•Estimation of compressive strength with non-destructive testing on concretes and eco-concretes.•New analytical estimation models considering the curing temperature and other factors.•Comparison of ...previous models with proposed models.•Improved models using oriented genetic programming.•A proposed model with a safety improvement.
The use of non-destructive testing for estimating the compressive strength of concrete has great advantages both short term and long term. In the case of eco-concrete with recycled materials, it is of particular interest, since its use in many regulations is conditional on further studies. In this research we analyze the applicability of the most common models for estimating compressive strength by combining nondestructive testing. Specifically it was applied to 11 concrete with cement CEM-I, 3 self-compacting concrete and 8 vibrated concrete. There are two reference concretes (one of each) and the rest of concretes either have changed water/cement ratio or they contain different percentages of recycled materials (recycled aggregate fine and coarse together, or biomass ashes). Destructive tests have been made (compressive strength) and non-destructive (ultrasonic pulse velocity and compressive strength) in all concretes, at different ages and different curing temperatures, obtaining a total of 181 data sets. New estimation models were proposed for compressive strength with factors such as the curing temperature, the temperature history, the density of the concrete and the quantity of additive. These models substantially improve the results obtained with the usual methods. Finally, using genetic programming, it has managed to obtain an equation that allows, safely, estimating compressive strength with the information of non-destructive testing. The equation obtained improves current predictions with the peculiarity that minimizes uncertain results.
•Meso-scale concrete models are constructed with numerical concrete modeling approach.•The geometrical shape and random distribution of aggregates are considered.•Mesh sensitivity and the influence ...of the ITZ are investigated.•The variation of micro-damage evolution and the macroscopic response is analyzed.•The statistic characteristics of concrete under tension and compression are discussed.
Meso-scale modeling approach is helpful for understanding the mechanism of both global nonlinear behavior and local failure pattern of concrete materials and structures. As a typical multiphase composite material, concrete is composed of randomly-distributed coarse and fine aggregates, mortar, interfacial transition zone (ITZ) together with initial pores or defects. The meso-scale mechanical behavior of concrete usually exhibits nonlinear and stochastic characteristics. In this paper, the influence of both distribution and geometrical shape of aggregates, the existence of ITZ, and meshing approach on macro stress-strain relationship, damage evolution and the final failure pattern of concrete are studied systematically. The results indicate that the effect of meso-scale structure of concrete on the nonlinear and stochastic characteristics of macro stress-strain curves and failure patterns is different when the concrete specimen is under tension and compression loadings. Comparatively, the geometrical parameters of coarse and fine aggregates have limited effect on the macroscopic tensile strength, but obvious effect on the post-tension segment and the compressive macro stress-strain curve. Moreover, the damage evolution and failure pattern of concrete under uniaxial tensile and compression are sensitive to the meshing approach. The existence of ITZ affects the tensile and compressive strength of concrete significantly. The statistic characteristics of macro stress-strain curves under both tension and compression loadings from the multi-scale simulation is compared with that from experimental studies and the rationality of the meso-scale simulation approach is validated.
The compressive strength of recycled concrete is acknowledged to be largely conditioned by the incorporation ratio of Recycled Concrete Aggregate (RCA), although that ratio needs to be carefully ...assessed to optimize the design of structural applications. In this study, Self-Compacting Concrete (SCC) mixes containing 100% coarse RCA and variable amounts, between 0% and 100%, of fine RCA were manufactured and their compressive strengths were tested in the laboratory for a statistical analysis of their strength variations, which exhibited robustness and normality according to the common statistical procedures. The results of the confidence intervals, the one-factor ANalysis Of VAriance (ANOVA), and the Kruskal–Wallis test showed that an increase in fine RCA content did not necessarily result in a significant decrease in strength, although the addition of fine RCA delayed the development of the final strength. The statistical models presented in this research can be used to define the optimum incorporation ratio that would produce the highest compressive strength. Furthermore, the multiple regression models offered accurate estimations of compressive strength, considering the interaction between the incorporation ratio of fine RCA and the curing age of concrete that the two-factor ANOVA revealed. Lastly, the probability distribution predictions, obtained through a log-likelihood analysis, fitted the results better than the predictions based on current standards, which clearly underestimated the compressive strength of SCC manufactured with fine RCA and require adjustment to take full advantage of these recycled materials. This analysis could be carried out on any type of waste and concrete, which would allow one to evaluate the same aspects as in this research and ensure that the use of recycled concrete maximizes both sustainability and strength.
