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Master of Science / Department of Civil Engineering / Asadollah Esmaeily / Reinforced concrete exceptional durability is a major reason why it is the most popular structural material in many infrastructures around the world. Most concrete structures serve for several decades; therefore problems of concrete durability gradually arise. To insure that concrete structures perform functionally, it is necessary to maintain and inspect them regularly. The durability of the reinforced concrete structures generally depends on four major factors: structure design and construction, maintenance, concrete aggregates, and environmental conditions. The most common causes of concrete deterioration are carbonation, design and construction errors, alkali-aggregate reactions, freeze-thaw cycles, and corrosion. Each type of concrete deterioration has its own signs and characteristics. Choosing the best repair technique to address concrete deterioration requires specific analysis and tests to find the cause of the deterioration and the extent of the damage. This study analyzes concrete structures inspection techniques to recognize the source of the problem and the part of the structure which has been affected. Choosing the most proper repair and strengthening techniques to prevent the structure from getting exposed to any further environmental and chemical are the next steps.

Thesis (MEng)--Stellenbosch University, 2015. / ENGLISH ABSTRACT: The building industry is responsible for a substantial contribution to pollution. The production of building materials, as well as the operation and maintenance of structures leads to large amounts of carbon-dioxide (CO2) being release in the atmosphere. The use of renewable resources and construction materials is just one of the ways in which the carbon footprint of the building industry can be reduced. Sisal fibre is one such renewable material. Sisal fibre is a natural fibre from the Agave Sisalana plant. The possibility of incorporating sisal fibre in a cement-based matrix to replace conventional steel and synthetic fibres has been brought to the attention of researchers. Sisal fibre has a high tensile strength in excess of polypropylene fibre and comparable to PVA fibre. Sisal fibre consists mainly of cellulose, hemi-cellulose and lignin. The disadvantage of incorporating sisal fibre in a cement-based matrix is the degradation of the composite. Sisal fibres tend to degrade in an alkaline environment due to changes in the morphology of the fibre. The pore water in a cement base matrix is highly alkaline which leads to the degradation of the fibres and reduced strength of the composite over time. Sisal fibre reinforced cement-based composites (SFRCC) were investigated to evaluate the durability of the composites. Two chemical treatments, alkaline treatment and acetylation, were performed on the fibre at different concentrations to improve the resistance of the fibre to alkaline attack. Alkaline treatment was performed by using sodium hydroxide (NaOH), while acetylation was performed by using acetic acid or acetic anhydride. Single fibre pull-out (SFP) tests were performed to evaluate the influence of chemical treatment on fibre strength, to study the fibre-matrix interaction and to determine a critical fibre length. A matrix consisting of ordinary Portland cement (OPC), sand and water were used for the SFP tests. This matrix, as well as alternative matrices containing fly ash (FA) and condensed silica fume (CSF) as supplementary cementitious material, were reinforced with 1% sisal fibre (by volume) cut to a length of 20 mm. The OPC matrix was reinforced with untreated- and treated fibre while the alternative matrices were reinforced with untreated fibre. Alternative matrices containing varying fibre volumes and lengths were also produced. Three-point bending- (indirect), direct tensile- and compression tests were performed on specimens at an age of 28 days to determine the strength of the matrix. The remainder of the specimens were subjected to ageing by extended curing in water at 24˚C and 70˚C respectively and by alternate cycles of wetting and drying, after which it was tested at an age of 90 days from production to evaluate the durability of the fibre. An increase in fibre volume led to a decrease in compressive strength and peak tensile strength. The optimum fibre length at a volume of 1% was 20 mm for which the highest compression strength was recorded. The combination of alkali treatment and acetylation was the most effective treatment condition, followed by alkali treatment at low concentrations of sodium hydroxide. At higher concentrations of sodium hydroxide, a significant reduction in strength was recorded. The addition of supplementary cementitious