Dans le cadre de l'installation GANIL / SPIRAL-1, un système innovant de source d'ions cible (TISS) a été conçu pour produire des nouveaux faisceaux d'ions radioactifs compétitifs et déterminer dans quelle mesure il est possible d'améliorer les performances de production par l’optimisation de l’ensemble cible-source (ECS) d’ions dédié à la production d'un faisceau d'ions radioactifs spécifique.L'isotope d'alcalin de courte durée de 74Rb ( demi-vie = 64.8 ms) a été choisi comme cas d’école.En étudiant et en améliorant les processus impliqués dans le fonctionnement de l’ECS, i.e. production dans la cible par réaction fusion-évaporation, implantation dans- et sortie du graphite, effusion et ionisation, une efficacité de transformation atome-ion de 75% est attendue, menant à un taux d'ions de l'ordre de 104 pps à la sortie de l’ECS.Les performances estimées permettraient à l'installation GANIL / SPIRAL-1 de délivrer des faisceaux de 74Rb et 114Cs avec des intensités compétitives, jamais atteintes dans d'autres installations.Le dispositif de production a été développé sur la base de mon travail, pour répondre aux objectifs de physique et aux conditions d'opération de SPIRAL-1. L'efficacité, la fiabilité et donc la simplicité étaient essentielles. Le système répond à ces exigences. / In the frame of GANIL/SPIRAL-1 facility, an innovative target ion source system (TISS) has been designed in order to produce new competitive radioactive ion beams, and to determine to which extent it is possible to improve the production performances by optimizing a target ion source system dedicated to the production of a specific radioactive ion beam.The short-lived alkali isotope of 74Rb (half-life = 64.8 ms) has been chosen as a challenging case study.By studying and improving the processes involved in the TISS designed, i.e. in-target production by fusion-evaporation, implantation in and release out of graphite, effusion, and ionization, an atom-to-ion transformation efficiency of 75% was predicted, leading to an ion rate of the order of 104 pps at the exit of the TISS.The estimated performances would allow the GANIL / SPIRAL-1 installation to deliver beams of 74Rb and 114Cs with competitive intensities never reached in other installations.The production device was developed based on my work, to cope with physics objectives and with the conditions of the SPIRAL-1 operation. Efficiency, reliability and thus simplicity were essential. The system meets these requirements.The instrumentation needed for tests existed or have been specifically designed. The TISS has been built, assembled and partly tested.In parallel, issues have been addressed to answer related questions about sticking times, resistivity, and emissivity, to get inputs for the design of the TISS.Expected results with alkali allow thinking that the transposition of the principle to the production of neutron-deficient short-lived isotopes of noble gases and eventually metals could be performing.

Copies of author's previously published articles inserted. Bibliography: leaves 195-230. In this study the soil aqueous phase chemistry of Zn and Cu in alkaline sodic soils are investigated. The chemistry of trace metal ions at indigenous concentrations in alkaline sodic soils are reported. Metal ions at low concentrations are measured by the graphite furnace atomic absorption spectrometry (GFAAS) technique.

A new design AMTEC converter model was developed, and its effectiveness as a design tool was evaluated. To develop the model, requirements of the model were defined, several new design models were successively developed, and finally an optimal new design model was developed. The model was created within Sinda/Fluint, with its graphical interface, Thermal Desktop, a software package that can be used to conduct complex thermal and fluid analyses. Performance predictions were then correlated and compared with actual performance data from the Road Runner II AMTEC converter. Predicted performance results were within 10% of actual performance data for all operating conditions analyzed. This accuracy tended to increase within operating ranges that would be more likely encountered in AMTEC applications. Performance predictions and parametric design studies were then performed on a proposed new design converter model with a variety of annular condenser heights and with potassium as a working fluid to evaluate the effects of various design modifications. Results clearly indicated the effects of the converter design modifications on the converter's power and efficiency, thus simplifying the design optimization process. With the close correlation to actual data and the design information obtained from parametric studies, it was determined that the model could serve as an effective tool for the design of AMTEC converters.

