JOURNAL OF CONCRETE RESEARCH
Year:2019 | Volume:12 | Issue:1 |Papers Count:10

Roller-compacted concrete (RCC) is a special blend of concrete that has essentially the same ingredients as conventional concrete but in different ratios, and without slump. It was shown that the application of fibers would enhance some properties of concrete, however in RCC, there was a lack of knowledge. The application of RCC gained a great attention for technical and economical benefits. In this paper, the effects of synthetic polymeric fibers on the physical and mechanical of fiber roller compacted concrete were experimentally investigated. To this means, three mixtures by using of micro and macro polymeric fibers made. The mechanical strength tests including compressive and flexural strength and also abrasion strength tests were carried out. The results showed that the application of fibers aid in these properties.

The use of concrete with ultra high performance reinforced steel fiber has been investigated in numerous studies. However, most research has focused on the use of steel fibers as reinforced. In this study, the replacement of steel fibers with polypropylene fibers and synthetic fibers (barchip) was investigated in the parameters of the mechanical properties of this concrete. The same use was also made of two hybrid fibers, as well as hybrid with steel fibers has been investigated. The total fiber content in each mixing plan is considered to be 1. 5% vol. A total of 28 specimens 70x70mm cubes and 14, 100x200mm cylindrical specimens were made for compressive strength testing and 28 specimens of 70x70x 350mm were used to test the flexural strength. The compressive and flexural strength tests were measured at 7 and 28 days. the compressive strength of samples made at 28 days of age at all levels of the fibers used was more than 100 MPa., the highest rupture stress relates to 28-day steel fiber reinforced specimens of 23. 9 MPa.

Pavement friction and skid resistance are some of the most important factors affecting road accidents frequency and severity. Texturing is using due to improve frictional properties of surface during the construction of concrete pavements. In this research, in order to create macrotexture on the concrete surface, techniques such as perpendicular and parallel brushing (according to traffic direction), turf and burlap dragging on fresh concrete, and grooving hardened concrete were used. Mean Texture Depth and Skid Resistance of concrete surface, were measured using Sand Patch and British Pendulum tests, respectively. The results showed that the pavement macrotexture affects skid resistance. For turf dragged, parallel brushed, and non-textured sample, R-squared of the obtained relation between British Pendulum Number and Mean Texture Depth was more than 0. 8. According to the results, among the created textures, perpendicular brushing of fresh concrete, with the average British pendulum number of 103. 5, reached the highest skid resistance and by increasing the Mean Texture Depth, the British Pendulum Number increases. The result also showed in non-textured samples increase/decrease in Mean Texture Depth result in high increasing/decreasing in British Pendulum Number.

Liquid storage tanks, as special structures, behave differently from ordinary structures in terms of dynamic behavior. These important structures are mostly constructed in two cylindrical and rectangular shapes, according to their use for storing various materials such as water and oil in the form of ground, elevated, buried and semi-buried. In this paper, an analytical method are presented for estimating the seismic responses of concrete cylindrical tanks subjected to horizontal and vertical earthquake. Effects of liquid-shell interaction on the dynamic response of cylindrical tanks, taking into account the flexibility of the wall that filled or partially filled has been studied. The velocity potential function with the satisfaction of boundary conditions is solved by separating the variables. The solution of the problem of the liquid-shell system is used to obtain the impulsive pressure using the Rayleigh-Ritz method, using the vibration modes of flexible shells (open top-bottom clamped) with suitable boundary conditions. Special attention was given to Cos-type vibration modes. The results of this proposed analytical method are compared with the results of previous studies. Then by using the proposed analytical method, a mechanical model is presented with considering the flexibility and mass of the tank wall for concrete cylindrical tank completely or partially filled with liquid. The results of proposed mechanical model and the accuracy of the resulting are compared with the analytical results and ACI 350. 3-06. Finally, the conclusion is that the estimating of the this standard are different from the analytical method, which is due to the inadequate consideration of the flexibility of the wall by this standard and the proposed mechanical model is in good agreement with analytical method in accuracy.

