Tribological interaction between polytetrafluoroethylene and silicon oxide surfaces

Cataloged from PDF version of article.We investigated the tribological interaction between polytetrafluoroethylene (PTFE) and silicon oxide surfaces. A simple rig was designed to bring about a friction between the surfaces via sliding a piece of PTFE on a thermally oxidized silicon wafer specimen. A very mild inclination (similar to 0.5 degrees) along the sliding motion was also employed in order to monitor the tribological interaction in a gradual manner as a function of increasing contact force. Additionally, some patterns were sketched on the silicon oxide surface using the PTFE tip to investigate changes produced in the hydrophobicity of the surface, where the approximate water contact angle was 45 degrees before the transfer. The nature of the transferred materials was characterized by X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). XPS results revealed that PTFE was faithfully transferred onto the silicon oxide surface upon even at the slightest contact and SEM images demonstrated that stable morphological changes could be imparted onto the surface. The minimum apparent contact pressure to realize the PTFE transfer is estimated as 5 kPa, much lower than reported previously. Stability of the patterns imparted towards many chemical washing processes lead us to postulate that the interaction is most likely to be chemical. Contact angle measurements, which were carried out to characterize and monitor the hydrophobicity of the silicon oxide surface, showed that upon PTFE transfer the hydrophobicity of the SiO2 surface could be significantly enhanced, which might also depend upon the pattern sketched onto the surface. Contact angle values above 100 degrees were obtained. (C) 2014 AIP Publishing LLC

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. Many structures built using BFSC could be prone to this mechanism, i.e. carbonation enhanced chloride induced corrosion, negatively affecting their durability. Focus of the research was given on the influence of carbonation on the chloride penetration resistance of BFSC mortars with varying slag content. In light of the characteristics from the overpass case, it was assumed that first there is a period of carbonation during sheltered exposure, and subsequently joint leakage causes exposure to chlorides. In order to identify the influence of slag content on carbonation, chloride penetration resistance and their coupled effect, mortars with twelve cement blends in a range of 0-70% slag were evaluated based on chloride migration coefficient, accelerated carbonation and electrical resistivity. This study shows that carbonation of BFSC mortars increases the porosity, consequently decreasing the chloride penetration resistance. Binders with 50% or more slag were found to have a significantly lower resistance after carbonation. Consequently, the chloride penetration resistance of a given concrete cover strongly depends on the duration of carbonation and the resulting carbonation depth, hence influencing its lifespan. The service life was estimated using a simplified model for the chloride penetration time of a combined carbonated and uncarbonated layer. It was found that mortar with a slag content between 35 and 50% that was carbonated before chloride exposure show the lowest influence of carbonation on the chloride penetration resistance. © Springer International Publishing AG 2018. Bam; Cement and Beton Centrum; et al.; Rijkswaterstaat - Ministry of Infrastructure and the Environment; Van Hattum en Blankevoort; VolkerInfr

Microscopy techniques for determining water-cement (w/c) ratio in hardened concrete: A round-robin assessment

Water to cement (w/c) ratio is usually the most important parameter specified in concrete design and is sometimes the subject of dispute when a shortfall in concrete strength or durability is an issue. However, determination of w/c ratio in hardened concrete by testing is very difficult once the concrete has set. This paper presents the results from an inter-laboratory round-robin study organised by the Applied Petrography Group to evaluate and compare microscopy methods for measuring w/c ratio in hardened concrete. Five concrete prisms with w/c ratios ranging from 0.35 to 0.55, but otherwise identical in mix design were prepared independently and distributed to 11 participating petrographic laboratories across Europe. Participants used a range of methods routine to their laboratory and these are broadly divided into visual assessment, measurement of fluorescent intensity and quantitative backscattered electron microscopy. Some participants determined w/c ratio using more than one method or operator. Consequently, 100 individual w/c ratio determinations were collected, representing the largest study of its type ever undertaken. The majority (81%) of the results are accurate to within ± 0.1 of the target mix w/c ratios, 58% come to within ± 0.05 and 37% are within ± 0.025. The study shows that microscopy-based methods are more accurate and reliable compared to the BS 1881-124 physicochemical method for determining w/c ratio. The practical significance, potential sources of errors and limitations are discussed with the view to inform future applications

