Hidro-abrazivna otpornost betona spravljenog sa recikliranim materijalima i ojačanog mikrovlaknima

Durability of hydraulic engineering structures in most part depends on the resistance of concrete surface to mechanical abrasion. Hydro-abrasion represents a surface damage caused by continuous removal of material due to impact of waterborne hard particles. This form of progressive deterioration of concrete surface occurs, to a varying extent, in almost all hydraulic engineering structures. Therefore, hydro-abrasion wear of concrete in general causes reduction of the service life of a hydraulic engineering structure, as well as an increase in operating costs due to the necessary maintenance, and the downtime of the structure during the repair period. In general, little is known about the onset of concrete hydro-abrasion and about development of this damage in time. Presently, there is no standard testing method for hydro-abrasive resistance of concrete in laboratory conditions, which can to a great extent simulate the natural abrasion conditions. Also, the factors representing the physical, mechanical and structural concrete characteristics, and which affect its resistance to wear have not been sufficiently investigated. In the literature is discussed mostly the hydroabrasive resistance of high-strength concretes made with natural crushed or river aggregate, which are reinforced by addition of the silica powder or micro-reinforcement, with a high quantity of cement (higher than 400 kg/m3). The potential for application of recycled materials and reduced quantity of cement for production of the concretes resistant to hydroabrasion is presented in a limited scope. The paper investigates and analyzes the potential for production of cement concrete in which the cement content has been partially replaced by fly ash or silica powder, and where fine river aggregate has been partially replaced by recycled granulated rubbers, with the addition of micro-fiber (steel or polypropylene), with the aim to increase resistance to hydro-abrasive action. Performances of these concretes in fresh and hardened state were compared with the performances of reference concrete without the partial replacement of cement and fine aggregate and without micro-fibers. The tests of hydro-abrasive resistance of concrete was performed on the original apparatus constructed in the Laboratory of building materials at the Faculty of Civil Engineering and Architecture of Niš, which facilitates that the jet of water and sand mixture impacts the concrete tests slab at a high speed. The parametric analysis of the effects of individual mechanical characteristics on the hydro-abrasive resistance of concrete was done. For the purposes of the research, a total of eighteen types of concrete was made, which can be divided into three groups of six mixtures. The first group consists of the concrete mixes where no partial substation of cement was performed, the second group consists of the concrete mixtures where cement was substituted by filter ash, in the amount of 20% in respect to the cement mass, while the third group consists of the concrete mixes where the cement was substituted with the silica powder in the amount of 10% in respect to the cement mass. Within each group, there is a mixture where a part of fine river aggregate was replaced by the granulated recycled rubber in the amount of 10% in respect to the aggregate volume, followed by two concrete mixtures where polypropylene fibers have been added (monofilament to one and fibrillated to the other mixture), then two mixtures where hooked end steel fibers were (in one those are fibers 30 mm long and in the other those are fibers 50 mm long) and finally, there is the concrete mix without partial replacement of the fine river aggregate with recycled rubber and without the addition of fibers. The conclusions are formulated based on the processed and systematized results of the experimental research, and the performed parametric analysis of the effects of individual mechanical characteristics on the hydro-abrasive resistance of concrete. In general, the most intensive effect regarding increase of hydro-abrasive resistance of concrete is achieved by the partial replacement of the fine river aggregate by the recycled granulated rubber. Also, concrete reinforcement with polypropylene fibers contributes to the considerable increase of hydro-abrasive resistance of concrete, whereby the better effect is achieved using the fibrillated fibers. The addition of steel fibers in concrete does not result in a considerable increase of the hydro-abrasion resistance. The concretes made with the partial replacement of cement by fly ash have lower hydro-abrasive resistance in comparison to the reference concrete, while with the addition of polypropylene fibers, their resistance becomes approximately equal to the resistance of the reference concrete. Partial replacement of cement with silica powder contributes to the increase of hydro-abrasive resistance of concrete. The parameters such as compressive strength, flexural strength, splitting tensile strength, tensile strength determined using „Pull-off“ method, static modulus of elasticity and resistance to abrasion wear can be the factors for prediction of hydro-abrasive resistance of concrete, due to the existence of strong. The behavior of concrete exposed to hydroabrasive action cannot be fully predicted based on the „Drop weight“ tests of the impact resistance of concrete. The accelerate abrasion test method implemented in the research is suitable for analysis of hydro-abrasive resistance of concrete