Alkali Silica Reaction in concrete is a chemical deterioration process occurring between alkalis in cement paste and reactive aggregates. ASR increases expansion and cracking as well as other durability problems such as freezing and thawing. It is most probable that concrete structure will collapse unless mechanical, mineral, or chemical preventive measures are taken against ASR or this problem is realized and solved in the design stage of the concrete structure or later on. Rather than ordinary preventive measures in which mineral admixtures are used, mechanical ones were investigated in this study. In the experiment done by using the accelerated mortar bar method, reinforced concrete specimens on which pre-stressing force was applied were examined. The effects of reinforcement ratio and pre-stressing force on ASR based expansion and cracking were studied. Expansion and cracking developments in time were followed, and the connection between these phenomena and the energy produced by ASR was made. By applying the same mechanical preventive measures on the specimens prepared by using different reactive aggregates, the effectiveness of these methods with respect to the degree of v ASR was investigated. Also, the methods in question were compared with traditional preventive measures (fly ash). The investigation results show that reinforcement and pre-stressing force play a significant role in diminishing the effects of ASR.Keywords:Alkali-Silica Reaction, Reinforced Concrete Specimen, Pre-stressed Concrete, Mechanical Preventive Measures, Energy of ASR

This thesis deals with the preparation of chemically and/or enzymatically modified cellulose. This modification can be either irreversible or reversible. Irreversible modification is used to prepare cellulose derivatives as end products, whereas reversible modification is used to enhance solubility in the preparation of regenerated cellulose. The irreversible modification studied here was the preparation of carboxymethyl cellulose (CMC) using extended mercerization of a spruce dissolving pulp. More specifically the parameters studied were the effect of mercerization at different proportions of cellulose I and II in the dissolving pulp, the concentration of alkali, the temperature and the reaction time. The parameters evaluated were the degree of substitution, the filterability and the amount of gel obtained when the resulting CMC was dissolved in water. Molecular structures of CMC and its gel fractions were analysed by using NIR FT Raman spectroscopy. It was found that the alkali concentration in the mercerization stage had an extensive influence on the subsequent etherification reaction. FT Raman spectra of CMC samples and their gel fractions prepared with low NaOH concentrations (9%) in the mercerization stage indicated an incomplete transformation of cellulose to Na-cellulose before carboxymethylation to CMC. Low average DS values of the CMC, i.e. between 0.42 and 0.50 were obtained. Such CMC dissolved in water resulted in very thick and semi solid gum-like gels, probably due to an uneven distribution of substituents along the cellulose backbone. FT Raman spectra of CMC samples and their gel fractions mercerized at higher alkaline concentration, i.e. 18.25 and 27.5% in the mercerization stage, indicated on the other hand a complete transformation of cellulose to Na-cellulose before carboxymethylation to CMC. Higher average DS values of the CMC, i.e. between 0.88 and 1.05 were therefore obtained. When dissolved in water such CMC caused gel formation especially when prepared from dissolving pulp with a high fraction of cellulose II. The reversible modification studied was the dissolution of cellulose in NaOH/ZnO. Here the effect of enzyme pretreatment was investigated by using two mono-component enzymes; namely xylanase and endoglucanase, used in consecutive stages. It was found that although the crystallinity and the specific surface area of the dissolving pulp sustained minimal change during the enzymatic treatment; the solubility of pulp increased in a NaOH/ZnO solution from 29% for untreated pulp up to 81% for enzymatic pretreated pulp. / Baksidetext Cellulose can be chemically and/or enzymatically modified. Irreversible modification is used to prepare cellulose derivatives as end products, reversible modification to enhance solubility in the preparation of regenerated cellulose. The irreversible modification studied here was the preparation of carboxymethyl cellulose (CMC) using extended mercerization of a spruce dissolving pulp. More specifically the parameters studied were the effect of mercerization at different proportions of cellulose I and II in the dissolving pulp, the concentration of alkali, the temperature and the reaction time. It was found that the alkali concentration in the mercerization stage had an extensive influence on the subsequent etherification reaction. The content of cellulose II had little effect on degree of substitution (DS) at low NaOH concentration, but tended to decrease DS at higher NaOH concentration in both cases compared with cellulose I. It was also found that the content of cellulose II correlates with the gel formation obtained when the CMC is dissolved in water. The reversible modification studied was the dissolution of cellulose in NaOH/ZnO. Here the effect of enzyme pretreatment was investigated by using two mono-component enzymes; namely xylanase and endoglucanase, used in consecutive stages. It was found that the solubility of pulp increased in a NaOH/ZnO solution from 29% for untreated pulp up to 81% for enzymatic pretreated pulp.