One novel lightweight building material was successfully manufactured using magnesium oxysulfate (MOS) cement as binder and foam and/or rice husk as inert fillers in this paper. It behaves much ...higher specific strength and lower environmental load than normal concretes prepared by Portland cement, natural sand and aggregates. To improve the mechanical strength and water resistance of MOS matrix, three kinds of weak acids were incorporated at low levels. On the basis of the modified MOS matrix, foam, rice husk, and the mixture of foam and rice husk were added at variable dosages as fillers to prepare MOS cement based lightweight materials. Mechanical strength, bulk density and compressive strength/volume deformation of these materials were evaluated after variable wetting-drying cycles. At the same time, phase composition and microstructure of typical samples were studied using X-Ray Diffraction (XRD), Fourier Transform Infrared Spectrometer (FTIR), Opto-digital Microscope (OM) and Scanning Electron Microscope (SEM) tests. Experimental results indicated that the addition of 0.5% citric acid by MgO weight can greatly enhance compressive strength and water resistance due to the formation of finer needle-like crystals of 5 × 1 × 7 phase instead of 3 × 1 × 8 phase. The higher incorporation of foam and rice husk decreases bulk density and mechanical properties of MOS cement pastes. With a density of as low as 1000 kg/m3, the compressive strength of MOS cement based lightweight materials can reach higher than 12 MPa. The wetting-drying treatment leads to a negative effect on both compressive strength and volume stability of these materials, due to the transformation of unreacted MgO into Mg(OH)2 crystals upon water immersion. On the other hand, the coupling effect of water-swelling and drying-shrinkage induces the formation of micro cracks in specimens. A proper addition of foam bubbles and rice husk presents the better resistance to wetting-drying cycles. Therefore, the MOS cement based lightweight material could be widely applied to inner partition walls and structural component, and its long-term service behavior should be further assessed.
The deterioration of cement-based materials due to chloride attack is a well-known phenomenon attracting extensive attention worldwide. However, the effects of chloride attack on the strength and ...long-term stability of stabilized/solidified heavy metal contaminated soils have not been studied in detail. In the present work, systematic tests were developed to investigate the effects of chloride attack on the strength and leaching characteristics of heavy metal contaminated soils treated with cement and fly ash. The test results suggested that increasing the NaCl concentration from 0% to 15% had a detrimental influence on the unconfined compressive strength (UCS) of solidified/stabilized contaminated soil. The UCS of the specimens decreased during the initial 7 days of soaking and then increased at longer soaking periods. In leaching tests, the concentration of Pb2+ in the leachate decreased with the increasing NaCl concentration, while that of Zn2+ and Cr3+ increased. The leached Pb2+ concentration reached its maximum level after 7 days of soaking and then decreased significantly at longer soaking times. In contrast, the leached Zn2+ and Cr3+ concentrations decreased significantly during short-term immersion. The leachability of the three contaminants followed the order: Zn2+ > Cr3+ > Pb2+. The mineralogical analysis performed by X-ray diffraction revealed that high concentrations of heavy metal contaminants significantly inhibit the development of hydration under such an aggressive chloride environment. The microstructure analysis performed by scanning electron microscopy indicated that Friedel's salt was formed during chloride attack, resulting in larger porosity and a looser structure of the solidified soils. More compact and uniform structures were observed for the specimens soaked for 28 and 90 days than that soaked for 7 days.
