Concentration monitoring and performance of a migratory corrosion inhibitor in steel-reinforced concrete

Inhibitor concentration depth profiles for concrete samples treated with a proprietary migratory corrosion inhibitor (of the Cortec MCI range) are presented. The treated concrete was cored and these cores were then sectioned and crushed before being immersed in distilled water to extract the available inhibitor. The amine concentrations were quantified using an ammonium-sensing electrode and were then related to the inhibitor concentration present. The inhibitor examined, reported to contain a combination of volatile amines and amino carboxylate compounds, was found to readily diffuse through concrete. The inhibitor was subjected to a 5-year trial and found to be effective in suppressing corrosion of steel reinforcement in the presence of high chloride concentrations. The concentration profiles indicate that only relatively low concentrations of inhibitor were required to achieve inhibition in this case.     

FLR technique: Exchange kinetics of ethanol/fluorescent dye with water in water-saturated cement paste examined by H- and H-NMR

A water-saturated cement paste (water-to-cement, w/c=1.0) confined in a solution of ethanol saturated with a fluorescent dye showed complete exchange of water after 2 days. The exchange rates of water and ethanol were independent of the presence of dye and determined to be 42×10⁻⁸ and 60×10⁻⁸ cm²/s, respectively. The exchange rate of dye was significantly smaller, by a factor of two to three. The exchange process resulted in a partitioning of the concentration of dye in ethanol within cement and bulk of (0.46+0.20). ¹H-NMR spectral analysis of the dye before and after exchange suggests the dye to be inert, i.e., it does not react with the cement paste. Furthermore, no indication of a reaction between ethanol and cement paste is observed.     

Change in pore structure and composition of hardened cement paste during the process of dissolution

An understanding about the dissolution phenomena of cement hydrates is important to assess changes in the long-term performance of radioactive waste disposal facilities. To investigate the alteration associated with dissolution, dissolution tests of ordinary Portland cement (OPC) hydrates were performed.Through observation of the samples after leaching, it was confirmed that ettringite precipitation increased as the dissolution of the portlandite and the C-S-H gel progressed. EPMA performed on cross-sections of the solid phase showed a clear difference between the altered and unaltered parts. The boundary between the two parts was termed the portlandite (CH) dissolution front. As the leaching period became longer, the CH dissolution front shifted toward the inner part of the sample. A linear relationship was derived by plotting the distance moved by the CH dissolution front against the square root of the leaching time. This indicated Ca ion movement by diffusion.     

The pore solution phase of carbonated cement pastes

Samples of hydrated cement pastes were exposed to atmospheres with various carbon dioxide concentrations at relative humidities controlled by different saturated salt solutions. When carbonated throughout their thickness, as indicated by the phenolphthalein test, they were resaturated with water and subjected to pore solution expression and analysis. The effects of the various carbonating environments on the pore solution composition and on aspects of the pore structure and mineralogy of the carbonated products are reported. Implications regarding the likely effects of different accelerated carbonation regimes on the corrosion behaviour of steel in concrete are discussed. In particular, it is shown that the use of saturated sodium nitrite solution to control the relative humidity of atmospheres with high concentrations of carbon dioxide may cause an evolution of gaseous oxides of nitrogen, which can result in the contamination of the pore solution with nitrite and nitrate salts.     

Carbonation around near aggregate regions of old hardened concrete cement paste

Analogous with most modern cities, waste disposal is a pressing issue due to limited landfill and public filling (land reclamation) areas in Hong Kong in which construction and demolition (C&D) waste forms the major source. Concrete, apportioning the largest portion of C&D waste, has the greatest potential for recycling. However, the knowledge on micro-structural behavior of concrete waste is immature to give adequate details on the macro-behavior of concrete waste. This paper attempts to examine the problems of recycling old concrete by investigating the microstructure and phase transformation of the concrete samples collected from buildings with 46 and 37 years of services. From the results of Fourier transform infrared spectroscopy (FT-IR) and scanning electron microscopy (SEM) examination, it is found that there are a lot of pores at the near layers of aggregate where carbonation of the hardened cement paste (HCP) is high. The pores may be generated as a result of poor workmanship such as insufficient concrete mixing time, trapping of air voids beneath coarse aggregate, inappropriate water to cement ratio, and the microclimate conditions such as humidity that affects the demand on water from the aggregate during mixing.     

