Nur Yazdani

Strength Enhancement of Cement Mortar with Carbon Nanotubes: Early Results and Potential

One of the most important areas of research in nanotechnology involves carbon nanotubes (CNTs) in commercial applications. Developing nanotechnology for cement and concrete has particular importance. The cement hydration chemistry and the physical behavior of hydration products have potential for manipulation through nanotechnology. A main objective of the development of cement-related nanotechnology is to produce stronger, tougher, lighter, and more durable products. Nanotubes were discovered in 1991 in Japan. They are available in single-wall and multiwall configurations. They have remarkable strength (>63 GPa) and a high Youngs modulus. Nanotubes are applied in nanoscale, electrical, biotechnological, and composite materials and in enhancing physical properties. There is great potential for CNT use as reinforcement for concrete in compression and tension. This use would eliminate reinforced concrete corrosion problems, with significant expected gain in strength and ductility and improved dispersion of reinforcement. An expanded study at the University of Texas at Arlington is exploring the effect of CNTs in cement mortar for different types and dosage rates of multiwall nanotubes. The initial results are encouraging but depend largely on the mixing technique and workability issues. Strong capillary CNT forces drew water and decreased the workability of the mortar. Large voids resulted, reducing test strengths. Uniform dispersion of CNTs is mandatory, and a sonication technique was adapted to disperse them. Scanning electron microscopy is being used to investigate CNT dispersion into mortar matrix.

Effect of Carbon Nanotube Size on Compressive Strengths of Nanotube Reinforced Cementitious Composites

Application of nanoscale science to construction material has already begun. In recent times, various nanofibers have raised the interest of researchers due to their exceptional mechanical properties and high potential to be used as reinforcement within cement matrix. Carbon nanotube (CNT) is one of the most important areas of research in the field of nanotechnology. The size and exceptional mechanical properties of CNT show their high potential to be used to produce high performance next generation cementitious composites. In this study, an attempt has been made to investigate the effect of size of CNTs on compressive strengths of CNT reinforced cement composites. Seven different sizes of multiwalled nanotubes (MWNTs) were used to produce MWNT-cement composites. A trend was observed regarding the effect of nanotube size on compressive strength of composites in most cases. MWNT with outside diameter (OD) of 20 nm or less exhibited relatively better performance. Smaller MWNT can be distributed at much finer scale and consequently filling the nanopore space within the cement matrix more efficiently. This in turn resulted in stronger composites.

Probabilistic fracture mechanics application to highway bridges

Abstract A probabilistic fracture mechanics model was developed for determining the risk of fatigue failure of steel highway bridges. A deterministic fracture mechanics calculation of crack growth with stochastic inputs for crack growth rate, fracture toughness, initial crack size and load history was used. Stress intensity expressions were compiled from literature for typical AASHTO category bridge details. Statistical distributions for input variables were derived from data reported in literature. The model was found to predict well the short service life of the Yellow Mill Pond Bridge in Connecticut.

Potential of carbon nanotube reinforced cement composites as concrete repair material

Carbon nanotubes (CNTs) are a virtually ideal reinforcing agent due to extremely high aspect ratios and ultra high strengths. It is evident from contemporary research that utilization of CNT in producing new cement-based composite materials has a great potential. Consequently, possible practical application of CNT reinforced cementitious composites has immense prospect in the field of applied nanotechnology within construction industry. Several repair, retrofit, and strengthening techniques are currently available to enhance the integrity and durability of concrete structures with cracks and spalling, but applicability and/or reliability is/are often limited. Therefore, there is always a need for innovative high performing concrete repair materials with good mechanical, rheological, and durability properties. Considering the mechanical properties of carbon nanotubes (CNTs) and the test results of CNT reinforced cement composites, it is apparent that such composites could be used conveniently as concrete repair material. With this end in view, the applicability of multiwalled carbon nanotube (MWNT) reinforced cement composites as concrete repair material has been evaluated in this study in terms of setting time, bleeding, and bonding strength (slant shear) tests. It has been found that MWNT reinforced cement mortar has good prospective as concrete repair material since such composites exhibited desirable behavior in setting time, bleeding, and slant shear.

Deficiency Analysis of Coastal Buildings toward Storm Damage Reduction

Engineering services provided by one of the writers to property owners, insurance companies, attorneys, and others have resulted in a compilation of case studies, based on more than 1,000 site inspections, suitable to generate a database regarding various aspects of building performance. The scope of those services typically included identifying the cause and origin of damage to residential and commercial structures, as well as an estimation of the magnitude of damage sustained by those structures. The majority of those damaged structures was located in proximity to a coastal region and experienced recent exposure to a storm or other weather event. Compilation of data from those case studies allowed identification and ranking of the occurrence of chronic building problems. It has become apparent that building-related deficiencies often exist as a common feature in similar structures. Some of those recurring deficiencies could be eliminated with alternate building design, better construction practices, or proper routine maintenance procedures. Where applicable, proposed remedial solutions are presented for specific building deficiencies or problems identified.

