Disposal of waste tyre rubber has become a major environmental issue in all parts of the world representing a very serious threat to the ecology. One of the possible solutions for the use of scrap ...tyre rubber is to incorporate it into concrete, to replace some of the natural aggregate. An estimated 1000 million tyres reach the end of their useful lives every year and 5000 millions more are expected to be discarded in a regular basis by the year 2030. Up to now a small part is recycled and millions of tyres are just stockpiled, landfilled or buried. The volume of polymeric wastes like tyre rubber and polyethylene terephthalate bottles (PET) is increasing at a fast rate. This paper reviews the tests performed to determine the compressive strength, flexural tensile strength, water absorption and water penetration of using rubber tyre waste concrete samples. Scanning Electron Microscopy (SEM) images were also presented in this paper. It was observed that the compressive strength, flexural tensile strength and depth of water penetration of the rubberized concrete were less than that of the control mix, while the abrasion resistance and water absorption (up to 10% substitution) exhibited better results than that of the control mix concrete. This paper also reviews the performance of concrete mixtures incorporating 5%, 7.5% and 10% of discarded tyre rubber as aggregate and cement replacements. Numerous projects have been conducted on replacement of aggregates by crumb rubbers but scarce data are found on cementitious filler addition. Hence to examine characteristics of tyre crumb-containing concrete, two sets of concrete specimens were made. In the first set, different percentages by weight of chipped rubber were replaced for coarse aggregates and in the second set scrap-tyre powder was replaced for cement. Selected standard durability and mechanical test were performed and the results were analysed.
Ever since the early days of Féret (1892) and Abrams (1919), concrete research has targeted at relating concrete composition to uniaxial compressive strength. While these activities were mainly ...characterized by empirical fitting functions, we here take a more fundamental approach based on continuum micromechanics. The loading applied at the concrete level, is first concentrated (“downscaled”) to maximum stresses related to cement paste volumes which are directly adjacent to the aggregates, i.e. to the interfacial transition zones (ITZ). These maximum stresses are further “downscaled” to the micron-sized hydrates, in terms of higher-order stress averages. The latter enter a Drucker-Prager failure criterion with material constants derived from nanoindentation tests. The model is successfully validated across the hydrate-to-concrete scales. Strength magnitude is governed by ITZ stress concentrations, and the water-to-cement ratio is its dominant mixture design parameter.
Steel slag has been used as supplementary cementitious materials or aggregates in concrete. However, the substitution levels of steel slag for Portland cement or natural aggregates were limited due ...to its low hydraulic property or latent volume instability. In this study, 60% of steel slag powders containing high free-CaO content, 20% of Portland cement and up to 20% of reactive magnesia and lime were mixed to prepare the binding blends. The binding blends were then used to cast concrete, in which up to 100% of natural aggregates (limestone and river sands) were replaced with steel slag aggregates. The concrete was exposed to carbonation curing with a concentration of 99.9% CO2 and a pressure of 0.10 MPa for different durations (1d, 3d, and 14d). The carbonation front, carbonate products, compressive strength, microstructure, and volume stability of the concrete were investigated. Results show that the compressive strength of the steel slag concrete after CO2 curing was significantly increased. The compressive strengths of concrete subjected to CO2 curing for 14d were up to five-fold greater than that of the corresponding concrete under conventional moist curing for 28d. This is attributed to the formation of calcium carbonates, leading to a microstructure densification of the concrete. Replacement of limestone and sand aggregates with steel slag aggregates also increased the compressive strengths of the concrete subjected to CO2 curing. In addition, the concrete pre-exposed to CO2 curing produced less expansion than the concrete pre-exposed to moist curing during the subsequent accelerated curing in 60 °C water. This study provides a potential approach to prepare concrete with low-carbon emissions via the accelerated carbonation of steel slag.