Magnetic resonance imaging of crack formation in hydrated cement paste materials

A magnetic resonance imaging (MRI) methodology is presented to spatially resolve cracking in hydrated cement paste. The method is based on the relatively long spin-lattice (T₁) and spin-spin relaxation (T₂) times (magnetic resonance signal lifetimes) associated with water-filled cracks. Water-filled capillary and gel pores have much shorter signal lifetimes. As a noninvasive method, the technique may be used to study crack formation and propagation in well-controlled laboratory test samples. The method is proven to be capable of spatially resolving crack structures with widths of tens of micrometers.     

Development of a galvanic sensor system for detecting the corrosion damage of the steel embedded in concrete structures: Part 1. Laboratory tests to correlate galvanic current with actual damage

The correlation between sensor output and corrosion rate of reinforcing steel was evaluated by laboratory electrochemical tests in saturated Ca(OH)₂ with 3.5 wt.% NaCl. In this paper, two types of electrochemical probes were developed: galvanic cells containing of steel/copper and steel/stainless steel couples. The corrosion behavior in saturated Ca(OH)₂ solution with and without 3.5 wt.% NaCl addition for the different electrodes was investigated by potentiodynamic test. Weight loss measurement and galvanic corrosion test were conducted to obtain the corrosion rate of reinforcing steel and the charge of sensor in saturated Ca(OH)₂ solution with 3.5 wt.% NaCl addition, respectively.

The results of the potentiodynamic test indicated the possibility of detecting an ingress point of chlorides by measuring the galvanic current. In galvanic corrosion tests, the galvanic current of steel/copper couple was higher than that of steel/stainless steel couple, i.e., the steel/copper sensor is more suitable for high resistance environment. The steel/stainless sensor showed a better linear correlation than the steel/copper sensor. Through the relationship between the sensor system output and the weight loss (mg/cm²) of steel, real corrosion damage of the steel embedded in concrete can be detected.     

Experimental study of mechanical behaviour of cement paste under compressive stress and chemical degradation

This paper presents experimental investigations of mechanical behaviour of a pure cement paste subjected to compressive stresses and chemical degradation. Two series of laboratory tests have been performed: decoupled and coupled chemical-mechanical tests. Hydrostatic and triaxial compression tests have first been realized respectively on sound and chemically degraded samples. The obtained results allow the characterization of basic mechanical responses of the tested cement paste and the identification of chemical degradation effects on the mechanical behaviour. In the coupled tests, the samples are simultaneously subjected to deviatoric stresses and chemical leaching by aggressive solution flow. Variations of deformation of cement paste samples are measured during chemical degradation process. The results obtained in these tests can be used for the validation of chemo-mechanical constitutive modelling.     

Rehydration and microstructure of cement paste after heating at temperatures up to 300 °C

This paper is concerned with the evolution of the microstructure of cementitious materials subjected to high temperatures and subsequent resaturation in the particular context of long-term storage of radioactive wastes, where diffusive and convective properties are of primary importance. Experimental results obtained by mercury intrusion porosimetry (MIP) are presented concerning the evolution of the pore network of ordinary portland cement (OPC) paste heated at temperatures varying between 80 and 300 °C. The consequences of heating on the macroscopic properties of cement paste are evaluated by measures of the residual gas permeabilities, elastic moduli and Poisson's ratio, obtained by nondestructive methods. Resaturation by direct water absorption and water vapour sorption are used to estimate the reversibility of dehydration. The results provide some evidence of the self-healing capacity of resaturated cement paste after heating at temperatures up to 300 °C.     

