Salah Altoubat

CREEP, SHRINKAGE, AND CRACKING OF RESTRAINED CONCRETE AT EARLY AGE

In this study, uniaxial restrained shrinkage tests were conducted on plain and fiber-reinforced concrete (FRC) samples to provide data on shrinkage stresses, shrinkage strain, and tensile creep at early age. The influences of water-cement ratio (w/c), fiber reinforcement, and curing conditions on restrained shrinkage behavior of concrete were investigated. It was found that tensile creep relaxed shrinkage stresses by 50% and doubled the failure strain capacity. Both the magnitude and time history of the shrinkage stress influence the time of cracking, which in this study occurred at approximately 80% of the static tensile strength. Steel fibers substantially delay the shrinkage cracking, but without influencing the stress at failure. Finally, it was found that sealing of the concrete specimens did not eliminate the early age shrinkage, and that wet curing effectively relaxed shrinkage stresses.

Tensile basic creep of early-age concrete under constant load

Viscoelastic behavior of early-age normal and high-strength concrete has been investigated. The study shows that concrete exhibits high tensile creep strain if loaded at an age less than or equal to 1 day. The investigations furthermore show that the creep strain is not proportional to the stress level in the specimen when loading occurs at 1 day. Creep experiments were also carried out on concretes with different w/c ratios and some qualitative comments are made. Finally, an approach for mathematical modeling of early-age creep for normal concrete was explored.

Shear Behavior of Macro-Synthetic Fiber-Reinforced Concrete Beams Without Stirrups

Twenty-seven large-scale beams were instrumented and tested under monotonic centerpoint loading to determine the effect of a newly developed high-modulus macrosynthetic fiber on the shear strength and failure behavior of longitudinally reinforced concrete (RC) beams without stirrups. Slender and short beams with respective shear span-depth ratios (a/d) of 3.5 and 2.3 were tested. The length of the beams varied between 1.9 and 3.2 m (75 and 126 in.), and the macrosynthetic fibers were added at volume fractions of 0.50, 0.75, and 1.0%. Deflection of the beam, strain in the concrete and in the flexural reinforcing bars, and the cracking pattern were monitored during the test at different stages of the monotonic loading until failure. The results showed that the addition of macrosynthetic fibers significantly improved the shear strength and ductility of the RC beams and modified the cracking and failure behavior. The ultimate shear strength of slender and short beams was increased up to 30% compared to the control RC beams.

TENSILE BASIC CREEP: MEASUREMENTS AND BEHAVIOR AT EARLY AGE

In this research, creep and shrinkage of concrete under wet and sealed curing conditions were studied to determine the tensile basic creep of concrete during the first days after casting. The common practice of sealing concrete to measure basic creep was found inaccurate because internal drying at this age is generally a significant factor. Instead, a moist cover was placed on the concrete samples to successfully suppress early-age shrinkage. A basic creep model based on solidification theory was implemented to provide insight on the behavior of plain and fiber-reinforced concrete. Results revealed a high rate of basic tensile creep in the first 20 hrs of limit. More importantly, the tensile basic creep was found sensitive to age at loading only within the first few days and age-independent after 5-6 days. Lastly, steel fiber reinforcement lowered the initial rate of tensile basic creep.

Effect of Synthetic Fibers on Structural Behavior of Concrete Slabs-on-Ground

This article studies whether newly developed synthetic macrofiber can enhance the strength properties of plain and fiber-reinforced concrete slabs-on-ground. The structural behavior of fiber-reinforced concrete slabs under interior and edge loading conditions were tested. The results of the load testing showed that the failure behavior of plain concrete slabs was significantly modified with the addition of synthetic fibers, but that the synthetic fibers did not change the tensile cracking load of the plain concrete slab. Strain gauges embedded in the concrete slabs indicated the fibers distributed the load-carrying capacity throughout the slab volume, increasing the concrete slab flexural and ultimate capacities.

Grip-Specimen Interaction in Uniaxial Restrained Test

Synopsis: Restrained tests are used to evaluate the risk of early age cracking and the cracking sensitivity of concrete mixtures. One test that has become common in recent years is the active uniaxial restrained test in which the length change due to shrinkage is recovered by applying external load to maintain the concrete sample at constant length. The length change is measured by linear variable differential transformer (LVDT), which is used as the control signal in this test. In such tests, the dog-bone geometry is used to grip the ends. To ensure a fully restrained test, the LVDT response to the loads and to shrinkage should reflect the deformation in the concrete sample. Therefore, the grip-specimen interaction should not interfere with the measurement of deformation, and this depends on the instrumentation and how the LVDT is attached to the concrete specimen. Some experiments in the literature have the LVDT attached to the steel grips, a practice vulnerable to possible error due to the interaction between the grip and the concrete. This study considered two methods of attaching the LVDT. First, the LVDT is attached to the steel grips; second, the LVDT is attached to the concrete within the zone of reduced cross-section. The results indicate that attaching the LVDT to the grips results in errant measurement of the shrinkage stress, creep, and elastic strains due to the grip-specimen interaction. The consequences will be false interpretation of fully restrained shrinkage and creep characteristics because the grip-specimen interaction leads to a partially restrained test. The study suggests mounting the LVDT to the concrete sample away from the grips to achieve a fully restrained test. Results for two concrete mixtures with w/c ratio of 0.51 and 0.56 are discussed for both methods of attaching LVDTs.

