The relative performances of mechanical, permeability, and corrosion resistance properties of different concrete types were compared. Concrete types were made from ordinary Portland cement (OPC), Portland pozzolana cement (PPC), and Portland slag cement (PSC). Compressive strength test, effective porosity test, coefficient of water absorption, short-term accelerated impressed voltage test, and rapid chloride permeability test (RCPT) were conducted on M30 and M40 grades of concrete designed with OPC, PPC, and PSC cements for 28- and 90-day cured concrete types. Long-term studies such as microcell and electrochemical evaluation were carried out to understand the corrosion behaviour of rebar embedded in different concrete types. Better corrosion resistant properties were observed for PSC concrete by showing a minimum current flow, lowest free chloride contents, and lesser porosity. Besides, PSC concrete has shown less coefficient of water absorption, chloride diffusion coefficient (CDC), and lower corrosion rate and thereby the time taken for initiation of crack extended.
Reinforced concrete is protected from corrosion due to the high alkalinity existing in the concrete environment and the cover concrete which acts as a protective barrier to the access of chloride ions. Apart from this, reinforced concrete structures when exposed to severe marine environments lead to the premature failure of the structures [
Curing plays a major role in affecting the strength and permeability characteristics of the concrete structures. Mainly while using supplementary cementitious materials, care should be taken for continuous curing of the structures in the initial stages since hydration reactions are slow in using pozzolanic materials. The other factors which influence the strength characteristics are fineness modulus, water-cement ratio (w/c), chemical composition, and so on [
Hence, the aim of the present investigation is to evaluate the corrosion resistant properties of rich and lean mix concrete made from Portland pozzolana cement (PPC) and Portland slag cement (PSC) under accelerated and normal exposure conditions. The results were compared with ordinary Portland cement (OPC) concrete. Further, in this investigation, two curing periods, two grades of concrete, and two different exposure conditions, namely, normal and accelerated exposure conditions, were carried out. In addition to these, short-term and long-term tests, porosity and permeability studies, and mechanical and electrochemical studies were also carried out for plain and blended cements, and the results were presented in a detailed manner.
Two mix proportions having characteristic compressive strength of 30 and 40 MPa concrete types were used for casting the concrete specimens. The details of concrete mix proportions are given in Table
Details of concrete mix proportions.
Grade | Type of cement | W/C ratio | Cement |
Fine aggregate |
Coarse aggregate (kg m−3) | Water |
---|---|---|---|---|---|---|
M30 | OPC | 0.55 | 338 | 607 | 1247 | 186 |
PPC | 0.55 | 338 | 607 | 1247 | 186 | |
PSC | 0.55 | 338 | 607 | 1247 | 186 | |
|
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M40 | OPC | 0.45 | 400 | 472 | 1247 | 180 |
PPC | 0.45 | 400 | 472 | 1247 | 180 | |
PSC | 0.45 | 400 | 472 | 1247 | 180 |
Chemical composition of cements.
Constituent | Weight/(%) | ||
---|---|---|---|
OPC | PPC | PSC | |
SiO2 | 21.0 | 32.0 | 31.0 |
Al2O3 | 5.50 | 10.0 | 10.5 |
Fe2O3 | 4.60 | 5.50 | 2.80 |
CaO | 63.0 | 44.0 | 47.0 |
MgO | 0.60 | 1.50 | 3.90 |
SO3 | 2.60 | 2.60 | 2.60 |
LOI | 2.70 | 4.40 | 2.70 |
Compressive strength test was carried out in concrete cubes of size
Percentage of water absorption is a measure of the pore volume or porosity in hardened concrete, which is occupied by water in saturated condition. Water absorption test was carried out as per ASTM C642-97 [
Coefficient of water absorption is suggested as a measure of permeability of water. This is measured by the rate of uptake of water by dry concrete over a period of 1 h. The concrete cube specimens of
Schematic of coefficient of water absorption test.
