INFLUENCE OF MIXTURE DESIGN, AGE, AND COOLING REGIME ON POSTFIRE MECHANICAL PROPERTIES OF LIGHTWEIGHT SELF-COMPACTED CONCRETE

The post-fire mechanical properties of concrete represent an important set of parameters in the analysis of structural behavior after the occurrence of a fire. In this study, four mixtures of lightweight selfcompacted concrete with different fillers were tested. The tests were conducted by considering the ages of the specimens, which, in this study, were two, three, and five years old. The results show the residual mechanical properties of the specimens after they were heated to target temperatures of 300, 500, and 700 °C. After exposure to high temperature, the specimens were either left to gradually cool until they reached room temperature or rapidly cooled by water. A comparative analysis was conducted on the compressive strength, mass, and dynamic modulus of elasticity of the specimens using the codified residual values given in EN 1994 1-2

Mechanism of vibratory action in the fresh and plastic concrete

Opisan je konstruirani vibracijski reometar koji omogućava kontinuirano zapisivanje ubrzanja klipa aparature koji oscilira u uzorku materijala. Dano je analitičko rješenje osciliranja klipa u pseudoplastičnom fluidu kojim se aproksimira ponašanje plastičnog svježeg betona. Oko svakog zrna stvara se sloj materijala približnih svojstava Newtonovu fluidu, čime se smanjuju otpori preoblikovanju i zbijanju uzorka. Opisano je djelovanje vibracije na plastični beton i slične materijale.The designed oscillatory rheometer, enabling continuous recording of acceleration data for piston oscillating in the sample, is described. An analytical solution for piston oscillation in the pseudoplastic fluid, approximating behavior of the fresh and plastic concrete, is presented. A material coating, with properties similar to those of a Newtonian fluid, forms around every grain and hence reduces sample resistance to remodeling and compaction. The influence of vibratory action on plastic concrete and similar materials is described

UTJECAJ STAROSTI I SKLADIŠTENJA CEMENTA NA NJEGOVU KVALITETU

Cement koji je nepropisno uskladišten nakon godine dana gubi i do 50% svoje čvrstoće. U radu su ispitana svojstva uskladištenih uzoraka cementa starih 30 godina. Radi usporedbe, ispitana su i dva nova cementa: jedan loše uskladišten, a drugi uzet iz tek otvorene vreće. Rezultati ispitivanja uspoređeni su s vrijedećim standardima i referentnim rezultatima iz vremena proizvodnje cemenata

Possibility of cement composite reinforcement by Spanish broom fibres

Danas se u svijetu posvećuje velika pozornost upotrebi celuloznih vlakana u kompozitnim materijalima. Pokušava se pronaći što bolja, ekološki prihvatljiva, jeftinija, obnovljiva i po mogućnosti lokalno dostupna zamjena klasičnim sintetičnim vlaknima. Kvaliteta prirodnih vlakana uvelike ovisi o njihovom porijeklu, načinu dobivanja, sastavu i fizikalnim svojstvima. U radu su dani rezultati ispitavanja mogućnosti upotrebe vlakna samonikle biljke brnistre ubrane na lokalnom području za potrebe ojačanja cementnog morta. Dobiveni rezultati pokazuju da kvaliteta vlakana brnistre ne zaostaje za poznatim vlaknima konoplje i lana.A considerable attention is currently paid worldwide to the use of cellulose fibres in composite materials. Attempts are made to find even better, environmentally more acceptable, less expensive, renewable and, if possible, locally available replacement for traditional synthetic fibres. The quality of natural fibres is greatly dependent on their origin, processing requirement, composition, and physical properties. Results obtained by testing possibilities of using fibres of the wild plant called Spanish broom, collected locally for cement mortar reinforcement, are presented in the paper. The results show that the quality of Spanish broom fibres is not inferior to the quality of known hemp and flax fibres

