This study addresses the effect of curing regimes on the mechanical properties, hydration and microstructure of ultra-high performance concrete (UHPC). The results demonstrate that the mechanical ...properties are strengthened by increasing curing temperature, but the flexural/tensile to compressive strength ratio shows an unusual increasing tendency with increasing temperature and compressive strength, which is opposite to normal concrete. The nano-mechanical properties are also enhanced by heat treatment. The ultra-high density phase is dominated hydrates. Microstructure observation indicates that heat treatment promotes the formation of additional hydrates with high-packing density and stiffness such as tobermorite and xonotlite, enhancement of transition zone around steel fiber, quartz and clinker, average chain length of hydrates and pozzolanic reaction between quartz/silica fume and Ca(OH)2. The evolution of hydrates and microstructure due to curing regimes and the presence of quartz play key roles in controlling the unusual behavior of the strength ratio and improvement of mechanical properties.
Graphene oxide (GO), which has been sucessfully used in cement composites as an admixture, can improve their mechanical and durability properties. This study examined and elucidated the effect of GO ...on the hydration products of a cement system in detail. The hydration precursors, morphology, composition, and chemical bonds of cement pastes were studied by SEM/BSE, XRD, LA-ICP-MS, and 29Si/27Al MAS-NMR, respectively. The experimental results indicated that the 28-day compressive strength of cement pastes (w/c = 0.35) was increased by 29% when admixing 0.02 wt% GO. There existed chemical reactions between the admixed GO and the cement hydration products. The improved strength actually was not only attributed to the improved hydration degree of cement, but also the newly found tobermorite-like hydrates and jennite-like hydrates due to the Ca ions consumption by the negatively charged GO. The polymerization of hydration products was also improved due to the addition of GO.
Display omitted
Machine learning techniques are widely used algorithms for predicting the mechanical properties of concrete. This study is based on the comparison of algorithms between individuals and ensemble ...approaches, such as bagging. Optimization for bagging is done by making 20 sub-models to depict the accurate one. Variables like cement content, fine and coarse aggregate, water, binder-to-water ratio, fly-ash, and superplasticizer are used for modeling. Model performance is evaluated by various statistical indicators like mean absolute error (MAE), mean square error (MSE), and root mean square error (RMSE). Individual algorithms show a moderate bias result. However, the ensemble model gives a better result with R
= 0.911 compared to the decision tree (DT) and gene expression programming (GEP). K-fold cross-validation confirms the model's accuracy and is done by R
, MAE, MSE, and RMSE. Statistical checks reveal that the decision tree with ensemble provides 25%, 121%, and 49% enhancement for errors like MAE, MSE, and RMSE between the target and outcome response.
Display omitted
•The effect of cement strength class on the compressive strength of cement mortar is examined.•ANN model accurately predicted compressive strength of cement mortar.•The results ...between ANN model and experiments results show a good agreement.•The performance of ANN model show improved with cement strength class considered.
An artificial neural network (ANN) study is presented to predict the compressive strength (Fc) of mortar mixtures containing different cement strength classes of CME 32.5, 42.5, and 52.5MPa. For this purpose, 54 mixtures considering six water/cement ratios (W/C) (0.25, 0.3, 0.35, 0.4, 0.45, and 0.5) and three sand/cement ratios (S/C) (2.5, 2.75, and 3) along with the abovementioned three types of cement strength classes have been constructed, and the results for a total of 810 specimens have been obtained. A comparative investigation was performed on two conditions of with and without considering the cement strength class as an input parameter in developed ANN-I and ANN-II models in order to obtain the optimum state.
The comparison of the proposed idealized ANN model with two other existing models indicates good precision and accuracy of the developed ANN model in predicting the compressive strength of the mortar and the deficiency of these existing models in situations where cement strength class is present as an input parameter.
•Mechanical changes of frozen coals under different liquid nitrogen treatments.•The freezing-thaw damage effect of LN2 and the principle of freezing enhancement.•The mechanics principle of liquid ...nitrogen changing the pore structure of coal.
Anhydrous liquid nitrogen cracking low-permeability coal seams can improve the efficiency of coalbed methane extraction. Under normal pressure, liquid nitrogen (-196℃) contacts the coal body, and the temperature stress and frost heave force generated will change the mechanical properties of the coal body, cause internal structural damage, and increase the seepage channel of coalbed methane. In order to study the influence of different liquid nitrogen freezing variables on the mechanical properties of coal, Brazilian splitting, uniaxial compression, and ultrasonic velocity measurement tests of saturated frozen coal were carried out. The results show that a single freezing of liquid nitrogen has a certain enhancement effect on the mechanical properties of coal: the tensile strength increases by 64.0%, the uniaxial compressive strength increases by 54.6%, but as the freezing time increases, the coal strength increases first and then decreases The strength of coal under the action of freeze–thaw cycles shows an exponential decline in varying degrees: tensile strength has dropped by 81.3%, uniaxial compressive strength has dropped by 68.9%, and the degree of decline is positively correlated with the number of freeze–thaw cycles. The change in mechanical strength changes the elastic stage. The freezing enhancement factor I and the freeze–thaw damage factor D are defined by the change of elastic modulus. I presents a quadratic function relationship that first increases and then decreases with freezing time, and D continues to decrease as the number of freeze–thaw cycles increases. The longitudinal wave velocity first increases and then decreases with the freezing times, and decreases with the increase of the number of freeze–thaw cycles. The porosity is negatively correlated with the wave velocity. The wave speed decreases, the porosity increases, and the internal pores of the coal are connected to form a fracture network, which accelerates the damage of the coal and reduces the mechanical properties of the coal. Through analysis, it is concluded that when liquid nitrogen acts on coal, the change in mechanical properties is the result of the combined action of multiple forces.
