Identification of various chemical phenomena in concrete using thermal analysis

Chemic~,1 changcs th,lt occur in concrctc at the microstrtrcture level due to environmental effects al-e normally identilied bv Sb'vl, XRD, thcnn;i1 alwly,;is and by chemical analysis_ In the [1rescnt investigation, the cbemical compounds fonned in various ehcmic<ll reactions ;:Jre quantified using thermal analysis_ The concrctc having characteristic compressivc strcngth or 20 Mr,l w;-rs subjeetcd to uncler-go various chemical reactions for;:J specified period. Three types of cements were uscd I(lr casting the conercte specimens Thc thermal an<llyses (DTA and TG) werc carricd out on [1owclercd concrete S<lInplcs p",:sing throu~h 75 pm sic\e dr-C\wn I.-r-oln the concrete specimens which wcrc subJccted to various ehcmieal phcnomenon. Chcmical compounds such ;:JS Ca(OIIh, gypsum, cllringite, calcium chloroaluminatc are estimatcd quantitatively. The type of Ily ;:Jsh blcnded with cement and complex aminc salt in Migr<lting Corrosion Inhibitor-(MCI) treated coneretc arc ahJ idcntilicd. Each chemical cOI11[1ound is identified by a distinct cndotherm present in the DTA (DiITerenti;d Tlrcl"llwl Analysis) plot ~lIlJ quantitative estirmtion has been Jone using the TG (Thermo-gravimetric) curve

Prediction of service life of concrete structures using corrosion rate model

The uncertainties that exist in the chloride diffusion model may lead to considerable error in the prediction of the time to initiation of corrosion in the service life estimation of concrete structures. An alternative approach based on polarisation resistance (Rp value) has been proposed. Enhancement of the service life of concrete made using blended cements was measured experimentally using the electrochemical impedance technique (EIS) in the presence of 0, 0·5 and 1% of added chloride. The time to initiation of corrosion (Ti) predicted from the Rp value agreed well with the value from the weight loss measurement, whereas it was overestimated by the chloride diffusion model. Using the Maaddawy mathematical model, the time to propagation (Tp) was obtained. From the results it was concluded that in the presence of 1% of added chloride, the time to failure Tf = (Ti + Tp) of Portland pozzolana cement (PPC) and Portland slag cement (PSC) concrete was four and ten times higher than that of ordinary Portland cement concrete, respectively, even in low-strength concrete of 20 MPa. Parameter Tf increased as the strength of the concrete increased. The Tf value predicted by the corrosion rate model agreed very well with the Tf from the weight loss measurement and cracks on the concrete surface. The reduced rate of diffusion of chloride due to pore constriction and the higher chloride binding ability of blended cements lead to the enhancement of the service life of concretes when exposed to marine atmospheric conditions. The estimation revealed that a rebar embedded at 25 mm cover in 40 MPa PPC and PSC concrete (0% Cl− added) could have a service life of more than 100 years. </jats:p

Analysis of the electrochemical phenomenon at the rebar–concrete interface using the electrochemical impedance spectroscopic technique

The corrosion rate of rebar during the various stages where it occurs, such as passivation, initiation of corrosion and severe corrosion, needs to be determined non-destructively for the maintenance, restoration and replacement of concrete structures. The double layer capacitance (Cdl) and the charge transfer resistance or polarisation resistance (Rp) of the corrosion processes have been associated with the slope of the low-frequency arc in the Nyquist plot, and this can be related to the electrochemical phenomenon that occurs at the steel–concrete interface. The present studies, based on electrochemical impedance spectroscopy (EIS) conducted on three different densities of concrete with addition of 0.5 and 1% chloride over a period of 1765 days, reveal that the capacitive behaviour of a lowfrequency arc with a slope more than �1 indicates the passive condition of rebar. Warburg diffusion behaviour with a slope exactly equal to �1 denotes the initiation of corrosion on the rebar. A slope of less than �1 is obtained when corrosion spreads uniformly on the rebar. Other electrochemical parameters such as Rp, Cdl and phase angle are correlated with the phenomenon occurring at the steel–concrete interface. An Rp value greater than 250 k� cm2 indicates the passive condition of rebar, whereas values of less than 230 and 14 k� cm2 indicate initiation and severe corrosion of the rebar respectively. Similarly, a Cdl value greater than 1000 �F/cm2 indicates the severe corrosion of rebar, whereas less than 100 �F/cm2 denotes the passive condition of rebar. If the rebar is in the passive condition, the phase angle is more than 308, whereas it is less than 208 under severe corrosion. The reduction of intrinsic chloride diffusivity owing to pore restructuring by pozzolanic reaction and adsorption of a greater amount of chloride ions into the interlayer of additional calcium silicate hydrate content are responsible for delayed initiation of corrosion in Portland pozzolana cement (PPC) and Portland slag cement (PSC) concretes, when compared with ordinary Portland cement (OPC) concrete