Regarding to advancement of civil engineering in recent decades, using high-strength concrete is so prevalent for constructing special projects. One of the main shortcomings of concrete is its tensile strength which causes cracking and brittle behavior. Today, to enhance concrete’ s performance and overcome this lack, a wide range of fibers including polymeric, steel, mineral and natural are used as well as classical solution for improving tensile strength of concrete: using steel bars. In this study, the effect of volume, length and shape of polyolefin fibers on mechanical characteristics and fracture properties of high-strength concrete is investigated. Three different fiber’ s length of 19, 30 and 50 mm are used at three different volume percentages of 0. 5, 1 and 1. 5 with two simple and reticular shapes. Conventional experimental tests of compressive strength, tensile strength, bending strength and modulus of elasticity are implemented to determine mechanical characteristics as well as U-notched prismatic beam to evaluate fracture properties. The results have shown that increasing the length of fiber enhances mechanical and fracture properties of concrete. Also, increasing the fibers’ volume causes an increasing the resistance time of concrete specimens against bending load. The simple-shape fiber shows higher bending strength in comparison with reticular fibers but fracture energy of reticular fibers is so greater than simple-shape fibers.

Silica aerogels as a nano material have significant properties including high porosity, high specific surface area, excellent heat insulation and low dielectric constant properties, low density and the potential to become an effective building insulation material. However, the implementation of silica aerogel as building insulation is restricted due to high manufacture in cost. Furthermore, despite having a relatively high compressive strength, it is very fragile due to its low tensile strength. In this study, carbon nanotube was used in order to overcome the undesirable properties of silica aerogel in concrete. Carbon nanotubes (CNTs) have been extensively studied as a reinforcement material for cement-based composites. The growing interest in this field comes from the desire to modify the cement matrix at the scale of their main compounds, taking advantage of the outstanding properties of CNTs. On the one hand, CNTs have ultra-high strength and stiffness, with Young’ s modulus up to 1 TPa and exceptional tensile strength in the range of 20– 100 GPa. On the other hand, they possess extremely high aspect ratios and surface are, as well as very low density. Therefore, CNTs are ideal candidates for cement nano-reinforcement, being potentially able to retain the propagation of small nano-cracks and improving some negative features of cement based materials, such as the low tensile strength and low strain capacity. In fact, CNTs have proven to enhance the fracture properties and early age strain capacity of cement pastes and mortars, reducing or preventing crack initiation. This study examined compressive strength, electrical resistivity, and penetration of chloride and thermal conductivity of concrete. To this end, 16 mix designs of concrete were considered and totally 192 samples were made. Silica aerogel was used in the amounts of 0, 4, 6, 8 percent of concrete and carbon nanotube was used in quantities of 0, 0. 03, 0. 05, . 07 percent weight of concrete. The results showed that combination use of silica aerogel and nanotube not only improves the compressive strength and durability of concrete but also enhances the performance of thermal conductivity of concrete. Also, the optimum percent of nanotube and silica aerogel in the mix design of concrete are 5% and 4% respectively.

Concrete and asphalt aggregates size distribution is one of the most important parameters in concrete and asphalt mix design that can affect the quality, durability, and strength of both concrete and asphalt. For evaluating the aggregates mix design, digital image analysis is a fast, reliable and indirect technique. In this study, based on one of the visual feature extractions methods (Gabor filters) and the neural networks, an algorithm was developed to determine the size distribution of digital images of concrete and asphalt aggregates. 100 images of concrete and asphalt aggregates were applied to train the neural network. Then, the results were compared with the results obtained by automatic estimation of aggregates size distribution by Split-Desktop software and sieving analysis. The results showed a general improvement in evaluating concrete and asphalt aggregates size distribution. Also, by using the proposed method, compare to automatic estimation of Split-Desktop, a reduction of 67% in error estimation was observed. Furthermore, this method showed also an improvement of 63% in evaluating of F10 to F100.