•Chloride attack has detrimental influences on the UCS of stabilized heavy metal contaminated soils.•Leachability of the heavy metal ions in the stabilized soils varies due to chloride attack.•Friedel's salt involved in consumption of some hydrates leads to the change of UCS and leaching under chloride attack.•The erosion of sodium chloride leads to a large porosity and loose structure of the solidified contaminated soils.
•Silica nano powder is used to increase the strength of concrete.•Eleven machine learning methods are used to estimate the UCS of nanosilica concrete.•A graphical user interface is developed for the ...concrete’s UCS estimation.
As nanotechnology developed, new materials emerged that might be employed to improve the mechanical properties of materials like concrete. Intensity factors are proportional to the kind of nanomaterial and its concentration. There is not yet a reliable prediction model for the uniaxial compressive strength (UCS) of nanomaterial-reinforced concrete. Access to such models is crucial for developing and evaluating nanomaterial-reinforced concrete structures. This study investigated the potential of eleven well-known machine learning (ML) algorithms to determine the most accurate and suitable ones to estimate the UCS of nanosilica-reinforced concrete. For this purpose, 460 data points were collected from experimental tests, including five input parameters and one target (UCS). The considered input parameters are the percentage of nanosilica in concrete cement (NS), sample diameter (D), sample length (L), porosity (n), and P-wave velocity (Vp). 80% of the data points were used for training and 20% for testing. The models were then verified using statistical analysis, and their behavior was compared to that in practice. Statistical analysis showed that all the models have achieved good accuracy compared to the experimental results, so the model with the lowest accuracy was the decision tree regressor (DTR) model, with a correlation coefficient of 0.68. However, comparing the behavior of the models with the practice mode by changing the value of one of the input parameters while keeping the values of other parameters constant showed that only the support vector regression (SVR) and null space SVR (NuSVR) models behave correctly. These two models were proposed as the most suitable for estimating the UCS of nanosilica-reinforced concrete. To further aid in the estimation of the UCS of nanosilica-reinforced concrete for engineering challenges, a graphical user interface (GUI) for the ML-based models was developed.
The frequency of tidal floods caused by seawater threatens coastal buildings. This study aims to see how using seawater for concrete treatment affects the resulting compressive strength. This study ...used additives such as Bestmittel to expedite hardening and boost compressive strength, as well as silica fume to increase porosity and prevent concrete from becoming porous owing to chloride ion intrusion from seawater. The findings of concrete compressive strength tests were achieved through laboratory testing using cylindrical test objects measuring 150 mm in diameter and 300 mm in height. The test specimens were ordinary concrete with 0.6% bestmittel and a Silica Fume combination with varying percentages of 15%, 20%, and 25% substituting partial cement. Meanwhile, the concrete treatment employs both fresh water and seawater. Pressure tests were performed at 14 and 28 days. At 14 days, concrete with fresh water treatment and a bestmittel content of 0.6% and silica fume concentrations of 15%, 20%, and 25% have compressive strengths of 25.53 MPa, 27.11 MPa, and 26.04 MPa, respectively. Meanwhile, it was 26.34 MPa, 27.61 MPa, and 26.75 MPa after 28 days of concrete age. At 14 consecutive days of concrete age, concrete with seawater treatment had a reduced compressive strength. 19.66 MPa, 22.13 MPa, and 23.07 MPa, respectively. Meanwhile, at 28 days, the pressures were 20.53 MPa, 24.77 MPa, and 25.53 MPa. Using 6% bestmittel followed by 15-20% SF can boost the strength of the concrete and let it survive reduced compressive strength due to seawater infiltration.
