Nakin Suksawang

Energy-Based Analysis of Nanoindentation Curves for Cementitious Materials

Energies dissipated during indentation are often employed to develop mathematical relations for the computation of nanoscale mechanical properties of a material. In this study, experimental nanoindentation curves pertaining to cement paste are extensively analyzed using the energy principle. As a result, energy ratios are found to be capable of accurately modeling the indentation behavior when used as parameters specific to a given nanoindentation curve. The efficacy of the elastic energy recovered during unloading in the determination of contact depth and initial unloading stiffness is also evaluated herein. While the expression representing the unloading curve yields an accurate value of contact depth, it generally overestimates the initial unloading stiffness. Considering this shortcoming of functional analysis-based expressions used for representing nanoindentation curves, a new procedure for the computation of initial unloading stiffness is proposed. The results obtained with this new approach are in reasonable agreement with those obtained experimentally.

Wind-Loading Effects on Roof-to-Wall Connections of Timber Residential Buildings

AbstractExtensive damage to residential wood-frame buildings caused by failures of roof-to-wall connections during extreme wind events underscores the need to improve their performance. Most of these connections use mechanical connectors, e.g., metal clips and straps (sometimes referred to as hurricane clips and hurricane straps). The allowable capacity of these connectors is based on results of unidirectional component tests that do not simulate multiaxial aerodynamic loading effects induced by high wind events. The objective of this research was to facilitate a better understanding of these loading effects on roof-to-wall connections of a typical low-rise gable roof residential structure subjected to combined impacts of wind and a potential breach of the building envelope. Large-scale experiments on a heavily instrumented building model generated multiaxial aerodynamic loading data on roof-to-wall connections for various wind angles of attack and internal pressure conditions. The results showed the seve...

Study of the Capability of Multiple Mechanical Fasteners in Roof-to-Wall Connections of Timber Residential Buildings

One of the most critical connections in wood frame construction is that of the roof rafter and the top plate of the wall. This type of connection typically uses mechanical fasteners, such as metal straps or clips fastened with nails. Manufacturers base the allowable capacity of connections with one fastener on results of tests performed on such connections. However, it is assumed in current design practice that the capacity of a connection with two mechanical fasteners is twice the capacity of a connection with a single fastener. Implicit in this practice is the assumption that the connection’s capacity is proportional to the number of fasteners per connection joint. This approach, based as it is on testing a single fastener per joint, disregards the fact that the failure modes of a connection joint may depend on the number of fasteners per joint. This paper presents results of tests that establish this fact. The results, based on testing with three types of wood (spruce pine fir, southern yellow pine, an...

EFFECT OF CURING METHODS ON EARLY-AGE AND DRYING SHRINKAGE OF HIGH-PERFORMANCE CONCRETE

Many engineers and agencies have observed that the field implementation of high-performance concrete (HPC) is highly dependent on curing and placing conditions. The effect of curing conditions on the early-age properties and long-term durability of HPC is not fully understood. There is a need to expand the knowledge of early-age properties and of the effect of pozzolanic material (like silica fume and fly ash) on drying shrinkage. Results are presented of a study performed to identify the effect of various curing methods on the early-age (autogenous) as well as drying shrinkage of normal and lightweight HPC. The study included a comparison of available analytical models for predicting early-age and drying shrinkage with results from tests performed on different mixes. HPC mixes were developed and evaluated as part of an overall study for the New Jersey Department of Transportation to develop and implement mix designs and technical specifications for HPC transportation structures, such as pavements and bridges. The effect of using three different curing methods on the early-age performance of HPC is presented. The curing conditions consisted of air-dry curing, burlap or moist curing, and use of a curing compound. Results show that moist (burlap) curing should be applied within 1 h after the placement of concrete to improve early-age performance. For very low water-to-cement plus pozzolan ratios, fly ash and lightweight aggregate improved the autogenous shrinkage performance. Moreover, current shrinkage models need to be revised to address HPC mixes.

Work-of-indentation as a means to characterize indenter geometry and load-displacement response of a material

Normalized indentation works, referred to as the total and elastic energy constants, have been shown to be effective in the representation and analysis of experimental load‐displacement data. However, their physical meaning, influencing factors, variation range and relationships with other nanomechanical quantities are not precisely known. In this study, the load‐displacement data obtained as a result of simulations of elastic and elasto-plastic indentations are extensively analysed to enhance our understanding concerning these two energy-based parameters. It has been shown that while the total energy constant describes the state of an indenter tip and the type of contact regime, the elastic energy constant characterizes the response of a material to indentation. In addition, their applications in the evaluation of key nanomechanical quantities such as the indenter tip radius, the nominal hardness and the contact depth are also discussed. (Some figures may appear in colour only in the online journal)

