The hydration of cement and water is an exothermic reaction capable of generating significant amounts of heat. Unaccounted for, the heat generated can alter the chemical reaction of the cement, producing massive cracks in the hardened concrete that sacrifice the integrity of the structural element. Alternatively, the heat produced can create thermal gradients capable of cracking the concrete and exposing the reinforcing steel to chlorides. To prevent either of these events from occurring, a software program known as ConcreteWorks was created as part of a previous research project funded by the Texas Department of Transportation. ConcreteWorks gives TxDOT engineers, contractors, and inspectors the ability to manage the structural design, mix proportions, and construction processes in order to minimize maximum concrete temperatures as well as temperature gradients. The free program has seen successful on several non-TxDOT projects, however, it has failed to become incorporated into standard TxDOT practices and specifications. The goal of this research, funded by TxDOT, was to promote widespread use and acceptance of the program within the DOT. In pursuit of this goal, a four-hour hands-on training class was developed and taught throughout the state of Texas, construction projects were selected for the use and validation of the software program, and a few modifications were suggested to make the program more helpful and easy to use. This thesis primarily focuses on the results of the validation of ConcreteWorks on mass concrete and precast applications. In total, four precast beams and two columns were instrumented. With regards to existing methods of predicting temperatures, the program was fairly accurate for mass concrete applications. The program was also very useful for precast elements; however, the lack of variables to match the model to the actual structure likely limits the software program from producing a more accurate prediction. / text

Thesis (Ph.D.)--Michigan State University. Dept. of Civil & Environmental Engineering, 2008. / Title from PDF t.p. (viewed on Apr. 2, 2009) Includes bibliographical references (p. 217-225). Also issued in print.

The aim of this thesis is to summarize the knowledge regarding reduction of negative impact of high volumes of fly ash in HVFA concretes using nanotechnology and experimentally verify the findings. To compensate the inferior early-age properties, it is possible to use active nanoparticles, such as nanosilica or nanolimestone. The first step of the experiment was the production of stable nanoparticle’s dispersions using ultrasonic homogenization and UV/Vis spectroscopy. In subsequent steps the influence of nanoparticle’s presence on cementitious materials’ properties was verified on cement pastes and mortars with 40 a 60 % of fly ash. The best variant was then used to produce nano-modified HVFA concretes. Even at a minimum dose, the positive effect on early-age properties indicates the usefulness of nanoparticles in technology of concrete. An important contribution of this thesis is also the acquired knowledge related to the nanoparticle’s behavior and handling.

