at the time of measurement. By Faraday’s equation, this can be extrapolated to direct metal sectional loss. The corrosion current values in GECOR are calculated from the polarization resistance Rp using the relation Icorr = B/Rp, where Icorr is given in lA/cm2 when Rp is given in k X/cm2 and B = 26 mV. The Icorr is directly proportional to corrosion rate through the relation, corrosion rate (lm/year) = 11.6  Icorr. This gives a tool for quantifying the average reduction of rebar diameter over time. The measurement of corrosion rate usually involves applying electrical signal through a connection to the steel bar. In GECOR, this signal is confined to the steel rebar in a circle with a diameter of 110 mm. There is evidence that this technique gives a more accurate measurement of the corrosion rate [27]. The GECOR device is quite valuable and versatile for the corrosion measurement of steel in concrete. See Fig. 3 for further illustration.

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200 mm 600 mm

10 mm Dia. deformed mild steel bars Lean Concrete

200 mm

W/C: 0.55 5% Admixed Cl200 mm

600 mm

(d(i)) Various intersection types and

200 mm

1000 mm x 1000 mm x 100 mm

(d(ii)) Various intersection types and binding wire materials

3. Nylon binding wire with plastic separators 4. Copper binding wire at the intersections 5. Steel binding wire at the intersections

(c) Steel reinforced concrete panels 3-5 (e) Casting of steel reinforced concrete structural members

(f) Bare steel bars sprinkled with 5% total chloride solution Fig. 2 (continued)

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The measurements were taken at the intersection point and in between them along the length of the intersecting reinforcement bars for both upper (U) and lower (L) intersecting bars. The final measurement value at the intersection was recorded as the average of the two upper and lower rebar separate measurements. Then, the physical observation/pictorial analysis of the corroded reinforcement bars and micro-nano scale investigations using SEM (scanning electron microscope) imaging and MIP (mercury intrusion porosimetry) techniques was also carried out after opening/breaking Panel 1. Finally, gravimetric mass loss determination will be carried out for all the Panels after three years elapse to attain full confidence in the experiment results; these results will be presented in future publications. This experimental scheme is intended to investigate the very high corrosion rate observed at the round steel rebar mesh intersection points in comparison to very low or no corrosion in other areas of the mesh in a typical reinforced concrete structure under controlled, simulated laboratory conditions and to clarify the mechanisms involved therein. The overall research program is summarized in Fig. 4.

3. Results and discussions 3.1. DC corrosion measurements The corrosion current (Icorr), corrosion half-cell potential (Ecorr) and concrete resistivity values were measured for the RC Panel 1 (Fig. 2a) specimen as shown in Fig. 5a–f. The experiment results of Panel 1 (Fig. 5) show that the corrosion measurements taken at the intersections of the steel bars (‘ab’ and ‘cd’) were much higher in magnitude than at the areas between them (a, b, c and d). These results confirm that the observation made in the real structure was a true phenomenon and must be explored in detail in terms of the rebar spacing, steel intersection, connection type, binding wire material, etc. The remaining RC Panels 2–5 (Fig. 2b and c) pertain to these parameters and the bare bar experiment specimens (Fig. 2f), which were prepared to observe day to day corrosion current, potential and resistivity variations, and are discussed in this paper below (Figs. 6–9). It was observed from the experimental results (Fig. 5) that the corrosion potential and the corrosion current for both the upper and lower steel bars were mostly higher at the intersection point compared to the areas between them. In the case of corrosion current measurements (Fig. 5a and b), the reading was sometimes four times greater at the intersection point compared to the magnitude of the corrosion current midway between the intersection and end of the rebars. This increase in corrosion current was found to be similar in both the upper and lower bars. It can be inferred that the corrosion rate at the intersections is higher, which may require modification of the rebar placement methodology in construction practices. Fig. 5c and d shows the experimental measurement of the corrosion half-cell potential. This measurement was found to be less consistent compared to the corrosion current measurements (Fig. 5a and b); however, the behaviour and increased corrosion at the intersection were found to have a trend similar to that of the corrosion current measurements for most of the cases. The variations observed with time can be attributed to the variation in moisture and surface contact, the logarithmic relation between corrosion current and potential, the corrosion resistivity, etc., which was also supported by the experimental measurements of concrete resistivity recorded for this study (Fig. 5e and f). The reason for the increase in the corrosion observed at the steel rebar mesh intersection points embedded in chloride contaminated concrete may be due to several reasons that require further research. One likely reason may be that the flow of corrosion current in two perpendicularly intersecting directions causes the concentration of current to focus on the rebar intersection area due to the direct electrical connectivity. Other possible factors leading to the increased corrosion rate at the steel rebar intersection could the overlapping corrosion currents and the corrosion potential contours from the two corroding rebars, leading to a concentration of the corrosion reaction in this area. Yet another possibility could be the accumulation of more