Development of Live Load Distribution Factor Equation for Girder Bridges

Live load distribution factor (LDF) equations are among the most important bridge design calculations because they provide the distributed moment and forces, which are needed for designing new or for evaluating existing bridges. The AASHTO load and resistance factor design specification is the current standard for bridge design. It uses a power function containing parameters that are often unknown when bridges are being designed and thereby allows the design procedure to be iterative. This iterative design procedure is perceived by many practicing engineers as impractical and has raised questions regarding the desired level of simplicity in bridge design calculations. Therefore, simpler but more accurate LDF equations are needed. An experimental program to evaluate the behavior of girder bridges was launched. The program consisted of live load testing of actual girder bridges. A parametric study was carried out by using a detailed three-dimensional finite element analysis to determine how sensitive various bridge parameters are to the LDF. On the basis of the parametric study, new simplified LDF equations are proposed for various types of girder bridges, including steel, prestressed, and spread box girder bridges. The equation format is simplified as a function of two bridge parameters only (girder spacing and span length) while an equal level of accuracy is maintained. Results are compared with those from other available equations and are shown to have excellent correlation with the more exact finite element method.

Triaxial Load Testing of Metal and FRP Roof-to-Wall Connectors

The allowable capacity of conventional roof-to-wall metal connectors is based on results of unidirectional component tests that do not simulate triaxial aerodynamic loading effects induced by high-wind events. The results of wind and wind-driven rain tests conducted at a full-scale facility were used to create a database on aerodynamic and aerohydrodynamic load effects on roof-to-wall connectors. Based on these results, three axial mean force components (triaxial mean loads) were combined into a series of resultant mean force vectors. A new test protocol was then developed for roof-to-wall connectors under simulated triaxial loading as opposed to simple uniaxial loading. The findings confirm that current testing methods tend to overestimate the actual load capacities of metal connectors. The performance of a nonintrusive roof-to-wall connector system using fiber-reinforced polymer (FRP) ties was also tested and compared with that of a traditional metal connector under simulated aerodynamic loads. The test...

EFFECT OF CURING METHODS ON DURABILITY OF HIGH-PERFORMANCE CONCRETE

Many state departments of transportation are currently either using high-performance concrete (HPC) or developing new mix proportions for the application of HPC to transportation structures, with emphasis on bridge decks. However, many state engineers have observed that curing methods and conditions in the field affect the behavior of HPC structures. Moreover, little is known about the effect of curing on the long-term durability of HPC. Therefore, it is necessary to understand the behavior of HPC under various curing conditions and durations and the effect of pozzolanic material such as fly ash and silica fume on rapid chloride permeability (RCP). These factors were studied as part of a project for the New Jersey Department of Transportation to develop and implement mix design and technical specifications for HPC transportation structures such as pavements and bridges. Several mixes were tested, and the best mix was selected on the basis of strength and shrinkage test performance. The long-term durability was assessed by tests for RCP, creep, and freeze–thaw behavior. Moreover, the effect on HPC of four curing methods—moist curing, air-dry curing, burlap wrap, and curing compound—was investigated. Moist-cured cylinders performed better than those cured with other methods, and a minimum of 14 days of cure was required for HPC to attain its full strength.

Field investigation and performance of bridge approach slabs

Bridge approach slabs are designed to function as a transitional roadway to the bridge deck, spanning the distance between the abutment and the road pavement. However, the number of rough riding approaches with heavy maintenance requirements is sufficient to convince highway agencies that a serious problem exists. The complaints usually describe a ‘bump’ that motorists feel when they approach or leave bridges. This bump results in reduction of steering response, distraction to the driver, amplified truck impact and dynamic response in bridge decks, and expense to maintenance operations. Approach slabs can lose their contact supports due to various reasons, including the settlement of soil and the bulging of embankments. The objective of this paper is to present results from a study to evaluate the analytical behaviour and the field performance of new design alternatives that could reduce or eliminate this problem. The new designs were analysed using comprehensive finite element (FE) models and were also built and instrumented at the Doremus Avenue Bridge in Newark, New Jersey, for testing and long-term monitoring. Various truck load tests were performed prior to opening the bridge to traffic. The results show that the FE models are accurate and that the new design alternatives have higher load carrying capacities than those of the existing design. In addition, results also show that long-term field monitoring using various sensors can lead to quantitative measurements of various factors that can lead to the cracking of the approach slabs.

The Development of High-Performance Concrete for Transportation Structures in New Jersey

A study was performed to develop high-performance concrete (HPC) mixes and specifications for transportation structures using resources that are readily available in New Jersey. A total of 87 mixes with the water-to-cementitious (w/cm) ratio ranging from 0.27 to 0.55, were developed. Both mineral and chemical admixtures—silica fume, fly ash, superplasticizer, and air-entraining agent—were used to improve the mechanical properties and durability of concrete. Out of these 87 mixes, three mixes with different compressive strengths were selected as base mixes. Both the mechanical properties and durability tests were performed on these mixes, which consisted of compressive strength, modulus of elasticity, drying shrinkage, creep, freeze-thaw durability, chloride permeability, and scaling resistance. Moreover, the effect of curing methods (dry, wet burlap, and compound) on the strength and durability of HPC were also investigated. Results show that the strength and durability of HPC could be enhanced with ternary blended mixes.