Cylindrical concrete specimens of size 50 mm diameter and 100 mm height were cast using M30 and M40 grades of concrete types. Rebar types of 12 mm diameter and 100 mm height were embedded centrally into the concrete specimens. During casting, the moulds were mechanically vibrated. After 24 h, the cylindrical concrete specimens were demoulded and cured for 28 and 90 days. Then the specimens were subjected to impressed voltage test. In this technique, the concrete specimens were immersed in 5% NaCl solution and embedded steel in concrete is made as anode with respect to an external stainless steel electrode serving as the cathode by applying a constant positive potential of 12 V to the system from a DC source. The variation of current is recorded with time. For each specimen, the time taken for initial crack and the corresponding maximum anodic current flow was recorded [
At the end of the 12 V impressed voltage test, the embedded steel specimens were broken and the sections were open, cleaned in pickling solution (concentrated HCl containing 20 g/L of stannous chloride and 15 g/L of antimony trioxide), then cleaned in distilled water, dried, and weighed as per ASTM G1-03 [
The resistance to chloride ion penetration in terms of total charge that passed in coulombs through OPC and blended concrete specimens after 28 and 90 days of moisture curing was measured as per ASTM C1202-12 [
The amount of chloride ion migrating through the concrete specimens after 28 and 90 days of curing was monitored by removing small amount of solution and determining the chloride concentration of these samples after 120 h. Chloride diffusion coefficients were calculated using Nernst–Einstein equation [
Concrete cubes of size
Schematic representation of concrete cube specimen used for potentiodynamic polarization and AC-impedance study.
The same cube specimens mentioned above were subjected to AC-impedance measurements. The real part (
A rectangular concrete (M40 grade) specimen of size 279 mm × 152 mm × 114 mm was designed as per ASTM G 109-07 [
Schematic view of macrocell specimen.
The specimens were mechanically vibrated. After 24 hours of setting, the specimens were demoulded and cured in distilled water for 90 days. Then the specimens were subjected to alternate wetting and drying cycles. One cycle consists of 15 days of wetting in 3% NaCl and 15 days of drying in open atmosphere. Measurements were carried out during the wetting cycle (15th day) as macrocell current showed maximum magnitude due to the low resistivity of the concrete. All the concrete specimens were subjected to 24 complete cycles of exposure period.
The compressive strength of OPC, PPC, and PSC concrete types after 28 and 90 days of curing for M30 and M40 grades of concrete are given in Figures
Comparison of compressive strength of M30 grade OPC, PPC, and PSC concrete types.
Comparison of compressive strength of M40 grade OPC, PPC, and PSC concrete types.
It was found from Figure
On the other hand, in M40 grade concrete (Figure
The magnitude of increase in compressive strength from 28 days to 90 days was found to be similar in M30 grade and M40 grade concrete types. Comparable compressive strength values were obtained for PPC and PSC concrete types when compared with OPC concrete.
Effective porosity and coefficient of water absorption values calculated for OPC, PPC, and PSC concrete types at 28- and 90-day cured specimens were reported in Table
Permeability parameters for OPC, PPC, and PSC M30 and M40 grade concrete types.
Grade | Type of cement | Effective porosity/(%) | Coefficient of water absorption/(m2s−1) × 10−7 | ||
---|---|---|---|---|---|
28 days | 90 days | 28 days | 90 days | ||
M30 | OPC | 15.15 | 10.72 | 3.20 | 2.30 |
PPC | 14.28 | 9.62 | 2.50 | 2.00 | |
PSC | 12.61 | 9.57 | 0.71 | 0.40 | |
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M40 | OPC | 13.28 | 9.43 | 3.00 | 1.90 |
PPC | 12.42 | 6.79 | 2.90 | 1.10 | |
PSC | 10.61 | 6.32 | 0.62 | 0.15 |
Among the three types of concrete used, PSC concrete showed least pores and lower coefficient of water absorption. The same trend is observed in both 28- and 90-day cured M30 and M40 grade concrete types. For example, in the case of PSC, 0.71 × 10−7 m2s−1 and 0.62 × 10−7 m2s−1 are the coefficient of water absorption values obtained for 28-day cured M30 and M40 grade concrete, respectively. Similarly, for M30 and M40 grade concrete types the coefficient of water absorption values obtained were 0.40 × 10−7 m2s−1 and 0.15 × 10−7 m2s−1 for 90-day cured concrete, respectively. The observed values of coefficient of water absorption was attributed to the impermeability nature of the pore structure refinement in PPC and PSC concrete types. The improvement in the permeability characteristics of PPC and PSC is due to the secondary hydration reaction.
