Modeling of the time-dependent behavior of cement paste has always been a difficulty. In the past, viscoelastic behavior of cementitious materials has been primarily attributed to the viscoelastic properties of C-S-H components. Recent experimental results show that C-S-H may not exhibit as much creep and relaxation as previously thought. This requires new consideration of different mechanisms leading to the viscoelastic behavior of cement paste. Thus the objective of this thesis is to build a computational model using finite element method to predict the viscoelastic behavior of cement paste, and using this model, virtual tests can be carried out to improve understanding of the mechanisms of viscoelastic behavior. The primary finding from this thesis is that the apparent viscoelastic behavior due to dissolution of load bearing phases is substantial. The dissolution process occurring during the hydration reaction can change the stress distribution inside cementitious materials, resulting in an apparent viscoelastic behavior of the whole cementitious materials. This finding requires new consideration of mechanisms of time-dependent behavior of cementitious materials regarding the dissolution process of cement paste.

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Underground mining can be summarized as the removal of economically viable volumes of rock which creates underground voids. In order to optimize ore extraction, a material is used to backfill these openings prior to creating any adjacent openings. The use of cemented paste backfill (CPB), a mixture of mine tails, water, and cement binder, has gained prominence as it not only provides a material that has engineered strength and can be deployed rapidly, but also decreases the surface storage volume of the mine tails. There is limited knowledge about the behavior of the stresses within the CPB during the filling of an underground opening, particularly during the early curing ages of the hydrating CPB which is critical to the design of fill barricades. This thesis presents a design procedure which can be used to determine the in situ stresses within the CPB. Three methodologies were used in the development of this design procedure. The first was to develop a laboratory testing method that determined the time-dependent consolidation characteristics and strength parameters of the hydrating cemented paste material. The second was to collect several field-data sets. The third methodology was to numerically model the CPB using Itasca’s FLAC3D, which incorporated the underground void’s geometry, backfilling strategy, and time-dependent backfill parameters in order to determine the in situ stresses of the CBP. This simulation allowed for the prediction of both total and effective stress throughout the stope. The model and the laboratory results were used to model the stresses in several test stopes so that a comprehensive comparison could be made between the model and field instrumentation results. Four case studies were examined using a total of six different field instrumentation datasets. The results from these case studies showed that the modeling approach, given some model calibration, is capable of quantitatively representing the important geomechanical aspects of paste filling and curing.

Measurement of the strength development of Cemented Paste Backfill in laboratory cast cylinders does not replicate the in situ strengths of CPB in mine stopes. The mass of CPB in a filled stope is large and temperature rises due to the heat of hydration of the cementing materials, thus accelerating the gain in strength, relative to laboratory specimens stored at ambient temperature. The purpose of this study was to determine the impact on strength development when CPB test cylinders were subjected to a temperature profile mimicking that in a large mass, such as a mine stope. Also, maturity (the integral of time and temperature during hydration of the CPB) was compared to actual strengths, and the maturity – strength concept used in concrete technology was applied. It was found that the strength- maturity relationship was applicable to CPB once the base line or datum temperature was adjusted.

Underground mining can be summarized as the removal of economically viable volumes of rock which creates underground voids. In order to optimize ore extraction, a material is used to backfill these openings prior to creating any adjacent openings. The use of cemented paste backfill (CPB), a mixture of mine tails, water, and cement binder, has gained prominence as it not only provides a material that has engineered strength and can be deployed rapidly, but also decreases the surface storage volume of the mine tails. There is limited knowledge about the behavior of the stresses within the CPB during the filling of an underground opening, particularly during the early curing ages of the hydrating CPB which is critical to the design of fill barricades. This thesis presents a design procedure which can be used to determine the in situ stresses within the CPB. Three methodologies were used in the development of this design procedure. The first was to develop a laboratory testing method that determined the time-dependent consolidation characteristics and strength parameters of the hydrating cemented paste material. The second was to collect several field-data sets. The third methodology was to numerically model the CPB using Itasca’s FLAC3D, which incorporated the underground void’s geometry, backfilling strategy, and time-dependent backfill parameters in order to determine the in situ stresses of the CBP. This simulation allowed for the prediction of both total and effective stress throughout the stope. The model and the laboratory results were used to model the stresses in several test stopes so that a comprehensive comparison could be made between the model and field instrumentation results. Four case studies were examined using a total of six different field instrumentation datasets. The results from these case studies showed that the modeling approach, given some model calibration, is capable of quantitatively representing the important geomechanical aspects of paste filling and curing.

