Husam Najm

Evaluation of Mechanical Properties for Self-Consolidating, Normal, and High-Performance Concrete

The use of self-consolidating concrete (SCC) is becoming increasingly popular in the United States. Several departments of transportation are currently accepting SCC mix design in some of their projects. Moreover, FHWA is also promoting SCC. However, because SCC is a relatively new material, its mechanical properties and durability are not fully understood. The main objective of this paper is to evaluate and compare the mechanical properties of SCC with normal-conventional concrete, as well as with high-performance concrete. The effect of supplementary cementitious materials on the mechanical properties of SCC is also investigated, and the experimental results of this study are compared to prediction equations. The results show that SCC has a lower modulus of elasticity compared with normal or conventional concrete but higher tensile splitting strength. SCC also has higher drying shrinkage and passes a higher charge (coulombs when using the rapid chloride permeability test).

Concrete bridge deck early problem detection and mitigation using robotics

More economical management of bridges can be achieved through early problem detection and mitigation. The paper describes development and implementation of two fully automated (robotic) systems for nondestructive evaluation (NDE) and minimally invasive rehabilitation of concrete bridge decks. The NDE system named RABIT was developed with the support from Federal Highway Administration (FHWA). It implements multiple NDE technologies, namely: electrical resistivity (ER), impact echo (IE), ground-penetrating radar (GPR), and ultrasonic surface waves (USW). In addition, the system utilizes advanced vision to substitute traditional visual inspection. The RABIT system collects data at significantly higher speeds than it is done using traditional NDE equipment. The associated platform for the enhanced interpretation of condition assessment in concrete bridge decks utilizes data integration, fusion, and deterioration and defect visualization. The interpretation and visualization platform specifically addresses data integration and fusion from the four NDE technologies. The data visualization platform facilitates an intuitive presentation of the main deterioration due to: corrosion, delamination, and concrete degradation, by integrating NDE survey results and high resolution deck surface imaging. The rehabilitation robotic system was developed with the support from National Institute of Standards and Technology-Technology Innovation Program (NIST-TIP). The system utilizes advanced robotics and novel materials to repair problems in concrete decks, primarily early stage delamination and internal cracking, using a minimally invasive approach. Since both systems use global positioning systems for navigation, some of the current efforts concentrate on their coordination for the most effective joint evaluation and rehabilitation.

Prediction of Early Age Normal Concrete Compressive Strength Based on Dynamic Shear Modulus Measurements

In this research, the relationship between the compressive strength, fc, and the dynamic shear modulus, Gd, of normal concrete at an early age was studied. To investigate the correlation between fc and Gd at an early age, different types of mixtures, including mortar and concrete, were prepared, and the corresponding fc and Gd values were measured every 12 h after initial mixing up to 72 h after casting. The influences of hydration age, water-to-cement (w=c) ratio, curing temperature, aggregate volume content, and maximum aggregate size on the Gdfc relationship of concrete were studied. The Gdfc relationship was then mathematically modeled by using multivariable power laws. The developed model is reasonably accurate to predict the early age compressive strength of concrete with variations in hydration age, aggregate content, and sizes. This type of model can be used directly in the field for the estimation of concrete strength when nondestructive testing (NDT) techniques are employed. DOI: 10.1061/(ASCE)MT.1943-5533.0000528.

Nanoscale materials for non-destructive repair of transportation infrastructures

Results related to the development of an inorganic matrix that is suitable for filling narrow cracks and thin delaminations on bridge decks are presented in this paper. Almost all the repair materials currently available for these types of repairs are organic polymer based matrices. These matrices create a discontinuity in the modulus of elasticity and water permeability. These discontinuities result in the failure of repairs within about five years. The matrix used in the current investigation has a modulus of elasticity and permeability characteristics that are similar to the concrete used in the bridge decks. The primary properties investigated were: bonding to cracked surfaces, flow characteristics, ease of application, and mechanical characteristics. This paper discusses these properties, matrix performance and matrix viability for use in automated nondestructive robotic delivery system to fill delaminations and narrow/hairline crack in bridge decks.

Assessment of AASHTO LRFD guidelines for analysis of regular bridges subjected to transverse earthquake ground motions

This paper evaluates the two procedures given in AASHTO LRFD specifications for the transverse analysis of regular bridges subjected to transverse earthquake ground motions. The Single Mode Spectral Analysis (SMSA) method and the Uniform Load (UL) method are evaluated for different bridge geometries, rigidities, and ground motion levels and were also compared to the sine shape function approximation and the computer methods. Two-span, three-span and four span continuous units were investigated. The effect of reduced column stiffness was also evaluated. The results from this study showed that for some bridges in certain seismic zones, the differences in column elastic shears and deformations from the UL method can be about 10% lower than those from the SMSA method and the other methods. Abutment elastic forces from the UL method were about 25% to 300% higher than those from the SMSA and other methods. The UL method, permitted by AASHTO LRFD may not be a conservative analysis method for predicting pier forces and deflections for certain bridges. The method also significantly over estimates abutment forces for longer bridges thus making them overdesigned and uneconomical.

