Wednesday, April 22, 2020

MECHANICAL PROPERTIES OF STEEL FIBER REINFORCED CONCRETE (SFRC)

MECHANICAL PROPERTIES OF STEEL FIBER REINFORCED CONCRETE (SFRC)



Toughness
Toughness is the total energy absorbed prior the complete separation of the specimen. It can be calculated as the area under the load-deflection curve plotted for beam specimen used in a flexure test. Although, it was well established that the steel fibers significantly improve concrete toughness and it is widely agreed that toughness can be used as a measure of the energy absorption of the material, there is a doubt about the way that SFRC toughness should be measured and used. Two methods to interpret and calculate the toughness of SFRC are widely used. The method in which the energy absorbed up to a certain specified deflection is normalized by the energy up to a point of fast cracking. The Japanese Institute of Concrete standards interprets the toughness in absolute terms, as the energy required to deflect the beam specimen to a mid-point deflection. The ASTM method evaluates the flexural performance of toughness parameters derived from SFRC in terms of areas under the load-deflection curve obtained by testing a simply supported beam under third-point loading. It provides for determination of a number of ratios called toughness indices that identify the pattern of material behavior up to the selected deflection. These indices are determined by dividing the area under the load-deflection curve up to a specified deflection by the area up to the deflection at first crack.

Abrasion and Skid Resistance

Knowledge of abrasion and wear resistance of concrete is essential especially for pavement due to the continuous nature of its loads. Difficulties might be encountered concerning of the wear and abrasion resistance, as the damaging action varies depending on the cause of wear, and no single test procedure is satisfactory in evaluating the resistance of concrete to the various conditions of wear.

                                                                         Skid Test

Tests on hydraulic structures, which have the same effect of wear on slabs under traffic loads, revealed that the abrasion resistance of SFRC is not improved over that of the plain concrete. Significant increases of abrasion resistance were found by other researchers, with about 15% higher resistance reported under drying, wet and frozen surface conditions.  Wear tests were carried out using a pair of hardened steel wheels running in a circular path under load on flat specimen slabs. It was found that for specific number of cycles, the SFRC exhibits average groove depths less than that of plain concrete, which in turn proves that the SFRC has a better wear resistance relative to an identical plain concrete. The skid resistance of SFRC was found to be same as that of the plain concrete at early stages prior the deterioration of the surface. In later stages, where abrasion and erosion of the surface had to take place, steel fiber reinforced concrete has an up to 15 % higher skid resistance relative to plain concrete. It can be concluded that the SFRC has better performance regarding its erosion, abrasion and skid resistance, but how much better is dependent on the case of application. 

Thermal Properties and Fire Resistance


                                                                                                        Fire resistance of concrete

There are three thermal properties that may be significant in the performance of concrete, coefficient of thermal expansion, specific heat and conductivity. Thermal expansion is seen to be the most relevant to the ground slabs applications especially for concrete subjected to thawing and freezing action. Specific heat and conductivity are normally relative to applications whereby thermal insulations are provided, or other applications such as rocket launch facilities or mass structures.
The effect of steel fibers on coefficient of expansion factor was studied using beam specimens that have various steel fibers content (ranges between 0 and 2 % by volume). Specimens were subjected to temperatures ranges between 38 and 66 degrees Celsius. Tests results indicate that the coefficient of thermal expansion factor was not significantly affected by fiber content. Tests on relatively dry SFRC specimens at ages of about 220 to 250 days and 27-degree Celsius temperature rise, revealed that addition of steel fibers marginally influence the thermal expansion coefficient. Just to give an indication, for SFRC containing 75 kg/m3 of enlarged-end steel fibers, the typical expansion coefficient is found to be 8.2 x 10-6 per degree Celsius. It can be seen from the above discussion that the expansion of SFRC is the same (if not less) than plain concrete for identical mixes. The author's opinion is that, the only hazard is the expansion coefficient of the steel fibers, in other words, large differences between thermal coefficients of steel fibers and paste might cause the interface layers between them to damage and damage in many surfaces in different dimensions might weaken the entire matrix.

Electrical Conductivity    


                                                                                                                       Electrical Conductivity
Steel fibers contents of up to 1 % by volume (80 kg/m3) has no significant effect on electrical conductivity, hence, wire guided vehicles may be operated without difficulties on SFRC floors, which can be taken as an advantage if compared with steel bars or mesh floors. It can also be beneficial where traffic devices are needed e.g. vehicle detection loops for traffic counting and classification. 

Durability

Porosity and permeability are primary factors affecting the durability of the concrete due to its effect on alkali-acid reaction, leaching characteristics, resistance to chloride or sulphate attack, reinforcement corrosion, and freezing and thawing characteristics. Initially SFRC mixes had high porosities and permeability due to the higher W/C used to increase the workability. Recently, reductions in W/C ratio are possible, which result in relatively low porosities and permeability’s. Tests indicated that the SFRC has permeability values typical of those for the plain concrete, therefore, apart from corrosion of steel fibers, the SFRC has the same durability (if not better) than the identical plain concrete.
Attention has to be given to the question of the corrosion of the steel fibers when added to concrete. Theoretically, one of the main problems associated with the use of steel fibers is their durability in concrete structures. In severe exposure condition, corrosion of steel fibers is more aggravated than that of steel bars, in other words, a significant decrease to the steel fibers diameter, contribute significantly to lessen the load capacity of the structure at service. In contrast, unlike steel bars, only limited expansion force develops due to the corrosion of steel fibers [14], which means less paste disruption and eventually minimal breakdown and weathering rates in comparison to conventional concrete reinforced by steel bars.

                                                             Concrete Durability
There is evidence that in practice, in good quality concrete, fibers corrosion does not penetrate into the concrete. Laboratory studies have shown that, stainless steel fibers can perform well even in a very aggressive type of exposure conditions while the carbon steel fibers invite the corrosion and cracks development. SFRC specimens exposed to a marine environment for about 10 years, show that the corrosion of fibers is limited to the surface of the un-cracked specimens and no noticeable reduction in flexural strength was found, whilst, for cracked specimens, corrosion does occur through the depth of the crack and reduction on flexural strengths were encountered. Under normal finishing processes very few fibers will be left exposed at the surface of slabs and any such fibers exposed to the surface is assumed to corrode and blow away under trafficking. It was found that the corrosion depth is usually confined to the first   5 mm, therefore, designs should consider cover depths of about 10 mm apart from recommending the knocking down of steel fibers while finishing the concrete surface. 



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