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Many (inter)national standards exist to evaluate the resistance of mortar and concrete to carbonation. When a carbonation coefficient is used for performance comparison of mixtures or service life prediction, the applied boundary conditions during curing, preconditioning and carbonation play a crucial role, specifically when using latent hydraulic or pozzolanic supplementary cementitious materials (SCMs). An extensive interlaboratory test (ILT) with twenty two participating laboratories was set up in the framework of RILEM TC 281-CCC 'Carbonation of Concrete with SCMs'. The carbonation depths and coefficients determined by following several (inter)national standards for three cement types (CEM I, CEM II/B-V, CEM III/B) both on mortar and concrete scale were statistically compared. The outcomes of this study showed that the carbonation rate based on the carbonation depths after 91 days exposure, compared to 56 days or less exposure duration, best approximates the slope of the linear regression and those 91 days carbonation depths can therefore be considered as a good estimate of the potential resistance to carbonation. All standards evaluated in this study ranked the three cement types in the same order of carbonation resistance. Unfortunately, large variations within and between laboratories complicate to draw clear conclusions regarding the effect of sample pre-conditioning and carbonation exposure conditions on the carbonation performance of the specimens tested. Nevertheless, it was identified that fresh and hardened state properties alone cannot be used to infer carbonation resistance of the mortars or concretes tested. It was also found that sealed curing results in larger carbonation depths compared to water curing. However, when water curing was reduced from 28 to 3 or 7 days, higher carbonation depths compared to sealed curing were observed. This increase is more pronounced for CEM I compared to CEM III mixes. The variation between laboratories is larger than the potential effect of raising the CO2 concentration from 1 to 4%. Finally, concrete, for which the aggregate-to-cement factor was increased by 1.79 in comparison with mortar, had a carbonation coefficient 1.18 times the one of mortar.
Supplementary Information
The online version contains supplementary material available at 10.1617/s11527-022-01927-7.
Moisture and ionic transport under non-saturated condition is an important, but poorly understood transport phenomena particularly for mature systems containing supplementary cementitious materials. This paper investigates the moisture and chloride profiles of 3-year old mortars containing Portland cement (OPC), slag and silica fume (SF) after long-term (30–48 months) wick action exposure in 1.09 M NaCl solution. Moisture profiles were measured with ¹H NMR relaxometry and chloride profiles with microXRF. The measured profiles were discussed in relation to moisture dependent material properties such as chloride diffusion coefficients, moisture diffusion coefficients, and desorption isotherms. Results show that the combination of different cementitious materials, e.g. the cementitious binder, is the key factor affecting chloride penetration depth. The cementitious binder also strongly affects chloride diffusion coefficient, moisture diffusion coefficient and chloride binding properties, which are all important parameters for the prediction of chloride ingress.
Electrical Resistance
(2018)
Electrical resistance is the ability for a material to impede the flow of electrical current. The resistance (R) is expressed in ohm (Ohm), its inverse is the conductance (C) is expressed in siemens (S). The resistance is function of geometry of tested body and measurement device. So one generally prefers the resistivity (ρΩ) expressed in ohm-meter (Ohm·m), or its inverse the conductivity (σ) in siemens per meter (S/m), representing the intrinsic material’s property.
ND-Methods - From a Surface
(2018)
Accelerated test methods are commonly used in order to predict concrete carbonation in natural concentrations. Here, specimens are carbonated at high CO 2 concentrations at a specified temperature and relative humidity. However, the transfer of laboratory results to field behaviour remains difficult because CO 2 transport is affected by the original moisture content of the specimens and additional moisture formed by the carbonation reaction. Therefore knowledge on moisture transport and content during carbonation is required. Specimens made with Ordinary Portland cement and a water/cement ratio 0.50 were exposed to 0.05, 2 and 10 vol.% CO 2 for 28 days. Single-sided NMR moisture profiles were determined before, during and after carbonation. It is shown that moisture content increases due to carbonation at high CO 2 10 %) in the beginning of the exposure. An increase in capillary pore water in front and behind the carbonation front could be observed even after 28d. During natural carbonation moisture changes are mainly due to the change in porosity produced by the carbonation reactions. It is shown that changes in phase composition and thus porosity dominate the carbonation process in cement-based materials. Therefore, the suitability of high CO 2 concentrations is limited for an accelerated test that reflects field condition. Single-sided 1 H NMR proved to be a valuable tool to investigate moisture transport in concrete non-destructively.