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.Structural EngineeringCivil Engineering and Geoscience

The characterisation, improvement and modelling aspects of Frost Salt Scaling of Cement-Based Materials with a High Slag Content

Blast furnace slag cement concrete is used extensively in a number of countries. In comparison with OPC, it is particularly well known for its excellent performance in marine environments. One dis-advantage of slag cement is its vulnerability to scaling under the combined load of freezing-thawing and de-icing salts. The current investigation was triggered by positive observations regard-ing certain grinding agents used in slag cement production to improve frost salt scaling resistance. The investigation was aimed at explaining the cause of this improvement, at finding alternative methods to improve scaling resistance and at developing a model that would be suitable for the simulation of frost salt scaling behaviour. The investigation conclusions are essentially confined to high slag cement, particularly type CEM III 42,5/B which has a 67% granulated slag content. The w/c ratio of the paste, mortar and concrete specimens is generally maintained at 0,45. Carbonation, known as the critical parameter in frost salt scaling, constituted the key area of inter-est. From previous investigations it is known that carbonation increases porosity and coarsens the pore system in slag cement paste while it actually does the reverse in OPC paste. In the light of lit-erature a new hypothesis has been suggested that the transition zones, which are the weakest points in normal-performance cement-based materials, critically determine frost salt scaling resis-tance. These zones are even more indicative in the case of slag cement pastes because of the sig-nificant amount of transition zones that can be weakened by carbonation unlike with low-slag ce-ment or OPC pastes. In the present investigation it was observed that carbonation causes significant slag cement paste shrinkage. It was especially the transition zones between non-reacted slag particles and hydration products that were found to be affected. Consequently this process leads to the paste having a coarser pore structure thus making it prone to greater water uptake when compared to non-carbonated slag cement paste or OPC paste. The new hypothesis was supported by findings emerging from the ESEM study. It was observed that frost salt scaling attack generates cracks in the microstructure which adhere to slag-matrix interfacial zones. This was confirmed by nano-indentation tests which demonstrated that carbonation creates a significant number of weak zones in the slag cement paste. In the case of OPC paste the picture that emerged was quite different. Natural air carbonation influences the mineral characteristics of cement pastes. The XRD study re-vealed that both slag cement paste and OPC paste possess various types of carbonate minerals, namely: calcite, aragonite and vaterite. However, accelerated carbonation creates overwhelmingly stable calcite phases in both types of cement pastes which are subsequently transformed from me-tastable carbonates. This observation draws attention to the role played by Ca(OH)2 in the good scaling resistance of OPC or low-slag cement systems. A curing regime, especially curing in lime water, appears to be favourable for slag cement materials. However, when compared to the effect of carbonation, the influence that the curing water quality has on scaling resistance is minimal. The contribution made by prolonged water curing to scaling resistance could have been greater but, as it was, the curing periods were limited to 5 weeks in the interests of remaining realistic and practi-cal. The main goal of the project was to investigate the improvements in frost salt scaling resistance in-stigated by chemical grinding agents on the basis of the various positive results gained from the preliminary tests. The intention was to study the effects that the chemicals had on the cement paste microstructure in order to understand frost salt scaling resistance in slag cement concrete and so as to contribute to structural improvements in that area. A microstructural comparative study was carried out on slag cement pastes that contain alkanola-mines/hydrocarboxylates (the best performing ones) and diethylene glycolâbased (the worst per-forming example) grinding agents. The most notable difference was in the pore structure of the paste samples. Alkanolamines/hydrocarboxylates-based grinding agents were found to produce smaller pore sizes when compared to the ones containing diethylene glycol. This is consequently likely to give rise to higher carbonation resistance, lower water uptake and, eventually, to higher frost salt scaling resistance. However, the improvement achieved by alkanolamines / hydrocarboxy-lates is not sufficient to enhance the scaling resistance of the slag cement materials investigated in similar detail to OPC pastes. Another technique that was investigated was sodium monofluorophosphate (Na-MFP) surface treatment. Remarkable improvements in frost salt scaling resistance were achieved by applying a 10% Na-MFP solution to the surface of the carbonated slag cement paste and concrete. The scal-ing resistance improved by about 95% after 7 freeze-thawing cycles. Evidence was found pertain-ing to the reaction between Na-MFP and metastable carbonates in the carbonated slag pastes. The application appears to significantly increase the tensile strength of the carbonated slag cement paste which is extremely favourable in terms of scaling resistance. The study finally resulted in the development of a new integrated model. The model mainly takes into account the glue-spall theory and the hypothesis developed in this thesis and it runs on the Delft Lattice Model platform. The model successfully demonstrates the experimental observations and the crack patterns created by the scaling action. The glue-spall theory suggests that cement-based material surface scaling derives from external ice layer cracking due to further cooling. Cooling consequently generates tensile stress due to the shrinkage of ice and causes cracking when the stress exceeds the tensile strength of the ice. This theory can explain many phenomena including the pessimum effect. On the basis of this theory, the new integrated model proved to be capable of simulating two important experimental observa-tions. Under identical conditions the model can predict higher surface scaling at a 3% salt concen-tration level in relation to higher and lower values. The effect of ice layer thickness is furthermore found to be crucial with respect to frost salt scaling. Under identical material and environmental conditions the thicker external ice layer creates more damage than thinner ice layers. This observa-tion was also successfully demonstrated with the new integrated model.Civil Engineering and Geoscience