In general, the primary hydration reaction in concrete is as follows:
In PPC and PSC concrete types, the Ca(OH)2 content reduction is due to the secondary hydration reaction. During the hydration in PPC, lime is consumed but in OPC lime is produced. This is the main advantage of using blended cements to decrease the permeability of the concrete thereby increasing the corrosion resistance properties [
The impressed voltage parameters of M30 and M40 grades of OPC, PPC, and PSC concrete types under 28 and 90 days of curing time were given in Table
Impressed voltage parameters for rebar in OPC, PPC, and PSC M30 and M40 grade concrete types.
Grade and type of cement | Maximum anodic current |
(%) |
Time to cracking |
Free chloride contents |
Weight loss |
Reduction in corrosion rate (%) |
---|---|---|---|---|---|---|
28-day cured concrete specimens | ||||||
OPC M30 | 77 | — | 07 | 4680 | 44.3210 | — |
PPC M30 | 65 | 15.6 | 08 | 3560 | 25.3120 | 42.9 |
PSC M30 | 43 | 44.2 | 10 | 3253 | 7.9140 | 82.1 |
OPC M40 | 72 | — | 10 | 3789 | 35.3020 | — |
PPC M40 | 56 | 22.2 | 12 | 3420 | 14.1410 | 59.7 |
PSC M40 | 39 | 45.8 | 14 | 3200 | 2.4650 | 93.0 |
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90-day cured concrete specimens | ||||||
OPC M30 | 74 | — | 08 | 4200 | 33.5780 | — |
PPC M30 | 58 | 21.62 | 09 | 3165 | 12.2290 | 63.6 |
PSC M30 | 43 | 41.89 | 11 | 2880 | 1.7030 | 94.9 |
OPC M40 | 60 | — | 14 | 3040 | 23.4530 | — |
PPC M40 | 43 | 28.30 | 18 | 2840 | 11.9420 | 49.0 |
PSC M40 | 26 | 56.66 | 20 | 2640 | 1.1470 | 95.1 |
The time taken for initial crack measured for 28- and 90-day cured OPC, PPC, and PSC concrete types was given in Table
Among the three concrete types, PSC concrete in M30 and M40 grades, 28- as well as 90-day cured specimens showed the longest time taken for initial crack. For example, for 28-day cured M30 and M40 concrete, time taken for initial crack observed was at 10 and 14 days, respectively. Similar trend was observed for 90-day cured concrete.
The better performance of PSC is due to the following reason. Aluminate phase is the most responsible one for binding the chlorides into complex forms. Fixing of chloride in concrete by the formation of complex aluminate phases has been reported by Karthick et al. [
The free chloride contents showed the lower values for PSC concrete. This can be attributed to the formation of free chlorides into complex form. The mechanism of the formation of complex, namely, Friedel salt, is by the precipitation method.
The corrosion rates of embedded steel in OPC, PPC, and PSC were given in Figures
Comparison of corrosion rate of rebar in OPC, PPC, and PSC concrete types obtained by impressed voltage test (28-day cured specimens).
Comparison of corrosion rate of rebar in OPC, PPC, and PSC concrete types obtained by impressed voltage test (90-day cured specimens).
The corrosion rates of steel calculated for 28 days cured M30 OPC, PPC, and PSC concrete types were found to be 0.0150, 0.0075, and 0.0060 mmpy, respectively. These results showed that blended cements, namely, PPC and PSC, considerably decreased the corrosion rate of embedded steel by 2 times and 2.5 times. Similar trend was also observed in M40 grade concrete. PPC and PSC concrete types showed 1.7 times and 3 times reduction in corrosion rate when compared to OPC concrete.
The corrosion rate values calculated for 90 days cured M30 OPC, PPC, and PSC concrete types were found to be 0.0110, 0.0060, and 0.0035 mmpy, respectively. These results showed that blended cements, namely, PPC and PSC, considerably decreased the corrosion rate of embedded steel by 2 and 3 times, respectively. A similar trend was also observed in M40 grade concrete. PPC and PSC concrete types showed 2.2 and 3.3 times reduction in corrosion rate when compared to OPC concrete.