materials also proved to be effective in mitigating degradation, especially in the cases where CSF was used. FA proved to be less effective in reducing the alkalinity of the matrix. However, the use of FA as fine filler resulted in higher strengths. Specimens manufactured by extrusion did not have superior mechanical properties to cast specimens. The conclusion was made that the use of sisal fibre in a cement-based matrix is effective in providing ductile failure. Chemical treatment and the addition of supplementary cementitious materials did improve the durability of the specimens, although degradation still took place. / AFRIKAANSE OPSOMMING: Die boubedryf is verantwoordelik vir 'n aansienlike bydrae tot besoedeling. Die produksie van boumateriale, sowel as die bedryf en instandhouding van strukture lei tot groot hoeveelhede koolstof dioksied (CO2) wat in die atmosfeer vrygestel word. Die gebruik van hernubare hulpbronne en boumateriale is maar net een van die maniere waarop die koolstof voetspoor van die boubedryf verminder kan word. Sisal vesels is 'n voorbeeld van 'n hernubare materiaal. Sisal vesel is 'n natuurlike vesel afkomstig vanaf die Agave Sisalana plant. Die moontlikheid om sisal vesels in 'n sement gebasseerde matriks te gebruik om konvensionele staal en sintetiese vesels te vervang, is tot die aandag van navorsers gebring. Sisal vesel het 'n hoër treksterkte as polipropileen vesels en die treksterkte vergelyk goed met die van PVA vesels. Sisal vesel bestaan hoofsaaklik uit sellulose, hemi-sellulose en lignien. Die nadeel verbonde aan die gebruik van sisal vesels in 'n sement gebasseerde matriks is die degradasie van die komposiet. Sisal vesels is geneig om af te breek in 'n alkaliese omgewing as gevolg van veranderinge wat in die morfologie van die vesel plaasvind. Die water in die porieë van 'n sement gebasseerde matriks is hoogs alkalies wat lei daartoe dat die vesel afgebreek word en die sterkte van die komposiet afneem oor tyd. Sisal vesel versterkte sement gebasseerde komposiete is ondersoek om die duursaamheid van die komposiete te evalueer. Twee chemiese behandelings, alkaliese behandeling en asetilering, is uitgevoer op die vesels teen verskillende konsentrasies om die weerstand van die vesels teen alkaliese aanslag te verbeter. Alkaliese behandeling was uitgevoer met natrium-hidroksied (NaOH) terwyl asetilering met asynsuur en asynsuurhidried uitgevoer is. Enkel vesel uittrek toetse is uitgevoer om die invloed van chemiese behandeling op veselsterkte te evalueer, om die vesel/matriks interaksie te bestudeer en om die kritiese vesellengte te bepaal. 'n Matriks wat uit gewone Portland sement (OPC), sand en water bestaan, is gebruik vir die enkel vesel uittrek toetse. Dieselfde matriks, sowel as alternatiewe matrikse wat vliegas (FA) en gekondenseerde silika dampe (CSF) as aanvullende sementagtige materiaal bevat, is versterk met 1% vesel (by volume) wat 20 mm lank gesny is. Die OPC matriks was versterk met onbehandelde- en behandelde vesels, terwyl die alternatiewe matrikse met onbehandelde vesels versterk is. Matrikse wat wisselende vesel volumes en lengtes bevat het is ook vervaardig. Drie-punt buigtoetse (indirek), direkte trek toetse en druktoetse is uitgevoer op proefstukke teen 'n ouderdom van 28 dae om die sterkte van die matriks te bepaal. Die oorblywende proefstukke is onderwerp aan veroudering deur verlengde nabehandeling in water teen 24˚C en 70˚C onderskeidelik en deur afwissilende siklusse van nat- en droogmaak waarna dit op 'n ouderdom van 90 dae vanaf vervaardiging getoets is om die duursaamheid van die matriks te evalueer. 'n Toename in vesel volume het tot 'n afname in druksterkte en piek treksterkte gelei. Die optimum vesel lengte teen 'n volume van 1% was 20 mm, waarvoor die hoogste druksterkte opgeteken is. Die kombinasie van alkaliese behandeling en asetilering was die mees effektiewe behandeling, gevolg deur alkaliese behandeling by lae konsentrasies natrium-hidroksied. Vir hoë konsentrasies natrium-hidroksied is 'n aansienlike afname in sterkte opgeteken. Die toevoeging van aanvullende sementagtige materiale was ook effektief om die degradadering van die vesels te verminder, veral in die gevalle waar CSF gebruik is. FA was minder effektief om die alkaliniteit van die matriks te verminder. Die gebruik van FA as fyn vuller het nietemin hoër sterkte tot gevolg gehad. Proefstukke wat deur ekstrusie vervaardig is, het nie beter meganiese eienskappe gehad as proefstukke wat gegiet is nie. Daar is tot die gevolgtrekking gekom dat sisal vesel in 'n sement gebasseerde matriks wel effektief is om 'n duktiele falingsmode te voorsien. Chemiese behandeling en die toevoeging van aanvullende sementagtige materiale het die duursaamheid van die proefstukke verbeter, alhoewel degradering steeds plaasgevind het.