EMABM 2015: Proceedings of the 15th Euroseminar on Microscopy Applied to Building Materials, Delft, The Netherlands, 17-19 June 2015

Structural EngineeringCivil Engineering and Geoscience

Lithium migration on mortar specimens with embedded cathode

Alkali-Silica Reaction (ASR) is a durability problem that affects numerous concrete structures woridwide. As the reaction progresses. it might lead to deleterious expansion and cracking. The addition of lithium into fresh concrete is already acknowledged as a preventive method. In existing structures, however, lithium would need to be driven into the concrete and the most effective way is through ionic migration. For migration to occur in reinforced concrete, power is supplied between an external electrode and the rebar. If the reinforcement is the cathode, it attracts positive ions, such as lithium ions. Several studies have been conducted on the use of lithium migration as a treatment for ASR. Nevertheless, so far, there is no agreement on the conclusions. It is necessary to better understand this process order to develop a possible treatment against ASR. In this work, a preliminary investigation on lithium migration into mortar specimens with embedded cathode is presented

Optimizing self-healing mechanism of slag cement concrete under natural carbonation (poster abstract)

Structural EngineeringCivil Engineering and Geoscience

Modeling of expansion and cracking due to ASR with a 3D lattice model

It is generally possible to consider modeling of ASR damage in concrete in two main groups: modeling of gel formation and its expansion; modeling of ASR related damage. In this paper, authors take an attempt to combine both: simulating the correct crack formation and the connected concrete expansion. It is aimed to simulate ASR damage in a cementitious material bearing reactive aggregates. The model that is used is a 3D lattice type model. It models concrete on a meso-scale in which particles embedded in a cement matrix are taken into account. The particle structure is obtained by CT-scanning of samples. With the model the concrete expansion can be simulated. One of the inputs in the model is the local expansion of the gel. For that the mechanical properties of the gel should be known, which are obtained from an experimental procedure developed by the authors.Structural EngineeringCivil Engineering and Geoscience