Possibilities of Biomass Wood Ash Usage in Geopolymer Mixtures

This paper presents the results of testing the physical and mechanical properties and durability of geopolymer mixtures. Fly ash was used as the basic binder, while its replacement was performed with wood biomass ash.The initial and final setting time were tested on the geopolymer paste, while flow value and bulk density were tested on the fresh mortar. Compressive and flexural strength and sulfate and freeze-thaw resistance were tested on hardened geopolymer mortar. Based on the most optimal results of mortar testing, the composition of concrete was determined, and slump, bulk density, compressive and flexural strength, penetration of water under pressure and sulfate and freeze-thaw resistance were tested. According to the results of compressive strength tests of geopolymer materials and concrete exposed to sulfate solution attack, the mixtures prepared with the addition of 20% wood biomass ash showed better sulfate resistance than the mixtures prepared only with fly ash

HIGH STRENGTH CONCRETES BASED ON THE CHOICE OF THE BEST PARTICLE SIZE DISTRIBUTION IN AGGREGATE

The requirements of the modern (high-end) construction industry demand the development of new types of concrete of high, and especially very high strength and with significantly improved properties in terms of durability. They provide new possibilities in the field of concrete technology of high strength and performance. When designing the composition of high-strength concrete (HSC), a special attention should be paid to the particle size distribution of aggregates, which should be chosen so as to achieve an "optimal" packing of the aggregate grains. The maximum grain size has been reduced to 2 mm. The Funk-Dinger formula was used to calculate the particle size distribution, which also takes into account fine particles of mineral powder additives. CEM I 52.5R, pure quartz sand, quartz filler, silica fume, powerful superplasticizer and low water/binder ratio were chosen for making HSC. In total, five different concrete mixtures were made. The paper presents the results of testing important properties of hardened concrete at ages from 1 day to 90 days and statistical processing of the obtained test results

Properties of Self-compacting Concrete Produced with Biomass Wood Ash

The demand of the contemporary society for renewable energy sources lead to the increase of the bio-power plants. Accordingly, the amount of ash generated by burning the biomass is increased, and its disposal becomes a large environmental problem. The paper presents the research of potential use of biomass wood ash as a partial replacement for coal fly ash (10%, 20%, 30% and 40% of mass) in production of self-compacting concrete (SCC). The effects of biomass wood ash on the properties of SCC in fresh and hardened states have been examined, as well as on the properties of durability. Test results indicated that the biomass wood ash slightly reduces the flowability and passing ability of SCC, while its addition enhances the viscosity of SCC and significantly prevents segregation and bleeding. SCCs with the contents of biomass wood ash up to 20% have approximately same mechanical strength as the reference mixture. Biomass wood ash has no negative effect on the resistance of concrete to the action of water under pressure, but a decrease of freeze/thaw resistance with de-icing salt is detected as its contents increases. The addition of biomass wood ash into SCC increases the drying shrinkage in the initial period of drying (up to 14 days), and it is decreased in a later phase

PROPERTIES OF STEEL-POLYPROPILENE HYBRID FIBERS REINFORCED CONCRETE

This paper present the results of mechanical properties of hybrid reinforced concrete made by adding polypropylene and steel fibers into concrete mixture. For the testing purposes were used steel fibers with hooked ends and monofilament polypropylene fibers. The total of 5 batches of concrete were made: concrete with addition of steel fibers, polypropylene fibers and their combination in amount of 0,5 % of the concrete volume. The test results show that concretes made by adding of 0.4% steel and 0.1% polypropylene fibers have better performance compared to other concretes

Morfološka i molekularna identifikacija izolata Alternaria alternata - patogena plodova kima u Srbiji