This study combined research available through literature with extensive experimental studies and substantial physical modelling to estimate the remaining ultimate life of large offshore reinforced concrete structures. Although much research has focussed on concrete degradation due to chloride ingress, corrosion of permanently submerged concrete is regarded as negligible due to the long-assumed apparent worst case of tidal or splash zone exposure. Around 350 specimens were tested with a further 200 exposed for further testing by future research groups. Specimens ranged in size from standard cubes to various beam lengths up to 1.5 metres, allowing for material and structural properties to be assessed. My original contribution to knowledge in the sector enhances the fundamental understanding of corrosion in subsea concrete, challenging the generally held belief of negligible corrosion. Results and modelling provides an improved ability to ultimately estimate the longevity of fully submerged offshore reinforced concrete. Throughout this thesis, the results from experimental works, carried out as a direct result of the lack of data or information in literature, are reported, assessed and then utilised to provide updated ultimate life estimations. With the current offshore concrete structures currently coming to the end of their service life, and the likelihood of further offshore development using concrete for the renewables sector, understanding the long-term degradation is vital in determining the most effective decommissioning and derogation options. The research carried out directly provides detailed information of the likely time-to-failure, allowing for an informed decision to be made on operational and decommissioning plans. Experimental work was carried out over four main phases; corrosion initiation due to bulk diffusion of chlorides (Phase I), corrosion propagation in low oxygen environments (Phase II), corrosion in statically and dynamically cracked sections (Phase III) and structural response of heavily corroded individual and lapped bar sections (Phase IV). Phase I work shows a marked difference between submerged exposures to seawater as opposed to NaCl solution, the unsuitability for accelerated testing with seawater and the likelihood of rapid initiation in offshore structures. Further experimental works through Phases II and III found that although exposed to low oxygen concentrations, reinforcement corrosion continued at significant rates. A variation between anode sizes on the reinforcement is noted, but critically the cross sectional area of the steel was still reduced, albeit in fewer locations. Corrosive products were visibly different, with fewer expansive products, if any, present. Additionally, this study further highlights the importance of cracking on corrosion, currently ignored by recent model codes, such as the fib Model Code 2010, up to 0.2mm crack width. A linear relationship was found between crack width and corrosion rates, with cracking above 0.1mm considered significant. The loss of cross sectional area due to propagation was determined for the given environment, and consequently further studies were initiated in an attempt to determine the relationship between this corrosion propagation and the reduced serviceability or ultimate life of concrete beams. Serviceability, defined by beam stiffness, was reduced due to bond loss along reinforcement. Most importantly, however, results prove that the loss of cross sectional area to be the critical influence on loss of ultimate life. Initial estimates on the remaining ultimate life of the large offshore structures support early rough work that the structures would last centuries. This thesis, however, has shown this is due to the ability of concrete structures with such large volumes of steel to continue to ultimately withstand loading at high corrosion percentages and not due to negligible corrosion, or long initiation periods, commonly suggested in submerged, low oxygen environments.

