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The objective of this research was to investigate the effects of twisted steel micro rebar (TSMR) fibers on 1) the mechanical properties of concrete used in bridge deck construction and 2) the early cracking behavior of concrete bridge decks. This research involved the evaluation of four newly constructed bridge decks through a series of laboratory and field tests. At each location, one deck was constructed using a conventional concrete mixture without TSMR, and one was constructed using the same conventional concrete mixture with an addition of 40 lb of TSMR per cubic yard of concrete. Regarding laboratory testing, the conventional and TSMR beam specimens exhibited similar average changes in height after 4 months of shrinkage testing. The electrical impedance measurements did not indicate a notable difference between specimens comprising concrete with TSMR and those comprising conventional concrete. Although no notable difference in behavior between conventional and TSMR specimens was apparent before initial cracking, the toughness of the TSMR specimens was substantially greater than that of the conventional concrete specimens. Regarding field testing, sensors installed in the bridge decks indicated that the addition of TSMR does not affect internal concrete temperature, moisture content, or electrical conductivity. The average Schmidt rebound number varied little between the TSMR decks and conventional decks; therefore, the stiffness of the TSMR concrete was very similar to that of conventional concrete. Distress surveys showed that the conventional decks exhibited notably more cracking than the TSMR decks. The TSMR fibers exhibited the ability to limit both crack density and crack width. For all of the decks, chloride concentrations increased every year as a result of the use of deicing salts on the bridge decks during winter. However, the chloride concentrations for samples collected over cracked concrete increased more rapidly than those for samples collected over non-cracked concrete. Although TSMR fibers themselves do not directly affect the rate at which chloride ions penetrated cracked or non-cracked concrete, the fibers do prevent cracking, which, in turn, limits the penetration of chloride ions into the decks. Therefore, the use of TSMR would be expected to decrease the area of a bridge deck affected by cracking and subsequent chloride-induced corrosion damage and thereby increase the service life of the bridge deck.

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While manufacturing fiber reinforced concrete, it is of utmost importance that quality and homogeneous fiber distribution are ensured. To verify that the fibers are evenly distributed in ready-mixed concrete, SS-EN 206 has introduced requirements for continuous control of fiber distribution. The requirement for test frequency was originally the same as for compressive strength. Since this frequency requirement would result in an unreasonably large workload, it has been opened for national adjustments. In order to proceed with this work, there is a need to develop documentation and investigate eventual impact on fiber distribution of the timing and the way the fibers are added to concrete. The purpose of this thesis is to study the possible impact of when and how fibers are added to the concrete. Based on the results, prepare a basis for recommendations for routine description. The hypothesis is that there are no measurable differences regardless how and when the fibers are added to concrete. The correctness of the foregoing is fully confirmed. Samples from real insitu steel fiber reinforced concrete deliveries were extracted at the beginning, middle and end of the discharge. The steel fibers were, thereupon, separated from the fresh concrete and washed. Thereafter, lab tests were performed; steel fibers of each sample were dried in a special oven and then weighted by a high accuracy weight scale. The rule of proportions was subsequently used, in order to compare the final result with the client’s order and study the deviation from the average. The results have shown that the variation between the three different methods of adding the fibers, namely, in the central concrete mixer plant, in the concrete mixer truck after concrete is filled, in the concrete mixer truck at the same time that concrete is filled, as well between start, middle and end of the discharge is negligible.  A questionnaire survey, focusing on different aspects about how a concrete company handles steel fiber reinforced concrete, was additionally performed in order to provide supplementary data to the literature study and the testing process. The initial hypothesis was once again verified; the differences between the methods used to produce steel fiber reinforced concrete as well as when the fibers are added to the concrete mix are insignificant, according to the surveyed companies.  Preliminary investigation, continuous control as well as careful procedures for addition and mixing are recommended for a more reasonable test frequency. Preliminary investigation includes a detailed documentation process from the manufacturer for adding and mixing fibers, before the concrete containing fibers is manufactured; fiber parameters (shape, length and type), duration of mixing, how fibers should be added, type of mixer that should be used. Continuous control includes tests that should be conducted if fibers are added to the concrete mixer truck and the acceptance criteria shall be according to B.5 in SS-EN 206.