Estimating the delayed strains in concrete, namely creep and shrinkage is very important to asses the condition of the structure. Time dependent deformations in concrete, both creep and shrinkage, play a critical role in prestressed concrete structures, such as bridge girders, nuclear containment vessels, etc. These strains result in lossess, through release of prestress, and thereby influence the safety of these structures. Recognizing the role of free and bound moisture movement is the primary ingredient responsible for the development of both creep and shrinkage stains as well as the degradation of concrete under high temperature, the present study has also examined the effects of high temperature on concrete degradation, experimentally and also analytically in the same modelling framework. Fire in concretes deteriorates mechanical properties of the material and lead to col-lapse under loads. Two types of spalling occur in concrete when exposed to high temperature, namely explosive and thermal spalling. Explosive spalling occurs once the hydrostatic stress (developed due to pore pressure) exceeds the tensile strength of the concrete. Where as thermal spalling of concrete happens due to degradation of material properties (elastic modulus, compressive and tensile strength) when exposed to high temperature due to decomposition of chemical bonds that release the bound water. The present study comprises of an experimental and analytical program to assess the levels of creep and shrinkage in different concrete under various loads and environmental conditions. Deformations due to high temperature in di erent concretes forms another component of the present study. Total six concrete mixes has been studied to investigate and asses the extent of creep and shrinkage taking place in the concretes under different environmental conditions, load level and age at loading. In total six mixes, three that are self compacted concrete mixes (35MPa, 55MPa and SCC70MPa), a high volume y ash concrete mix ( 45 MPa) and two normal concrete mixes (35 MPa and 45 MPa) have been considered in this study. To study the high temperature effects, the same mixes considered in the creep and shrinkage study and in addition a heavy density concrete mix (25 MPa) is used. A normal concrete having a 28 day uniaxial compressive strength of 45 MPa after proper curing, referred to as M45 concrete, was one of the six mixes. Likewise a heavy density concrete designated as H25, corresponding to a 28 day uniaxial compressive strength of 25 MPa was another mix that was studied and was made using iron ore aggregate and iron ore sand. A concrete having high volume y ash replacing cement designated as F45 offered a 28 day strength of 45MPa. Three self-compacting concretes with uniaxial compressive strengths of 35, 55 and 70 MPa were designated as SCC35 SCC55 and SCC70, respectively is studied for creep, shrinkage and high temperature effects. F45 concrete shows lower creep strain when compared to normal M45 concrete, under similar casting, curing and testing condtions. This is due to increase in stiffness of y ash based concretes with time. Where as in shrinkage it is observed that a little higher strain takes place in F45 at initial ages than in M45 concrete mix for the same conditions. But in the later age, F45 concrete shows a decreasing rate of shrinkage strain. This is because, water to cement ratio of y ash concrete is higher than the M45 concrete. The SCC35 concrete shows higher creep and shrinkage than M35 concrete even though both the concretes have the same water cement ratio. This difference comes from the aggregate cement ratio (a/c). The lower the aggregate cement ratio, the higher the creep and shrinkage. M35 concrete has a higher aggregate cement ratio than the SCC35. Concretes exposed to higher temperature and lower humidity shows higher creep and shrinkage due to its higher rate of drying. An analytical model has been developed to simulate the drying phenomena in concrete based on poromechanics. The hydration effects of blended cements is considered while developing the model. This models prediction of degree of hydration, temperature and relative humidity is used to model creep and shrinkage in concrete. To model creep and shrinkage, micro prestress solidi cation theory is implemented and validated with the present experimental results. The model is able to predict the drying phenomena of concrete realistically. Further, a benchmark problem reported in the literature is solved through this model and validated through a comparison with the experimental results (beam detection due to creep and shrinkage). Under high temperature tests, H25 concrete shows better resistance for all the ranges of temperatures. This may be because of the hematite aggregate having a high melting point and strong interfacial transition zone (ITZ) properties between aggregate and cement mortar. The SCC70 shows poor performance against explosive spalling at both the ages (28 and 365 days) due to its lower permeability when exposed to high temperature. The intensity of explosive spalling is higher in SCC70 concrete tested at 28 days than at 365 days of age. This is because of variation in moisture content. SCC70 concrete failed due to explosive spalling at temperature of 398oC when tested at 28 days and failed at 575oC when tested at 365 days. This indicates the amount of moisture content in the concrete plays an important role while causing explosive spalling. F45 concrete shows a poor resistance against temperature beyond 500oC in its residual properties. SCC55 contains cement and y ash and shows higher residual properties when compared to normal vibrated M45 mix under similar high temperature conditions. Two geopolymers pastes prepared with y ash and metakaolin as a complete cement replacement were studied for passive re protection capability. The study shows MF70 mix (containing 70% y ash and 30% metakaolin) gives better resistance against heating than MF50 mix (50% each of metakaolin and y ash). Hence y ash geopolmer is a choice of material for passive re protection. An analytical model has been developed based on poromechanics to simulate high temperature e ects in concrete. Two type of spalling is considered while modelling the high temperature e ects of concrete, namely explosive and thermal spalling. Explosive spalling is estimated based on the hydro static stress (Biotech efficient times the pore pressure). If the hydrostatic stress increases beyond the tensile strength of concrete then explosive spalling occurs. Where as the thermal spalling is estimated based on the stresses developed due to applied mechanical and thermal loading. To validate this model, two benchmark problems from the literature have been solved and validated with the reported results. This model is able to predict pore pressure and temperatures gradients accurately, and this in turn helps to predict explosive and thermal spalling realistically in concrete under elevated temperature conditions.