Mechanism of vibratory action in the fresh and plastic concrete

Opisan je konstruirani vibracijski reometar koji omogućava kontinuirano zapisivanje ubrzanja klipa aparature koji oscilira u uzorku materijala. Dano je analitičko rješenje osciliranja klipa u pseudoplastičnom fluidu kojim se aproksimira ponašanje plastičnog svježeg betona. Oko svakog zrna stvara se sloj materijala približnih svojstava Newtonovu fluidu, čime se smanjuju otpori preoblikovanju i zbijanju uzorka. Opisano je djelovanje vibracije na plastični beton i slične materijale.The designed oscillatory rheometer, enabling continuous recording of acceleration data for piston oscillating in the sample, is described. An analytical solution for piston oscillation in the pseudoplastic fluid, approximating behavior of the fresh and plastic concrete, is presented. A material coating, with properties similar to those of a Newtonian fluid, forms around every grain and hence reduces sample resistance to remodeling and compaction. The influence of vibratory action on plastic concrete and similar materials is described

Cotton knitted fabric waste as reinforcement in cement screed

Cement screed consists of cement, sharp sand and water, laid as a thin layer over the concrete subfloor. Although the screed is strong, it can be additionally reinforced with fibres, most often polypropylene fibres. In this study, cotton knitted fabric waste (CKFW) was obtained from a local factory producing underwear to reinforce the cement screed. A total of eight mixtures were made: reference mixture, screed reinforced with polypropylene fibres and six mixtures reinforced with CKFW. CKFW was added in 1,25 %, 2,50 % and 5,00 % of the total volume. Before adding to the mixtures, the first CKFW group was saturated with tap water, whereas, the second CKFW group was saturated with dispersion that improves adhesion. The density and air content of the fresh mixtures were tested, and the compressive strength and flexural strength were determined when the specimens were 28 days old. The specific fracture energy is determined based on the curve from the plot of load vs. displacement. The CKFW increases the ductility of cement screeds, although it has no significant effect on the compressive and flexural strength of the specimens. The CKFW that was saturated with tap water achieved better results

EFFECT OF FILLER FROM RECYCLED CONSTRUCTION WASTE ON PROPERTIES OF SELF-COMPACTING CONCRETE IN FRESH AND HARDENED STATES

Construction waste is becoming a growing concern in the modern world. Waste recycling reduces the need for landfills and preserves natural resources. This paper investigates the possibility of using construction waste in self-compacting concrete. Fillers of various particle sizes were prepared by grinding several types of waste directly without any sorting or separation. Eighteen mixtures of concrete were prepared by varying the types and proportions of fillers and other additives, and they were tested for workability in fresh state, compressive strength, and dynamic modulus of elasticity after 7 and 28 days of hardening. Eventually, the mixtures were also tested for water absorption. Results indicated that fresh concrete samples did not meet all the requirements of self-compacting concrete, and therefore guidelines were proposed for improving the composition of concrete

Determination of rheological properties of fresh concrete and similar materials in a vibration rheometer

A vibration rheometer has been developed for the purpose of determining the viscosity coefficient and the yield value of fresh concrete under vibration. The main parts of the apparatus, a test specimen and a vibration source form a unitary oscillatory system whose parameters can be measured with sufficient precision. Two types of fine grained reference material have been prepared and examined; one with a high coefficient of viscosity and the other with a high yield value. The rheological properties of reference materials have been determined in a capillary tube viscometer. Since there is no analytical solution to the flow in the vibration rheometer, the constants of the vibration rheometer have been determined by experiment, for each position of the apparatus piston within the measuring range. The parameters of the flow depend on the maximum acceleration of the vibration source. An increase in acceleration causes an increase in the yield value as well as a reduction in the plastic viscosity coefficient of the material specimen. A testing of fresh mortar has been carried out as well. The obtained results have been compared with the impact on reference materials, which makes the results of our research applicable to fresh vibrated concrete

Experimental Testing of the Effects of Fine Particles on the Properties of the Self-Compacting Lightweight Concrete

The self-compacting lightweight concrete (SCLC) is a combination of the Self compacting concrete (SCC) and the Lightweight concrete. It combines all the good properties of those two materials and is extremely convenient for the construction of buildings that require low mass and do not require high compressive strength, for example restoration works in old structures (e.g., replacement of wooden floors), prefabricated elements that require transportation, and for structures and elements where the concrete surface should be visible. In this paper the effect of the amount of fine particles on the properties of the self-compacting lightweight concrete (SCLC) in the fresh and hardened state was explored. For this purpose, sets of specimens with different combinations of admixtures of silica fume, fly ash, and filler were prepared and tested. Slump flow and flow time of fresh concrete, as well as the dynamic elastic modulus and compressive strength of hardened concrete, were measured at different ages of concrete. The processes of manufacturing and methods of testing are described, as well as the obtained results