The use of recycled aggregates in concrete opens a whole new range of possibilities in the reuse of materials in the building industry. This could be an important breakthrough for our society in our endeavours towards sustainable development. The trend of the utilisation of recycled aggregates is the solution to the problem of an excess of waste material, not forgetting the parallel trend of improvement of final product quality. The utilisation of waste construction materials has to be related to the application of quality guarantee systems in order to achieve suitable product properties. Therefore the complete understanding of the characteristics of new material becomes so important in order to point out its real possibilities. The studies on the use of recycled aggregates have been going on for 50 years. In fact, none of the results showed that recycled aggregates are unsuitable for structural use. Only having inadequate number of studies in durability aspects, made recycled aggregates to be preferred just as stuffing material for road construction. My thesis, aimed to focus on the possibility of the structural use of recycled aggregate concrete based on a better understanding of its microstructure.To begin with the characteristics of the aggregates were established, to study their possible application in concrete production. After analysis, the dosage procedure was carried out in order to produce four concrete mixtures using different percentages of recycled coarse aggregates (0% (HC), 25% (HR25), 50% (HR50) and 100% (HR100)) with the same compression strength. Raw coarse aggregates (granite) and sand (crushed limestone) were used in the different concrete mixes.Macroscopic and microscopic examination were carried out in HC, HR25, HR50 and HR100 concretes in order to observe the durability effects. The macroscopic examination determined the aggregates distribution, composition, the contaminants and aureoles around adhered mortar. Microscopic examination was carried out by Optical light transmitted microscope Leica Leitz DM-RXP, using Fluorescence Thin Sections, in order to analyse the cement paste, the new and old interfacial transition zones, secondary reactions as well as damage. Original aggregates and cement paste, interfacial transition zones and alkali silica gel produced due to reactive aggregates present in adhered mortar were analysed by SEM and EDX-maps. Beside macro and micro observations, shear failure behaviour of recycled aggregate concretes was studied. Shear failure test was found more appropriate, as concrete properties had more influence in this type of failure behaviour compared to the flexural failure where the reinforcement plays the important role. Sixteen beam specimens were cast and the structural behaviour of these beams was analysed using four different transversal reinforcements for each kind of concrete. An analytical prediction of the experimental results are carried out using a numerical model (Modified Compression Field Theory), using the codes AASHTO LRFD, CSA, Eurocode-2 and expressions proposed in the Spanish code EHE-99.Organic and inorganic compounds were found to be released from waste materials through leaching and dispersed into the soil and surface water. The leaching of these compounds were measured employing different codes, the two Dutch codes (NEN 7341 and 7345) and the European Normative (EN 12457-2).Some recommendations are given as to the aggregates characteristics to be used in concrete mixes, taking into account the European standards for recycled aggregates. Also suggestions are made for the production process of concrete using recycled aggregate. Mechanical properties of recycled aggregate concrete are studied and they are compared with that of conventional concrete. Based on the durability of the concrete, some suggestions are proposed with respect to possible alkali silica reaction between new cement and original fine aggregates. It is also determined that the effect of the use of recycled aggregate on the beams' shear strength depend on the percentage of coarse aggregate substituted. The applicability of concrete recycled aggregate with respect to its environmental behaviour is demonstrated. In conclusion, some suggestions for future studies are made which would help us in the evolution of our understanding in this field.