Modelling elasticity of a hydrating cement paste

Concrete is a complex multi-scale composite involving multi-physics processes. As it is the only evolving component of concrete, the cement paste has a major influence on the mechanical properties of concrete at early age. This paper focuses on the increase of the elastic properties of a cement paste during its hydration. The homogenization theory for disordered media is used in order to estimate the evolution of the effective elastic moduli of the hydrating paste. The morphological model refers to two types of C-S-H (calcium silicate hydrates, main hydration products of Portland cements) distinguished by many authors: inner products or high density C-S-H build up layers surrounding the anhydrous particles, while the outer products or low density C-S-H play the role of a porous matrix.The simulations of the effective Young's modulus at late age during hydration and at the end of hydration prove to be in excellent agreement with the experimental results available in the literature.     

Stress relaxation of foamed high-alumina cement paste

A series of stress relaxation tests on foamed high-alumina cement pastes with different relative densities under various temperatures and imposed fixed strains were conducted to study the effects of relative density and imposed strain on the stress relaxation rates of foamed high-alumina cement pastes. At the same time, the activation energy for stress relaxation of foamed high-alumina cement paste was determined from experimental results. Experimental results on the stress relaxation rates of foamed high-alumina cement pastes are also compared to a theoretical expression obtained from a cell-edge relaxation-bending model. Consequently, the microstructural coefficients included in the theoretical expression for describing the stress relaxation rates of foamed high-alumina cement pastes are found. Furthermore, the stress relaxation rates of foamed high-alumina cement pastes can be predicted from the theoretical expression once their relative density and the imposed strain are known.     

A study on chloride-induced depassivation of mild steel in simulated concrete pore solution

The effect of chloride ions on passivity breakdown of steel in simulated concrete pore (SCP) solution was studied using electrochemical techniques. In this regards, the sensitivity of cyclic potentiodynamic parameters such as ΔE (difference between E_pit and E_rep), i_peak and AC-impedance parameters like R_ct, R_f, C_i, R_ad and C_ad to chloride ion concentration was investigated. Adsorption of OH⁻ ions on the metal surface in free chloride SCP solution and also displacement of those ions by Cl⁻ ions were demonstrated in high frequency part of Nyquist plots. In addition, a severe decrease in resistance of interfacial reaction (R_f+R_ct) was observed through breakdown of passive film in the [Cl⁻]/[OH⁻] ratio of 0.6. The interfacial capacitance, C_i, was abruptly raised when localized corrosion changed to general one.     

Methods of obtaining pore solution from cement pastes and mortars for chloride analysis

Two techniques for the recovery of pore solution from cement mortars are examined: pore solution expression and miscible displacement using a high pressure permeameter. In the former, the pore solution is expressed from the mortar by crushing; in the latter, it is eluted from the mortar over 30 min by miscible displacement with water. Experimental results are presented for a range of cement pastes and mortars into which known amounts of chloride ion have been incorporated by using sodium chloride solution as the mix water. The results show that both eluted and expressed solutions exhibit a decrease in chloride ion concentration as the cement matrix ages, with the elution method showing a greater sensitivity to mix composition. Both methods show a decrease in chloride concentration as the water: cement ratio of the mix is increased. Overall, the high pressure elution method is capable of recovering a significantly higher proportion of the incorporated chloride. The application of these techniques to pore solution analysis is discussed.     

Cement paste colouring in concretes

Tests have been carried out with chemicals that give rise to coloured compounds through a selective reaction with certain mineral species present in cement paste. In the case of carbonated concretes (pH≅9), the reaction with potassium ferrocyanate dyes the cement paste a strong blue colour that disappears when the concrete is treated again with a slightly more alkaline solution. Both in carbonated concretes and in young concretes with an alkaline reserve (pH≅12.5), the tannic acid dyes the cement paste dark brown.     

