O. Copuroglu

Modelling of Effect of ASR on Concrete Microstructure

In the present study the damage mechanism of Alkali-Silica-Reaction (ASR) in concrete bearing reactive volcanic rock is investigated. A combined numerical and experimental research is presented. The mechanism of ASR is investigated on a concrete microbar specimen by using various microscopy techniques. A meso mechanical model based on lattice theories is used as a starting point. Variables in the model are the properties of the concrete components on the meso-level, like mechanical properties of cement paste, aggregates, dissolved aggregate, bond properties and finally the properties of the gel. It is found that current numerical model is able to simulate ASR cracks similar to the experimental observations.

Reinforcement and healing of asphalt mixtures by rejuvenator encapsulation in alginate compartmented fibres

This paper explores the potential use of compartmented alginate fibres as a new method of incorporating rejuvenators into asphalt pavement mixtures. The compartmented fibres are employed to locally distribute the rejuvenator and to overcome the problems associated with spherical capsules and hollow fibres. The work presents proof of concept of the encapsulation process which involved embedding the fibres into the asphalt mastic mixture and the survival rate of fibres in the asphalt mixture. To prove the effectiveness of the alginate as a rejuvenator encapsulating material and to demonstrate its ability survive asphalt production process, the fibres containing the rejuvenator were prepared and subjected to thermogravimetric analysis and uniaxial tensile test. The test results demonstrated that fibres have suitable thermal and mechanical strength to survive the asphalt mixing and compaction process. The CT scan of an asphalt mortar mix containing fibres demonstrated that fibres are present in the mix in their full length, undamaged, providing confirmation that the fibres survived the asphalt production process. In order to investigate the fibres physiological properties and ability to release the rejuvenator into cracks in the asphalt mastic, the environmental scanning electron microscope and optical microscope analysis were employed. To prove its success as an asphalt healing system, compartmented alginate fibres containing rejuvenator were embedded in asphalt mastic mix. The three point bend tests were performed on the asphalt mastic test samples and the degree to which the samples began to self-heal in response was measured and quantified. The research findings indicate that alginate fibres present a promising new approach for the development of self-healing asphalt pavement systems.

Influence of the alkali-silica reaction on the mechanical degradation of concrete

The alkali-silica reaction (ASR) is an important problem that has yet to be completely understood. Owing to the complexity of this phenomenon, a number of studies have been conducted to characterize its kinetics, its impact on the material, and its structural consequences. This paper focuses on the deteriorating impact of ASR on concrete material, not only in terms of concrete swelling but also in consideration of the induced mechanical degradation. The relationships between concrete expansion and various engineering properties, which are key parameters in structural assessments, are investigated. First, new mechanical test results are presented. Second, available literature data on the evolution of engineering properties of ASR-affected concrete under free-expansion conditions are collected and statistically analyzed. The elastic modulus was found to be the best indicator for identifying the progression of ASR in concrete. Conversely, the evolution of compressive strength was observed to potentially mask damage resulting from the ASR. The tensile behavior of the affected concrete was better represented by the splitting tensile test.

Modeling of a self-healing process in blast furnace slag cement exposed to accelerated carbonation

In the current research, a mathematical model for the post-damage improvement of the carbonated blast furnace slag cement (BFSC) exposed to accelerated carbonation is constructed. The study is embedded within the framework of investigating the effect of using lightweight expanded clay aggregate, which is incorporated into the impregnation of the sodium mono-fluorophosphate (Na-MFP) solution. The model of the self-healing process is built under the assumption that the position of the carbonation front changes in time where the rate of diffusion of Na-MFP into the carbonated cement matrix and the reaction rates of the free phosphate and fluorophosphate with the components of the cement are comparable to the speed of the carbonation front under accelerated carbonation conditions. The model is based on an initial-boundary value problem for a system of partial differential equations which is solved using a Galerkin finite element method. The results obtained are discussed and generalized to a three-dimensional case.

Quantitative Energy-Dispersive X-Ray Microanalysis of Chlorine in Cement Paste

AbstractEnergy dispersive X-ray spectroscopy (EDS) is a microanalysis technique for material characterization that can provide accurate quantification of elemental composition while maintaining high spatial resolution. EDS microanalysis studies on cementitious materials can face several challenges essentially due to the complex chemical and mineralogical characteristics of cement hydration products. Furthermore, EDS microanalysis is widely carried out in “standardless analysis” mode, which relies on the internal standards of the microanalysis software. This can lead to highly erroneous analysis results because standardless analysis typically normalizes total to 100% without taking chemically bound water into account. In the case of quantifying chlorine content of a cement paste, such normalization is unacceptable because it can lead to overestimation. For accurate determination of elemental concentrations, X-ray spectra collected from minerals or glasses with known chemical compositions are necessary as r...