It is quite well known that fracture behavior of concrete is complex and is influenced by several factors. Apart from material properties, geometric parameters influence fracture behavior and one notable phenomenon is size effect. The existence of the size effect in concrete is well known and various attempts to model the behavior is well documented in literature. However the approach by Bazant to describe the size effect behavior in concrete has received considerable attention. The major advantage of developing the size effect law for concrete is the ability to describe the fracture behavior (namely failure strength) of large size structures inaccessible to laboratory testing. The prediction of size effect is done on the basis of laboratory testing of small size geometrically similar structures. In all the models developed earlier heterogeneity of concrete has not been quantitatively simulated. Hence, the complete description considering heterogeneity in concrete is attempted using the lattice model to understand size effect behavior in concrete. In the present study, a detailed description of the heterogeneity in concrete is at- tempted by 3D lattice structure. Analytical treatment to gain insights to fracture behavior is difficult and hence a numerical approach capable of handling the het- erogeneous nature of the material is adopted. A parametric study is performed to understand the influence of various model parameters like mesh size, failure criterion, softening model. The conventional size effect studies for 2D geometrically similar structures are performed and a comparison is done with experimentally observed behavior. The variation of fracture process zone with respect to structure size is observed as the reason for size effect. The influence of variation in properties of ag- gregate, matrix and interface are studied to explain the deviation in pre-peak and post-peak response. A statistical study is performed to establish the size dependence of linear regression parameters (Bf ‘t and D0) which are used in Bazant size effect law. An analytical framework is also proposed to substantiate the above results. Size effect in concrete is generally attributed to the effect of depth viz. the dimension in the plane of loads. However although the effect of thickness viz. a dimension in a plane perpendicular to that of the loads is not considered in concrete. The same is quite well known in fracture of metals. Therefore the variation in grading of aggregates along with the influence of thickness on fracture behavior is analysed. To understand the thickness effect a comparison of 2D and 3D geometrically similar structures is performed to understand the effect of thickness on fracture parameters. Heterogeneity is a matter of scale. A material may be homogeneous at a coarser scale while at a finer scale it is heterogeneous. Hence only way to capture the effect of the behavior at micro level on the behavior at meso level particularly in a heterogeneous material like concrete is by a multi-scale modelling. The best numerical tool for multiscale model of a heterogeneous material is lattice model. The heterogeneous nature of concrete is not just due to the presence of aggregates but is evident right from the granular characteristics of cement. The hydration of cement grain leads to the development of products with varying mechanical and chemical properties. As the micro-crack initiation and development of thermal cracking is observed at the micron level, understanding of hydration behavior in concrete can be thought of as a pre-requisite for complete understanding of fracture behavior. The properties of matrix and interface observed during hydration modelling can also be used as an input for fracture predictions at upper scale models (eg. mesoscale). This can also be used to study the coupling of scales to understand the multi-scale fracture behavior in concrete. A numerical model is hence developed to study the hydration of concrete. Due to the existence of complex mechanisms governing the hydration behavior in con- crete and the large number of parameters affecting its rate, the hydration of a grain is assumed to proceed in isolation. A single particle hydration model is developed to study the hydration of isolated grain. A shrinking core model usually used to describe the burning of coal is adopted as a base model for analytically describing the hydra- tion behavior. The shrinkage core model in literature is modified to be applicable to hydration of cement matrix. The effect of particle diameter as well as changing water concentration is incorporated into the model whereas the influence of reduction in pore sizes as well as the effect due to embedding of particles and the constraint due to hydration of neighbouring particles is accounted using correction factor. The effect of temperature on rate of hydration is considered to be independent of the physical and chemical aspects of grain. Hence a temperature function developed using Arrhe- nius equation and activation energy is incorporated separately. The porous nature of reaction products affects the diffusivity leading to the development of tortuous path for flow of water through the hydrated portion. Knowing the tortuosity it is possible to obtain the diffusivity which in turn can be used as an input to the lattice model. An algorithm is developed to determine the tortuosity in diffusion of water through the reaction products. The tortuosity depends on the distribution of pores in the hydrated system. This requires the use of simulation technique to generate the initial position of voids. A simulation technique is also required to generate the initial con- figuration of hydrating cement system. In order to generate the initial configurations of such systems a numerical technique to generate a large scale assembly of particles is proposed. In the present work, parameters of Bazant's size effect law Bf’t and D0 are shown to depend on structure size and heterogeneity. The span to thickness ratio of the structure increases fracture energy and also substantially influences the response of structure. The variation in failure load occurring due to the heterogeneous nature of the material is shown to follow a normal distribution. The fracture behavior of a material is seen to be influenced strongly by the variation in the strength of matrix and interface. The model proposed to describe the hydration process of cement can be used to determine the properties of matrix and interface. The degree of hydration as well as the embedded centre plane area can be adopted as a measure of strength of matrix and interface. The understanding of the hydration process and the wall effect around the aggregate surface can possibly improve our ability to predict the strength of interface. The material strength of the interface is certainly a necessary input to the lattice model. Infact experimental determination of interface strength is a lot more complicated than the present numerical approach. The only weakness of the present numerical approach is the assumption regarding certain empirical constants which of course may be improved further. Understanding of material behavior can be further improved if a molecular dynamics approach is adopted to describe the hydration behavior of cement. The approach via molecular dynamics is suggested as a problem for future research.

Plasticizing and superplasticizing admixtures are the key components of nearly every concrete. These admixtures improve workability of mortar and fresh concrete which lead to facilitation of depositing and compaction. Adding of plasticizers or superplasticizers enables to change properties of fresh and hardened concrete, especially to reduce water-cement ratio, thereby increasing strength and durability of concrete or to improve other properties. The main aim of this thesis is to monitor the effect of plasticizing admixtures and their dosage on the rheology and hydration of concrete with emphasis on the development of hydration temperature. The other goal is to examine their strength qualities. Behaviour of plasticizing admixtures is analysed at first on cement pastes, as simple systems, and then verified on concrete.