Influence of lithium-based products proposed for counteracting ASR on the chemistry of pore solution and cement hydrates

Low- and high-alkali cement pastes were made with or without LiNO₃ or a Li-bearing glass. The [Li]/[Na+K] molar ratio was kept constant to 0.74. The specimens were stored at 23, 38, and 60 °C in sealed containers. After 3, 7, 28, and 91 days, their pore solutions were extracted and analysed, and their residual water contents were obtained by drying. The Li glass was found to react quite slowly, and the corresponding [Li⁺] in solution progressively increased with time, temperature, fineness (as-received glass vs. ground glass), and the [Na⁺+K⁺] concentration in solution. This glass increased the pH by about 0.1, and by about 0.2 after it was finely ground. In contrast, LiNO₃ decreased the pH by about 0.1, despite significantly increasing the [Na⁺+K⁺] in the pore solution. The higher the total %Na₂O_e content (including Li) in the original mixtures, the higher the total alkali content incorporated in the cement hydrates. The [Li⁺]-[Na⁺+K⁺] ratio in solution was about half of the initial ratio (0.74), while this ratio in the cement hydrates was always over 1.1. Li is the alkali most preferentially incorporated into the cement hydrates, while K is the least.     

Generalization of the possibility of eliminating the filtration step in the determination of acid-soluble chloride content in cement and concrete by potentiometric titration

In this work, it is demonstrated that it is possible to eliminate the filtration step in the determination of the acid-soluble chloride content of cement and concrete by potentiometric titration, irrespective of the method used for detecting the end-point of the titration. Extensive analytical results are presented corresponding to several types of cement and to a broad range of chloride concentrations in concrete. Some requirements to avoid errors in such analytical determinations are discussed.     

The influence of different drying methods on cement paste microstructures as reflected by gas adsorption: Comparison between freeze-drying (F-drying), D-drying, P-drying and oven-drying methods

Drying of cement pastes is required prior to microstructure investigation by means of gas adsorption technique. An ideal drying method, which would give reproducible results that could perfectly remove only the non-bound water and, at the same time, preserve the microstructure, unfortunately does not exist. The different drying methods used affect the microstructures in different ways. However, an effective water removal and, less damaging drying method between the common methods used would be still of outstanding importance for sample preparation. Many drying methods have been investigated in the past for such a purpose, and a good agreement for the best drying method does not exist. The so-called D-drying method is being used in many laboratories as the “best” method for drying cement pastes. The surface areas and pore size distributions results of the current work confirm that D-drying (D-Drying C^− t) is a relatively good preservation and effective drying method, and that Freeze-drying gives slightly better results compared to D-drying (C^− t) and other methods. However the short time versions of some of these methods indicate the presence of very few “micropores”, which are not present with prolonged drying times. The outgas level is also a very important variable affecting the gas adsorption measurements especially in the case of short duration drying conditions, as indicated by the results of this work.     

Potential pitfalls in assessing chloride-induced corrosion of steel in concrete

Electrochemical assessment of the condition of embedded reinforcing bar (rebar) in concrete is being carried out increasingly routinely, both in the laboratory and in the field. However, because of the perceived need to produce results very rapidly, the results may not, in fact, be representative of the actual behaviour of the rebar. This paper describes some of the pitfalls the authors have encountered in their own work and have, therefore, analysed and quantified, together with others which have appeared in the literature.     

X-ray absorption study of drying cement paste and mortar

X-ray absorption was used to observe water evaporation with hydration time in paste and mortar specimens, with the aim of studying the influence of water/cement (w/c) ratio, presence of aggregates, curing conditions on drying during early hydration. For the samples subjected to surface drying immediately after mixing, there exists a moisture gradient within the internal part of the specimen. However, obvious top-down drying only occurs within a small zone near the surface for early age cement pastes and mortars. The evaporation rate of water is very high in the first day after casting and is drastically reduced afterwards due to the formation of a microstructure that greatly improves specimens resistance to moisture loss. Mortars reveal a slightly lower evaporation rate since the aggregate increases the length of the transport route because of a larger tortuosity. However, the effect of sealed curing is much more important than the tortuosity effect of the aggregates.