Concrete repair materials are being widely used to restore and extend the service life of structures. While most cement-based repair materials are compatible with concrete structures, their durability properties do not attract much attention which it deserves from researchers. Since repair materials can deteriorate like conventional concrete, the search for reliable, long-lasting concrete repair materials is becoming more intensive. Amongst other factors, concrete permeability and chloride diffusivity within concrete are believed to play a major role in determining the durability and success of the repair. These two parameters determine the penetration rate of aggressive substances into concrete and how fast degradation could take place. A number of test methods have been proposed to study these two factors, and the commonly used test methods are water penetration, surface/bulk electrical resistivity, rapid chloride permeability (RCP), and half-cell potential. However, the relationship between each durability test method and their correlation with compressive strength measurement have not been fully understood. So, in this study, we aim for using multiple testing techniques, destructive and non-destructive, to evaluate the durability of concrete repair materials as well as correlating different test methods. Three types of commercially available cement-based materials are tested and evaluated, and results have indicated that cementitious concrete mortar (termed as Mix M) amongst others has the best durability performance which means low water permeability, high resistivity, and compressive strength. Whereas, the flexural performance of Mix M still needs some improvement in terms of flexural strength and flexural toughness. For various durability testing methods, surface resistivity is found to have a strong linear relation and a polynomial relation to bulk resistivity and water permeability respectively. No relationship is established between concrete resistivity and compressive strength, though high-strength concrete tends to have a high resistivity in our study. RCP test results do not correlate well with resistivity measurements, which requires further study to overcome its heating and binding effect when measurements are being taken. Half-cell potential method is used for validating test results but it reveals no difference for materials with different permeability and resistivity. A model is proposed to counteract temperature’s effect while calculating the coefficient of diffusion, which indicates the concrete to resist chloride diffusion. It is found that this model can shift the RCP measurement slightly closer to its theoretical prediction but the difference between them is still large. Therefore, further research is required for acquiring more raw data from RCP measurements as the regression analysis input. In addition, a more comprehensive model that involves more correction factors for binding effects, etc., is also needed. / Graduate / 2020-04-30

Highway air pollution is a significant environmental threat that has staggering implications for human health worldwide. Photocatalytic materials have the potential to reduce air pollution levels near major highways using ultraviolet radiation. This project, funded by the Texas Department of Transportation, evaluated photocatalytic efficiency and durability of several commercially-available photocatalytic coatings for use on concrete structures near highways. The research presented in this thesis involved obtaining concrete highway barriers and creating concrete slabs for outdoor testing and laboratory chamber testing. Three commercially-available coatings were applied to the specimens for testing: Keim Soldalit ME paint, TxActive Stucco Cement, and Pureti Clean. Field sites were set up near major highways in Houston and Austin, Texas. Durability and photocatalytic efficiency were regularly monitored at the field sites using ion chromatography and spectrophotometry. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were conducted on samples from each of the specimens taken before and after placement at the field sites in order to understand durability of the photocatalytic materials that were exposed outdoors. SEM results from this research project revealed that the photocatalytic material in the TxActive stucco and Keim paint remained in their original distribution after the exposure period, while the photocatalytic material in the Pureti Clean product appeared to decrease. XRD results remained fundamentally consistent for all coatings. Ion chromatography results showed that TxActive specimens had the highest surface levels of nitrates and nitrites between rainfall events, which indicates photocatalytic activity. Spectrophotometry results revealed a decrease in brightness for the Keim paint-coated specimens and no change or an increase in brightness for the TxActive stucco over time. The spectrophotometry results indicate how many surface contaminants are accumulating on the surface of a specimen, and thereby how efficiently sunlight is reaching the surface and activating the photocatalytic process. Results obtained from this research project may be influential in the selection of a means for reducing highway pollution in Texas. / text

Corrosion of reinforcing steel in concrete due to chloride ingress is one of the main causes of the deterioration of reinforced concrete structures, particularly in marine environments. It is therefore important to develop a reliable prediction model of chloride ingress into concrete, which can be used to predict the chloride concentration profiles accurately to help to assess the service life for reinforced concrete structures. Cementitious materials are porous media with a highly complex and active chemical composition. Ionic transport in cementitious materials is a complicated process involving mechanisms such as diffusion, migration, ionic binding, adsorption and electrochemical interactions taking place in the pore solution of the materials. The process is dependent on not only the microstructural properties of the materials such as porosity, pore size distribution and connectivity but also the electrochemical properties of the pore solution including ionic adsorption and ion-ion interactions. This thesis presents a numerical study on the multi-component ionic transport in concrete with the main focus on the microscopic scale. This study first investigated the impact of the Electric Double Layer (EDL) on the ionic transport in cement-based materials. The EDL is a well-known phenomenon found in porous materials, which caused by the surface charges at the interface between solid surfaces and pore solutions. The numerical investigation is performed by solving the multi-component ionic transport model with considering the surface charges for a cement paste subjected to an externally applied electric field. The surface charge in the present model is taken into account by modifying the Nernst-Planck equation in which the electrostatic potential is dependent not only on the externally applied electric field but also on the dissimilar diffusivity of different ionic species including the surface charges. Some important features about the impact of surface charge on the concentration distribution, migration speed and flux of individual ionic species are discussed. Then a new one-dimensional numerical model for the multi-component ionic transport in concrete to simulate the rapid chloride migration test is proposed. Advantages and disadvantages of the traditional methods used to determine the local electrostatic potential, i.e. electro-neutrality condition and Poisson’s equation, are illustrated. Based on the discussion a new electro-neutrality condition is presented, which can avoid the numerical difficulties caused by the Poisson’s equation, and remain the non-linearity of the electric field distribution. This model with the new electro-neutrality condition is employed to simulate the RCM test to prove its applicability. The new model is promising in solving the multi-component ionic transport problems especially in microscopic scale. Lastly, a one-dimensional numerical investigation on the chloride ingress in a surface-treated mortar with considering the penetration of sealer induced porosity gradient was performed. The numerical model was carefully treated to apply governing equations of ionic transport to this situation of two pore structures, with every parameter clearly defined on the microscopic scale.