Pyrite (FeS2) is commonly present in complex sulphide ores in significant amounts. After the enrichment of such ores by flotation, pyrite is either produced as a separate concentrate and sold to acid manufactures or removed and disposed off as tailing. Due to lack of demand from manufacturers, most of pyrites is usually disposed off as tailing. Therefore, pyrite is usually a waste from complex sulphide ores. Yet, it may be a remediation additive for calcareous soils and calcareous- alkali soils deficient in Fe and other micronutrients such as Cu, Zn and Mn. Waste pyrite may be also an alternative amendment to gypsum because of the production of sulphuric acid which is effectively used in the reclamation of calcareous alkali soils. The effectiveness of adding waste pyrite and sulphuric acid produced from waste pyrite to calcareous-alkali soil (Sarayk&ouml / y-Ankara) and calcareous soil (Gaziantep) was studied under laboratory conditions. Pure gypsum was also used as an amendment for the comparison of the effectiveness of waste pyrite in the reclamation of alkali soils. Gypsum, powder waste pyrite and sulphuric acid were applied to the soil with reference to the gypsum requirement (GR) of the soils. Greenhouse pot tests were carried out with wheat as test plant to determine the effect of waste pyrite treatment on the plant yield (wheat) and on the amount of micronutrient (Fe, Cu, Zn, Mn) essential for plant growth. Hazard potential of pyritic tailings in terms of heavy metal contamination was also taken into account. The results showed that the soil pH and exchangeable sodium percentage (ESP), indicators of alkalization, decreased upon pyrite addition to calcareous- alkali soils of Sarayk&ouml / y-Ankara. It was also found that pyritic tailings were effective in the increasing level of essential micronutrients (Fe, Cu, Zn and Mn) for plant growth in both soils. This was ascertained by the dry matter yield of the plants in the green house pot tests. Heavy metal toxicity caused by pyrite which is a rightful concern remained well below the legal limits in the soils. Thus, it was concluded that the application of pyritic tailings promoted rapid amelioration of calcareous-alkali soil (Sarayk&ouml / y-Ankara) and calcareous soil (Gaziantep) with no deleterious heavy metal contamination.

Xylanases are xylan-degrading enzymes, belong to glycosyl hydrolases (GH). Xylanases from the two major families 10 (GH10) and 11 (GH11) catalyze the hydrolysis of internal β-, bonds of xylan backbone. Xylan is the second most abundant polysaccharide in nature. Nearly one third of the dry weight of the higher plants is xylan and therefore, xylanases have an important role in biomass conversions. Currently, the most effective application of xylanases is in prebleaching of kraft pulp to minimize the use of environmentally hazardous chemicals in the subsequent treatment stages. In recent years, therefore, attention is focused on to isolate and/or engineer the xylanases for the industrial requirements. The desirable roperties of xylanases in paper industry are stability and activity at high temperatures and alkaline pH. While he factors responsible for the thermal stability of GH10 xylanases have been analyzed, factors governing the alkaline stability of GH10 xylanases remain poorly understood. The present thesis reports the crystal structures of an alkali thermostable GH10 extracellular endo-xylanase (BSX) from an alkalophilic organism, Bacillus sp. NG-27 in free and xylosaccharides-bound form. The enzyme was purified from the native organism and crystallized. The structure was solved by molecular replacement method. The 2.2 Å crystal structure of the native BSX enzyme is the first structure of an alkali thermostable GH10 family xylanase from an alkalophilic organism. It has unveiled unique protein properties that can form the basis for improving the thermal, alkaline stability and activity by site directed mutagenesis. The comparative study, especially in relation to GH10 xylanases, deciphered important structural features which are likely to be responsible for the alkaline stability of the enzyme. The work exemplifies the mechanism of adaptation of enzymes to function under polyextreme conditions through changes in the nature and composition of solvent-exposed residues. As apparent from the comparative study, the enhanced stability of the protein can be attributed to the surface rich in acidic residues and less number of solvent-exposed Asn as seen in BSX. This situation which may be roughly described as “acidic residues outside and Asn inside”, is a notable feature of alkali-stable GH10 xylanases from alkalophilic organisms. In addition, the candidate has carried out the comprehensive database analysis of the occurrence of C-H…O hydrogen bonds in helices and helix termini of globular proteins. The study provides a compelling evidence that the main-chain Cα and the side-chains CH which participate in C-H…O hydrogen bonds collectively augment the cohesive energy and thereby contribute together with the classical N-H…O hydrogen bonds and other interactions to the overall stability of helix and therefore of proteins. Chapter 1 starts with a brief introduction of xylanases, their classifications and overall folds. At present, a little more than a dozen of crystal structures of GH10 xylanases are known and described in the literature. A brief