High Strength Structural Concrete Research Articles

Tensile Fatigue Properties of Ordinary Plain Concrete and Reinforced Concrete under Flexural Loading.

Many bridge structural components are subjected to repetitive vehicle load and temperature gradient action. The resulting cyclic tensile stresses within the structures could cause premature fatigue failure of concrete, dramatically impairing structural components' durability and sustainability. Although substantial knowledge of fatigue properties on low-strength pavement concrete and high-strength structural concrete has been obtained, research on the most widely used normal-grade ordinary concrete in bridge engineering is still ongoing. Therefore, a four-point bending fatigue test of 97 C50 concrete specimens under a constant amplitude sinusoidal wave was conducted in the laboratory, the flexural fatigue behavior of plain and reinforced concrete specimens was studied, and the cyclic deformation evolution of concrete under fatigue loading was obtained. The empirical fatigue S-N equations of concrete with a failure probability p of 0.1~0.5 were derived through statistical analysis of the test results. The fatigue life of the tested specimens exhibited a two-parameter Weibull distribution. In addition to the maximum stress level Smax, the stress ratio R is also a key factor affecting the flexural fatigue life of concrete N. The semi-logarithmic and logarithmic equations were almost identical at the tested stress levels, the latter predicting longer fatigue life for Smax < 0.70. The restraining effect from steel reinforcement slightly lengthened the concrete's fatigue cracking initiation life. The insight into concrete flexural fatigue properties from this study not only contributes to a better understanding of structural concrete, but also provides a basis for the practical evaluation of concrete or composite bridge decks.

Performance evaluation of lightweight insulating plaster for enhancing the fire endurance of high strength structural concrete

Structural concrete is a widely used building material due to its versatile characteristics including higher compressive strength and longevity. However, when exposed to fire, concrete experiences faster degradation in its mechanical properties and is also susceptible to spalling. To overcome this problem, the possibility of lightweight plaster application on High Strength Concrete (HSC) is explored in the presented investigation. Insulation plasters, namely Sand Plaster (SP), Gypsum Perlite Plaster (GPP), and Gypsum Mineral Wool Plaster (GMP) were developed. This investigation evaluates the compressive strength, bond strength, and shear strength of concrete which is exposed to the standard fire temperature. Varying cooling conditions which involved air and water were adopted to cool the concrete specimens after the elevated temperature test. Further, the damaged concrete and the plaster were examined to analyse the physical changes. Analysis of the study reveals that more number of denser surface cracks and higher mass loss was observed for reference and SP specimens. Temperature penetration at the core of cube, bond, and shear specimens is less for the GPP and GMP specimens when compared with the SP specimens. At higher temperatures (986 °C), the reference and SP specimens show a lower bond and shear strength with higher slip values. Specimens insulated with GPP and GMP exhibited a low-temperature penetration at the core portion. Also, the results of the study reported the higher residual compressive, bond, and shear strength compared to other specimens.

Fire resistance of corroded high-strength structural concrete

Purpose The exposure of reinforced concrete structures such as high-rise residential buildings, bridges and piers to saline environments, including exposure to de-icing salts, increases their susceptibility to corrosion of the reinforcing steel. The exposure to fire can further deteriorate the structural integrity of corroded concrete structures. This combined effect of corrosion damage and fire exposure is not generally addressed in the structural concrete design codes. The synergistic combination of the effects of corrosion and fire forms the basis of this paper. Design/methodology/approach Concrete beam specimens with different strengths were prepared, moist-cured and corroded with impressed current. Later, they were “crack-scored” for corrosion evaluation, after which half were exposed to fire in a gas kiln. The fire damage was evaluated by nondestructive testing using ultrasonic pulse velocity. Next, all specimens were tested for residual flexural strength. They were then autopsied, and the level of corrosion was determined based on mass loss of the reinforcement. Findings For corroded specimens, the flexural capacity loss because of fire exposure increases as the compressive strength increases. In general, the higher the crack score, the higher the corresponding mass loss, unless some partial/segmental debonding of the reinforcement occurred. The degree of corrosion increases with decreasing compressive strength. The residual moment capacity, based on analytically determined capacities of uncorroded and nonfire-exposed beams, was significantly lower than those of uncorroded beams exposed to fire. Originality/value The combined effects of corrosion and fire on the mechanical properties of structural concrete are relatively unknown, and no guidance is available in the existing design codes to address this issue. Accordingly, the findings of the paper are expected to be valuable to both researchers and design engineers and can be regarded as the initial investigation on this topic.

Optimum concentration of Bacillus megaterium for strengthening structural concrete

Five different cell concentrations of Bacillus megaterium (10×105 to 50×105cfu/ml) were introduced in structural concrete to achieve the optimum concentration of bacteria. The significant increase in the strength was obtained in the case of 30×105cfu/ml at different ages. The strength of highest grade of bacterial concrete had improved (24%) as compared to lowest grade (12.8%) due to calcification mechanism. Microbial calcite precipitation was quantified using X-ray diffraction analysis, visualized by scanning electron microscopy and analyzed by energy dispersive spectrometer. It was found that the optimum concentration of B. megaterium had a positive effect on high strength structural concrete.

Experimental Study on the Constitutive Relation of SPHSC under Uniaxial Compression Loading

Steel fiber reinforced polymer high strength structural concrete (SPHSC) is a new composite material which is used in long-span bridge structures invented by this research team. Based on the new composite material, the experiments of stress-strain full curves under uniaxial compression were carried out. The stress-strain full curves were achieved from the simple improved method. By the method of theoretical derivation and experimental data fitting, the constitutive equation of SPHSC which contains only two undetermined parameters of compressive strength and parameter of steel fiber was obtained. The study is the theoretic basis of the new material which was widely use in civil engineering.

NCHRP Project 12-56, Application of Load and Resistance Factor Design Specifications to High-Strength Structural Concrete: Shear Provisions

The load and resistance factor design (LRFD) specifications in the current AASHTO LRFD Bridge Design Specifications limit the compressive cylinder strength to 10 ksi in relationships for calculating shear design strength, whereas concrete compressive strengths close to twice this level are commercially available in some parts of the United States. To overcome this limitation, allowing the same-size structural sections to span longer distances or carry higher loads, NCHRP has funded Project 12-56, Application of the LRFD Bridge Design Specifications to High-Strength Structural Concrete: Shear Provisions. In the experimental component of this project, 12 prestressed concrete bridge girders 52 ft long and 6 ft deep are being subjected to a uniformly distributed load until failure in shear. The primary variables in this study are concrete strength (ranging from 10 to 18 ksi), the maximum shear design stress (700 to 2,500 psi), strand anchorage details (straight, unbonded, and draped), and end reinforcement de...

Some characteristics of high-strength structural concrete

The 68-storey Scotia Plaza tower in Toronto is an outstanding example of the use of concrete technology to achieve high-performance high-strength concrete. Cementitious hydraulic slag, silica fume, and a superplasticizer were combined with CSA type-10 Portland cement and high-quality aggregates to produce very workable high-strength concrete. During the course of construction, data were published suggesting the possibility of the strength regression of some silica fume concretes after long exposure to low humidity, the determinations being made on standard test cylinders. Tests were, therefore, made at ages of 1 year and 2 years on specimens drilled from columns in the structure. This technical note gives details of the laboratory examination and testing of these specimens. Key words: high strength, slag, silica fume, permeability, rapid chloride permeability, petrographic examination, superplasticizers.

Shrinkage and creep properties of high-strength structural concrete

Shrinkage and creep properties of high-strength structural concrete