(a) GECOR Corrosion Meter [25]

(b) Standard ASTM Half-Cell Potential Measurement Fig. 3. Corrosion measurement technique.

moisture and/or mortar around the intersection area due to the congestion of steel rebars due to the two perpendicular sides adversely affecting the quality of the concrete matrix and the chloride content surrounding the rebar intersection. This issue can also eventually result in an increased corrosion rate at the intersection of reinforcement bars compared to the other areas of the steel reinforcement mesh embedded in reinforced concrete. The other Cases (2–5) show similar results with some limitations and implications for future research. The increased number of steel bars in Panel 2 exhibits relatively less of a difference in the corrosion magnitudes between the intersections and the rest of the lengths of bars; yet, the general behaviour still shows the tendency toward a higher corrosion potential at the intersections. The reason for the relatively lesser difference of the magnitude between the intersections and other areas is thought to lie in the overlapping effect of the corrosion potential and current contours

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Fig. 4. Flow chart of this research program.

due to the closer points of measurements compared to Case 1, with just a single intersection in the middle of the panel. The introduction of an insulating material (i.e., a plastic separator) between the two bars (upper and lower) at the point of intersection with a nylon binding wire (Case 3) has a significant effect on the corrosion rate. It separates the overlapping area, making the intersection electrically disconnected so that the flow of corrosion current is not superimposed. The measured corrosion current and potential at the intersection as well as other areas of steel bars were not very different in contrast to the high corrosion observed at the intersection point of rebars in Case 1 with electrical conductivity connection, which verifies the explanation given above (Fig. 6). Thus, the present steel rebar placement, fixing, detailing and bending schemes may need to be revised in such a way that there is no electrical contact between the intersecting rebars, thereby ensuring better corrosion protection. Panel 4 with copper binding wire still showed a high corrosion rate at the intersection points, which is similar to what was observed for Panel 1. The copper binding wire is highly resistant to corrosion but failed to prevent the high corrosion at the intersection points, and these results were similar to those of steel binding wire, as shown for Panel 1. Thus, the binding wire material has a significant effect on the high corrosion rate observed at the steel rebar intersection points with the condition that a direct electrical disconnection is present. Other factors causing this phenomenon could be the galvanic corrosion developed between two dissimilar metals, such as copper and steel. In Panel 5 with steel binding wire, a dual phenomenon was observed. The right half of the panel containing two intersections showed a higher corrosion magnitude in comparison to other points in the steel rebar mesh, while the left half of the Panel 5 containing the other two intersections showed a lower corrosion current and potential in comparison to other points in the steel rebar mesh. This difference could be due to the formation of a macro-cell within the panel, causing each half of the panel to behave opposite from the other. Further investigation is needed to clarify the observed phenomenon, and this topic is an ideal aim for future

research. Overall, the corrosion current measurements make more sense than the potential and resistivity values because the corrosion potential measurement is a relatively approximate measurement compared to the measurement of corrosion current density, which is a much more reliable and accurate method of corrosion rate assessment. The same was also observed and reported in this paper. In addition to the concrete panels, an effort was also made to investigate the bare steel bars sprinkled with 5% chloride solution, as shown in Fig. 2f, which was performed to visually observe the difference in the corrosion rate at the steel intersection points and other areas because the bars embedded in concrete Panels 1–5 could not be observed by the naked eye. However, the rebar surface corroded uniformly throughout the length of the bars, and the difference in the corrosion rates could not be observed. It was concluded that the high corrosion rate at the steel intersection points could only be investigated by embedding rebars in concrete; therefore, the bare bar investigation was discontinued. 3.2. SEM imaging analysis Panel 1 was broken, and the corroded steel bars were carefully removed from the concrete panel and their physical appearance was observed. Fig. 9 shows the condition of the steel rebars removed from Panel 1. The steel bars did corrode more around the intersection point compared to the other areas of the steel bars. Especially in case of the horizontal steel bar, the severely corroded brown area can be clearly observed to be concentrated at the intersection. However, in the case of vertical bars, the more severely corroded brown area starts from the center and extends up to the middle. For a more in-depth, micro-scale investigation of the reasons for this observation, eight representative concrete samples were taken from areas in contact with the rebar at the intersection and at the midpoints of rebar intersections and at the extreme ends, as shown in Fig. 10. The concrete specimens taken from Panel 1 were examined with SEM, and the results are discussed in the following sections.