AbstractCollapsible soils such as loess, which are naturally unsaturated, are a type of problematic soil that has high resistance in dry conditions but when wet and saturated suddenly collapses, ...creating hazards for structures built on them. Therefore, the stabilization of such soils with cement can be effective. Because cement production is costly and one of the most important causes of air pollution in the world, replacing a portion of cement with an environmentally friendly additive such as natural zeolite (NZ) is of great importance. The present study aimed to quantify the influence of cement and zeolite as well as recycled polyester fiber (PET) on the unconfined compressive strength (UCS) of loess soil using a series of unconfined compression tests. The results indicated that the UCS of samples stabilized with 4% and 8% cement were substantially enhanced by increasing zeolite replacement to an optimum value of 10% and 30%, respectively. The highest resistance was achieved in mixtures with a water content (ω) equal to 1.2 times optimum water content (ωopt). Additionally, soil stabilization with a combination of cement, zeolite, and recycled polyester fiber significantly increased UCS. The addition of PET to a zeolite-cement-loess mixture caused an increase in failure strain. To overcome the brittle behavior of cemented loess, the combination of PET and zeolite in a cement-loess mixture is very effective.
To enhance the sensitivity of graphene aerogel‐based piezoresistive sensors by weakening their compressive strength while keeping their elasticity, lightweight and lamellar graphene aerogels (LGAs) ...with high elasticity and satisfactory electrical conductance networks are fabricated by bidirectional‐freezing of aqueous suspensions of graphene oxide in the presence of small amounts of organic solvents, followed by lyophilizing and thermal annealing. Because of the lamellar structure of the LGA, its compressive strength along the direction perpendicular to the lamellar surface is much lower than those of both isotropic and unidirectionally aligned graphene aerogels with similar apparent densities, leading to an ultrasensitive LGA‐based piezoresistive sensor with a high sensitivity of −3.69 kPa−1 and a low detection limit of 0.15 Pa. The ultrahigh sensitivity and low detection limit of LGA‐based piezoresistive sensor contribute to detecting subtle pressure at room temperature and in liquid nitrogen with ability to detect dynamic force frequency and sound vibration. Besides, thanks to the fewer junction points between the graphene lamellae, LGAs slices can be integrated as a wide‐range and sensitive bending sensor, which can detect arbitrary bending angles from 0° to 180° with a low detection limit of 0.29°, and is efficient in detecting biosignals of wrist pulse and finger bending.
Soft yet elastic lamellar graphene aerogels are fabricated by bidirectional‐freezing graphene oxide suspension with the addition of ethanol, followed by freeze‐drying and thermal annealing. An aerogel‐derived piezoresistive sensor exhibits both a high sensitivity of −3.69 kPa−1 and a low detection limit of 0.15 Pa, while the aerogel‐based bending sensor shows a wide detecting range and a low detection limit of 0.29°.
Hydrogels with high strength and ductility are normally prepared from synthetic polymers, and most protein‐based hydrogels are soft and brittle. Here, a strong, ductile gelatin hydrogel is prepared ...by simply soaking a virgin gelatin gel in an ammonium sulfate solution. The polymer chains in the covalent, crosslink‐free network can freely move to homogeneously distribute stress, and more importantly, the highly kosmotropic ammonium sulfate ions greatly enhance the hydrophobic interactions and chain bundling within the gelatin gels. As a result, the treated hydrogels have an extraordinary ultimate strength (compressive and tensile strains of over 99% and 500%, respectively, and stresses of 12 and 3 MPa) superior to that of common protein gels. The physical crosslinks introduced by the Hofmeister effect can rapidly absorb energy and sustain large deformations via decrosslinking and dissociation, which result in energy dissipation and antifatigue properties. The effects of the gelatin and (NH4)2SO4 concentrations on the hydrogel mechanics are evaluated, and the possible strengthening mechanism is discussed. The effect of various ions in the Hofmeister series on the gelatin hydrogel is also investigated. Kosmotropic ions enhance the mechanical properties, whereas chaotropic ions soften and dissolve the gel.
A gelatin‐based, strong, ductile hydrogel is prepared by soaking a virgin gelatin hydrogel in ammonium sulfate solutions. Chain bundling, hydrophobic interactions, and microphase separation regions induced by the Hofmeister effect endow the hydrogels with superior mechanical properties.
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