Ultra-high performance concrete (UHPC) is a recently developed concrete gaining a lot of interest worldwide, and a lot research has been conducted to determine its material properties. UHPC is known for its very high strength and high durability. Association Francaise de Genie Civil (AFGC) has defined UHPC as a concrete exhibiting compressive strength greater than 150 MPa (22 ksi). To utilize the full compressive strength of UHPC, complementary tension reinforcement is required. A recent research study to find light weight yet high strength alternative deck systems for Florida movable bridges demonstrated that a composite UHPC and high strength steel (HSS) reinforcement deck system is a viable alternative. However, failure modes of the deck system observed during experimental testing were shear failures rather than flexural failures. Interestingly, the shear failures were ductile involving large deformations and large sectional rotations. The purpose of this research is to quantify the sensitivity of UHPC structural member mechanical response to different shear and normal stress demands, and investigate the underlying failure modes. An experimental investigation on small-scale prisms without reinforcement, prisms reinforced with ASTM Grade 60 steel, and prisms reinforced with high strength steel was carried out to capture load-deflection behavior as well as modes of failure of the UHPC specimens. Numerical analysis based on modified compression field theory (MCFT) was developed to verify experimental results at the section level, and further verification using continuum methods was performed using MCFT/DSFM (disturbed stress field method) based finite element analysis software (VecTor2). Results from the numerical analysis could reasonably predict the load-displacement as well as the failure modes of the experimental specimens. Obvious flexural failure was observed on unreinforced UHPC specimens where wide crack opening gradually widened at the bottom fiber of the concrete to the loading position. Whereas UHPC-Grade 60 steel specimens experienced ductile flexural failure with similar wide crack opening after the rebar yielded. On the other hand, UHPC-MMFX specimens largely failed in shear from a diagonal tension crack and crush of concrete top fiber.

Shear failure in concrete beams are often brittle in nature and potentially dangerous without adequatereinforcing measures. In design of concrete, it is commonly recommended to install transversalreinforcement along the shear span to induce a more ductile structural response, improving the shearcapacity all together and providing sufficient warning prior to collapse. However, it is more frequentlybeing assessed whether analogous performance can be achieved in fiber reinforced concrete beamswithout stirrups, and multiple attempts in literature confirm that it is possible. This alternative technologyintroduces need for better understanding of the modeling aspects of FRC in numerical simulations, as it isbecoming more common for engineers to resort to the finite element method in quality assurance ofstructures.In this thesis, the possibility of predicting shear failure numerically in simply supported fiber reinforcedconcrete beams with flexural bar reinforcement but without stirrups was investigated by means ofnonlinear finite element analysis, using the software package ATENA 2D Engineering. The ultimate aimwas to, as accurately as possible by means of numerical analyses on representative FE-models, replicatethe results from physical three-point-bending tests on simply supported FRC beams of various sizesperformed by Minelli et al. (2014). These beams were merely equipped with flexural reinforcement andexhibited shear failure.This thesis revolved around development and comparative assessment of material models for FRC basedon the smeared crack approach, adopting two different strategies: (1) The first strategy was to calibratematerial parameters based on results from 3PBT on notched FRC beams that were carried out prior totesting of the reinforced FRC beams, as reported by Minelli et al. (2014). Nonlinear finite element analysiswas used on representative FE-models for the notched 3PBT specimens, from which material parameterswere obtained iteratively by employing inverse analysis methods proposed by Červenka Consulting s.r.o.(2). The second strategy comprised of utilizing recommended constitutive relations from designrecommendations in SS812310 and RILEM TC 162-TDF. All of the constructed material models werefinally coupled with the FE-models that represented the beams with flexural reinforcement for evaluationof their performance based on their consistency with experiment data.It was found that the material models that were generated from inverse analysis in general would haveyielded successful predictions for the occurrence of shear failure in the reinforced FRC beams, providedthat the governing post-cracking residual tensile parameters were processed with respect to relevantassumptions as to describe uniaxial tensile behavior. However, although it was possible to utilize theproposed calibration method to replicate the load-displacement data for the notched 3PBT specimens withsufficient conformity, it was not possible to arrive at only one unique solution. Instead, multiple outcomescould be obtained based on the initial choice for the input value of the uniaxial tensile strength, leading tothe conclusion that experience and the engineering judgment of the user is of high importance whenadopting this method.Regarding the material models that were derived from constitutive relations in design recommendations,satisfactory estimates for the shear capacity could be obtained from the FE-models that were based onrecommendations by RILEM. The models that were based on SS812310, on the other hand, demonstratedover-stiff behavior and they were unable to provide accurate graphical visualizations of characteristicshear cracking, although the obtained load bearing capacity overall matched the experiment data in caseswhen size effects seemingly had a minor influence. An important observation from the comparison ofthese material models was that the initial drop in tensile strength during crack initiation within an elementis crucial in modeling of FRC, as it accounts for a more realistic behavior through a gradual transitionfrom aggregate bridging mechanisms of PC to the added fiber bridging mechanisms of FRC. Forsituations with high residual tensile strengths in relation to tensile strength at crack initiation, theguidelines in SS812310 become less practical for predicting shear failure by means of NLFEA.

