Hans Beushausen

Prediction of Corrosion Rate in RC Structures - A Critical Review

Corrosion rate is one of the most important input parameters in corrosion-induced damage prediction models for reinforced concrete (RC) structures. Its accurate assessment and/or prediction is therefore required if the damage prediction models are to be reliably used to predict both the rate and severity of damage and to plan for maintenance of these structures. However, it has not been assigned the level of importance it deserves especially with respect to its prediction. In most cases, instantaneous measurements or constant predicted corrosion rate values are used in damage prediction models hence neglecting its time-variant nature while in some cases, salient factors that affect corrosion rate such as cover cracking and concrete quality and not taken into consideration during the model development. The direct consequence of this may be under- or overestimation of the severity and the time to corrosion-induced damage such as for example cover cracking, and hence service life of the structure. This paper presents a critical review of some of the available corrosion rate prediction models focusing mainly on chloride-induced corrosion. In addition, proposals for the improvement of these models are made.

Suitability of Various Measurement Techniques for Assessing Corrosion in Cracked Concrete

A critical evaluation of results from three corrosion assessment methods used in an experimental study to investigate the influence of cracks on the rate of chloride-induced corrosion is presented in this article. The objective was to determine the suitability and reliability of the corrosion assessment methods for application in cracked concrete. Beam specimens (100 x 100 x 500 mm [3.9 x 3.9 x 19.7 in.]) with crack widths of 0.7 and 0.4 mm (0.028 and 0.016 in.), as well as incipient cracks, were made using two binder types (100% CEM I ordinary portland cement [OPC]) and 50/50 OPC/ slag blend), two water-binder ratios (w/b) (0.40 and 0.55) and a constant cover of 40 mm (1.6 in.) to steel reinforcing. The specimens were subjected to repeated cycles of 3-day wetting with 5% NaCl, and 4-day air drying for 32 weeks. Half-cell potential (HCP), resistivity (Wenner probe), and corrosion rate (coulostatic) measurements were taken at the end of each 3-day wetting period. Chloride conductivity tests were also performed on companion cube specimens at 28 and 90 days. No significant differences in resistivity values were noted between cracked and uncracked specimens, despite the fact that the corrosion rate increased with increasing crack width. The insensitivity of resistivity measurements to the presence of cracks was attributed to the fact that measurements were taken after every 3-day wetting period, when the specimens would have been saturated with salt solution. The chloride conductivity and resistivity measurements gave a similar indication of the corrosion rates. HCP and corrosion rate measurements showed a good correlation, and they gave a clear indication of corrosion in cracked reinforced concrete (RC) specimens, although for practical applications, HCP measurements need to be complemented with other corrosion assessment techniques. For the resistivity assessment technique—in addition to concrete quality and magnitude of resistivity—correction factors are required to relate the resistivity of uncracked concrete to corrosion characteristics of cracked concrete. A methodology for the application of correction factors is proposed.

A new approach for the mix design of (patch) repair mortars

The failure of most repairs in concrete structures is mainly manifested in the form of cracking and/or debonding of the repair layer. These two repair mechanisms could lead to the continuous corrosion of reinforcing steel in concrete. Cracking and debonding have been attributed to the high differential shrinkage that exists between the concrete substrate and repair layer as well as the lack of adherence, which commonly results from poor workmanship. It is crucial that restrained shrinkage mechanisms of patch repairs be understood so that the abovementioned effects can be prevented. The performance of patch repairs can be improved through the optimization of their mix design. Thus, it is common to use admixtures, non-cementitious compounds and polymers. However, promising technologies such as expansive admixtures that counteract shrinkage have found little application to date. Polymers do not chemically react with the cementitious matrix. They, however, have been reported to modify the microstructure in cementitious materials, consequently, resulting in improved properties with respect to mechanical, chemical and durability aspects. They increase chemical interactions with mineral phases, which improves the bond between the mortar and the substrate. Besides this, expansive cements are types of cement whose volume increases, unlike plain Portland cement where the volume typically decreases with time. This property relates to the use of expansive admixtures like quicklime, which exhibits an enormous volume increase upon hydration. This property leads to the reduction of the shrinkage, which is increased by the combination of the quicklime to a shrinkage-reducing admixtures (SRA). Using polymer technology and expansive admixtures, it is possible to develop a new approach for the mix design of repair mortars that combines the positive effects of polymer latex (increasing of bond strength) and expansive admixture (reducing the shrinkage) properties to reduce cracking and debonding failures in patch repairs.