مجله تحقیقات بتن ایران
سال شانزدهم شماره 1 (بهار 1402)

The aim of this study is to determine the effect of replacing polyvinyl alcohol (PVA) fibers, fly ash (FA), and silica aggregate with polypropylene (PP) fibers, ground blast furnace slag (GBFS), iron furnace dust, limestone powder (LSP), natural sand, and microsilica to improve the mechanical properties and ductility of Engineered Cementitious Composites (ECC). Twelve different mixtures of ECC were designed and prepared. For each mixture, flexural and compression specimens were made and tested. The combination of microsilica and GBFS increases the strength and ductility of the composite, making it a viable alternative to fly ash. Replacement of silica sand with LSP that contains the appropriate composition can enhance ECC. The best results were achieved in ECC when the cementitious materials ratio was 1.25. By increasing the percentage of PP fibers from 1 to 1.5, the flexural strength increased by 65 percent, the middle span deflection of the flexural specimen increased by 21.7 percent, and the optimal amount of PP fibers to initiate hardening was 1.5 percent.

Carbon nanotube, a product of chemical exfoliation of graphite, is a suitable additive for use as nanoreinforcement in cement-based materials due to its high aspect ratio, good water dispersibility and excellent mechanical properties. In the present study, the effect of volume fraction, aspect ratio, distribution orientation and interaction between surfaces on the mechanical properties of cement matrix reinforced with carbon nanotubes using multi-scale modeling was investigated. To Model in the Abaqus software, with the conceptual understanding of the volume representative element, a developed MATLAB and Python scripts were applied. To observe the interphase behavior between the matrix and fillers, the cohesive surface theory was used. Also, the output results of molecular dynamics modeling was used to determine the cohesive surface parameters. Modeling was done in the states of full and limited bonding between two phases in nano-compsite with compressive axial loading. The cement models with 0, 0.5, 1, and 1.5 vol% with aspect ratios of 10 and 20 were evaluated and discussed. Furthermore, the distribution effect was studied by defining the nanotubes to be parallel, perpendicular and random regarding the force direction. The results showed that increasing the volume fraction of CNTs improves the yield strength and toughness of the samples. Increasing the CNT aspect ratio from 10 to 20 leads to an increase of elastic limit and an improvement of plastic behavior of the next matrix. Finally, the cohesive modeling of the interactions of matrix and CNT eventuated in 3 to 6% reductions per 0.5 and 1% CNT/cement composites.

Simultaneous use of pozzolans and geopolymers to improve the properties of concrete is one of the relatively new areas of concrete technology. In this study, slag and zeolite pozzolans were used as a substitute for different percentages of cement weight in concrete. Alkaline activator was also used to reduce the negative effects of high percentages of pozzolan replacement and to investigate their interactions. Based on the results of the defined tests, it was found that the addition of activator to the concrete containing pozzolan had a significant role in recovering part of the loss of properties due to high consumption of pozzolan. By adding activator to the design containing 60% slag, an increase of 52.1% in compressive strength and 44.7% in electrical resistivity for 28 days was observed. Also, the mix design containing 60% zeolite, which had the highest decrease in compressive strength by 59.3%, led to the improvement of thermal insulation properties of concrete up to 14.3% compared to the control mix design.

Compressive strength of concrete is considered as one of its most important properties and usually gives an overview of the quality of concrete, because the strength depends directly on the microstructure of the cement paste. Evaluation of compressive strength of concrete is done by destructive and non-destructive methods. Non-destructive methods, with a much fewer number of tests, can provide a good estimate of the compressive strength of concrete. In this research, non-destructive ultrasonic pulse velocity and electrical resistivity methods were used to estimate the compressive strength of normal concrete tests with three different water-to-cement ratios, and mathematical models were proposed to estimate the compressive strength were compared. SPSS statistical software was used to analyze the test data. Different linear and nonlinear mathematical models related the relationship between the parameters of electrical resistivity, ultrasonic pulse velocity and compressive strength for each of the water-to-cement ratios as well as 28-day age were extracted by software to estimate the compressive strength. The results showed that the combination of ultrasonic pulse velocity and electrical resistivity methods for estimating compressive strength has a higher accuracy compared to one method alone. For this purpose, the modified form of the exponential function with a coefficient range of 0.63-0.83 and the mean absolute value of relative error 2.3-6.5% and the polynomial function with the range 0.63-0.89 and the mean absolute value of relative error is 3.2-7% had better performance.