To mitigate autogenous shrinkage that may cause early-age cracking of alkali-activated fly ash-slag (AAFS) concrete, internal curing using superabsorbent polymers (SAP) is employed in this study. ...AAFS pastes with different SAP dosages (0–0.5%) and slag replacement ratios to fly ash (15–30%) are investigated. Experimental results indicate that with the addition of SAP workability of fresh paste is improved while compressive strength is comparatively reduced. As SAP dosage increases from 0.2% to 0.5%, chemical shrinkage and autogenous shrinkage of AAFS pastes are reduced by around 18% to 45% and 76% to 85%, respectively. Internal curing of SAP is found to lower the heat peak and shift the peak to the right. This indicates the slower hydration rate corresponding to the lower chemo-mechanical deformation (chemical shrinkage), which contributes to the mitigation of autogenous shrinkage. Therefore, internal curing by means of SAP is an efficient method for mitigating autogenous shrinkage in AAFS pastes.
The effect of fly ash on the hydration of calcium sulfoaluminate cement was investigated. Increasing fly ash contents accelerated the hydration of calcium sulfoaluminate cement due to the filler ...effect. Dissolution rims around fly ash particles after 90days indicated a reaction degree of the fly ash of approximately 20 to 30% as estimated by various independent methods. The contribution of fly ash to the hydration reactions resulted in the formation of C-S-H, in an increase of the strätlingite content and in the destabilization of monosulfate.
The mortar with 7.5 mass-% fly ash reached a higher compressive strength than the reference without fly ash when the water to cement ratio was kept constant. Up to 15 mass-% of fly ash could be added without strength loss.
This study proposed the use of hydration agent (HA) and seeds to improve the hydration and carbonation of reactive magnesium cement (RMC)-based concrete formulations. Hydration of RMC was evaluated ...by isothermal calorimetry. Water absorption and compressive strength results were used to assess the mechanical performance of RMC-based concrete samples. Quantification of hydrate and carbonate phases was performed via XRD and TGA. Formation and morphology of carbonates were observed via BSE and SEM. In addition to increasing the utilization of RMC in the carbonation reaction and facilitating early strength development, the use of HA formed large carbonate phases, while the addition of seeds improved sample microstructures via the development of dense carbonate networks. The improvements in morphology, microstructure and carbonate content in samples involving the simultaneous use of HA and seeds resulted in 56% lower water absorption values and 46% higher 28-day compressive strengths (70MPa) in comparison to the control sample.
This article presents the results of tests on walls made of AAC (autoclaved aerated concrete) strengthened with horizontal reinforcement in the form of strips of mesh rolled out from a roll, ...consisting of steel cords with an interwoven fiberglass roving was used. The effect of the applied strengthening was determined based on the results of tests on masonry models subjected to compression, and diagonal compression. Specimens were tested according to guidelines of PN‐EN 1052‐1:2000 and ASTM ES519‐81 code in case of compression and shear test respectively. The results of the tests were also compared with the results of tests of AAC masonry walls without reinforcement, reinforced with truss‐type reinforcement.
Abstrakcyjny
W artykule przedstawiono wyniki badań dotyczące wpływu zbrojenia spoin wspornych stalowym zbrojeniem rozwijanym z rolki na nośność murów z ABK. Modele badawcze podano badaniu wytrzymałości na ściskanie oraz ukośne ściskanie zgodnie z wymaganiami norm PN‐EN 1052‐1:2000 oraz ASTM ES519‐81. Wyniki badań porównano z wynikami murów bez zbrojenia, zbrojonych zbrojeniem typu kratowniczka.
This work focuses on the fabrication of hybrid bio-composites using green epoxy as the matrix material, hemp (H) and sisal (S) fibre mats as the reinforcements. The hybrid composite with sisal/hemp ...fibres were fabricated by cost effective hand lay-up technique, followed by hot press with different stacking sequences. Static properties of the composites such as tensile, compressive, inter-laminar shear strengths (ILSS) and hardness were examined. The physical properties such as density, void content, water absorption and thickness swelling were also analyzed. The experimental results indicate that hybrid composites exhibited minor variation in tensile strength when the stacking sequence was altered. The hybrid composite with the intercalated arrangement (HSHS) exhibited the highest tensile modulus when compared with the other hybrid counterparts. Hybrid composites (SHHS and HSSH) offered 40% higher values of compressive strength than the other layering arrangements. HHHH sample exhibited the highest ILSS value of 4.08 MPa. Typical failure characteristics of the short beam test such as inter-laminar shear cracks in the transverse direction, micro-buckling and fibre rupture were also observed.
•Hybrid biocomposites with varying the stacking sequences of hemp and sisal fibre mats in green epoxy matrix was fabricated.•Effect of varying sequences on the tensile, compression and inter-laminar shear strengths were investigated.•Significant improvements were seen on the tensile and compressive modulus of the composites when compared with the matrix.•Statistical analysis was performed by One-way ANOVA method.
PDF
