Microstructural and bulk property changes in hardened cement paste during the first drying process

This paper reports the microstructural changes and resultant bulk physical property changes in hardened cement paste (hcp) during the first desorption process. The microstructural changes and solid-phase changes were evaluated by water vapor sorption, nitrogen sorption, ultrasonic velocity, and ²⁹Si and ²⁷Al nuclear magnetic resonance. Strength, Young's modulus, and drying shrinkage were also examined. The first drying process increased the volume of macropores and decreased the volume of mesopores and interlayer spaces. Furthermore, in the first drying process globule clusters were interconnected. During the first desorption, the strength increased for samples cured at 100% to 90% RH, decreased for 90% to 40% RH, and increased again for 40% to 11% RH. This behavior is explained by both microstructural changes in hcp and C–S–H globule densification. The drying shrinkage strains during rapid drying and slow drying were compared and the effects of the microstructural changes and evaporation were separated.     

Characterisation of pre-industrial hybrid cement and effect of pre-curing temperature

This study aimed to determine the physical-mechanical, mineralogical and microstructural properties of a pre-industrially manufactured hybrid cement (HYC) containing 5% alkaline activator and less than 30% clinker. The effect of the initial curing temperature (25 ± 1 or 85 °C for 20 h) on hydration kinetics and the development of compressive strength were also explored. The hydration products formed were characterised using XRD, SEM/EDX and ²⁷Al and ²⁹Si MAS-NMR. The findings showed that pre-industrial hybrid cement sets when hydrated with water and hardens to a 28-day mechanical strength of 35 MPa. The main reaction product formed was a mix of cementitious gels: C-(A)-S-H and C-A-S-H. Curing at 85 °C for 20 h, shows a behaviour similar to OPC, inhibited ettringite formation and generated more polymerised gels, enhancing 3-day but not 28- or 90-day mechanical strength.     

Uncertainty quantification and seismic fragility of base-isolated liquid storage tanks using response surface models

Seismic response of base-isolated liquid storage tank is represented using response surface model (RSM) to consider the uncertainty in the isolator parameters. The effectiveness of RSM to represent the probability distributions of the peak seismic response quantities of the base-isolated liquid storage tank is studied in the framework of Monte Carlo (MC) simulation. Broad and slender configurations of the tanks isolated by lead–rubber bearing (New Zealand – NZ system) characterized with non-linear force-deformation behavior is considered in the present study. The influence of the uncertain isolator parameters on the seismic response of the base-isolated liquid storage tanks is investigated. Subsequently, seismic fragility of the base-isolated liquid storage tanks is evaluated using the RSM based MC simulation. The RSM estimates the non-linear seismic response of the base-isolated liquid storage tanks with sufficient accuracy. It is observed that the uncertainties in the isolator parameters significantly influence the peak response quantities of the base-isolated liquid storage tanks. The effectiveness of the base isolation technique, in terms of the reduced probability of failure, is observed by comparing the fragility curves for the fixed-base and base-isolated liquid storage tanks. It is also observed that increase in the isolation time period decreases the probability of failure for the base-isolated liquid storage tanks. It is concluded that the peak ground acceleration (PGA) of the earthquake ground motion can be included in the RSM to reduce the computational efforts for seismic fragility analysis.     

Simulating RC beams with unbonded FRP and steel prestressing tendons

A partial interaction based analysis to simulate the behaviour of RC beams with prestressed unbonded tendons is proposed. Unlike bonded reinforcement, the strain developed in unbonded reinforcing tendons under bending is uniform along the length of the member and is thus member dependant. Conventional analysis techniques incorporate correction factors and empirical components in defining the strain developed in both the unbonded and bonded reinforcement. Being semi-empirical, the post-cracking analysis cannot directly simulate the effects of tension-stiffening on the untensioned bonded reinforcement. Accordingly, this paper presents a segmental moment–rotation approach for simulating the behaviour of RC beams with unbonded prestressed reinforcement, such that the mechanics of the approach removes the reliance on empiricisms in defining the reinforcement and unbonded tendon behaviour. Validated against experimental results, the approach is shown to accommodate concrete creep, shrinkage and reinforcement relaxation, thus enabling prestressing losses to be quantified.     

Modeling of mechanical response of FRP composites in fire

A thermomechanical model is presented for predicting the time-dependent deflections of cellular FRP slab elements subjected to mechanical loading and fire from one side. The model comprises temperature-dependent mechanical property sub-models for the Young’s modulus, viscosity and coefficient of thermal expansion. Two different thermal boundary conditions were investigated: with and without liquid-cooling of the slab elements in the cells. A finite difference method was used to calculate the deflection at each time step. Deflections resulting from stiffness degradation due to glass transition and decomposition of the resin dominated over those resulting from viscosity and thermal expansion. The predicted total deflections compared well with the measured results over a test period of up to 2 h. The failure mode of the non-cooled specimen could be explained.     