The mechanical behavior of rock is significantly affected by water. To further study the influence of water on the mechanical behavior of rocks, uniaxial compressive loading experiments were ...performed on black sandstone specimens (collected at a site in Longchang, Sichuan Province, China) with different water contents. The water absorption was first tested to study the evolution of the water content in the specimens. Then, the mechanical behaviors of the specimens after different durations of soaking in water (from dry to saturation) were tested under uniaxial compression. The results showed that, in the stress-strain curves for the specimens, the initial slope and strength decrease with increasing soaking duration and that the curves are strongly nonlinear in the prepeak region. Furthermore, with increasing water content, the elastic modulus and uniaxial compressive strength decrease, while the Poisson's ratio increases. A comprehensive set of predictive empirical relationships is established between the mechanical properties and the water content/soaking duration for the black sandstone. As the soaking duration increases, the failure pattern gradually changes from tensile to shear-dominated failure. A novel observation is that, for two specimens with the same water content, the strength of a specimen with a homogeneous water distribution is lower than that of a specimen with a non-homogeneous water distribution. The results suggest that the water soaking duration, water distribution and loading-saturation sequence of a rock should all be considered in experiments to gain a deeper understanding of the mechanical responses of rocks with different water contents.
•The UCS and elastic modulus decrease with the soaking duration of black sandstone.•Relationships between mechanical properties and the water content are established.•Tensile failure transitions to shearing in partially to fully saturated specimens•The water distribution in black sandstone affects the strength.
Calcareous sands are known as problematic soils in nature and challenge geotechnical engineers in many practical projects. Microbially induced calcite precipitation (MICP) is an innovative soil ...improvement technique that uses biomineralisation processes to induce cementation in-situ. The work described in this paper investigates the strength, deformation, and microstructure characteristics of biocemented calcareous sand under different cementation solution to sample volume ratios. A series of laboratory experiments was conducted, including unconfined compressive strength tests, splitting, tensile (i.e., Brazilian) strength tests, and consolidated drained triaxial tests. The results indicate that an exponential function reasonably describes the unconfined compressive strength and splitting tensile strength with increasing cementation solution to sample volume ratios. The tangent modulus at 50% peak strength increases exponentially with an increase in cementation solution to sample volume ratio, whereas it increases linearly with an increase in strength. The strength parameters for this MICP-improved soil, including the peak cohesion and friction angle, are derived to facilitate engineering design. Microstructure analyses are used to illustrate the physical basis for the increase in strength and stiffness with increases in the calcite content, as demonstrated using the cementation solution to sample volume ratio.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
•Fracture toughness of Be12Ti was calculated with the samples which sintered at various temperature.•Microstructure observations by EBSD were performed.•Compression tests were carried out at various ...temperature.•Average compressive strength was 1692 MPa at RT. Compressive strength tend to decrease over 700˚C.
This purpose of study is to establish the material database of neutron multiplier for the JA DEMO design. In the previous study, we reported the effects of sintering conditions as temperature and pressure on hardness and sintered density mainly in the sintering temperature from 1050 °C to 1200 °C. In this study, the microstructure observation and compressive tests were carried out. As results of microstructure observations, the pores were almost disappeared, and the grain size increased with increase in sintering temperature. Compression tests were carried out from room temperature (R.T.) to 1000 °C with the samples sintered at 1200 °C. The compressive strength at R.T. was approximately 1.69 GPa. With the increase in testing temperature, there was a tendency for the strength to decrease from 800 °C to 1000 °C. In addition, the load-compressibility indicated that the yield point appears at 850 °C.
The uniaxial compressive strength (UCS) of rock is an important geotechnical parameter for engineering applications. However, how to determine the UCS simply and accurately has drawn the attentions ...of may researchers. To date, different kinds of indirect methods have been invented to determine the UCS of rocks, and among these methods, estimation of the UCS based on the Schmidt hammer rebound value (Hr) was commonly adopted. In this paper, an insightful analysis of the literature related to UCS estimation using the Schmidt hammer test was conducted, and three stages for the development of UCS estimation using Hr were classified. The drawbacks and merits of different kinds of techniques were analyzed in detail. Then, a data set containing the data for different rock types was collected from references, and to obtain an objective empirical formula, the simulated annealing-gene expression programming (SA-GEP) method was employed to establish the correlation between UCS and Hr. Based on the calculation results, the L-type Schmidt hammer was suggested for use in UCS estimation, and the corresponding empirical formula was established. To confirm validity of the empirical formula, the Schmidt hammer tests and uniaxial compressive tests were conducted separately, the experimental results were in a good agreement with the proposed empirical formula, implying that the proposed empirical formula can be applied in engineering practice.
