Length Effect at Testing Splitting Tensile Strength of Concrete

Tensile strength of concrete is the basic property when estimating the cracking resistance of the structure and when analysing fracture processes in concrete. The most common way of testing tensile strength is the Brazilian method. It has been noticed that the shape and size of specimens influence the tensile splitting strength. The experiments were performed to investigate the impact of cylinder’s length on tensile concrete strength received in the Brazilian method. During the experiment the tensile concrete strength was tested on two different sizes cylindrical specimens: 150 mm × 150 mm and 150 mm × 300 mm. Experiments were performed in two stages, with two types of maximum aggregate size: 16 mm and 22 mm. The software “Statistica” was used to perform the broad scale statistical analysis. When comparing test results for shorter and longer specimens, the increase of tensile splitting strength tested on shorter cylinders was observed (approximately 5%). However, when performing deeper statistical analysis, it has been found that the length effect was not sensitive to the strength of the cement matrix and the type of aggregate but was influenced by the aggregate size. Further experiments are needed in order to perform a multi-parameter statistical analysis of scale effect when testing the splitting tensile strength of concrete.

Study on Mechanical and Frost Resistance Properties of Slag and Macadam Stabilized with Cement and Fly Ash

China is a large country in terms of coal production and consumption. The fly ash and slag produced by thermal power plants pose a great threat to the environment. To reduce the adverse effects of fly ash and slag on the environment, a mixture of slag and macadam stabilized with cement and fly ash was prepared as pavement base material. Compaction tests, unconfined compressive strength tests, splitting strength tests, frost resistance tests, and ultrasonic tests were performed on the mixture. The results show that with an increase in slag replacement rate, the unconfined compressive strength and splitting strength decreased. However, the adverse influence of the slag replacement rate on unconfined compressive strength and splitting strength of specimens gradually weakened with increasing curing time. The frost resistance of the mixture first increased and then decreased with an increase in the slag replacement rate. When cement content was 5% and the slag replacement rate was 50%, the frost resistance of the mixture was the best. Regression analysis of the ultrasonic test showed that the ultrasonic test can effectively characterize the strength of the mixture and the internal damage degree under freeze–thaw cycles. In conclusion, the slag replacement rate of the mixture is recommended to be ~50%, which has preferable mechanical and frost resistance performance.

Mechanical Properties of Furnace Slag and Coal Gangue Mixtures Stabilized by Cement and Fly Ash

The mechanical properties and strength formation mechanism of cement–fly-ash-stabilized slag–coal gangue mixture were examined using an unconfined compressive strength test, splitting strength test, triaxial test, and scanning electron microscopy to solve the limitations of land occupation and environmental pollution that is caused by fly ash from the Xixia District thermal power plant in Yinchuan, slag from the Ningdong slag yard, and washed coal gangue. Its performance as a pavement base mixture on the road was investigated. The results demonstrated that as the slag replacement rate increased, the maximum water content increased while the maximum dry density decreased. The addition of slag reduced the unconfined compressive strength and splitting strength of the specimens; furthermore, the higher the slag substitution rate, the lower the unconfined compressive strength and splitting strength of the specimens. As the cement content increased, the specimen’s unconfined compressive strength increased. Based on the principle of considering the mechanical properties and economic concerns, the slag replacement rate in the actual construction should be ~50% and should not exceed 75%. Based on the relationship between the compressive strength and splitting strength of ordinary concrete, the relationship model between the unconfined compressive strength and splitting strength of cement–fly-ash-stabilized slag–coal gangue was established. The failure mode, stress–strain curve, peak stress, and failure criterion of these specimens were analyzed based on the triaxial test results, and the relationship formulas between the slag substitution rate, cement content, peak stress, and confining pressure were fitted. As per the SEM results, the mixture’s hydration products primarily included amorphous colloidal C-S-H, needle rod ettringite AFt, unhydrated cement clinker particles, and fly ash particles. The analysis of the mixture’s strength formation mechanism showed that the mixture’s strength was the comprehensive embodiment of all factors, such as the microaggregate effect, secondary hydration reaction, and material characteristics.

