HISTORY OF CONCRETE

HISTORY OF CONCRETE

                                                           Fig: Portland cement

The time period during which concrete was first invented depends on how one interprets the term “concrete.”  Ancient materials were crude cements made by crushing and burning gypsum or limestone. Lime also refers to crushed, burned limestone. When sand and water were added to these cements, they became mortar, which was a plaster-like material used to adhere stones to each other.  Over thousands of years, these materials were improved upon, combined with other materials and, ultimately, morphed into modern concrete. Today’s concrete is made using Portland cement, coarse and fine aggregates of stone and sand, and water.  Admixtures are chemicals added to the concrete mix to control its setting properties and are used primarily when placing concrete during environmental extremes, such as high or low temperatures, windy conditions, etc. The precursor to concrete was invented in about 1300 BC when Middle Eastern builders found that when they coated the outsides of their pounded-clay fortresses and home walls with a thin, damp coating of burned limestone, it reacted chemically with gases in the air to form a hard, protective surface. This wasn’t concrete, but it was the beginning of the development of cement.

                                     Fig:  History of concrete

Early composite materials typically included mortar-crushed, burned limestone, sand and water, which was used for building with stone, as opposed to casting the material in a mold, which is essentially how modern concrete is used, with the mold being the concrete forms. As one of the key constituents of modern concrete, cement has been around for a long time. About 12 million years ago in what is now Israel, natural deposits were formed by reactions between limestone and oil shale that were produced by spontaneous combustion. However, cement is not concrete. Concrete is a composite building material and the ingredients, of which cement is just one, have changed over time and are changing even now. The performance characteristics can change according to the different forces that the concrete will need to resist. These forces may be gradual or intense, they may come from above (gravity), below (soil heaving), the sides (lateral loads), or they might take the form of erosion, abrasion or chemical attack. The ingredients of concrete and their proportions are called the design mix.

HISTORY OF STEEL FIBER REINFORCED CONCRETE

A French gardener by name Joseph Monier first invented the reinforced concrete in the year 1849. If not for this reinforced concrete most of the modern buildings would not have been standing today. Reinforced concrete can be used to produce frames, columns, foundation, beams etc. Reinforcement material used should have excellent bonding characteristic, high tensile strength and good thermal compatibility. Reinforcement requires that there shall be smooth transmission of load from the concrete to the interface between concrete and reinforcement material and then on to reinforcement material. Thus the concrete and the material reinforced shall have the same strain.

                                                                                      Fig:  Steel Fiber reinforced concrete

The steel bars are reinforced into the concrete. The bars have a rough, corrugated surface thus allowing better bonding with steel rebar’s the concrete gets extra tensile strength. The compression strength, bending also shown marked improvement thermal expansion characteristic of steel rebar’s and concrete shall match. The rebar shall have cross sectional are equal to 1% for slabs and beams, this can be 6% in case of columns. The concrete has alkaline nature, this forms a passivating film around the bars thereby protecting it from corrosion. This passivating film will not form neutral or acidic condition. Carbonation of concrete takes place along with chloride absorption resulting in failure of steel rebar. By comparing the tension capacity of steel bars and concrete + steel reinforcements the reinforced concrete can be called as under reinforced (tensile capacity of bars in less than concrete + bar) it is over reinforced (tensile capacity of steel is greater than concrete + steel tensile strength. The over reinforced fails without giving prior warning and under reinforced fails but gives a deformation warning before it fails. Therefore it is better to consider an under reinforced concrete. The long process of inventing modern steel fiber reinforced concrete started in 1874, when A. Bernard, in California, patented the idea of strengthening concrete with the help of the addition of steel splinters (Maidl 1995). Another 36 years passed before Porter in 1910 mentioned the possibility of applying short wire to concrete. This was supposed to improve homogeneity of concrete reinforced by thick wire. In 1918, in France, H. Alfsen patented a method of modifying concrete by long steel fibers, long wooden fibers, and fibers made of other materials. According to him, the addition of such fibers was to increase tensile strength of concrete (Maidl 1995). Alfsen was the first to mention the influence of coarseness of the surface of fibers onto their adhesiveness to matrix, and it was also he who paid special attention to the problem of anchorage of fibers. After these first patents, there were numerous others, but generally they concerned different shapes and probable applications of readymade SFRC.