Analysis of the electrochemical phenomenon at the rebar–concrete interface using the electrochemical impedance spectroscopic technique

The corrosion rate of rebar during the various stages where it occurs, such as passivation, initiation of corrosion and severe corrosion, needs to be determined non-destructively for the maintenance, restoration and replacement of concrete structures. The double layer capacitance (Cdl) and the charge transfer resistance or polarisation resistance (Rp) of the corrosion processes have been associated with the slope of the low-frequency arc in the Nyquist plot, and this can be related to the electrochemical phenomenon that occurs at the steel–concrete interface. The present studies, based on electrochemical impedance spectroscopy (EIS) conducted on three different densities of concrete with addition of 0·5 and 1% chloride over a period of 1765 days, reveal that the capacitive behaviour of a low-frequency arc with a slope more than −1 indicates the passive condition of rebar. Warburg diffusion behaviour with a slope exactly equal to −1 denotes the initiation of corrosion on the rebar. A slope of less than −1 is obtained when corrosion spreads uniformly on the rebar. Other electrochemical parameters such as Rp, Cdl and phase angle are correlated with the phenomenon occurring at the steel–concrete interface. An Rp value greater than 250 kΩ cm2 indicates the passive condition of rebar, whereas values of less than 230 and 14 kΩ cm2 indicate initiation and severe corrosion of the rebar respectively. Similarly, a Cdl value greater than 1000 μF/cm2 indicates the severe corrosion of rebar, whereas less than 100 μF/cm2 denotes the passive condition of rebar. If the rebar is in the passive condition, the phase angle is more than 30°, whereas it is less than 20° under severe corrosion. The reduction of intrinsic chloride diffusivity owing to pore restructuring by pozzolanic reaction and adsorption of a greater amount of chloride ions into the interlayer of additional calcium silicate hydrate content are responsible for delayed initiation of corrosion in Portland pozzolana cement (PPC) and Portland slag cement (PSC) concretes, when compared with ordinary Portland cement (OPC) concrete. </jats:p

Quality assurance tests for corrosion resistance of steel reinforcement

Nowadays, for enhancement of durability of reinforced concrete (RC) structures in marine environment, different protective systems, namely coating to steel/concrete surface, corrosion resistant rebars, addition of inhibitor admixtures, etc are adopted. The efficiency of each system should be ensured by conducting appropriate quality assurance tests. This paper discusses the various test methods and presents the appropriate recommendations for corrosion resistance of reba

Enhanced corrosion resistance of rebar embedded in fly ash added concrete: Role of "k" factor

In Indin, the abundnnt availability of fly ash necessitates the increase in the addition of lly ash in pozzolana ceinent from 25 to 35%. Higher rcplacemcnt level of tly ash reduced the development of eorly oge strength of concretc. By adopting the cemcnt cfficicncy factor (k) when dcsigning the fly ash added concrctc, eithcr equal or more compressive stl'ength has becn attaincd. In the prescnt study, by adopting this factor, the concretes having strength of20 and 35 MPa were designcd at 40% replaccmcnt of tly ash. Thc enhanced COITosion resistance of rebm was evaluated by conducting chloride migration test, potcntial-timc studics and 90 days chloride ponding test. Results were compared with Ordinary Portli1l1d cement (OPC) and Fly ash addcd concrete (FAC) without k factor. The studies reveal that the diffusion co-efficient of chloride is Icss in FAC-with k than that of ope ,md FAC without k eoncretcs. 1n 1% chloride contaminated concretc, the rebar in FAC-with k shows more passivc potential than the other two concretes. Under 90 days chloride ponding test, the corrosion resistance ofrebar in FAC is 3-7 times more than that of the rebar in OPC concrete. The reduced chloride ion penetration inneascd the cOITOsion resistance of rcbar. The 40% I'eplacement of fly ash reduced the alkalinity of FAC-without k whereas the 01-1 ions concentration increases in FAC-with-k and maintained the passivity of rebnr in presence of chloride. Hence in addition to early strength development, the enhanced corrosion resistance of rebar in FAC-with k is mainly because of reduced chloride ions penctration and increased OW ions concentration

Longterm corrosion performance of rebar embedded in blended cement concrete under macro cell corrosion condition