mention about these structures and their optimum pH and temperature is outlined under a separate section. In view of the industrial importance of the study enzyme, the potential industrial and biotechnological applications of xylanases are detailed in this Chapter. A section is dedicated to describe the present study enzyme BSX, an alkali thermostable endo-xylanase from an alkalophilic bacterium, Bacillus sp. NG-27. BSX has a molecular mass of ~41 kDa and is optimally active at 343 K and at a pH of 8.4. The alkaline thermostability of the wild type BSX is likely to be industrially important. At the end, the scope of the present work is detailed. Chapter 2 presents the purification of xylanase (BSX) from Bacillus sp. NG-27, the crystallization of the native and xylosaccharides-bound BSX, the X-ray diffraction data collection on these crystals and processing of the data. Repeated attempts to crystallize the protein expressed in the chloroplast of transgenic tobacco plant were unsuccessful. However, crystallization was achieved with the protein sample purified from the native source by hanging drop vapour diffusion method. Crystals were grown at both acidic (4.6) and basic pH (8.5). The corresponding crystallization conditions are 0.2 M MgCl2, 0.1 M sodium acetate pH 4.6 and 20% PEG 550 MME and 0.1 M aCl, 0.01 M MgCl2, 0.1 M Tris-HCl pH 8.5 and 15% PEG 8000. Crystals grown at acidic pH were not suitable for X-ray diffraction study. Subsequently, crystal obtained at a basic pH of 8.5 was used for X-ray data collection and it diffracted X-rays to better than 2.2 Å at the home source at cryo-temperature (100 K). Native BSX crystals belong to monoclinic space group C2 with unit cell parameters a = 174.5 Å, b = 54.7 Å, c = 131.5 Å and β = 131.2°. Crystals of xylosaccharides-bound enzyme were grown in a slightly modified crystallization condition of native, 0.1 M NaCl, 0.2 M MgCl2, 0.1 M Tris-HCl pH 8.5 and 15% PEG 8000 and the enzyme was incubated with xylan prior to setting up the crystallization. Crystals belong to primitive orthorhombic space group P212121 with unit cell parameters a = 59.2 Å, b = 83.8 Å and c = 174.4 Å. A data set was collected using synchrotron radiation of wave length 1.0 Å from a cryo-cooled crystal at Spring-8 BL26B1 beam line, Japan. The Matthews coefficient VM for native and xylosaccharides- bound crystals was calculated to be 2.8 and 2.7 Å3 Da-1, respectively, suggesting two molecules in each crystal asymmetric unit. No twinning was detected in both the datasets and the overall quality of the data sets was found to be good. Chapter 3 details the application of molecular replacement method to the structure solution of native and xylosaccharides-bound BSX, the course of iterative model building and the refinement carried out, and the quality of the final protein structure models. The native-enzyme structure solution was obtained by the molecular replacement method using as a search model the crystal structure (PDB code 1hiz) of the closest homologous, extracellular xylanase (GSX) from Geobacillus stearothermophilus. No non- crystallographic symmetry (NCS) restraint was applied between the two independent molecules in the crystal asymmetric unit at the final round of refinement. The final positional refinement of native BSX converged to R factors of R = 19.4% and Rfree = 23.5% for data between 20.0 to 2.2 Å. The final native model consists of 5704 protein atoms, two Mg2+ ions and 721 solvent water molecules. The final native model was taken as the search structure for the xylosaccharides-bound BSX and a solution with a correlation coefficient of 70.7% and an R-factor of 32.1% was obtained from the molecular replacement calculation. Unlike the native structure refinement, NCS restraint was imposed at all stages of the refinement. Bound xylosaccharides were clearly visible inthe difference Fourier electron density maps. The last round of refinement gave a model with R and Rfree of 21.8% and 25.7%, respectively. The final xylosaccharides-bound model consists of 5766 protein atoms, four Mg2+ ions, 85 atoms belong to bound xylosaccharides and 523 solvent water molecules. No residues were found in the disallowed region of the Ramachandran (φ, ψ) map for both the structures. Chapter 4 describes the native and xylosaccharides-bound BSX crystal structures and the structural comparison of BSX with other GH10 family xylanase crystal structures for which the optimum temperature and pH are known in the literature. BSX folds as the ubiquitous (β/α)8-barrel, a common structural superfold characteristic of GH10 xylanases. The two active site glutamic acid residues, Glu149 and Glu259, are located on opposite sides of the active site cleft and their side-chains are at a distance of 5.5 Å apart suggesting the enzymatic reaction takes place by the retaining mechanism. From the structural superposition of other xylotriose-bound xylanase structures on to the xylosaccharides-bound BSX, structural plasticity in the xylotriose binding can be inferred, implying that the xylose recognition at the subsite -3 displays plasticity and is less specific as opposed to that at -1 and -2 subsites. The stacking interaction of one of the xylose moieties of the xylobiose with the Trp235 seen in BSX provides, for the first time, a structural evidence for the direct involvement of Trp235 in xylosaccharides binding. The crystal structure revealed