The primary cause of corrosion in transportation structures is due to chlorides which are applied to bridge decks as deicing salts. The direct cost of corrosion damage to the country's infrastructure is approximately $8.3 billion per year. One of the most common corrosion abatement methods in the United States is the barrier protection implemented through the application of fusion bonded epoxy coatings. The purpose of this study was to investigate various coating and exposure parameters to determine their effects on the corrosion of reinforcing steel. The parameters investigated were: chloride content at the bar depth, coated bar corroded area, corrosion product color under the coating, epoxy coating adhesion, coating color, coating damage (holidays and holes), coating thickness, TGA, DSC and EDS analysis and SEM coating cracking investigation. This was accomplished by testing new coated bar specimens as well as specimens extracted from 27 bridge decks located in Virginia. This study demonstrated the following: The extracted ECR coating samples presented extensive cracking compared to the new ECR samples in which the coating cracking was limited to only one sample. The DSC results showed that both the extracted samples as well as new samples are not fully cured during the manufacturing process. The coating degree of curing data also showed that the bars are insufficiently and unevenly heated prior to the application of the powder coating. Additionally, the samples investigated presented significant permanent adhesion loss with little or no epoxy coating residue present on the bar surface, while the EDS analysis showed that once adhesion is lost, corrosion will proceed unimpeded under the coating even in the absence of chlorides. The parameters that presented a direct correlation with the observed corrosion activity were the number of holidays and the number of damaged areas per unit length of bar. This indicates that the passivation of the bare steel exposed to the concrete pore solution at the breaches in the epoxy coating is not the same as a bare bar under similar exposure conditions allowing it instead to corrode at lower concrete chloride concentration levels than bare bars. The results also show a distinct loss of quality control in the handling and possibly storage of new coated bars. The new ECR samples had significantly higher damage density than the samples extracted from concrete even though the coating is damaged during the placement of the concrete, while there was no change in the number of holidays and cure condition. Finally, the data presented further evidence that while limited, the non-destructive corrosion assessment methods available for bare steel reinforced structures may also be used on ECR reinforced structures. In particular, the corrosion rate measurements correlated reasonably well with the chloride concentrations at bar level. This indicates that while the chlorides may not influence the corrosion activity under the coating, they do influence the corrosion activity at breaches in the coating. / Ph. D.