Sustainable service life design of reinforced concrete structures relies on accurate input values. However, in the field of carbonation induced corrosion some input parameters and statistical distributions still need to be validated for worldwide climate conditions. Furthermore, many well-published literature data is not considered due to different storage conditions. At the suggestion of CEN/TC 104/SC1/WG1 the database “CarboDB” was created providing open access to extensive information on concrete carbonation under different storage conditions. The natural carbonation coefficient as well as the minimum concrete cover can be calculated for chosen situations. CarboDB provides reliable data on concrete carbonation in order to increase existing knowledge on concrete carbonation. The database is available online at http://carbodb.bgu.tum.de//. By registration further contribution is possible and appreciated. New insights can be gained by merging several sources. For natural carbonation, testing only up to 140 days underestimates the carbonation progress of concretes with limestone fillers and high amount of ground granulated blast furnace slag.
Binders with large portions of carbon-intensive Portland cement replaced by supplementary cementitious materials (e.g. fly ash) are more susceptible to carbonation mainly due to their lower CO2 buffering capacity. This conclusion is usually drawn from accelerated experiments at elevated CO2 levels involving processes that seriously differ from natural carbonation. The resulting presence of H2O reactant in the pore system and the carbonated microstructure itself may be very different. In this paper, these phenomena were investigated for High-Volume Fly Ash (HVFA) mortar via carbonation tests at ±0.04% CO2 (natural carbonation), 1% CO2 and 10% CO2. Internal humidity sensor monitoring and 1H NMR relaxometry revealed the highest water vapour and liquid water contents after carbonation at 10% CO2. Carbonation at 10% CO2 results in a coarser pore structure than carbonation at 1% CO2, and this probably due to a higher degree of C–S–H carbonation.
While in the field of reinforcement corrosion service life prediction is well-established, there is still need for a user-friendly design model in the field of freeze-thaw deicing salt attack. This attack is one of the major causes of damage to concrete structures in cold climates. The degree of moisture during the first freeze-thaw-cycles exceeds moisture saturation by capillary suction and diffusion. When a critical degree of saturation is reached before freezing damage occurs immediately. As a consequence, depending on the concrete resistance (e.g. pore structure of the concrete) and the severeness of the freeze-thaw load (e.g. minimum temperature, moisture offer and salt concentration), a certain number of freeze-thaw cycles is needed before damage occurs (initial phase). The main exposure parameters influencing the initial phase as well as the subsequent evolution of damage (deterioration phase) are the minimum temperature as well as the salt and moisture content. These factors were determined by single-sided NMR, gravimetry and mercury intrusion porosimetry respectively. Three different concrete compositions were exposed to freeze-thaw deicing salt load at different minimum temperatures. To investigate the influence of the initial moisture content concrete specimens were continuously exposed to freeze-thaw load while other specimens from the same batch were exposed to alternating freeze-thaw exposure with intermediate dry periods. It was found that the minimum temperature and intermediate dry periods had a significant influence on the initial phase as well as on the degradation phase. In addition, higher chloride content in the near-surface concrete reduced the scaling rate. The chloride content under freeze-thaw exposure with 3% NaCl solution in laboratory significantly exceeded typical contents due to capillary suction and diffusion. Increasing the minimum temperature increased the chloride content. The investigations form the basis for service life prediction of concrete towards freeze-thaw deicing salt attack.
