Showing papers on "Compressive strength published in 2013"

Synthesis and characterization of red mud and rice husk ash-based geopolymer composites

Abstract: A new type of geopolymer composite was synthesized from two industrial wastes, red mud (RM) and rice husk ash (RHA), at varying mixing ratios of raw materials and the resulting products characterized by mechanical compression testing, X-ray diffraction, and scanning electron microscopy to assess their mechanical properties, microstructure, and geopolymerization reactions. Prolonged curing significantly increases the compressive strength and Young’s modulus, but reduces the ductility. Higher RHA/RM ratios generally lead to higher strength, stiffness, and ductility, but excessive RHA may cause the opposite effect. The compressive strength ranges from 3.2 to 20.5 MPa for the synthesized geopolymers with nominal Si/Al ratios of 1.68–3.35. Microstructural and compositional analyses showed that the final products are mainly composed of amorphous geopolymer binder with both inherited and neoformed crystalline phases as fillers, rendering the composites very complex composition and highly variable mechanical properties. Uncertainties in the composition, microstructure, the extent of RHA dissolution, and side reactions may be potential barriers for the practical application of the RM–RHA based geopolymers as a construction material.

Cementation of sand soil by microbially induced calcite precipitation at various degrees of saturation

Abstract: A newly emerging microbiological soil stabilization method, known as microbially induced calcite precipitation (MICP), has been tested for geotechnical engineering applications. MICP is a promising technique that utilizes the metabolic pathways of bacteria to form calcite precipitation throughout the soil matrix, leading to an increase in soil strength and stiffness. This paper investigates the geotechnical properties of sand bio-cemented under different degrees of saturation. A series of laboratory experiments was conducted, including sieve analysis, permeability, unconfined compressive strength, consolidated undrained triaxial, and durability tests. The results indicate that higher soil strength can be obtained at similar CaCO3 content when the treatment is performed under a low degree of saturation. The experimental results are further explained with a mathematical model, which shows that the crystallization efficiency, i.e., actual volume of crystals forming at the contact point where they contribute the most to strength, can be calculated from the degree of saturation and grain size. Fine sand samples exhibited higher cohesion, but lower friction angle than coarse sand samples with similar CaCO3 content. The results also confirm the potential of MICP as a viable alternative technique for soil improvement in many geotechnical engineering applications, including liquefiable sand deposits, slope stabilization, and subgrade reinforcement. The freeze-thaw and acid rain resistance of MICP-treated sand has also been tested.

Modification of cement-based materials with nanoparticles

Abstract: This is a summary paper on the work being done at the Center for Advanced Cement-Based Materials at Northwestern University on the modification of cement-based materials with nanoparticles, specifically nanoclays, calcium carbonate nanoparticles, and nanosilica. The rheological properties of clay-modified cement-based materials are investigated to understand the influence of nanoclays on thixotropy. The influence of the method of dispersion of calcium carbonate nanoparticles on rate of hydration, setting, and compressive strength are evaluated. And an in-depth study on the mechanisms underlying the influence of nanosilica on the compressive strength gain of fly ash–cement systems is discussed. The motivation behind these studies is that with proper processing techniques and fundamental understanding of the mechanisms underlying the effect of the nanoparticles, they can be used to enhance the fresh-state and hardened properties of cement-based materials for various applications. Nanoclays can increase the green strength of self-consolidating concrete for reduced formwork pressure and slipform paving. Calcium carbonate nanoparticles and nanosilica can offset the negative effects of fly ash on early-age properties to facilitate the development of a more environmentally friendly, high-volume fly ash concrete.

Long-term mechanical and durability properties of recycled aggregate concrete prepared with the incorporation of fly ash

Abstract: This paper presents the findings of a long-term study on the mechanical and durability properties of concrete prepared with 0%, 50% and 100% recycled concrete aggregate that were cured in water or outdoor exposure conditions for 10 years. The recycled aggregate concrete (RAC) was prepared by using 25%, 35% and 55% class-F fly ash, as cement replacements. It was found that, after 10 years, the compressive strength and modulus of elasticity of the concrete prepared with 100% recycled concrete aggregate was still lower than that of the control concrete. Over this period, the highest gain in compressive strength and modulus of elasticity was recorded for the concrete mixture prepared with 55% fly ash. Fly ash improved the resistance to chloride ion penetration but it also increased the carbonation depth of the concrete.