a metal binding site, found at the C-terminal end of catalytic domain. The presence of metal binding site was not anticipated from earlier theoretically modeled structure and biochemical studies. Further, we have shown experimentally the requirement of Mg2+ ion for the enzyme activity. We havedescribed a novel WP sequence-structure-interaction motif which is present in the (+) side of the active site region and presumably helps in the efficient binding of the carbohydrate moiety of the xylan in the active site cleft of BSX. The structural comparison of BSX with other GH10 xylanases solved to date and characterized to be active at a pH close to neutral was done for the first time. The comparative study revealed the essential structural features which may responsible for the alkaline stability of GH10 xylanases.Briefly, the alkalophilic GH10 xylanases from alkalophilic organisms have surface abundant in acidic residues, the heat and alkaline susceptible residue Asn depleted on the protein surface and increased number of salt bridges. Our study has unveiled the role of the nature and composition of protein surface amino acids in the adaptation of enzymes to polyextreme conditions. The observations reported in the thesis provide important lessons for engineering alkaline stability in xylanases for industrial applications and in general for the understanding of alkaline stability in related proteins. A comparison of the surface features of the BSX and of halophilic proteins allowed us to predict the activity of BSX at high salt concentrations, which we verified through experiments. This offered us important lessons in polyextremophilicity of proteins, where understanding structural features of a protein stable in one set of extreme conditions provided clues about the activity of the protein in other extreme conditions. Chapter 5 summaries the important findings of the present study from the crystal structural analysis of BSX and its comparison with non-alkalophilic GH10 xylanases. Separate sections are made on conclusions and future prospects for the study on BSX. Chapter 6 describes the comprehensive database analysis of C-H…O hydrogen bond in helices of globular proteins. The C-H…O hydrogen bonds found in helices are predominantly of type 5 → 1 or 4 → 1.Our analysis reveals that the Cγ and Cβ hydrogen atom(s) are frequently involved in such hydrogen bonds. A marked preference is noticed for aliphatic β-branched residue Ile to participate in 5 → 1 C- H…O hydrogen bonds involving methylene Cγ1 atom as donor in α-helices. In addition, C-H…O hydrogen bonds are present along with helix stabilizing salt bridges and to some extent compensate for the side-chain conformational entropy loss. Our analysis highlights that a multitude of local C-H…O hydrogen bondsformed by a variety of amino acid side-chains and Cα hydrogen atoms occur in helices and more so at the helix termini. A majority of the helix favouring residues, Met, Glu, Arg, Lys, Leu and Gln which also have large side-chains with more donatable CH groups, have significant propensity to form side-chain to main-chain C-H…O hydrogen bonds in helix. The large side-chains are marked by their ability to shield from the solvent the polar atoms of the peptide backbone and at the same time participate in weak cohesive C-H…O interactions in the helix. This chapter also details the identification for the first time a novel chain reversal motif stabilized by 1 → 5 Cα-H…O interactions. The importance of these hydrogen bonds with respect to helix stability is discussed in the final section of the chapter. Appendix A details the crystallographic and structural analyses oftwares used for the present thesis work. Appendix B describes, in addition to the crystal structure analysis of BSX, the work carried out by the candidate on a comparative study of a thermostable xylanase from Thermoascus aurantiacus, solved in our laboratory at atomic 1.11 Å (293 K) and ultrahigh 0.89 Å (100 K) resolutions. From the comparison, we have for the first time pointed out the possibility of plasticity of ion pairs in proteins with water molecules mediating some of the alternate arrangements. The αβ-loops are relatively less flexible than the βα-loops. The β-strands are least affected structurally with the increase in temperature. Thus the TIM barrel fold in the study enzyme, though having a single domain, may be dissected into parts based on the relative flexibility and described as having a rigid core constituted by the β-barrel and a less rigid exterior formed by the surrounding α-helices. Appendix C presents the crystallization and the preliminary X-ray characterization work done by the author of the thesis on an alkali thermostable cellulase enzyme from Thermomonospora sp. The protein is an extracellular enzyme with molecular mass of 14.2 kDa and interestingly, has the dual activity for both cellulose and xylan. The primary structure of the enzyme is not known. The enzyme was purified from the source organism and crystallized. A complete diffraction data set was collected and processed to 2.3 Å in an orthorhombic space group P212121. Appendix D contains tables which give details about the analysed 5 → 1 Cα- H…O hydrogen bonds in helices and a novel chain reversal motif with 1 → 5 Cα-H…O hydrogen bonds. Appendix E encloses reprints of publications which have resulted from the work reported in the thesis.