Measurement of the strength development of Cemented Paste Backfill in laboratory cast cylinders does not replicate the in situ strengths of CPB in mine stopes. The mass of CPB in a filled stope is large and temperature rises due to the heat of hydration of the cementing materials, thus accelerating the gain in strength, relative to laboratory specimens stored at ambient temperature. The purpose of this study was to determine the impact on strength development when CPB test cylinders were subjected to a temperature profile mimicking that in a large mass, such as a mine stope. Also, maturity (the integral of time and temperature during hydration of the CPB) was compared to actual strengths, and the maturity – strength concept used in concrete technology was applied. It was found that the strength- maturity relationship was applicable to CPB once the base line or datum temperature was adjusted.

The focus of this research was to investigate the use of functionalized/non-functionalized multi walled carbon nanotubes (MWCNTs) as reinforcements for the Portland cement paste. The unique geometrical characteristics of the carbon nanotubes (CNTs), as well as its unique mechanical properties such as high strength, ductility and stiffness, were the vital motivation for this study. In this research, we combined this unique material (CNTs) with concrete which is the most used man-made material. When compared to other composite materials, a limited amount of research has been conducted on the CNTs/cement composites. In order to investigate how the aspect ratio of functionalized/non-functionalized MWCNTs affects the mechanical properties of cementitious composites, ten different mixes of the MWCNTs/cement composites were prepared and tested. The different batches had a fixed water/cement ratio of 0.4, and variations of MWCNTs length, concentration and surface treatment. The cement nanocomposites were cast in small-scale specimens (beams) for the three-point flexural testing. Four major mechanical properties were evaluated at ages of 7, 14, and 28 days from the casting day: the maximum flexural strength, ultimate strain capacity (ductility), modulus of elasticity, and modulus of toughness. The results for the different nanocomposite batches were compared with the plain cement (reference) batch. The mechanical testing results showed that at 28 days almost all of the MWCNTs composites increased the flexural strength of the cement nanocomposites. At 28 days, the long MWCNTs increased the flexural strength more than the short MWCNTs. In general, the ultimate strain (ductility) of the short MWCNTs nanocomposites was higher than the ultimate strain of the long MWCNTs nanocomposites. The flexural strength of short 0.2 percent MWNT and long 0.04 percent MWNT (OH) increased by 269 percent and 83 percent, respectively, compared to the plain cement sample at 28 days. The highest ductility at 28 days for the short 0.1 percent MWNT and the short 0.2 percent MWNT was 86 percent and 81 percent, respectively. Clear evidence was obtained from the SEM images for micro-crack bridging; many of the MWCNTs were stretching across the micro-cracks. In conclusion, CNTs as nano reinforcements, can effectively improve certain mechanical properties of the cement paste composites.

Zugl.: Dresden, Techn. Universiẗat, Diss., 2008.

Universiẗat, Diss., 2003--Freiburg (Breisgau).

The Subtext project seeks to make programming fundamentally easier by altering the nature of programming languages and tools. This paper defines an operational semantics for an essential subset of the Subtext language. It also presents a fresh approach to the problems of mutable state, I/O, and concurrency.Inclusions reify copy & paste edits into persistent relationships that propagate changes from their source into their destination. Inclusions formulate a programming language in which there is no distinction between a program’s representation and its execution. Like spreadsheets, programs are live executions within a persistent runtime, and programming is direct manipulation of these executions via a graphical user interface. There is no need to encode programs into source text.Mutation of state is effected by the computation of hypothetical recursive variants of the state, which can then be lifted into new versions of the state. Transactional concurrency is based upon queued single-threaded execution. Speculative execution of queued hypotheticals provides concurrency as a semantically transparent implementation optimization.