Evaluation of nano/micro composites for nondestructive repair of delaminated structures

Results of an experimental evaluation of nano/micro inorganic composites are presented in this paper. Alkali alumino silicates matrices reinforced with nano/micro fibers were used to repair (glue) fractured concrete prisms and test them in three point bending. Further, shear strength of matrices were also obtained using push-up tests. The variables evaluated were mix composition, temperature and specimen size. It is observed that flexural tensile strength of 1000 psi can be achieved from the developed matrices. In some instances when repaired broken prisms were tested, the failure occurred by creation of a new fracture surface. The developed matrices had the fluidity to fill very thin delamination, which can be pumped to reach delamination through small drilled holes. The results show that the compositions obtained in this study have excellent potential for application involving the repair of delamination.

Response of laterally loaded group shafts for bridge foundations in cohesionless soils using a 3D FE soil-structure model

The lateral stiffness of bridge foundations has a major effect on its response to lateral loads. Response values such as maximum moments, maximum displacements, and plastic hinge location below ground need to be accurately predicted for a safe and sound design. Many approaches have been used to model foundation lateral stiffness such as the Winkler beam (spring models) and the concept of length of fixity to simplify the design. This paper discusses the response of laterally loaded group shafts using three-dimensional finite element (FE) soil–structure models for single shaft–soil system and group shaft–soil system in cohesionless soils. Parameters investigated include soil modulus, shaft slenderness, shaft–soil interface, soil effective zone, shear interaction, and support conditions at the bottom of the shaft. Results from this investigation provide criteria for the effective volume of soil that needs to be included in the FE analysis for single and group shaft analysis. It also showed that shaft spacing...

EVALUATION OF DESIGN LEVEL EARTHQUAKES OF NCHRP PROJECT 12-49 FOR SEISMIC DESIGN OF BRIDGES IN NEW JERSEY AND THE NORTHEAST UNITED STATES

NCHRP Project 12-49 is intended to develop comprehensive specifications for seismic design of bridges considering all aspects of the design process including: (1) design philosophy and performance criteria, (2) seismic loads and site effects, (3) analysis and modeling, (4) design requirements, and (5) detailing. These new specifications will be nationally applicable with provisions for all seismic zones. Performance levels for life safety and operational are defined for the rare earthquake event (MCE) and for the Expected earthquake event. While the ratio of the accelerations from the two events is about 1.5 to 2 in the Western United States, this ratio is about 3 to 4 in the Eastern United States. The maximum considered earthquakes (MCE) in New Jersey based on the proposed NCHRP 12-49 provisions are significantly higher than current design level earthquakes based on current AASHTO LRFD criteria. Maximum accelerations for relatively stiff bridges in soft soils located in North New Jersey from NCHRP 12-49 are approximately twice those from current LRFD accelerations. The current guidelines for seismic design of bridges in New Jersey are based on the 1998 LRFD Specifications (w/interims 1999-2003). The specs also require Safety Evaluation performance level as well as Functional Evaluation level for “critical” bridges including bridges carrying Turnpike traffic. Because of the several significant changes in the design earthquake and design criteria provided in the new provisions, there are questions on how these new provisions will affect the design and performance of bridges in states nationwide as well as the retrofit of existing bridges and the associated costs; in particular, the return period and soil factors. The current recommended return period for the MCE in NCHRP 12-49 is 2500 years. This return period seems conservative for the Northeastern US especially for “non critical” bridges. The soil factors were based on seismic records from the Western US and were higher for soft soils. Hence, there is a need to evaluate the impact of the maximum accelerations in new seismic design provisions proposed in NCHRP Report 12-49 on the seismic design and detailing of bridges in New Jersey and the Northeastern US and whether the use of such high accelerations and high soil factors in areas of low seismic risk such as New Jersey is justified.

IMPACT OF NCHRP PROJECT 12-49 PROVISIONS ON THE SEISMIC DESIGN OF BRIDGES ON VERY SOFT SOILS IN NEW JERSEY

This paper evaluates the impact of newly recommended LRFD seismic design guidelines (NCHRP Project 12-49) on the seismic design of bridges on soft soils in New Jersey. Current LRFD provisions are mostly based on the Division I-A Seismic Design of the AASHTO Standard Specifications (1996). NCHRP Project 12-49 was initiated to address the inadequate performance of highway bridges in recent earthquakes and the deficiencies in the current seismic code and to develop comprehensive specifications for seismic design of bridges considering all aspects of the design process. The intent of the new specifications is to be nationally applicable with provisions for all seismic zones. Because of the several significant changes in the design criteria and approach provided in the new provisions, there are questions on how these new provisions will affect the design and performance of bridge in states in the eastern United States as well as the retrofit of existing bridges. There are also questions about the impact of new...