•Different kinds of UCS evaluation methods using the Schmidt hammer test were reviewed in detail.•A new empirical formula for estimating UCS using the L-type Schmidt hammer were proposed.•The proposed empirical formula was verified by laboratory tests.
To effectively stabilize coarse sand, bentonite was introduced in microbially induced carbonate precipitation (MICP) grouting. Varying concentrations (0, 20, 40, and 80 g/L) of bentonite were added ...to bacterial suspensions (BSs), which were magnetically stirred to form bacterial-bentonite suspensions (BBSs). Then, coarse sand specimens were treated with BBSs and cementation solutions (CSs) to different cementation levels. The results showed that the addition of bentonite could increase the volume fractions of the precipitates consisting of calcium carbonate (CaCO
3
) and bentonite. The permeability decreased exponentially as the volume fraction of precipitates increased. As the active precipitates increased when a lower concentration (e.g., 20 g/L) of bentonite was added to the MICP grouting, the unconfined compressive strength (UCS) was substantially improved. However, detrimental effects were observed for specimens treated with a high concentration of bentonite. These results indicate that the effective concentration of natural clay aggregates used in MICP grouting was different for different engineering applications, e.g., seepage control and strength enhancement. The current work provides an encouraging method of improving the MICP technique.
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Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
•Machine learning techniques predict compressive strength of concrete made with RAC.•GB_PSO is the best Machine Learning model for prediction of compressive strength.•Mix design of concrete made with ...RAC can be estimated with aided of PDP 1D.
To reduce the environmental impact of construction and demolition waste of concrete, recycled concrete aggregate (RCA) has been widely utilized in concrete. The compressive strength of recycled concrete is one of the most important parameters governing the quality of concrete. The compressive strength is determined from the compression test, which requires a huge amount of materials as well as consumes cost and time. Thus, to solve those limitations, this study focused on evaluating the compressive strength of concrete made from RCA using different single and hybrid models of machine learning. Six machine learning models including Gradient Boosting (GB), Extreme Gradient Boosting (XGB), Support Vector Regression (SVR), and three hybrid models of those single models with Particle Swarm Optimization (PSO) namely GB_PSO, XGB_PSO, and SVR_PSO were used to estimate the compressive strength of recycled concrete. The input variables for modeling consisted of cement content, water content, aggregate content, natural aggregate content, recycle concrete aggregate content, sand content, water absorption rates of natural aggregate and RCA. The results of this study show that hybrid models performed better than single models in terms of prediction accuracy. The results indicated that the GB_PSO has the highest prediction accuracy with R = 0.9356, RMSE = 5.5604 MPa, and MAE = 4.2882 MPa. The results of feature importance analysis and partial dependence plots (PDP) analysis revealed that the most important variable effect on compressive strength of concrete made with RAC is cement content, whatever performance strategies of concrete made with RAC. From the results of PDP, the quantity of each material can be computed easily for the designed compressive strength. In the end, this study provides a systematic evaluation of the compressive strength prediction of recycled concrete and has a significant contribution to literature and practice.
In the present investigation, a simple and cost-effective method is described for preparing lightweight open cell aluminum foam by impregnating of aluminum powder into a substrate polyurethane foam ...template. Aluminum powder impregnated polyurethane foam was converted into open cell aluminum foam by several heat treatments inert as well reduce atmospheric pressure. The different concentrations of slurries of aluminum powder in acetone were used in combination with substrate foams to prepare open cell foams of relative density in the range of 0.08–0.22. The open cell aluminum foam prepared by this sacrificial template techniques is found to possess a compressive strength of 6.2 MPa. Electromagnetic interference shielding of aluminum foam was measured in X-band frequency region (8.2–12.4 GHz) using waveguide method. The electromagnetic interference shielding of aluminum foam with relative density of 0.22 is found −44.6 dB. This technique is fast, very economic and highly reproducible which may help the commercialization of such open cell aluminum foam for various applications.
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•Open cell aluminum foams were inventively fabricated by PU foam template.•The obtained aluminum foam demonstrates the compressive strength of 6.2 MPa.•Aluminum foam possess EMI shielding of −44.6 dB with 0.22 relative density only.•This technique is very simple, economical and highly reproducible.•The aluminum foam can use in the fields of civil and military aerospace industry.
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