مجله تحقیقات بتن ایران
سال پانزدهم شماره 2 (تابستان 1401)

Salt scaling of concrete pavements is known as a serious problem in cold regions. In the present study the effect of water-binder (W/B) ratio, silica fume and air entraining on the salt scaling resistance, compressive strength and electrical resistance were investigated. Microstructure of the concretes was assessed by mercury intrusion porosimetry and scanning electron microscope. Water-binder ratios were 0.35, 0.40 and 0.45. Silica fume was used, by 8% weight of cement, in the predetermined mixtures. The results showed that the best salt scaling resistance was correspondent to the air entrained silica fume specimens. By reducing the water-binder ratio, the amount of surface scaling was decreased. The MIP results declared that silica fume refined the microstructure and also decreased the total porosity and critical pore radius compared with those for the reference specimens. Furthermore, the MIP results of the silica fume specimens declared that the pores smaller than 10 nano were increased, due to the effective pozzolanic reaction. The scanning electron microscope images showed that the concrete containing silica fume had a denser gel compared with those for the reference ones. Introducing silica fume into the specimens had remarkable effect on the later age electrical resistance of the concrete. It was concluded that the highest electrical resistance belonged to the durable specimens. The results of the present study clarify how the micro structure may influence the salt scaling resistance of concrete pavement, this achievement will surely reduce the cost of rehabilitation of concrete pavements in the cold regions.

Relationship between the compressive strength of concrete in terms of the age of concrete is set out in the American Concrete Institute Regulations for Type I and Type III Cement. In this study, the relationship between compressive strength in terms of the age of concrete containing Portland Cement Type II has been experimentally investigated. The relationship has also been explored for crumb rubberized concrete. For this purpose, three cement contents: 350, 400 and 450 kg / m³, as well as two water-to-cement ratios of 0.5 and 0.65, and crumb rubber with ratios of 5 to 20% by the Volume of sand have been used. In the relations obtained, α and β values for the ordinary concretes containing cement type II are 3.13 and 0.91, respectively, while for the crumb rubberized concrete are 3.32 and 0.9, respectively. In this study, the relationship between compressive strength in terms of the age of concrete containing Portland Cement Type II has been experimentally investigated. The relationship has also been explored for crumb rubberized concrete. For this purpose, three cement contents: 350, 400 and 450 kg / m³, as well as two water-to-cement ratios of 0.5 and 0.65, and crumb rubber with ratios of 5 to 20% by the Volume of sand have been used. In the relations obtained, α and β values for the ordinary concretes containing cement type II are 3.13 and 0.91, respectively, while for the crumb rubberized concrete are 3.32 and 0.9, respectively.

The aim of this study is the experimental and numerical investigation of the simultaneous effect of PET and steel fibers on the rheological properties, flexural strength, and compressive stress-strain curve of self-compacting concrete. For this purpose, 30 mixing designs by response surface method (RSM) were designed by combining different percentages of PET, steel fibers, stone powder, and superplasticizer variables. The results demonstrate that with increasing the percentage of PET and steel fibers, the viscosity of concrete decreased. Also, the highest flexural strength was obtained from the combination of 0.4% fibers, 8% PET, and 1% superplasticizer. The stress-strain curves plotted for the mixtures showed that the steel fibers had little effect on the ascending part of the stress-strain curve and improved the stability on the descending part of the curve. PET particles also reduce the slope of the ascending part of the stress-strain curve. In general, Steel fibers and PET caused the soft failure of the samples so that PET increased the strain corresponding to the maximum compressive strength, and steel fibers increased the rupture strain of concrete.

This study aims to investigate several well-known stress-strain relationships for tensile concrete in literature and introduces the most appropriate one for predicting flexural behaviour of reinforced concrete hollow core slabs in post cracking phase. Despite comprehensive experimental and numerical researches that are conducted to understand the performance of hollow core slabs, limited information exists about post cracking behavior and specifically the tension stiffening characteristic of this system. In the current study some relationships that were proposed by other researchers for reinforced concrete in tension and flexure are concerned. Numerical analyses were performed by a program based on a layered approach which was specifically developed for analysis of voided slabs. Different tension relationships for concrete from previous literature were implemented in the code and the numerical results were compared with experimental results. Eventually, the most appropriate tension relationship of concrete for predicting post cracking flexural behavior of hollow core slabs will be proposed.