The present research investigates the effects of nano-silica on strength parameters of sand-cement mortar at high temperatures. For this, sand cement mortar replacing 5, 10 and 15 wt% of cement with nano-silica was prepared. The mortar, having been processed at 3, 28 and 90-day ages, was subjected to 25, 100, 200, 400, 600 and 800 ºC, respectively. Effects of high-temperature rates on the physical and mechanical properties of the sand-cement mortar were examined by macrostructural experiments of compressive strength, weight loss and water uptake, as well as microstructural experiments using XRD and SEM. The research found that the macrostructural behavior of sand-cement mortar was highly dependent on microstructures and nanostructure cementitious changes when subjected to heat. At 600 ºC, the initial portlandite was fully degraded, which caused the CaO to form as water exited. At 800 ºC, in addition to alite (C3S) and Belite (C2S), β- Wollastonite was formed from the degradation of the C-S-H nanostructure. The addition of nano-silica improved the strength properties of the sand cement mortar against heat, with the compressive strength of the 28-day samples without nano-silica experiencing a 57% weight loss as the temperature rose to 800 ºC, decreasing from 31.1 MPa to 13.3 MPa. On the other hand, the compressive strength of the sand cement mortar samples containing 15% nano-silica experienced lesser strength loss (52%) at 800 ºC, decreasing from 40.2 MPa to 19.2 MPa.

Principal recycling and reuse of industrial waste is one of the most important human challenges to protect the environment. In recent years, due to the unique properties of pervious concrete, the use of pervious concrete pavement as a good alternative to asphalt procedures has been considered. In this study, in order to assess a new method for the manufacture of high -environmentally friendly concrete pavements with aluminum slag replacement with 0, 5, 10, 15 and 30 % cement weight, the concrete samples were made and the tests including compressive strength, bending strength, water absorption percentage, permeability and porosity done. The results showed that aluminum slag replacement with 5 % of cement weight, compressive strength, bending resistance and water absorption percentage compared to the control sample did not change significantly. But using the replacement of aluminum slag with 10, 15 and 30 percent of the cement weight, compressive strength and bending strength comparing to the control sample were decreased by 16, 24 and 50% and by 6, 15 and 24 %, respectively. By replacing aluminum slag with 10 and 15% by weight of cement, the permeability comparing to the control sample was increased by 61 and 97%, respectively. It was also observed that due to the high porosity of the samples containing aluminum slag replacement with 30 % of the weight of the cement, water passed immediately and quickly.

Today, construction of large structures is growing quickly. For this reason, it is necessary to find the material with a rather low weight and high strength. For this purpose, Steel-Concrete-Steel (SCS) sandwich structures were proposed. SCS structures are composed of two steel layers and one concrete layer. Due to their low weight and high strength and flexibility, they have become popular among engineers. In the present research, first, three specimens of push-out test of strip shear connector were modeled and validated using ABAQUS finite elements software. Then, since the present equations to predict the shear strength of the shear connectors are complicated and are not so precise, the authors proposed an equation taking the effects of different geometrical parameters and the concrete's compressive strength in to account. For this purpose, using the experimental design, 17 specimens were designed and modeled. Then, an equation was proposed using the Genetic Expression Programming Algorithm (GEP) to predict the system's shear strength. Finally, the performance of the proposed equation was evaluated using the error parameters.

Considering the low concrete resistance to cracking and high volume of aggregate used in it, this study has addressed the effects of increasing the volume of wavy steel fibers (0.15, 0.3 and 0.45%) and volume of coarse aggregates (30, 40, 50 and 60%) in relation to the total volume of aggregates on the properties of fresh, hardened self-compacting concrete as well as on the fracture mechanics. To this end, different tests were performed on: 1) fresh concrete (slump-flow, T-50, J-Ring, Sieve analysis and L-BOX tests), 2) hardened concrete (compressive strength, tensile strength and modulus of elasticity tests) and 3) fracture mechanics (Mode I fracture toughness test on ENDB specimens) using 108 cylindrical specimens. Results showed that increasing the coarse aggregate volume improves the rheological properties and increasing the wavy steel fiber volume reduces the efficiency and flowability of the self-compacting concrete, but improves the compressive strength in concretes containing 50 and 60% coarse aggregate volume. Increasing this volume, will slightly reduce the tensile strength of only the fiber reinforced self-compacting concretes containing 60% coarse aggregates. Increasing the coarse aggregate volume by 30 to 60% improves the modulus of elasticity of specimens containing a fixed fiber percent and increasing the volume of wavy steel fibers, at 30 and 40% coarse aggregate volumes, increases the fracture toughness (Mode I) of the self-compacting concrete specimens containing wavy steel fibers.