A New Look at Tensile Creep of Fiber-Reinforced Concrete

The study reveals that additional steel fibers with a volume fraction of 0.50% influences the individual components of tensile creep in different manners. Steel fibers reduce the initial rate of tensile basic creep, but increase long term basic creep capacity. That suggests that fibers provide more stress relaxation in time. The study attributes this to the ability of the fibers to control microcracking, distribute internal stresses more uniformly and engage greater volume of the matrix in stress transfer. To avoid confusion in interpreting the stress relaxation of FRC from total tensile creep and drying creep test results, the study suggests dividing the stress relaxation mechanisms into beneficial and detrimental components. This study demonstrates an approach for testing tensile creep of concrete in order to isolate the mechanisms responsible for basic and drying creep.

Instrumentation and Analysis of High-Performance Concrete Bridge Decks

The use of high-performance concrete (HPC) for transportation structures was the subject of a 3-year study that involved field investigation, laboratory experiments, analysis, and modeling. The field study involved instrumentation and analysis of six HPC bridge decks. The laboratory component characterized early-age thermal, shrinkage, creep, and cracking behaviors. A three-dimensional finite element model was used in conjunction with material models to analyze and predict creep and shrinkage behavior and to investigate structural and material interactions. This paper focuses on the field component of the project and discusses the instrumentation, deformation measurements, and analysis of bridge decks in Illinois. The bridges were instrumented to understand the development of shrinkage and thermal stress in concrete bridge decks with the use of various materials and structural components. The results indicate that the stress development due to daily temperature cycles and long-term temperature changes are relatively small compared with the stress development due to drying shrinkage. According to model simulations, a 15% to 40% reduction in shrinkage would reduce the stress level enough to prevent most cracking. Although drying shrinkage is the major driving force for stress development, the interaction of concrete shrinkage and structural restraint influences the magnitude of the stress and is linked to the propensity for early-age cracking.

Experimental Study of In-Plane Shear Behavior of Fiber-Reinforced Concrete Composite Slabs

AbstractThis study investigates the in-plane shear behavior of composite slabs reinforced with different types of secondary reinforcements. Experimental results of 20 large-scale composite slabs constructed with two different deck profiles (reentrant and trapezoidal) are presented. The slabs were instrumented and tested in a cantilever diaphragm configuration under monotonic in-plane shear to assess and compare the effect of secondary reinforcement on their in-plane shear capacity. Five types of secondary reinforcements were considered: Conventional WWM of sizes A142 and A98, synthetic macrofibers at dosage rates of 3.0 and 5.3  kg/m3, and hooked-end steel fibers at a dosage rate of 15.0  kg/m3. Tests were carried out for both strong and weak orientation of decking. The load-deflection and load-strain responses were measured, and the cracking pattern and sequence were observed. The results showed that fibers notably improved the in-plane shear behavior (strength and ductility) of the composite slabs. Stee...

Elliptical Leaf Spring Shock and Vibration Mounts with Enhanced Damping and Energy Dissipation Capabilities Using Lead Spring

We present an enhancement to the existing elliptical leaf spring (ELS) for improved damping and energy dissipation capabilities. The ELS consists of a high tensile stainless steel elliptical leaf spring with polymer or rubber compound. This device is conceived as a shock and vibration isolator for equipment and lightweight structures. The enhancement to the ELS consists of a lead spring plugged vertically between the leaves (referred to as lead-rubber elliptical leaf spring (LRELS)). The lead is shown to produce hysteretic damping under plastic deformations. The LRELS isolator is shown to exhibit nonlinear hysteretic behavior. In both horizontal directions, the LRELS showed symmetrical rate independent behavior but undergoes stiffening behavior under large displacements. However, in the vertical direction, the LRELS behavior is asymmetric, exhibiting softening behavior in compression and stiffening behavior in tension. Mathematical models based on the Bouc-Wen model, describing the hysteretic behavior of the proposed isolator, are developed and numerically calibrated using a series of finite element analyses. The LRELS is found to be effective in the in-plane and vertical directions. The improved damping and energy dissipation of the LRELS is provided from the hysteretic damping of the lead spring.