During 2013, samples of infected seed grown caraway were collected from commercially grown crops in the experimental field in Mošorin and analyzed for the presence of plant pathogenic fungi. In all samples of seeds, weaker germination and a high seed infection 100%, with phytopathogenic fungi of the genus Alternaria, was found. From the infected caraway seed, monosporial cultures, based on morphology, were identified as Alternaria alternata, and their pathogenicity proved on artificially inoculated caraway and parsley seedlings. Molecular identification was performed utilizing polimerase chain reaction (PCR) with primer pair ITS1/ITS4 and by amplification of the ITS region of ribosomal DNA. Gene sequences of selected isolates CC1 (KP822948), CC2 (KP822949) i CC3 (KP822950) exhibited 100% nucle­otide identity with the sequences of 31 isolates of A. alternata deposited in the GenBank. The presence of a large number of phytopathogenic fungi on caraway seed should be further investigated in order to clarify their relationships and relative significance.Tokom 2013. godine, na oglednom polju u Mošorinu, prikupljeni su uzorci zaraženog semena gajenog kima i analizirani na prisustvo fitopatogenih gljiva. U svim uzorcima semena, ustanovljena je slabija klijavost i visok stepen zaraze fitopatogenim gljivama iz roda Alternaria, 100%. Iz zaraženog semena izolovane su monosporijalne kulture, čija je patogenost potvrđena pojavom simptoma na veštački inokulisanim klijancima kima i peršuna, a na osnovu morfoloških svojstava identifikovane su kao Alternaria alternata. Molekularna identifikacija obavljena je primenom lančane reakcije polimeraze (PCR, Polymerase Chain Reaction) uz korišćenje prajmera ITS1/ITS4 i amplifikaciju ITS regiona ribozomalne DNK. Sekvence gena odabranih izolata CC1 (KP822948), CC2 (KP822949) i CC3 (KP822950) pokazale su 100% nukleotidne identičnosti sa sekvencama 31 izolata A. alternata deponovanih u GenBank bazi podataka. Prisustvo većeg broja vrsta gljiva na semenu kima, zahteva dalja ispitivanja njihovih međusobnih odnosa i relativnog značaja

Impact of crushed mineral aggregate on the pumpability of concrete during transport and placement

In the spirit of the sustainable buildings, and with the goal of protection of river courses, in the near future an already announced directive ordering closing down of a large number of river aggregate dredging operations will be adopted. For that reason, usage of crushed mineral aggregate in concrete mixes is increasing. Irrespective of downsides of the fined crushed mineral aggregate, such as the presence of fine particles bordering the upper permissible limit and the unfavorable shape of the grain of the course aggregate for obtaining liquid consistency required for the pumpable concrete, the demanded pumpability of concrete during transport and placement has been achieved. By adding admixtures to concrete, the required concrete properties, such as: frost resistance, simultaneous frost and salt resistance and water tightness have been achieved

The effect of intravitreal administration of bevacizumab on macular edema and visual acuity in age-related macular degeneration with subfoveolar choroidal neovascularisation

Background/Aim. Age-related macular degeneration (AMD) is a leading cause of the loss of central visual acuity in population older than 70 years. We can distinguish wet and dry form of AMD. The aim of the study was to present our early results in treatment of the wet (neovascular) form of AMD with intravitreal administration of bevacizumab. Methods. The study included 39 patients. Each patient underwent a complete ophthalmological examination, fluorescein angiography (FA) and optical coherence tomography (OCT). All the patients received 1.25 mg of intravitreal bevacizumab (0.05 mL of commercial phial of Avastin®). The total of three doses was given with a one-month interval between doses. Results. Among 39 patients, 24 were women and 15 men. The average best corrected visual acuity (BCVA) was improved from 0.09 before the therapy to 0.24 after the administration of all the three doses of bevacizumab (p < 0.001). The average central macular thickness (CMT) measured by OCT was improved from 474 μm in the beginning to 341 μm after the administration of all the three doses of the drug (p < 0.001). There were no side effects. Conclusions. Our short-term experience indicates that intravitreal administration of three doses of bevacizumab in one-month intervals between the doses leads to a significant reduction of macular edema and improvement of BCVA in patients with neovascular AMD

Influence of used waste cathode ray tube glass on alkali silicate reaction and mechanical properties of mortar mixtures

Rapid transition of electronic device manufacturing industry has led towards the increase of glass waste quantities, which are still being speculated. This resulted in increasing research on the use of waste glass in many different industries. In this study, the impact of using grounded waste cathode ray tube (CRT) glass as aggregate replacement (AR) on the alkali-silica reaction (ASR), mechanical properties and structure and microscopy of mortar were examined and reported. Crushed waste CRT aggregate was used to replace natural limestone aggregate of 0, 25, 50, 75 and 100% in mortar bars. ASR expansion values of mortar with added waste glass were investigated and tested for observation period according to Ultra-accelerated mortar-bar test. The results showed that the increase of AR percentage resulted in higher susceptibility to ASR. Mechanical properties and microscopy of mortar mixtures showed the potential of using waste CRT glass, due to the small difference between tested mixtures