Alkali-silica reaction (ASR) is a common deterioration mechanism responsible for numerous concrete durability issues. Since ASR was first discovered in the 1940's, a significant number of investigations have been carried out in order to understand its mechanisms. However, due to the complexity of the reaction and to the numerous factors that affect its development, many aspects still remain unexplained. The research described in this document was funded by the Texas Department of Transportation (TxDOT), and it focused on a specific type of reactive aggregates, known as 'pessimum'; they present an unexpected behavior with respect to the relation between the amount of material present in the mixture and the extent of ASR related damage. The main objective of this investigation was to determine a method for identifying aggregates that exhibit the 'pessimum' behavior by means of a short-term testing regime. Modified versions of the Accelerated Mortar Bar Test (AMBT) and the Concrete Microbar Test (CMBT) were considered for this purpose. In addition, the behavior of a selected group of 'pessimum' aggregates in the Concrete Prism Test (CPT) and the Chemical Method was evaluated. The petrographic characteristics for a reduced number of the aggregates studied were linked to their performance in the ASR tests. The results obtained from the experimental program conducted were combined with results from previous investigations performed at UT Austin to draw conclusions about the overall behavior of ‘pessimum’ aggregates. ‘Pessimum’ aggregates were successfully identified with a modification proposed to the AMBT. As for their behavior, it was found that depending on the amount of reactive constituents present in each test, these aggregates are classified as reactive (for low chert contents) or as non-reactive (for chert contents above the 'pessimum' proportion). Whether these aggregates will generate durability problems depends on the amount of reactive silica in the concrete mixture. / text

Concrete bridges in Texas have developed large cracks in bent caps and pretensioned trapezoidal bridge girders. The bridges show premature concrete deterioration due to alkali-silica reaction (ASR) and delayed ettringite formation (DEF). There is concern that deterioration due to ASR/DEF may lead to a loss of structural capacity. However, there are no quantitative guidelines to relate the level of concrete deterioration due to ASR/DEF to structural performance. Using such guidelines, the need for rehabilitation of beams with ASR/DEF cracking can be assessed. The goal of this research was to determine the shear capacity of pretensioned trapezoidal box girder specimens exhibiting varying degrees of ASR and/or DEF cracking and to use the shear testing results to evaluate the severity of the problem that may exist in Texas bridge structures. To achieve this goal, beams that were severely deteriorated due to ASR/DEF over a period of more than ten years were transported to the University of Texas for testing to failure. Both severely deteriorated and uncracked beams were tested in shear. The test results were used to evaluate the shear performance of trapezoidal box beams affected by ASR/DEF. In addition, three different types of forensic analyses were conducted on the beams to understand the nature of the ASR/DEF cracks and severity of the deterioration. After testing, it is found that the shear capacity of the test specimens was not significantly reduced even with heavy ASR/DEF cracking. Assessment using current US design provisions for bridges or buildings (ACI 318-08 and AASHTO LRFD 2008) and the proposed provision from an earlier project (TxDOT Project 5253) yielded conservative estimates of strength. Results from forensic analyses provided a qualitative indication of ASR/DEF damage but did not correlate with the observed levels of ASR/DEF deterioration. / text