For reinforcement corrosion to occur, a critical quantity of chloride is required at the steel/concrete interface. In OPC, the critical chloride is reached in a very short time, whereas it took more time for PPC and PSC concrete types as evidenced by the time taken for initial crack. The considerable reduction in corrosion rate of the blended cements was attributed to the secondary pozzolanic reaction in blended cements which favoured more C-S-H gel formation and interconnected the micropores present in the blended cements. Moreover, the PSC showed the least corrosion rate values because, in addition to the secondary pozzolanic reaction, the higher Al2O3 content adsorbed more free chloride and formed chloroaluminate complex [
Better corrosion resistant properties were observed for PSC concrete that exhibited the minimum current flow, lowest free chloride contents, and lower corrosion rate.
The amount of charge that passed (coulombs) through OPC, PPC, and PSC concrete types cured after 28 and 90 days was reported in Table
Rapid chloride permeability test (RCPT) parameters for OPC, PPC, and PSC M30 and M40 grade concrete types.
Grade | 28 days | 90 days | ||||
---|---|---|---|---|---|---|
Charge passed (coulomb) | Free chloride contents |
Chloride diffusion coefficient (cm2s−1) × 10−10 | Charge passed (coulombs) | Free chloride contents |
Chloride diffusion coefficient (cm2s−1) × 10−10 | |
OPC M30 | 669 | 749 | 2.4 | 630 | 579 | 1.9 |
PPC M30 | 281 | 681 | 1.8 | 220 | 528 | 1.5 |
PSC M30 | 226 | 481 | 1.0 | 200 | 380 | 1.4 |
OPC M40 | 626 | 540 | 1.8 | 600 | 520 | 1.4 |
PPC M40 | 261 | 532 | 1.5 | 180 | 483 | 0.8 |
PSC M40 | 204 | 596 | 0.5 | 200 | 393 | 0.6 |
The chloride diffusion coefficient (CDC) calculated for 28- and 90-day cured OPC, PPC, and PSC concrete types was reported in Table
In PSC M30 and M40 grade 28- as well as 90-day cured concrete showed the least chloride diffusion coefficient (CDC) values. For example, in M30 grade 28-day cured concrete the CDC values are found to be
Potentiodynamic polarization curves obtained for the corrosion of rebar in various types of cement concrete were shown in Figure
Potentiodynamic polarization parameters for the corrosion of rebar embedded in OPC, PPC, and PSC M40 grade concrete (90-day cured specimens).
System | Various grades of concrete | |||||
---|---|---|---|---|---|---|
M30 | M40 | |||||
|
|
Corrosion rate (mmpy) × 10−3 |
|
|
Corrosion rate (mmpy) × 10−4 | |
OPC | −397 | 6.9922 | 8.0082 | −301 | 2.1496 | 2.491 |
PPC | −303 | 1.6902 | 1.9589 | −263 | 1.6902 | 1.9589 |
PSC | −299 | 1.4448 | 1.674 | −258 | 1.4448 | 1.674 |
Potentiodynamic polarization curves for TMT rebar embedded in OPC, PPC, and PSC M30 (a) and M40 (b) grade concrete types (90-day cured specimens).
The corrosion potential of rebar in M30 concrete types from OPC, PPC, and PSC are −397, −303, and −299 mV, respectively. PSC concrete improves the corrosion resistance of rebar when compared to OPC concrete. There is a difference of −97 mV which moves in the positive direction for PSC cement concrete. The corrosion rates of rebar types in OPC, PPC, and PSC are
In M40 grade concrete, the measured corrosion potential was −301, −263, and −258 mV versus SCE for OPC, PPC, and PSC, respectively. The variation in potential for M40 PPC and PSC concrete types when compared to OPC was found to be less than M30 grade concrete. The corrosion current density for M40 concrete follows the same order as M30 concrete:
The PSC and PPC concrete types exhibit lower corrosion current density and lowest corrosion rate when compared to their OPC concrete counterpart.