Behaviour of cement concrete at high temperature

Abstract: The paper presents the impact of high temperature on cement concrete. The presented data have been selected both from the author's most recent research and the published literature in order to provide a brief outline of the subject. The effect of a high temperature on concrete covers changes taking place in cement paste, aggregates, as well as the interaction of these two constituents, that result in changes of mechanical and physical characteristics of concrete. This paper presents the effects of a high temperature on selected physical properties of concrete, including colour change, thermal strain, thermal strains under load, and transient thermal strains. In addition, changes to mechanical properties are discussed: stress-strain relationship, compressive strength, and modulus of elasticity. Moreover, the phenomenon of explosive spalling and the main factors that affect its extent are analysed in light of the most recent research. Interest in the behaviour of concrete at a high temperature mainly results from the many cases of fires taking place in buildings, high-rise buildings, tunnels, and drilling platform structures. During a fire, the temperature may reach up to 1100 ◦ C in buildings and even up to 1350 ◦ C in tunnels, lead- ing to severe damage in a concrete structure (1). However, in some special cases, even much lower temperature, may cause explosive destruction of concrete, thus endangering the bear- ing capacity of the concrete element. Nevertheless, concrete is considered a construction material that satisfactorily preserves its properties at high temperature. Owing to concrete's fairly low coefficient of thermal conductivity, the movement of heat through concrete is slow, and thus reinforced steel, which is sensitive to high temperature, is protected for a relatively long period of time. When concrete is heated under conditions of fire, the increase in temperature in the deeper layers of the ma- terial is progressive, but because this process is slow, signifi- cant temperature gradients are produced between the concrete member's surface and core inducing additional damage to the element. Fundamental issues related to the impact of high temperature on concrete involve identification of the complex changes that take place in concrete while heated. This con- cerns both the physical and chemical changes taking place in the cement matrix, as well as the phenomena involved in mass movement (gases and liquids). The analysis is complicated due to the fact that cement concrete is a composite consisting of two substantially different constituents: cement paste and aggregates. The effects of the various changes taking place in heated concrete are the alterations of its physical, thermal, and mechanical properties. Research has demonstrated (1, 2), that changes in the strength of concrete as a function of tem- perature are related to, inter alia, concrete composition the type of aggregate used, the water/cement ratio, the presence of pozzolana additives, etc. Important factors are also the rate of heating and the time of concrete exposure to high tempera- ture. The increase in temperature results in water evaporation, C-S-H gel dehydration, calcium hydroxide and calcium alu- minates decomposition, etc. Along with the increase in tem- perature, changes in the aggregate take place. Due to those changes, concrete strength and modulus of elasticity gradu- ally decreases, and when the temperature exceeds ca. 300 ◦ C, the decline in strength becomes more rapid. When the 500 ◦ C threshold is passed, the compressive strength of concrete usu- ally drops by 50% to 60%, and the concrete is considered fully damaged. The Eurocode method of calculating the load- bearing capacity of reinforced concrete members subjected to a fire is based on this assumption. In the 500 ◦ C isotherm method, sections of concrete surface where the temperature had exceeded 500 ◦ C are omitted from the calculations (3).

Microwave-sintered 3D printed tricalcium phosphate scaffolds for bone tissue engineering.

