Amal R. Jayapalan

Influence of Additions of Anatase TiO2 Nanoparticles on Early-Age Properties of Cement-Based Materials

The performance and properties of cement-based materials can potentially be altered by the addition of nano-sized inclusions. In this study, the effect of chemically nonreactive anatase TiO2 nanoparticles on early-age hydration of cement was investigated. First, the effects of different percentage addition rates of TiO2 to portland cement on early-age behavior were examined through isothermal calorimetry and measurements of chemical shrinkage. On the basis of accelerations in hydration observed in TiO2 portland cements, additional experiments were performed with tricalcium silicate (C3S), the main strength-giving mineral component of portland cement, to determine whether the influence of TiO2 could be adequately described by a kinetic model that relies on boundary nucleation theory. Comparison of the experimental results and the modeling showed that (a) an increase in addition rates of TiO2 accelerates the rate of cement hydration and (b) the heterogeneous nucleation effect rather than the dilution effect was dominant. The result of the boundary nucleation model reinforces the concept of the heterogeneous nucleation effect and demonstrates that the surface area provided by nano-TiO2 particles increases the rate of hydration reaction. This research forms the foundation for future studies aimed at optimizing photocatalytic and other nanoparticle-containing cements.

Effect of Nano-sized Titanium Dioxide on Early Age Hydration of Portland Cement

The effect of nano-scale non-reactive anatase titanium dioxide (TiO2) on early age hydration of cement was experimentally studied. Isothermal calorimetry was performed on cement pastes with two different particle sizes of TiO2 at replacement levels of 5, 7.5 and 10%. The addition of TiO2 to cement increased the heat of hydration and accelerated the rate of reaction at early stages of hydration. This increase was found to be proportional to the percentage addition and the fineness of TiO2. These results demonstrate that the addition of non-reactive nano-scale fillers could affect the rate of cement hydration by heterogeneous nucleation.

Nonlinear Wave Modulation Spectroscopy Method for Ultra-Accelerated Alkali-Silica Reaction Assessment

In order to predict potential aggregate alkali-silica reactivity, there was development and assessment of an ultra-accelerated testing method using advanced nonlinear ultrasonic techniques. There was observation of the nonlinear interaction of propagating acoustic waves being affected by very early alkali-silica reaction (ASR) gel formation during standard accelerated mortar bar exposure (ASTM C1260 or AASHTO T 303). There was observation, furthermore, of a clear distinction of varying reactivity in the nonlinearity parameter for three different aggregates. Compared to a 14-day standard accelerated mortar bar test method test period, aggregate reactivity could be distinguished in as early as four days through the spectroscopic technique. Study results suggests that further development of this method could result in very rapid screening of aggregates for alkali reactivity and very early detection of ASR damage in the laboratory.

Micro- and Nanoscale Characterization of Effect of Interfacial Transition Zone on Tensile Creep of Ultra-High-Performance Concrete

Ultra-high-performance concretes (UHPCs) are nano- to microstructurally optimized construction materials whose use presents significant opportunities for improving the performance of prestressed bridge girders. In UHPC girders, transverse shear reinforcement may be eliminated because of the high tensile strength of the material achieved through the use of short dispersed steel fibers as part of the mix. Use of the concretes tensile strength requires that the long-term tensile performance be understood to avoid brittle shear failure in service. The scope of the present study was characterization of the tensile creep of UHPCs under different thermal treatment regimens, with complementary assessment of the underlying mechanisms by characterization by nanoindentation and scanning electron microscopy. In this study, tensile-creep tests were conducted for a period of 1 year with UHPCs subjected to three different moist thermal curing regimes (i.e., early curing at 90°C, early curing at 60°C, and curing at 23°C). The effects of the curing conditions were further examined by nanoindentation and scanning electron microscopy, with particular emphasis being placed on the influence of thermal curing on the fiber–matrix interface. On the basis of the findings of this multiscale study, it is proposed that an enhanced fiber–cementitious matrix interfacial region, created by thermal curing, contributes significantly to the observed reductions in tensile-creep deformation.

Photocatalytic Efficiency of Cement-based Materials: Demonstration of Proposed Test Method

While there is an increasing interest in using photocatalytic cement-based materials, a lack of an appropriate standard procedure for testing the photocatalytic activity and efficiency of such materials makes comparisons among materials and data from different sources challenging. Current standard test methods are not appropriate for assessing photocatalytic nitrogen oxide conversion by cementitious materials because the standards do not consider surface properties of cementitious materials, nor are the test conditions appropriate for evaluation of cementitious materials. Therefore, the main objective is to propose a standard procedure for assessing the photocatalytic activity of cement mixtures with TiO₂ additives and to define a parameter-the photocatalytic efficiency factor (PEF)-that could be used to compare the photocatalytic activity of different TiO₂ cement mixtures, even for extended exposure conditions. The use of PEF is demonstrated, and values of PEF ranging from 8.40 to 51.55 µmol·h⁻¹·m⁻² (0.78 to 4.79 µmol·h⁻¹·ft⁻²) was observed for mixtures with TiO₂ up to 15% replacement.