Impedance diagrams obtained for the frequency range 0.01 Hz to 30 kHz at the OCP for the corrosion of rebar in different concrete types were shown in Figure
AC-impedance parameters for the corrosion of rebar embedded in OPC, PPC, and PSC M40 grade concrete (90-day cured specimens).
System | Various grades of concrete | |||||
---|---|---|---|---|---|---|
M30 | M40 | |||||
|
|
Corrosion rate (mmpy) × 10−3 |
|
|
Corrosion rate (mmpy) × 10−4 | |
OPC | 3.464 | 7.531 | 8.728 | 1.399 | 1.865 | 2.161 |
PPC | 3.885 | 6.715 | 7.782 | 1.594 | 1.637 | 1.897 |
PSC | 4.399 | 5.930 | 6.873 | 1.714 | 1.522 | 1.764 |
Nyquist plots for TMT rebar embedded in OPC, PPC, and PSC M30 (a) and M40 (b) grade concrete types (90-day cured specimens).
From the table, it is found that
Nyquist plots clearly indicated that rebar types embedded in PSC and PPC concrete performed better than OPC. The corrosion rates of rebar in various concrete types at the end of 24 months’ exposure in M30 and M40 grade concrete types are as follows:
From the results it is observed that PSC concrete showed the higher charge transfer resistance and lowest corrosion rate when compared to PPC and OPC concrete types.
The half-cell potential of steel measured periodically against SCE with time for OPC, PPC, and PSC for M30 and M40 grade 90-day cured concrete types was shown in Figures
Potential versus number of cycles of exposure for rebar in OPC, PPC, and PSC M30 grade concrete types (90-day cured specimens) under macrocell condition.
Potential versus number of cycles of exposure for rebar in OPC, PPC, and PSC M40 grade concrete types (90-day cured specimens) under macrocell condition.
The macrocell current measured periodically with time for M30 and M40 grade OPC, PPC, and PSC concrete types is shown in Figures
Macrocell current versus number of cycles of exposure for rebar in OPC, PPC, and PSC M30 grade concrete types (90-day cured specimens) under macrocell condition.
Macrocell current versus number of cycles of exposure for rebar in OPC, PPC and PSC M40 grade concrete types (90 days cured specimens) under macrocell condition.
From Figure
The total integrated current calculated (ASTM G109) with time for M30 grade OPC, PPC, and PSC concrete types is illustrated in Figure
Total integrated current versus number of cycles of exposure for rebar in OPC, PPC, and PSC M30 grade concrete types (90-day cured specimens) under macrocell condition.
Total integrated current versus number of cycles of exposure for rebar in OPC, PPC, and PSC M40 grade concrete types (90-day cured specimens) under macrocell condition.
In the case of OPC system, the availability of chloride ions that occupied the defect position and reacted with ferrous ions initiated the corrosion process. But in the case of PPC and PSC concrete types, the availability of free chloride is significantly reduced due to the formation of chloroaluminate complexes [
The following conclusions were drawn from the above investigation: Mechanical properties are not altered by using blended cements, but corrosion resistant properties are greatly improved. The effective porosity and coefficient of water absorption values showed the decreasing order as follows: OPC > PPC > PSC. Among the three types of concrete used, PSC concrete showed the least pores and lower coefficient of water absorption. Better corrosion resistant properties were observed for PSC concrete types by showing a minimum current flow, lowest free chloride contents, and lesser corrosion rate, and thereby the time taken for initiation of crack period was extended in accelerated impressed voltage technique. In the case of M30 grade concrete cured for 28 days, the corrosion rate was considerably decreased by 2 and 2.5 times for PPC and PSC, respectively. In the case of M40 grade concrete cured for 28 days, PPC and PSC concrete types showed 1.7 and 3 times reduction in corrosion rate when compared to the OPC concrete. RCPT indicated that both 28- and 90-day cured M30 and M40 grade concrete types showed the decrease in CDC values as follows: OPC > PPC > PSC. Macrocell corrosion studies revealed that PPC and PSC concrete types are graded as very low chloride permeability concrete in both M30 and M40 grade concrete types. Electrochemical studies also proved beyond doubt that PSC concrete performed better than PPC and OPC concrete types.
The authors declare that there are no conflicts of interest regarding the publication of this paper.
This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (no. 2015R1A5A1037548).