An Experimental Study for Quantitative Estimation of Rebar Corrosion in Concrete Using Ground Penetrating Radar

Corrosion of steel rebar in reinforced concrete is one the most important durability issues in the service life of a structure. In this paper, an investigation is conducted to find out the relationship between the amount of reinforced concrete corrosion and GPR maximum positive amplitude. Accelerated corrosion was simulated in the lab by impressing direct current into steel rebar that was submerged in a 5% salt water solution. The amount of corrosion was varied in the rebars with different levels of mass loss ranging from 0% to 45%. The corroded rebars were then placed into three different oil emulsion tanks having different dielectric properties similar to concrete. The maximum amplitudes from the corroded bars were recorded. A linear relationship between the maximum positive amplitudes and the amount of corrosion in terms of percentage loss of area was observed. It was proposed that the relationship between the GPR maximum amplitude and the amount of corrosion can be used as a basis of a NDE technique of quantitative estimation of corrosion.

Blast Capacity and Protection of AASHTO Girder Bridges

AASHTO has specified probability-based design methodology and load factors for designing bridge piers against ship impact and vehicular collision. Currently, no specific AASHTO design guideline exists for bridges against blast loading. Structural engineering methods to protect infrastructure systems from terrorist attacks are required. This study investigated the most common types of concrete bridges on the interstate highways and assessed the capacities of the critical elements. A 2-span 2-lane bridge with Type III AASHTO girders was used for modeling. AASHTO Load and Resistance Factor Design methods were used for bridge design. The girders, pier caps and columns were analyzed under blast loading to determine their capacities. This study determined the blast capacities of the AASHTO girders, pier caps and the columns, and the required standoff distance of explosion from the columns that may possibly protect the bridge from failure.

VALIDATION OF AASHTO BEARING STIFFNESS FOR STANDARD PRECAST CONCRETE BRIDGE GIRDERS

Precast American Association of State Highway and Transportation Officials (AASHTO) concrete bridge girders are often supported at the ends of elastomeric bearing pads. The bearing pad-bridge girder interface defines support boundary conditions that may affect the performance of the bridge. In this study, finite element (FE) modeling was used to validate AASHTO bearing stiffness specifications. Stiffness characteristics of the Florida Department of Transportation (FDOT) type bearing pads were theoretically determined under varying elastomer shear modulus values. FE models of AASHTO Type III and V girders were subjected to simulated static truck loads. Vertical and horizontal spring elements, simulating new bearing pads, were incorporated at the ends of the girder models. A full section of a bridge on U.S. Highway 27 was also modeled, and the results were compared with field tests. In general, the restraint effects of the bearing pads were beneficial to the performance of the girders and the bridge. The beneficial effect, however, was small for new bearing pads, and more pronounced under drastic increases in bearing stiffness because of aging and colder temperatures. Such dramatic increase in bearing stiffness may be considered in design. Current FDOT type bearing pads are serving the main purpose of their application, which is to provide vertical support with minimum horizontal restraint force to the girders, thus allowing horizontal movement and rotational movements.

Effect of Steam Curing on Concrete Piles with Silica Fume

Silica fume is a common addition to high performance concrete mix designs. Florida Department of Transportation (FDOT) allows only a 72-hour continuous moist cure process for concrete containing silica fume. FDOT allows steam curing only for regular concrete not containing silica fume. Accelerated curing has been shown to be effective in producing high-performance characteristics at early ages in silica-fume concrete. However, the heat greatly increases the moisture loss from exposed surfaces, which may cause shrinkage problems. An experimental study was undertaken to determine the feasibility of steam curing of FDOT concrete with silica fume in order to reduce precast turn around time. Various steam curing durations were utilized with full-scale precast prestressed pile specimens. The concrete compressive strength and shrinkage were determined for various durations of steam curing. Results indicate that steam cured silica fume concrete met all FDOT requirements for the 12, 18 and 24 hours of curing periods. No shrinkage cracking was observed in any samples up to one year age. It was recommended that FDOT allows the 12 hour steam curing for concrete with silica fume.

Aggregate-Based Modulus of Elasticity for Florida Concrete

The current empirical model for the Florida Department of Transportation (FDOT) concrete modulus of elasticity (E) is a function of concrete compressive strength and unit weight. Recent testing shows that this model consistently underestimates E for FDOT concrete. This underestimation may lead to construction problems caused by the overprediction of camber and the deflection of concrete structural members. E values for the typical concrete mix used in FDOT projects were experimentally determined at various concrete ages. Regression analyses were used to find the E models that best fit the generated data, and such models were compared with existing E models from the literature. It was found that the aggregate type and specific gravity play significant roles in influencing the E value of concrete. The best-fit E models for FDOT concrete, together with suggested modification factors for various aggregate types, are recommended.