Cementitious composites have high compressive strength and modulus of elasticity, but relatively low tensile strength, toughness and ductility. They are amenable to enhancement through nanotechnology due to the physical behavior and size of hydration products. Carbon Nanotube (CNT) is a very efficient reinforcing agent due to its extremely high aspect ratio and ultra-high strength. So, there is a high potential to utilize CNT in producing new cement based composite materials. In this study, a parametric experimental investigation was undertaken to determine optimum mix dosage of CNT for cement mortar. Different dosage rates of surface treated Multi-Walled Nanotubes (MWNT), water/cement ratios and plasticizer amounts (as surfactant for the MWNT) were investigated through compressive and flexural strength determinations. A mixing technique was proposed to address the issues related to dispersion of nanotubes within the cement matrix. The proposed mixing technique and design mix significantly enhanced the compressive and flexural strengths, as compared to control samples with no MWNT.

In the customary way of clay brick production by burning soil in Bangladesh and other developing countries, a good number of bricks (approximately 13%) are severely overburnt due to uncontrolled distribution of temperature in the kiln. These bricks are dark red in color, severely distorted, and swollen. These bricks are considered totally useless and are not allowed in any construction work. In this study, the feasibility of utilizing these overburnt bricks as coarse aggregates in highway pavement projects was explored. Routine laboratory tests such as the Los Angeles abrasion, water absorption, specific gravity, and unit weight, were used to compare the properties of the overburnt bricks with the best possible quality picked or pick-Jhama (Type A) bricks from the same stockyards in Bangladesh. It was found that overburnt brick aggregates are much stronger, less absorptive, and denser in general than the ones from the picked or pick-Jhama (Type A) bricks. Therefore, these bricks may be conveniently and ec...

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.

Highway bridges may encounter various extreme events during their design lives, and the effect of accidental, natural, or artificial fire on bridges is one of the least investigated hazard types. To bridge this knowledge gap, a full-scale one-span prestressed concrete composite bridge was tested under a combined hydrocarbon pool fire in an open environment and simulated AASHTO live load. This is believed to be the first testing of its kind in the world. The objective was to study the response of a concrete bridge superstructure, the carbon fiber-reinforced polymer (CFRP) strengthening, and associated fireproofing protection against the fire hazard. Unlike the standard fire tests, which are conducted in a controlled environment, the 60-min event resulted in fluctuating and high-intensity fire temperatures on the girders due to the prevailing wind. This characteristic showed that results from standard fire tests should not be used to justify the fire performance of concrete bridge components. Of the three tested girders, the girder with CFRP and no fireproofing sustained severe damage, indicating that the nation’s concrete bridges with such girders are highly vulnerable and unprotected for extreme fire events. The fireproofing was helpful in lowering the temperature at the CFRP-concrete interface, preserving the integrity of the CFRP bonding, the concrete substrate, and the prestressing steel.

The feasibility of strengthening deteriorated or under-capacity concrete structures with external carbon-fiber-reinforced polymer (CFRP) laminates has been widely validated in the literature. However, there is a lack of knowledge on the in situ long-term performance and age-related environmental degradation of the mechanical properties of the laminates. The current study involved the immersion of coupons from a common new CFRP laminate in heated water at 23, 45, and 60 °C for 224 days. The coupons were then tested for residual tensile properties, such as tensile capacity and elastic modulus, using ASTM D3039 (2017) specifications. The CFRP tensile capacity and elastic modulus decreased by a maximum of 33% and 26%, respectively, for 224 days of exposure. Based on the test data, an age-based long-term prediction model with excellent reliability for CFRP laminate tensile capacity was developed. The model was then calibrated with test results from old CFRP coupons collected from an existing CFRP laminate retrofitted concrete bridge. The calibrated model output was then compared with the environmental reduction factor from ACI 440.2R-17 and a few other common sources. It was found that the ACI specified a reduction factor of 0.75, which does not consider the CFRP age and overestimates the design tensile strength of the CFRP laminate by approximately 13%, which may compromise the structural safety of retrofitted bridges. The reduction factors from the other guidelines varied between 0.51 and 0.85.

Prestress losses assumed for bridge girder design and deflection analyses are dependent on the concrete modulus of elasticity (MOE). Most design specifications, such as the American Association of State Highways and Transportation Officials (AASHTO) bridge specifications, contain a constant value for the MOE based on the unit weight of concrete and the concrete compressive strength at 28 days. It has been shown in the past that that the concrete MOE varies with the age of concrete. The purpose of this study was to evaluate the effect of a time-dependent and variable MOE on the prestress losses assumed for bridge girder design. For this purpose, three different variable MOE models from the literature were investigated: Dischinger (Der Bauingenieur 47/48(20):563–572, 1939a; Der Bauingenieur 5/6(20):53–63, 1939b; Der Bauingenieur, 21/22(20):286–437, 1939c), American Concrete Institute (ACI) 209 (Tech. Rep. ACI 209R-92, 1992) and CEB-FIP (CEB-FIP Model Code, 2010). A typical bridge layout for the Dallas, Texas, USA, area was assumed herein. A prestressed concrete beam design and analysis program from the Texas Department of Transportation (TxDOT) was utilized to determine the prestress losses. The values of the time dependent MOE and also specific prestress losses from each model were compared. The MOE predictions based on the ACI and the CEB-FIP models were close to each other; in long-term, they approach the constant AASHTO value. Dischinger’s model provides for higher MOE values. The elastic shortening and the long term losses from the variable MOE models are lower than that using a constant MOE up to deck casting time. In long term, the variable MOE-based losses approach that from the constant MOE predictions. The Dischinger model would result in more conservative girder design while the ACI and the CEB-FIP models would result in designs more consistent with the AASHTO approach.