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36 days 40 days

U

43 days 50 days 57 days

L

L

64 days 71 days 85 days

U

96 days

Panel 1 with copper binding wire

103 days

(c) Panel 1, lower reinforcement bar (1-L)

(a) Panel 1, lower reinforcement bar (1-L)

(b) Panel 1, upper reinforcement bar (1-U)

(d) Panel 1, upper reinforcement bar (U)

(e) Panel 1, lower reinforcement bar (1-L)

(f) Panel 1, upper reinforcement bar (1-U) Fig. 5. Half-cell potential, corrosion current and concrete resistivity measurements of Panel 1.

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v w x

Panel 2 Copper wires at the Intersection (c) Panel 2, lower reinforcement bar (2-L)

(a) Panel 2, lower reinforcement bar (2-L)

(b) Panel 2, upper reinforcement bar (2-U)

(d) Panel 2, upper reinforcement bar (2-U)

(e) Panel 2, lower reinforcement bar (2-L)

(f) Panel 2, upper reinforcement bar (2-U) Fig. 6. Half-cell potential, corrosion current and concrete resistivity measurements of Panel 1.

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Panel 3 Plastic insulating separator with nylon wires at Intersections

(c) Panel 3, lower reinforcement bar (3-L)

(a) Panel 3, lower reinforcement bar (3-L)

(d) Panel 3, upper reinforcement bar (3-U)

(b) Panel 3, upper reinforcement bar (3-U)

(e) Panel 3, lower reinforcement bar (3-L)

(f) Panel 3, upper reinforcement bar (3-U) Fig. 7. Half-cell potential, corrosion current and concrete resistivity measurements of Panel 1.

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Panel 4 Copper wires at the Intersection

(c) Panel 4, lower reinforcement bar (4-L)

(a) Panel 4, lower reinforcement bar (4-L)

(d) Panel 4, upper reinforcement bar (4-U)

(b) Panel 4, upper reinforcement bar (4-U)

(e) Panel 4, lower reinforcement bar (4-L)

(f) Panel 4, upper reinforcement bar (4-U) Fig. 8. Half-cell potential, corrosion current and concrete resistivity measurements of Panel 1.

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Panel 5 Steel wires at the Intersection

(c) Panel 5, lower reinforcement bar (5-L)

(a) Panel 5, lower reinforcement bar (5-L)

(b) Panel 5, upper reinforcement bar (5-U)

(f) Panel 5, upper reinforcement bar (U)

(d) Panel 5, upper reinforcement bar (5-U)

(e) Panel 5, lower reinforcement bar (5-L)

Steel rebars removed from Panel1

Fig. 9. Steel rebars removed from Panel 1.

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Fig. 10. Panel 1: Numbering showing the respective samples.

Fig. 11. (a) Concrete specimen away from Intersection at 1 mm and (b) concrete specimen near Intersection at 1 mm.

Fig. 12. (a) Concrete specimen away from Intersection at 10 lm and (b) concrete specimen near Intersection at 10 lm.

Fig. 12 (continued)

Fig. 11 (continued)

The multi-scale SEM image analysis (Figs. 11–18) shows that specimens taken at different places of the panel exhibit an evident difference in their constituents and the structure of concrete matrix close to the reinforcing bars. The concrete closer to an intersection (points 5, 6, 7 and 8) tends to be much more porous than the concrete near the other parts of bar (points 1, 2, 3 and 4) and the concrete far from the rebars. The flakiness of the concrete near the intersections is due to increased corrosion products and decreased silica gel constituents. Easy spalling of the concrete from the rebars, especially close the intersection point, is also proof of

Fig. 13a. Concrete specimen away from Intersection at 1 mm.

the more deteriorated and weakened concrete and the increased amount of corrosion. The disturbed silica gel structure caused by excessive corrosion on joints is caused by the damage to the integrity of the concrete. Flaky structure(s) and more closely bound particles exhibiting better density and more durability are farther away from intersections. Closer to the bar and gradually approaching the point of

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Fig. 13b. Concrete specimen near Intersection at 1 mm.

Fig. 14a. Concrete specimen away from Intersection at 10 lm.

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Fig. 15a. Concrete specimen away from Intersection at 1 mm.

Fig. 15b. Concrete specimen near from Intersection at 1 mm.

Fig. 16a. Concrete specimen away from Intersection at 10 lm.