Abstract: We report here the fabrication of three dimensional (3D) interconnected macro porous tricalcium phosphate (TCP) scaffolds with controlled internal architecture by direct 3D printing (3DP), and high mechanical strength by microwave sintering. TCP scaffolds with 27%, 35% and 41% designed macro porosity having pore sizes of 500 μm, 750 μm, and 1000 μm, respectively, have been fabricated via direct 3DP. These scaffolds are then sintered at 1150 °C and 1250 °C in conventional electric muffle furnace as well as microwave furnace. Total open porosity between 42% and 63% is obtained in the sintered scaffolds due to the presence of intrinsic micro pores along with the designed pores. A significant increase in compressive strength, between 46% and 69%, is achieved by microwave sintering as compared to conventional sintering as a result of efficient densification. A maximum compressive strength of 10.95 ± 1.28 MPa and 6.62 ± 0.67 MPa is achieved for scaffolds with 500 μm designed pores (~400 μm after sintering) sintered in microwave and conventional furnaces, respectively. An increase in cell density with a decrease in macro pore size is observed during in vitro cell-material interactions using human osteoblast cells. Histomorphological analysis reveals that the presence of both micro and macro pores facilitated osteoid like new bone formation when tested in the femoral defect on Sprague-Dawley rats. Our results show that bioresorbable 3D printed TCP scaffolds have great potential in tissue engineering applications for bone tissue repair and regeneration.

Microstructure and mechanical behavior of porous Ti-6Al-4V parts obtained by selective laser melting.

Abstract: Rapid prototyping allows titanium porous parts with mechanical properties close to that of bone tissue to be obtained. In this article, porous parts of the Ti-6Al-4V alloy with three levels of porosity were obtained by selective laser melting with two different energy inputs. Thermal treatments were performed to determine the influence of the microstructure on the mechanical properties. The porous parts were characterized by both optical and scanning electron microscopy. The effective modulus, yield and ultimate compressive strength were determined by compressive tests. The martensitic α' microstructure was observed in all of the as-processed parts. The struts resulting from the processing conditions investigated were thinner than those defined by CAD models, and consequently, larger pores and a higher experimental porosity were achieved. The use of the high-energy input parameters produced parts with higher oxygen and nitrogen content, their struts that were even thinner and contained a homogeneous porosity distribution. Greater mechanical properties for a given relative density were obtained using the high-energy input parameters. The as-quenched martensitic parts showed yield and ultimate compressive strengths similar to the as-processed parts, and these were greater than those observed for the fully annealed samples that had the lamellar microstructure of the equilibrium α+β phases. The effective modulus was not significantly influenced by the thermal treatments. A comparison between these results and those of porous parts with similar geometry obtained by selective electron beam melting shows that the use of a laser allows parts with higher mechanical properties for a given relative density to be obtained.

Influence of concrete strength and confinement method on axial compressive behavior of FRP confined high- and ultra high-strength concrete

Abstract: This paper presents an experimental investigation on the effect of concrete compressive strength and confinement method on confined high and ultra high-strength concrete (HSC and UHSC) specimens. A total of 55 fiber reinforced polymer (FRP) confined concrete specimens were tested under monotonic axial compression. All specimens were cylinders with 152 mm diameter and 305 mm height and confined by carbon FRP (CFRP). Three different concrete mixes were examined, with average compressive strengths of 35, 65 and 100 MPa. The effect of the confinement method was also examined with FRP-wrapped specimens compared to FRP tube-encased specimens. Axial and lateral behavior was recorded to observe the axial stress–strain relationship and lateral strain behavior for concentric compression. Ultimate axial and lateral conditions are tabulated and the complete stress–strain curves have been provided. The experimental results presented in this paper provide a performance comparison between FRP-confined conventional normal-strength concrete (NSC) and the lesser understood area of FRP-confined HSC and UHSC. The results of this experimental study clearly indicate that above a certain confinement threshold, FRP-confined HSC and UHSC exhibits highly ductile behavior, however for the same normalized confinement pressures, axial performance of FRP-confined concrete reduces as concrete strength increases. The results also indicate that ultimate conditions of FRP-wrapped specimens are similar to those confined by FRP tubes, however a performance difference is evident at the transition region. The performance of 10 existing stress–strain models were assessed against the experimental datasets and the performance of these models discussed. The results of this model assessment revealed the need for further development for stress–strain models developed specifically for FRP-confined HSC or UHSC.