This research was conducted to characterize the flexural and tensile characteristics of thin, very high-strength, discontinuously reinforced concrete panels developed by the U.S. Army Engineer Research and Development Center. Panels were produced from a unique blend of cementitous material and fiberglass reinforcing fibers, achieving compressive strength and fracture toughness levels that far exceeded that of typical concrete.The research program included third-point flexural experiments, novel direct tension experiments, implementation of micromechanically based analytical models, and development of finite element numerical models. The experimental, analytical, and numerical efforts were used conjunctively to determine parameters such as elastic modulus, first-crack strength, post-crack modulus and fiber/matrix interfacial bond strength. Furthermore, analytical and numerical models implemented in the work showed potential for use as design tools in future engineered material improvements.

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[pt] A fluência em compósito cimentício reforçado com fibra (CRF) no estado pré-fissurado tornou-se um importante tópico de estudo recentemente. Isso se deve ao fato de que o comportamento dependente do tempo do CRF e a estabilidade a longo prazo de fissuras sob cargas de flexão sustentadas ainda são pouco compreendidas. Este trabalho busca explorar o uso de fibras de aço e PP para definir sua influência na fluência, analisando a evolução das aberturas de fissuras com o tempo. O material foi inicialmente caracterizado sob testes de flexão de três e quatro pontos em amostras prismáticas com entalhe. Para os testes de fluência, os corpos de prova foram pré-fissurados a 0,5 mm e testados sob carga constante durante 45 dias. Para entender os mecanismos relacionados, testes de fluência também foram realizados em fibras e em uma configuração de arrancamento. Analisando os resultados dos testes de fluência, verificou-se que a taxa de COD (crack opening displacement) é uma ferramenta interessante para avaliar o comportamento a longo prazo do CRF fissurado e para definir um critério de estabilidade. Além disso, verificou-se que o concreto incorporando fibras sintéticas apresenta maiores deformações de fluência do que o reforçado com fibras de aço. Isso pode ser explicado pelas diferentes características das ligações fibra-matriz, analisadas pelos testes de arrancamento monotônico e de carga sustentada, e pela resposta de compressão do compósito. Finalmente, as propriedades residuais das amostras ensaiadas por fluência foram determinadas por testes monotônicos de flexão. / [en] Creep in pre-cracked fiber reinforced cementitious composites has become an important topic of study recently. This is due to the fact that the time dependent behaviour of FRC and long term stability of cracks under sustained bending loads are still poorly understood. This work seeks to explore the use of steel and PP fibers in order to define their influence on creep, by analyzing the crack opening displacement rate in FRC specimens. The material was first characterized under three and four-point bending tests in notched prismatic specimens. For the creep tests, the specimens were pre-cracked to 0.5 mm, and then tested under constant load during 45 days. In order to better understand the related mechanisms, creep tests were also carried on single fibers and in a fiber pullout configuration. Analyzing the creep tests results, it was verified that the COD rate is an interesting tool to evaluate the long-term behaviour of the cracked FRC and to define a stability criterion. In addition, it was found that concrete incorporating macro synthetic fibers presents higher creep deformations and higher creep rate than concrete reinforced with steel fibers. This can be explained by the different fiber-matrix bond characteristics, analyzed by monotonic and sustained load pullout tests, and by the compression response of the composite. Finally, the residual properties of creep tested specimens were determined by monotonic flexural tests performed in the FRC specimens after the creep tests.