For material testing, qualification and subsequent assessment fast, simple, reliable and economic methods are needed. This paper provides an overview of different techniques to determine the depth of carbonation in cementitious materials. The easiest and cheapest way is to remove a core or a fragment of concrete cover and spray a colour indicator on the fresh fractured surface (by splitting). Here, 1 % phenolphthalein in 70 % ethanol is most widely used. However, this only indicates in which area the concrete has a pH value below 8.2-10, but not the actual carbonation front which is on average about 20 % higher. Since phenolphthalein is carcinogenic, alternative colour indicators were investigated in the laboratory. 0.5 % Curcumin showed similar colour change to phenolphthalein. In addition, 0.1 % thymolphthalein proved to be an alternative. Scattering within one measurement was lower than for phenolphthalein and the results were closer to the carbonation depths determined by different analytical methods (TGA and LA-ICP-MS). Furthermore, the transition pH-value of thymolphthalein is in the area of 9.3-10.5 which is closer to the actual carbonation front, allowing a more accurate assessment of the risk of reinforcement corrosion.
Lebensdauerbemessung
(2021)
Christoph Gehlen, Till Felix Mayer, Charlotte Thiel und Christian Fischer beschreiben die Lebensdauerbemessung von Beton. Umfassend werden Transport‐ und Schädigungsmodelle vorgestellt, die die Mechanismen der Carbonatisierung, der Chlorideindringung, der Korrosion und der korrosionsinduzierten Rissbildung im Stahlbeton beschreiben. Die damit einhergehende Verbundthematik wird erläutert. Auch wird auf alle Mechanismen der Betonkorrosion eingegangen. Die Bemessung in Bezug auf die Dauerhaftigkeit und damit Abschätzungen für die Lebensdauer von Beton wird in drei unterschiedlich genauen Formaten geführt. Neben dem vollprobabilistischen Format wird beschrieben, wie Bemessungen auch auf Grundlage von Nachweisen basierend auf Teilsicherheitsbeiwerten durchgeführt werden können. Daran anschließend wird aufgezeigt, welche deskriptiven Bemessungsansätze sich daraus ableiten lassen. An einigen exemplarisch herausgegriffenen Objektbeispielen wird die Lebensdauerbemessung konkret vorgeführt.
With steadily rising CO2 concentrations in the ambient air and fast-changing concretecompositions with reduced clinker contents, the availability of reliable and accelerated concrete car-bonation tests is of crucial importance to design durable structures. This paper focuses on the effectsof moisture under accelerated conditions and the effects of different CO2 exposure conditions. Mor-tar prisms incorporating three different cement types were cured and stored at either 50% or 65%relative humidity (RH). Afterwards, the prisms were carbonated at different ambient humidities(50, 57 and 65%), different CO2 concentrations (0.04, 1 and 3 vol.%) and complemented by a seriesof tests at increased gas pressure (2 barg). High-resolution test methods were used to explain theunderlying carbonation mechanisms. The results show that pre-conditioning for two weeks—ascurrently suggested by the European Standard—seems to be too short because the initial inner mois-ture content severely affects the carbonation rate. Relative humidity during carbonation of 57% ledto higher carbonation rates compared to 50% and 65%. In addition, climate data needs to be period-ically (preferably permanently) recorded in research experiments and in laboratory testing to ensurefair interpretation of experimental results.
Nuclear Magnetic Resonance (NMR) enables to quantify the hydrogen nuclei in a sample and thus indirectly the water content. Here, hydrogen nuclei are put in oscillation (resonance) by applying a magnetic field and additional irradiation with pulsed electromagnetic waves of certain frequencies. In case all hydrogen nuclei of a sample are exclusively bound in water molecules, the resulting initial amplitude of the detected NMR signal is directly proportional to the water content. The subsequent attenuation of the oscillation is caused by relaxation mechanisms and provides information on the mobility and thus the binding states of the hydrogen nuclei. Therefore even the pore size distribution can be determined in completely water-saturated samples. If the magnetic field is graded, images can be created based on spatially resolved nuclei density distributions (MRI). Depending on the research objective, tailored choice of equipment and settings can be used to qualitatively and mostly quantitatively analyze almost every moisture-related problem. The article gives an overview of the existing NMR methods, equipment, influencing parameters, calibration techniques, applications and limitations.