Fig. 14b. Concrete specimen near from Intersection at 10 lm.

overlapping the upper and lower reinforcements leads to the deteriorated and porous concrete, which is shown by SEM images (Figs. 11–18) at different scales. 3.3. Mercury intrusion porosimetry (MIP) Pores, or voids, are unavoidably produced in concrete during hydration/curing. Their introduction must be limited due to their

detrimental effects on the mechanical properties and fluid uptake by the cured concrete. MIP was used in this research to determine the void content in concrete at the ITZ of steel and the concrete interface and to analyze the interfacial transition zone of concrete responsible for the development of a good or bad quality passive layer and its successive breakdown at the intersection of steel bars, as well as at (see Table 2) the midpoints. In the MIP method, mercury is forced to penetrate into porous concrete samples under stringently controlled pressure. MIP can be used for accurate

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Fig. 16b. Concrete specimen near Intersection at 1 mm.

Fig. 18a. Concrete specimen near Intersection at 10 lm.

Fig. 17a. Concrete specimen away from Intersection at 1 mm.

Fig. 18b. Concrete specimen away from Intersection at 10 lm.

Table 2 MIP results for concrete specimens at the intersection and mid points. Specimen ID

Location

Total intrusion volume (ml/g)

1 2 3 4

Mid Mid Mid Mid

0.082 0.061 0.057 0.076

Average value 5 6 7 8

Four Mid points Intersection Intersection Intersection Intersection

0.069 0.088 0.094 0.063 0.079

Average value

Four intersection points

0.081

point point point point

Fig. 17b. Concrete specimen near from Intersection at 1 mm.

measurement of critical parameters, such as the total intrusion volume, pore area and diameter, density, and porosity of concrete. Small sample sizes are required, typically 1–5 g, and MIP also has a rapid turnaround time. Despite of MIP’s various advantages, it is not easy to obtain reproducible results for pore size distribution of complex systems such as concrete [28]. Therefore, averaged results of intrusion volume are presented in this paper. The intruded volume in steel–concrete ITZ at the intersection points (5–8) was 0.081 ml/g on the average and at the less corrosion mid points of steel bars (1–4)

the averaged intruded volume was 0.069 ml/g. This means that porosity near the severely corroded intersection areas was more than the less corroded mid steel bar areas. The higher porosity may have contributed to the observed higher corrosion rate at the steel rebar intersection points (5–8) as compared to other areas (1–4) in reinforced concrete specimens under experimentation. 4. Conclusion Field observations of the high corrosion rate at the intersection points of steel rebar mesh in the foundations of an actual RC struc-

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ture were confirmed by experimental measurements in the laboratory. Several parameters that could influence the corrosion rate at the steel rebar intersections were investigated in this paper, including different types of connection joints, their spacing and various binding wire material types. Similar to the field observations, high corrosion rates were observed at some of the intersection points. However, this phenomenon varied from one case to another. To understand the phenomenon at the microstructural level of the steel–concrete interfacial transition zone, SEM imaging and MIP analysis were employed. The high corrosion rate observed at the steel intersection points appeared to be due to the coupled effects of the corrosive binding wire material, electrical connectivity, reduced c/c steel bar spacing and poor concrete microstructure at the rebar intersections. Further investigation is required for a better understanding of the effect of these factors individually, and this topic remains as a potential area of future research. Acknowledgments This paper is a part of a research project supported through the NPST program by King Saud University, Project No. 09-NAN 67402. The help of engineers and technicians at the Center of Excellence for Concrete Research & Testing (CoE-CRT) at King Saud University is highly appreciated. References [1] ACI Committee 201. Durability of concrete. ACI manual of concrete practice, Part I. USA: American Concrete Institute; 1992. [2] Hussain Raja Rizwan, Tetsuya Ishida. Enhanced electro-chemical corrosion model for reinforced concrete under severe coupled actions of chloride and temperature. Constr Build Mater 2011;25(3):1305–15. [3] Abdulrahman Alhozaimy, Raja Rizwan Hussain, Rajeh Al-Zaid, Abdulaziz AlNegheimish. Coupled effect of ambient high relative humidity and varying temperature marine environment on corrosion of reinforced concrete. Constr Build Mater 2011;28(1):670–9. [4] Hussain SE, Rasheeduzzaffar, et al. Factors affecting threshold chloride for reinforcement corrosion in concrete. Cem Concr Res 2006;25:1543–55. [5] Al-amoudi et al. Effectiveness of corrosion inhibitors in contaminated concrete. Cem Concr Compos 2004;25:439–49. [6] Saricimen H et al. Effectiveness of inhibitors in retarding rebar corrosion. Cem Concr Compos 2005;24:89–100. [7] Rasheeduzafar et al. Corrosion of reinforcement in concrete structures in the middle east. ACI Concr Int 1985;6(9):48–55.

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