Cross-Well Radar. I: Experimental Simulation of Cross-Well Tomography and Validation

This paper explains and evaluates the potential and limitations of conducting cross-well radar (CWR) in sandy soils. Implementing the experiment and data collection in the absence of any scattering object, and in the presence of an acrylic plate [a representative of dielectric objects, such as dense nonaqueous phase liquid (DNAPL) pools, etc.], as a contrasting object in a water-saturated soil is also studied. To be able to image the signature of any object, more than one pair of receiving and transmitting antennas are required. The paper describes a method to achieve repeatable, reliable, and reproducible laboratory results for different transmitter-receiver combinations. Different practical methods were evaluated for collecting multiple-depth data. Similarity of the corresponding results and problems involved in each method are studied and presented. The data show that the frequency response of a saturated coarse-grained soil is smooth due to the continuous and dominant nature of water in saturated soils. The repeatability and potential symmetry of patterns across some borehole axes provide a valuable tool for validation of experimental results. The potential asymmetry across other borehole axes is used as a tool to evaluate the strength of the perturbation on the electromagnetic field due to hidden objects and to evaluate the feasibility of detecting dielectric objects (such as DNAPL pools, etc.) using CWR. The experimental simulation of this paper models a real-life problem in a smaller scale, in a controlled laboratory environment, and within homogeneous soils that are uniformly dry or fully water saturated, with a uniform dielectric property contrast between the inclusion and background. The soil in the field will not be as homogeneous and uniform. The scaling process takes into consideration that as the size is scaled down; the frequency needs to be scaled up. It is noteworthy that this scaling process needs to be extensively studied and validated for future extension of the models to real-field applications. For example, to extend the outcome of this work to the real field, the geometry (antenna size, their separation and inclusion size) needs to be scaled up back to the field size, while soil grains will not. Therefore, soil, water, and air coupling effects and interactions observed at the laboratory scale do not scale up in the field, and may have different unforeseen effects that require extensive study.     

Modeling, design and thermal performance of a BIPV/T system thermally coupled with a ventilated concrete slab in a low energy solar house: Part 2, ventilated concrete slab

This paper is the second of two papers that describe the modeling and design of a building-integrated photovoltaic-thermal (BIPV/T) system thermally coupled with a ventilated concrete slab (VCS) adopted in a prefabricated, two-storey detached, low energy solar house and their performance assessment based on monitored data. The VCS concept is based on an integrated thermal-structural design with active storage of solar thermal energy while serving as a structural component - the basement floor slab (∼33 m²). This paper describes the numerical modeling, design, and thermal performance assessment of the VCS. The thermal performance of the VCS during the commissioning of the unoccupied house is presented. Analysis of the monitored data shows that the VCS can store 9-12 kWh of heat from the total thermal energy collected by the BIPV/T system, on a typical clear sunny day with an outdoor temperature of about 0 °C. It can also accumulate thermal energy during a series of clear sunny days without overheating the slab surface or the living space. This research shows that coupling the VCS with the BIPV/T system is a viable method to enhance the utilization of collected solar thermal energy. A method is presented for creating a simplified three-dimensional, control volume finite difference, explicit thermal model of the VCS. The model is created and validated using monitored data. The modeling method is suitable for detailed parametric study of the thermal behavior of the VCS without excessive computational effort.     

Integrating Industry Experts into Engineering Education: Case Study

An advanced grade control course is described as a model for industry involvement in special topics educational offerings. Several unique aspects of these courses were the lack of textbooks on the topic, little previously developed course content, and the lack of faculty expertise in the subject area. In light of these challenges the course was successfully implemented by selecting industry experts and coordinating their efforts into a cohesive learning experience. The writers worked with representatives from McAninch Corporation, Ziegler Caterpillar, and other organizations to develop the courses at Iowa State University. Since the courses were the first known offerings of their kind, qualitative interviews were conducted to identify appropriate educational outcomes and objectives necessary to expedite the process of becoming an advanced grade control specialist. These outcomes and objectives can be applied to a variety of educational delivery systems and were helpful in determining suitable course material, topics, and structure.     

Optimal design of passive viscous damping systems for buildings under seismic excitation

Nowadays, it is known that through the use of energy dissipation devices, the seismic performance of buildings can be improved. However, for efficiency and structural safety, the locations and sizes of these devices need to be properly defined. In this work, a procedure to optimally define the damping coefficients of added linear viscous dampers to meet an expected level of performance on buildings under seismic excitation is proposed. The performance criterion is expressed in terms of a maximum interstory drift, which is one of the most important limitations provided by the seismic design codes. For a given level of performance, the effectiveness of the damper distribution obtained by means of different objective functions is also assessed. Knowing that the main contribution to the total uncertainty is due to the excitation and with the aim of achieving robust results, the most appropriate approach to model the excitation is through a stationary stochastic process characterized by a power spectral density compatible with the response spectrum defined by the seismic design code. Accordingly, the structural response is obtained in the frequency domain. Through numerical examples, on planar and three-dimensional steel buildings with coupled lateral and torsional vibrations, the proposed procedure is verified.     

Optimizing tradeoffs among housing sustainability objectives

The sustainability of housing units can be improved by integrating green building equipment and systems such as energy-efficient HVAC systems, building envelopes, water heaters, appliances, and water-efficient fixtures. The use of these green building measures often improves the environmental and social performances of housing units; however they can increase their initial cost and life cycle cost. This paper presents a multi-objective optimization model that is capable of optimizing housing design and construction decisions in order to generate optimal/near-optimal tradeoffs among the three sustainability objectives of maximizing the operational environmental performance of housing units, maximizing the social quality of life for their residents, and minimizing their life cycle cost. The model is designed as a multi-objective genetic algorithm to provide the capability of optimizing multiple housing objectives and criteria that include minimizing carbon footprint and water usage during housing operational phase, maximizing thermal comfort, enhancing indoor air and lighting quality, improving neighborhood quality, and minimizing life cycle cost. An application example is analyzed to illustrate the use of the developed model and evaluate its performance. The results of this analysis illustrate the novel capabilities of the model in generating 210 near-optimal tradeoff solutions for the analyzed housing example, where each represents an optimal/near-optimal and unique tradeoff among the aforementioned three sustainability optimization objectives of maximizing the operational environmental performance of housing units, maximizing the social quality of life for their residents, and minimizing their life cycle cost. These novel capabilities of the developed model are expected to improve the design and construction of housing units and maximize their overall sustainability.