The analysis of German weather data from 1996 to 2019 reveals a development in climate towards freeze-thaw exposure conditions which are detrimental to concrete durability. This trend may intensify freeze-thaw attack in future and lead to more internal and/or external damage in concrete structures. This contribution considers the inclusion of real climate conditions in laboratory simulations. Weather data were recorded and compared with depth-resolved measurements of humidity and temperature in concrete samples stored at the median site of an expressway. The strength of attack depends mainly on the amount of damaging freeze-thaw cycles (minimum temperature <-5°C in combination with moisture). In addition, the salt content of the contact water, minimum temperature and thickness of the ice layer on the concrete surface are decisive. Experiments show that internal and to a minor degree also external damage increase due to lower heating rates. The exposure of the sample itself (i.e. compass direction and removal of snow) affects the resulting damage evolution significantly. In order to save resources and enable the production of more sustainable concrete structures, current accelerated tests need to be adapted to real climatic conditions. 1. Introduction Adaptation to climate change and the associated extreme weather events represents a crucial challenge for our society. The durability of concrete, the most frequently used building material, is highly dependent on the local environment of concrete structures. In the case of a freeze-thaw de-icing salt attack, this is given by essentially the number of frost-thaw cycles (FTC) relevant to damage, the minimum temperature, the thickness of the ice layer, the moisture conditions and the concentration of salt. By investigating the interdependencies, the depth of scaling of the concrete surface per winter period can be predicted as a function of parameters accounting for the different effects of the concrete composition (air content, w/b ratio and binder type), curing time, carbonation and the duration and formation of damaging freeze-thaw cycles, [1, 2]. In order to quantify the individual effects and map them in an engineering model, a large number of laboratory tests are necessary. For this purpose, a known freeze-thaw cycle usually has to be modified. While it is already known that the cooling rate [3] and holding time at minimum temperature [4] affect scaling development, we investigated the effect of different heating rates on scaling on freeze-thaw de-icing salt attack.
The determination of durability-relevant material resistances of concrete is of great importance. They serve as input to engineering models to predict the durability of structures under real environmental conditions. The natural resistances have to be determined in time-consuming experiments, since the processes in nature are very slow. This is particularly important for new materials where long-term experience is not yet available. Thus, accelerated testing is required. Only that way new materials can be evaluated regarding their durability and subsequently be used in practical applications. For carbonation, a new R3 accelerated test method is presented in this contribution. An automated carbonation pressure chamber was developed. It consists of a pressure vessel, automated in such a way that it can apply gas overpressure of various intensities up to 8 bar to mortar and concrete samples. Simultaneously, it can control and regulate the ambient CO2 concentration from 0 to 99.5% in a fully automated and continuously variable procedure. Experiments were carried out with varying combinations of gas overpressure at CO2 concentrations of 3 vol.-% to achieve the most time-efficient carbonation of mortars and concretes. Mortars with different material compositions were used to evaluate the general suitability of the test procedure with the developed equipment. The automated carbonation pressure chamber enables reliable carbonation testing with a total duration under accelerated conditions of only 7 days.
Im Rahmen einer nachhaltigen Entwicklung hat sich u.a. Deutschland dazu verpflichtet, klimaschädigende Emissionen zu reduzieren.
Luftschadstoffe werden zu erheblichen Teilen durch den Straßenverkehr verursacht. Die potentiellen Umweltwirkungen, die bei der Herstellung und Nutzung eines 1 km langen Autobahnabschnitts mit vier
verschiedenen Oberbauarten wurden in einer Ökobilanz ermittelt und miteinander verglichen. Durch Berücksichtigung der Verkehrsbelastung während einer dreißigjährigen Nutzungsdauer werden die Auswirkungen des Treibstoffverbrauchs verdeutlicht. Mögliche Einsparpotentiale
werden dargelegt.
Concrete in cold climates is not only exposed to cold weather while concreting but also to freeze-thaw cycles during its lifetime. Thus not only the concrete production needs to be adjusted to the environmental conditions but also the mixture has to be durable and resist the environmental impacts. Commonly an air entraining agent is used to interrupt the capillary pore system and to provide further space for the freezing water to expand. The downside of this approach is a loss in compressive strength. Within this paper the effect of simultaneous use of air entraining agent and calcium nitrate is discussed. It is shown that calcium nitrate can counteract strength loss due to the air entraining agent without reducing the durability gain in the same extent. A certain combination of calcium nitrate and air entraining agent can also be used to increase the freeze-thaw-resistance.