Carbstone Pavers: A Sustainable Solution for the Urban Environment

To reduce CO2 emissions from the building industry, one option is to replace cement in specific applications with alternative binders. The Carbstone technology is based on the reaction of calcium- and magnesium-containing minerals with CO2 to form carbonate binders. Mixes of carbon steel slag and stainless-steel slag, with tailored particle size distributions, were compacted with a vibro-press and subsequently carbonated in an autoclave to produce carbonated steel slag pavers. The carbonated materials sequester 100–150 g CO2/kg slag. Compressive and tensile splitting strength of the resulting pavers were determined, and the ratio was found to be comparable to that of concrete. The environmental performance of the Carbstone pavers, with an average tensile splitting strength of 3.6 MPa, was found to be in compliance with Belgian and Dutch leaching limit values for construction materials. In addition, leaching results for a concrete mix made with aggregates of crushed Carbstone pavers (simulating the so-called “second life” of pavers) demonstrate that the pavers can be recycled as aggregates in cement-bound products after their product lifetime.

DAMAGE CHARACTERISTICS OF RECYCLED ASPHALT MIXTURES UNDER THE EROSION EFFECT OF SNOW-MELTING SALT

The erosion effect of snow-melting salt will degrade the durability of recycled asphalt pavement, but the damage characteristics of recycled asphalt mixture triggered by the erosion effect of snow-melting salt remain unclear. To solve the snow-melting salt-induced durability degradation of asphalt pavement, two commonly used snow-melting salts, NaCl and CaCl2, were selected to carry out the saline water immersion, salt-drying and -wetting cyclic and salt-freezing and -thawing cyclic splitting tests on recycled asphalt mixture, and the attenuation laws of splitting strengths and its damage characteristics under the erosion effect of snow-melting salts were analyzed. Results demonstrate that with the increase in soaking time, salt-drying and -wetting cycles and salt-freezing and -thawing cycles, the splitting strength of the recycled asphalt mixture maintain a declining trend, and the attenuation rate of splitting strength is elevated. The damage degree of the recycled asphalt mixture presents a nonlinear growth trend during saline water immersion, salt-drying and -wetting cycles, and salt-freezing and -thawing cycles. Under the same conditions, the damage degree after the action of NaCl solution is higher than that after the action of CaCl2 solution, and meanwhile, within the range of test concentration, the damage degree after the action of low-concentration saline solution is higher than that after the action of high-concentration saline solution. Conclusions provide a significant reference for the composition design and maintenance decisions of recycled asphalt pavement materials in cold regions. Keywords: road engineering; salt erosion; recycled asphalt mixture; damage characteristics; splitting strength

Verification of the shear performance of mortise and tenon joints with top and bottom notches at the beam end

Shear experiments on mortise and tenon joints with top and bottom notches in the beam end were conducted with the length of the tenon as a variable. In addition, material experiments were performed to investigate the structural performance of the fracture modes of the joint. The experimental results show that when the lower notch at the beam end experiences splitting, the shear forces were identical for different tenon lengths and did not decrease. In addition, the deformation performance of the joint up to yielding was largely due to the compressive deformation perpendicular to the grain of the wood on the sides of the tenon. Based on the theory of the calculation method of the splitting strength of a notched beam, a formula for the splitting strength when the end of the beam is moment-resisting was proposed. It was confirmed that the proposed formula could estimate the shear force that caused the splitting fracture of the lower notch. In addition, the increase in shear force after the lower notch experienced splitting fracture was due to the increase in clamping force of the beam end due to rotational deformation.

MECHANICAL PROPERTIES OF RECYCLED COARSE AGGREGATE CONCRETE REINFORCED WITH STEEL FIBERS

This paper presents the combined influence of natural aggregates (NA) replacement with recycled concrete aggregates (RCA) and incorporating steel fiber reinforcement on the mechanical properties of normal-strength (30 MPa) concrete mixes. Hooked-end steel fibers were added in a 2% volumetric fraction to promote 100% RCA replacement. Fine aggregates were in the form of locally-abundant desert dune sand. Mechanical properties of 28-day concrete samples were assessed, including compression strength, tensile splitting strength, elastic modulus, flexural stress, and flexural toughness. For plain concrete mixes, the replacement of NA by RCA resulted in 18, 27, and 5% reductions in the respective design compression strength, elastic modulus, and tensile splitting strength. Nevertheless, the addition of steel fibers could restore the aforementioned properties by up to 90, 77, and 164%. Compared to the control mix made with NA, the flexural strength of the plain RCA-based concrete mix decreased by 33%, while the flexural toughness increased by 100%. In turn, the corresponding flexural properties of RCA concrete mix reinforced with steel fibers were 2 and 56 times those of the control made with NA. Findings provide evidence of the ability to produce concrete made with 100% RCA and reinforced with steel fibers with comparable compression properties and improved tensile and flexural performance compared to those of NA-based concrete.