One of the important subjects in the concrete structure field is rehabilitation. The goal of rehabilitation is made equivalent between the structural capacity and the seismic demand of it. In some cases, the structural capacity should be increased so that it equals to the seismic demand. This increasing structural capacity is called strengthening. The strengthening is including two methods: By increasing stiffness (adding bracing, shear wall etc. ) or increasing strength (steel jackets etc. ). One method of strengthening is the approach of using a special concrete with high capabilities. Experimental studies have been carried lately about strengthening with HPFRCC on beams, columns, slabs and the other structural elements and the strength of this concrete has been confirmed in strengthening. In the following, the experimental study on strengthening of weak one way slabs with precast high performance fiber concrete laminates will be investigated. In this paper, we use HPFRC laminates. The location of laminates is the variable parameters, too. The aim is reaching the best method of strengthening weak one-way slabs with HPFRC. The experimental evidences show that using high-performance fiber concrete laminates is caused to increase bearing capacity (about two times larger than the reference slab), absorbed energy (about three times larger than the reference slab) and ductility (about twenty times larger than the reference slab). In addition, pasting the laminates into tensile, compression and sides' zone of the RC slabs has the best result.

Graphene Nano sheets have the planar structure. These nanoparticles are hydrophobia so the water molecules cannot adsorbed on graphene surface due to the absence of surface tension. Low quantity application of these materials due to high value of specific surface and low density can have significant effects on the concrete properties without having any considerable effect of concrete weight. However, because of hydrophobia nature of these nano particles, its uniform dispersion in concrete body is an essentially problem. In this research, it is attempted to study the effects of Graphene Nano sheets on the concrete properties. For this goal and due to non-uniform dispersion of these nanoparticles, graphene oxide nano sheets were first prepared from the nano graphene according to modified Hummer method and then, after uniform dispersion in concrete, graphene is recovered through heat. In this laboratory investigation the graphene oxide nano sheets are used in the concrete mix designs with ordinary and autoclaved curing condition. According to the obtained results from the x-ray analysis, the graphene oxide nano sheets are successfully converted to graphene nano sheets in the concrete by applying modified Hummer synthesizing method. The results show that the mechanical properties of the specimens which contain graphene oxide in the autoclaved curing condition, increases considerable. Also, the usage of these nanoparticles could improve the water absorption properties of concrete specimens.

In the present study, after obtaining an optimal mixing plan and making concrete samples, in order to investigate changes in the amount of replacements of diatomite instead of Silica Fume, with the aim of reducing the price of RPC production, six mixing plans were considered. These six mixing plans are containing 0, 25, 50, 75 and 100 percent diatomite powder, and the final design is the same as 100% diatomite powder with an increase of 14% water due to the consistency correction. The used diatomite powder is from Mamaghan mines, which is used without cook and raw and cheap. On the other hand, the available building materials collection and usable in RPC is increased. The flow table test is used for checking consistency and workability of RPC. Samples were cured in water at 25 and 50 degrees Celsius at the age of 7, 28 and 90 days, then pressure and tensile tests where done, and also water absorption at the age of 28 days and water absorption during the curing. The results indicate that the highest compressive strength for concrete with 25% diatomite powder is obtained, which compressive strength is 87. 2 MPa for 90 days. Silica Fume and Diatomite powder are both highly reactive and chemically active silicon. Therefore, we expected that there was no problem in terms of compressive strength and tensile strength during the above-mentioned substitutions. The results of the experiments clearly showed that diatomite powder could be used as a substitute for silica fume in the manufacture of RPC, and also the results indicated that the curing temperature had a significant effect on the initial resistance of RPC, so that with increasing temperature the compressive strength, tensile strength and water absorption also increase.