The concrete does not possess sufficient resistance towards the permeation of aggressive ions when it is exposed to marine environment. When mineral admixtures are added, they impart certain impermeability to concrete by improving the physical structure by pozzolanic reaction. The reduction of concentration of OH ion occurs either by consumption of free Ca(OH)2 by pozzolanic reaction or by dilution of cement alkalis due to replacement with mineral admixture. This causes a substantial reduction in the threshold value of chloride in mineral admixed concrete. In the present investigation, the corrosion resistance of rebar embedded in concrete made with Portland pozzolana cement (PPC), Portland slag cement (PSC) was studied for 847 days under macro cell corrosion condition with on comparison using ordinary Portland cement (OPC). Concrete having characteristic compressive strength of 20, 30 and 40 MPa were taken for evaluation. Potential and macro cell corrosion current were measured periodically and corrosion rates were determined by weight-loss method. This long term experiment revealed that in 20 MPa concrete, the corrosion rate of rebar in PPC and PSC concrete was 9 and 10 times lower than the rebar in OPC concrete: In the case of 30 MPa concrete, the corrosion rate of rebar in PPC and PSC concrete was 17 and 6 times lower respectively and in 40 MPa concrete it was 1.6–2.5 times less than the rebar in OPC concrete. The reduction of chloride ion content in blended cement concrete was varied from 1.4 to 3.1 times less than the OPC concrete among all the three concretes studied. Reduction of alkalinity in 20 MPa concrete at the rebar level in PPC and PSC concrete is 6 and 10 times lower, respectively, than in OPC concrete. In the case of 30 and 40 MPa concrete it was 2 and 1.6 times lower. The reduction in alkalinity did not accelerate the corrosion rate of rebar in blended cement concretes even in presence of higher amount of chlorides. The apparent chloride diffusion co-efficient of blended cement concretes was 1.6–1.8 times lower than that of OPC concrete. The combined effect of higher chloride complexing ability and reduction of chloride ion diffusivity of blended cement concretes made them to perform better in terms of corrosion protection of reinforcing steel

Embeddable corrosion rate-measuring sensor for assessing the corrosion risk of steel in concrete structures

New and rehabilitated concrete structures need assessment of corrosion rate (CR) of rebars more precisely to ensure efficiency of protection against corrosion. For assessing the corrosion status, different sensors have been developed to reduce the risk of undetected corrosion problems. An embeddable CR-measuring sensor is embedded either closer to the rod during the construction of a new structure or in drilled holes in old structures so as to generate reliable data. In the present paper, the accuracy in predicting the corrosion current (Icorr) by embeddable sensor was assessed by comparing with other conventional electrode arrangements. Icorr was assessed under passive and active conditions of rebar. The presence of a cross bar and the effect of cover on Icorr were also studied. Using linear polarization resistance (LPR) technique and electrochemical impedance spectroscopic (EIS) technique, the CR was determined and compared with the weight-loss method. Results concluded that by using electrochemical impedance technique, developed embeddable sensor is able to predict the CR very close to the CR determined from gravimetric method. The deviation from an ideal linear curve and the higher interfacial capacitance of steel in concrete caused the LPR method to underestimate the CR of rebar. The presence of the cross bar increases the Icorr of the main bar by 4 times in a passive state of the rebar and it has no effect if the rebar is in an active state. The embedded sensor, though having smaller counter electrode (CE), polarizes the rebar up to 2 times the length of CE under both active and passive states

Application of harmonic analysis in measuring the corrosion rate of rebar in concrete

The corrosion rate (CR) of rebar embedded in cement mortar, concrete and cement extract is determined using harmonic analysis technique (HA). Simultaneously using other electrochemical techniques such as impedance spectroscopy (EIS) and Tafel extrapolation (TET), the CR was determined and compared with the weight loss method. CR obtained from HA is comparable to that of EIS provided that the Stern–Geary constant (B value) obtained from HA is used in the calculation. In concrete, comparable corrosion rates are obtained between TET and HA only under active condition of the rebar whereas under passive state, the corrosion current (icorr) by TET is 10 times lower than that of HA. A good agreement is obtained between the HA and weight loss method. The outcome of the result suggests that HA is capable of providing a higher degree of accuracy than that of EIS and TET in the determination of icorr in the medium like rebar in concrete having very low rate of corrosion

Determination of migration efficiency of amino alcohol based migrating corrosion inhibitor through concrete

The migration efficiency of an amino alcohol based migrating corrosion inhibitor (MCI) through hardened concrete of two different strengths (20 and 30 MPa) with two different thicknesses (25 and 40 mm) has been studied using diffusion test cell arrangement. The corrosion current Icorr was measured using Tafel extrapolation technique. There is a 20 times reduction in Icorr of steel in presence of MCI compared to that of bare steel. The migration efficiency decreases with an increase in thickness as well as strength of concrete. The density of concrete influences the diffusion rate of MCI. The diffusion rate is two times higher in 20 MPa concrete than that of 30 MPa concrete. The studies have indicated that the passive layer formed on the rebar surface is not stable in presence of 1% of chloride in 20 MPa concrete. Because of higher chloride threshold level the MCI has performed better in 30 MPa concrete. The results conclude that if higher protection efficiency is needed where structure is exposed to high chloride environments, it is more appropriate that the MCI shall be added at the rate of 2?45 kg m23 along with concrete/ repair mortar than applied on the concrete surface