O presente trabalho contempla um estudo experimental de análise do comportamento de concretos projetados por via seca, através de ensaios indicadores dos principais parâmetros de durabilidade, constantes de especificações nacionais e internacionais. A pesquisa envolve a avaliação comparativa entre quatro tipos de cimento especialmente formulados para uso em concreto projetado, em relação a outros dois tipos de cimento tradicionalmente empregados na composição de misturas projetáveis, no que se refere ao atendimento aos requisitos relativos à durabilidade, normalmente especificados para o material. Os concretos projetados, elaborados com consumos de cimento de 300 kg/m³, 350 kg/m³, 400 kg/m³ e 450kg/m³ foram inicialmente caracterizados, no estado fresco, com base em ensaios de consistência e depois produzidas amostras, em condições similares às de obra e estas submetidas aos ensaios balizadores de parâmetros de durabilidade adotados neste estudo, a saber: Absorção por Imersão e Fervura, Índice de Vazios, Massa Específica Real; Absorção de Água por Capilaridade; Migração de Íons Cloreto; Resistividade Elétrica; Penetração de Água sob Pressão e Carbonatação Acelerada. Como alguns dos ensaios supracitados não são destrutivos, foi possível aproveitar corpos-de-prova, que foram depois submetidos a ensaios de resistência à compressão axial e à tração através da compressão diametral, que também contribuíram para caracterização do material. De um modo geral, os resultados foram satisfatórios e atenderam às especificações vigentes. Contudo, os concretos elaborados com cimentos especiais, formulados com granulometria mais fina e adição de metacaulim, apresentaram indicadores de durabilidade consideravelmente superiores aos dos concretos elaborados com cimentos convencionais ou mesmo cimentos especiais formulados com aditivos, inclusive para consumos relativamente baixos. Apesar de não ter sido o foco principal deste trabalho, vale ressaltar os surpreendentes resultados dos ensaios de resistência mecânica, obtidos para os concretos elaborados com cimentos especiais formulados com granulometria mais fina e adição de metacaulim, que atingiram valores de resistência à compressão nunca antes observados, fato que pode tornar o material bastante promissor. / The present work involves an experimental study with dry-mix shotcrete aiming at the behavior analysis, through indicative testing of the main durability parameters of national and international specifications. The research involves the use of four cement types especially formulated for use in shotcrete use in comparison with two other cements that are traditionally used in mix production for shotcrete spraying. The shotcrete mixtures were elaborated with cement consumptions of 300 kg/m³, 350 kg/m³, 400 kg/m³ and 450kg/m³. They were characterized initially, in the fresh state, in terms of stiffness tests, and later, cores were extracted to produce samples, in similar conditions and submitted to ordinary durability tests: Water Absorption, Volume of Voids, Specific Weight; Water Absorption by Capillarity; Chloride Migration; Electric Resistivity; Water Penetration under Pressure and Accelerated Carbonatation. As some of the tests are not destructive, some of the samples was used for compressive and tensile (Brazilian Test) strength determination, what also contributed to additional characterization of the material. In general, the results were very satisfactory and they fit to the specifications requirements. However, the shotcretes elaborated with special cements, formulated with finer gradation and metakaolin addition, presented a much better behavior in terms of durability indicators comparing to the shotcretes produced with conventional cements and cements formulated with accelerators, even when, relatively low consumption of cement was used. Despite not being the main focus of this work, it is worth mentioning the surprising results of the testing of mechanical resistance, obtained for the shotcretes elaborated with special cements formulated with finer gradation and metakaolin addition, which reached resistance values to the compression never observed, making it a promising material.

In marine environments and where de-icing salts are applied, the degradation of reinforced concrete structures due to chloride induced corrosion of the reinforcement is a major problem. The expansive nature of the corrosion process results in cracking of the concrete and eventually spalling. In order to select suitable remedial measures it is necessary to make an assessment of the residual strength and the residual life. In order to investigate the effect of corrosion on bond strength of the reinforcement, specimens comprising square prismatic sections containing steel reinforcement in the four corners have been subjected to a wet-dry cycle and corrosion has been accelerated by polarising the bars. The research has studied the change of bond strength with level of corrosion for 12 mm and 16 mm bars with concrete cover of 1 and 3 times the bar size. The bond strength is assessed by means of pull out tests and the corresponding extent of corrosion has been assessed in terms of the mass loss. Observations and measurements of the form of the corrosion (pit dimensions and loss of bar diameter) are also presented. The relationship between bond strength and surface crack width has been investigated. Results show that the surface crack width may be a good indicator of residual bond strength. In addition, the influence on bond strength of concrete compressive strength, reinforcement cover, bar position and bar size on the change of bond strength has been explored. It should be noted that all conclusions drawn in this project are based on tests on specimens without shear reinforcement (unconfined) and that accelerated corrosion (by impressed current) has been adopted. Consequently, care should be exercised in applying these results directly to structures in the field. Additional research is needed to assess the influence of impressed current on crack patterns and the effect of shear reinforcement.