FABRICATION AND CHARACTERIZATION OF LTCC BASED ON TALK GLASSES

In this work, fabrication of low temperature co-fired (LTCC) with different Talc content is performed. Density and mechanical characterization of the prepared samples has been examined. Three series of low temperature co-fired ceramics have been prepared. The glass part of these composites contains talc to enhance the mechanical properties. The densities, hardness and splitting strength are measured for these series to monitor the effect of composition on their properties. It is shown that the third series (Base3; Flint 25%, talc 35%, Pota ash 8%, Soda ash 12%, and Boric Acid 20%) of the higher talc content shoes the better densities and mechanical properties. The results are explained in terms of better balance of the ceramic-class content that enhances the measured properties.

Research on the Performance of Regenerant Modified Cold Recycled Mixture with Asphalt Emulsions

In order to study the mechanical properties and effect of a regenerant on a cold recycled mixture with asphalt emulsions (CRMEs), the moisture susceptibility, high-temperature performance, low-temperature performance, dynamic mechanical properties and durability of CRMEs were analyzed and evaluated by immersion splitting strength tests, freeze-thaw splitting strength tests, rutting tests, semi-circle bending tests, uniaxial compression dynamic modulus tests and indirect tensile tests. Scanning electron microscopy (SEM) was used to analyze the micromorphology of CRMEs modified with regenerant. Finally, a comprehensive evaluation system of five different CRMEs was established based on the efficacy coefficient method to quantitatively analyze the comprehensive performance of the CRMEs. The test results showed that the regenerant can significantly improve the water immersion splitting strength, freeze-thaw splitting strength fracture energy density, and fatigue resistance of CRMEs. However, the addition of regenerant affected the high-temperature performance of the cold recycled mixture. The dynamic modulus of the CRMEs first increased and then decreased with regenerant content increasing. When the regenerant content was 8%, the dynamic modulus of the CRMEs was the highest. Adding styrene-butadiene rubber (SBR) latex can improve the high-temperature performance of CRMEs, but the moisture susceptibility, low temperature performance and fatigue resistance of the cold recycled mixture were not significantly improved, and the dynamic modulus of the mixture was reduced. Based on the efficacy coefficient method, the optimal content of regenerant is 8%. Regenerant are potential modifiers for cold recycled mixture that they can significantly improve the dynamic mechanical properties and durability.

Effect of All-Component Recycled GFRP on Physical-Mechanical Properties and Microstructures of Concrete

Large inventory and non-degradability made waste glass fiber reinforced plastics (GFRP) a heavy burden to environment. They are increasingly reclaimed through mechanical crushing and used as aggregate replacement in concrete. However, reuse of all-component recycled GFRP (rGFRP) was still limited due to the inconsistent influences of powder and fiber on cementitious materials. In this study, mortar and concrete with two different gradations of all-component rGFRP at 10 wt%, 20 wt% and 30 wt% were investigated with mechanical tests, ultrasonic pulse velocity inspection, Depth-of-Field optical microscopy, Scanning Electron Microscopy (SEM) and micro-CT. It revealed that the splitting strength of cement mortar was significantly increased while 10 wt% of rGFRP was added, whereas the compressive and flexural strength were barely affected. For concrete, the initial and final setting time were prolonged by the addition of 30 wt% rGFRP up to 93.8% and 124.3%, respectively. The mechanical strength of concrete increased with rGFRP content firstly, and then decreased, due to the reduced dispersity of rGFRP and compactness of mortar. When 10 wt% of rGFRP was added, the 28-day compressive, flexural and splitting strength of concrete were optimized to 25.8 MPa, 4.25 MPa and 3.02 MPa, respectively. The failure pattern analysis indicated that rGFRP can restrain crack propagation, reduce crack width and improve the integrity of fractured concrete. The results suggested the potential feasibility of rGFRP as fine aggregate replacement, and provided solid experimental references for practically reusing rGFRP in cementitious materials.