Chloride ingress of carbonated blast furnace slag cement mortars

In the Netherlands civil engineering structures, such as overpasses, bridges and tunnels are generally built using blast furnace slag cement (BFSC, CEM III/B) concrete, because of its high resistance against chloride penetration. Although the Dutch experience regarding durability performance of BFSC concrete has been remarkably good, its resistance to carbonation is known to be sensitive, especially when the used slag percentage is high. In a field investigation on a highway overpass damage was found in sheltered elements such as abutments and intermediate supports, which was attributed to chloride induced corrosion enhanced by carbonation that occurred prior to the chloride exposure.

Revealing the Dark Side of Portlandite Clusters in Cement Paste by Circular Polarization Microscopy

Plane and crossed polarization are the two standard light modes in polarized light microscopy that are widely used to characterize crystalline and amorphous phases in cement-based materials. However, the use of the crossed polarized light mode has been found to be restrictive for studying birefringent phases quantitatively due to the extinction phenomenon that arises depending on the crystal orientation. This paper introduces circular polarization microscopy as an alternative technique to overcome the extinction problem during the examination of cementitious materials’ microstructure with optical microscopy. In order to evaluate the feasibility of this technique, selected optical and micromorphological features of portlandite clusters were investigated in cement paste. Image analysis results showed that compared to the conventional crossed polarization technique, circular polarization offers significant advantages when portlandite quantification is of interest, and it stands out as a promising low-cost alternative to backscattered electron microscopy.

Experimental and numerical study on cement paste degradation under external sulfate attack

External sulfate attack is one of the situations that may cause gradual but severe damage in cementitious materials, which may lead to cracking, increased permeability and strength loss. In this paper, thin-walled hollow cement paste cylinders with a wall thickness of 2.5mm were made considering the slow penetration process of sulfate ions under continuous immersion condition. Three types of longitudinal restraints were applied on the hollow cement paste cylinders by means of a spring and steel bars through the specimens in order to facilitate non-, low- and high-restraint conditions. Strain gauges were glued on the steel bars so as to increase the accuracy of the measurements. During the immersion tests, specimen expansion and generated stress were monitored. Additionally, sulfur element mapping was generated by EDS (energy dispersive X-ray spectrometry). Expansion behaviours of the hollow cement paste cylinders were simulated under the aforementioned restraint conditions which were carried out based on the Delft lattice fracture model. The expansion was assumed to be realized upon formation of ettringite inside the nanopores of the cement hydration products. Local expansion stresses were computed by employing the crystallization pressure theory. A comparison between the simulation and the experimental results showed reasonable correlation and tendency for further exploration of our approach.

Influence of Anolyte on Lithium Migration in Concrete

Alkali-silica reaction (ASR) affects concrete structures worldwide. During this deleterious process, alkali and hydroxyl ions react with reactive siliceous components of the aggregate, producing a hygroscopic gel. Once the gel absorbs water from the surrounding cement paste, it swells. Consequently, the reaction might lead to expansion and cracking of concrete elements. Lithium is known to prevent those detrimental effects. In fresh mixtures, the incorporation of lithium-based admixtures as a preventive method has been acknowledged for years. However, in hardened concrete lithium ions need to be driven into it. Ionic migration seems to be the most effective method, when compared to other transport mechanisms. Even though several investigations have been conducted on the use of electric field to transport lithium ions into concrete, so far, there is no agreement on the findings. The present paper aims to investigate the influence of the type and concentration of lithium solution used as anolyte.

Feasibility of Using Low CO 2 Concrete Alternatives in Extrusion-Based 3D Concrete Printing

In conventional concrete, replacing high-volume (more than 45%) of ordinary Portland cement (OPC) by supplementary cementitious materials (SCMs) is not a novel CO2 reduction method, whereas rarely in 3D printable concrete. This study attempts to explore the feasibility of using SCMs in 3D printable concrete. Initially, the existing binder mixes, required fresh properties and a research method of 3D printable concrete are investigated by reviewing the relevant papers. Additionally, the constraints and opportunities of using SCMs in 3D printable concrete are illustrated and summarized. Finally, it has been found that up to 45% of cement can be replaced by a blend of fly ash and silica fume. The essential fresh properties of 3D printable concrete include extrudability, workability, open time, buildability and structural build-up, which are influenced by the binder mix, particle size distribution, water to binder ratio, binder to aggregate ratio, admixture addition, the dosage of reinforced-fibers, etc. On the other hand, there are many limitations to develop SCMs-based 3D printable concrete, such as few relevant studies, a lack of the certificated standard, massive related-parameters and the shortage of common SCMs. For the first three problems, it can be solved with the development of 3D printable concrete. For the last one, calcined clay is one potential alternative for developing sustainable 3D printable concrete in the areas where are in short supply of fly ash and silica fume.