In recent years, the use of synthetic fibers to improve mechanical properties and reduce the shrinkage of fresh and hardened concrete, increase energy absorption and impact resistance of concrete, and replace thermal reinforcement with fibers have greatly expanded. The use of various types of fiber has led to the improvement of some properties and the weakening of other properties. Simultaneous use of several types of fibers with designed ratios can enhance the mentioned properties simultaneously and prevent possible weaknesses. In this study, the effect of fibers with various volumetric percentages on compressive, flexural and tensile strength of the samples at different ages compared to the control sample has been measured. The simultaneous combination of steel, barchip and macro-synthetics fibers with different ratios, compressive performance, flexural and tensile performance of concrete has been tested. Among the constructed concretes, the best performance of reinforced concrete with steel fibers in different tensile, compressive, and bending strengths has shown that these fibers perform better than other constructed samples. In hybrid fiber concrete, the best performance in bending and compressive strength and tensile strength has been related to hybrid fiber concrete consisting of macrosynthetics fibers and steel fibers.

This experimental research has investigated in particular the durability of fully recycled concrete containing natural zeolite (NZ), microsilica (SF) and fly ash (FA) pozzolans. Recycled concretes consist of recycled concrete aggregates and contain different percentages of replacement of SF, FA and NZ. A total of 11 mixing designs were constructed, including plain reference concrete (RC), 100% recycled concrete without pozzolans, and 9 mixing designs of fully recycled concretes containing the mentioned pozzolanic materials as separate applications. To evaluate and compare some mechanical properties and especially properties related to the durability of this type of concrete, in the age range of 28- to 91-day, compressive strength (CS), in the age range of 28-day the ultr-pulse velocity, and in the age range of 180-day immersion water absorption, capillary water absorption, electrical resistivity and electrical conductivity of specimens made of this type of concretes, were measured. The results showed that although the complete replacement of recycled aggregates in this type of concrete leads to a decrease in their CS compared to the RC, but the replacement of pozzolanic materials such as SF in fully recycled concrete can lead to gain more CS than RC. Also, the rate of loss of durability properties in fully recycled concretes without pozzolans is much higher than the loss of mechanical properties compared to the RC. However, due to changes in the chemical structure of recycled concretes from pozzolanic materials such as NZ, some of the durability properties of these concretes did not change significantly compared to RC.

Pozzolans are alumina-silicate materials, partially replacing cement to improve many durability features of concrete. They can also help the environment and contribute to sustainable development through reducing the production of cement, culminating in the decrease of carbon-dioxide. One of the important applications of pozzolans is in the production of self-consolidating concrete (SCC), a concrete that has been receiving extensive attention nowadays due to its properties. On the other hand, one of the durability features is the resistance of concrete against scaling due to freezing and thawing in the presence of salt that generally causes distress in the surface of concrete. In some previous studies, it was observed that binary (one pozzolan along with cement) and ternary (two pozzolans along with cement) blends decreased resistance against salt scaling despite providing considerable improvements in durability characteristics of concrete. There has been limited studies on scaling resistance of ternary and quaternary blends (three pozzolans along with cement) done to date. Hence, the purpose of this study was to investigate the effect of binary, ternary and quaternary blends on SCC scaling. For this purpose, 0, 10 and 20 percent of trass and pumice, beside 10% of silica fume was used in 9 mixtures in addition to two mixtures with 3% air-entrainment. The results illustrated that a ternary mix had the best performance among non-air-entrained mixtures. Overall, binaries and ternaries illustrated good scaling resistance compared to quaternaries. In addition, air-entrainment significantly increased resistance against salt scaling.

In the current study, mechanical properties and durability of polymer modified mortars by Styrene Butadiene Resin (SBR) containing glass powder and slag is studied. Different mix design with and without SBR is regarded by consideration of different amount of glass powder and slag individually or simultaneously, and furthermore, hybrid or individual presence on glass, polypropylene, and steel fibers is investigated. In all of the fiber-reinforced mixes, tensile and flexural strengths were increased. Also, the presence of SBR improved these strengths. The maximum of flexural strength was observed in polymer modified mortars contained hybrid fibers of glass, polypropylene, and steel. In these mixes with presence of 10% glass powder, 10% slag and, 5% slag and 5% glass powder, the flexural strength increased 36.05, 31.84 and 41.06% , respectively. Polymer modified mortars showed higher T_150^D toughness and the presence of SBR significantly increased the energy absorption of mixes. Moreover, simultaneous use of glass powder and slag enhanced the energy absorption. The use of SBR drastically improved the durability against acidic environment. The effect was observed in weight loss, and residual strength of samples after exposure the sulfuric acid environment. The residual compressive strength after exposure to acid in polymer modified mortars contained polypropylene and steel fibers with presence of 10% glass powder, 10% slag and, 5% slag and 5% glass powder, were respectively, 33.26, 33.42 and 33.76% higher than control sample.