Multi-scale investigation of the effect of thermal treatment on the tensile creep of ultra-high performance concrete: preliminary assessment

The effect of thermal treatment on the tensile creep of steel-fibre reinforced, nano-engineered ultra-high performance concrete (UHPC) was examined through short-term creep testing, nanoindentation and scanning electron microscopy (SEM). UHPC thermally treated at 90%C for 48 hours prior to loading showed a decrease of 73% in the tensile creep coefficient and 77% in the specific creep at seven days as compared to companion samples subjected to ordinary curing. Nanoindentation showed an average decrease of 63% in the modulus of elasticity of bulk paste of UHPC with no thermal treatment. Also, the presence of a more porous and lower modulus zone around steel fibres in the case of non-thermally treated UHPC was evident by nanoindentation measurements and SEM. These preliminary measurements suggest that the differences in the interface zone between fibres and UHPC cementitious matrix may have a significant effect on the tensile creep performance.

Applications in Complex Systems

Laser scanning confocal microscopy (LSCM) is widely used in biological, semiconductor, geological and other material science fields. For most non-opaque applications, interior structures can be imaged. LSCM can perform optical sectioning by acquiring images point by point and then resolve the images using a computer to provide a 3-D profile of the sample (Corle, 1996). In the past ten to fifteen years, researchers have begun to develop LSCM techniques for applications with opaque samples. LSCM technology has allowed collection of scattered, reflected or fluorescence photons to provide depth profiling of opaque samples. More importantly, advances in software for LSCMs have allowed 3-D models to be developed that provide researchers an

CHARACTERIZATION OF DISTRIBUTED DAMAGE IN MORTARS USING A NONLINEAR ACOUSTIC TECHNIQUE

In this work, a developed nonlinear acoustic method—nonlinear impact resonance acoustic spectroscopy (NIRAS) is used to characterize the distributed microcracks in mortars. A nonlinear parameter related to hysteresis effect of cement‐based materials is selected as the representative parameter of NIRAS technique. The NIRAS technique is applied in differentiating four different groups of mortar samples with varying damage state. Results from NIRAS technique are compared to the results from linear pulse velocity measurements and show a substantial better sensitivity, particularly at the early stage of damage. In addition, the variation trend of nonlinear parameter defined in NIRAS technique is correlated to accompanying petrographic analysis of the micro‐structure of mortar samples.

RAPID ASSESSMENT OF ALKALI‐SILICA REACTION DAMAGE IN CEMENT MORTARS BY NONLINEAR ACOUSTIC TECHNIQUE

In this work, a novel nonlinear acoustic method—nonlinear impact resonance acoustic spectroscopy (NIRAS) is developed to detect and differentiate alkali‐silica reaction (ASR) in mortar samples. The shift of resonance frequency of slender mortar bars is observed with the progress of material nonlinearity caused by ASR damage. The relative resonance shift is proportional to the amplitude of impact applied on samples and slope of this linearity represents the hysteresis parameter of materials. The developed NIRAS technique is then used to distinguish aggregates with varying alkali‐reactivity and it is found that NIRAS method is able to separate those aggregates very well particularly in the early age. Those results indicate a great potential of proposed NIRAS technique for the rapid assessment of ASR damage in practice.

ASSESSMENT OF ALKALI‐SILICA REACTION DAMAGE IN MORTARS WITH NONLINEAR ULTRASONIC TECHNIQUES

In this work, a nonlinear ultrasonic modulation technique is employed to assess the damage state of portland cement mortar samples induced by alkali‐silica reaction (ASR). Due to the nonlinear interaction of propagating waves caused by distributed microcracks that are agitated from its equilibrium state, the ultrasonic responses of samples produce sideband frequencies around the frequency of propagating waves. The amplitude of the sidebands depends on the amplitude of the input signals and is particularly sensitive to the state of damage evolved in the sample. Therefore, the development of internal microcracks with increasing duration of exposure to aggressive conditions can be quantitatively related to the variation of external ultrasonic measurements. The ultrasonic results are compared with results from standard ASR expansion measurements (ASTM C 1260), and a proportionally increasing relation was found in the early stages. In addition, aggregates with different alkali‐reactivity (i.e., low reactivity ...