                                                                                                       Fig:  Concreting

For instance, the patent from 1927 worked out in California by G.C. Martin, regarded the production of SFRC pipes. In 1938, N. Zitkewic patented a way to increase the strength and impact resistance of concrete by adding cut pieces of steel wire (Jamrozy 1985). Steel fibers, patented in 1943 by G. Constancinesco, were already very similar to the ones used at present. The patent, apart from different shapes of fibers, contained information about the kind and dispersion of cracks during loading of SFRC elements and it made mentioned of the great amount of energy which is absorbed by SFRC under impact. The largest number of patents concerning the use of steel fibers to modify concrete have been submitted in the USA, France, and Germany in the years following. Wide applications of fiber reinforced composites in civil engineering were limited for a long time by lack of reliable methods of examination and mainly by the sudden progress of traditional rod reinforcement.  Concrete is most widely used construction material in the world due to its ability to get cast in any form and shape. It also replaces old construction materials such as brick and stone masonry. The strength and durability of concrete can be changed by making appropriate changes in its ingredients like cemetitious material, aggregate and water and by adding some special ingredients. Hence concrete is very well suitable for a wide range of applications. However concrete has some deficiencies as listed below:

1) Low tensile strength

2) Low post cracking capacity

3) Brittleness and low ductility

4) Limited fatigue life

5) Incapable of accommodating large deformations

6) Low impact strength

The presence of micro cracks in the mortar-aggregate interface is responsible for the inherent weakness of plain concrete. The weakness can be removed by inclusion of fibers in the mixture. Different types of fibers, such as those used in traditional composite materials can be introduced into the concrete mixture to increase its toughness, or ability to resist crack growth. The fibers help to transfer loads at the internal micro cracks. Such a concrete is called fiber-reinforced concrete (FRC). The concept of using fibers in order to reinforce matrices weak in tension is more than 4500 years old.

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APPLICATION OF SFRC

APPLICATION OF SFRC

Steel Fiber Reinforced Concrete for Industrial Flooring

Warehousing demand is increasing day-by-day particularly in developing. Warehouse floor is important element of whole structure and floor should have particular properties to service the needs. The features desired for warehouse floors are - ’Vertical storage racks, material handling equipment, impacts & abrasions during material handling, long term serviceability need, etc. SFRC floors are now choice for Warehouses in India like elsewhere in world. Concrete is multifaceted material; it can derived to chosen properties by technology inductions.

There are so many ways to enhance concrete properties to suit desired properties. Thanks to technological developments. Warehouse floor demands hard, durable, tough, smooth floor with long term serviceability. As normal concrete can’t have these properties, advancements like reinforced concrete, Ferro cements, fiber reinforced (poly & metal fibers); polymer concrete has evolved over period of time. 

Steel Fiber Reinforced Concrete Pavement

Steel fibers have used in concrete since the early 1900s.The early fibers were round and smooth and the wire was cut or chopped to the required lengths. The use of straight, smooth fibers has largely disappeared and modern fibers have either rough surfaces, hooked ends or are crimped or undulated through their length. Modern commercially available steel fibers are manufactured from drawn steel wire, from slit sheet steel or by the melt-extraction process which produces fibers that have a crescent-shaped cross section. Steel fibers have equivalent diameters (based on cross-sectional area) of from 0.15 mm to 2 mm and lengths from 7mm to 75 mm.

Aspect ratios generally range from 20 to 100. (Aspect ratio is defined as the ratio between fiber length and its equivalent diameter, which is the diameter of a circle with an area equal to the cross-sectional area of the fiber). Steel fibers have high tensile strength (0.5–2GPa) and modulus of elasticity (200GPa), a ductile/plastic stress-strain characteristic and low creep. Concretes containing steel fiber have shown to have substantially improved resistance to impact and greater ductility of failure in compression, flexure and torsion. It has been extensively used for overlay roads, airfield pavements and bridge decks.

SPRAYED CONCRETE

The addition of fibers increases the ductility of sprayed con­crete. For instance, if the sprayed concrete lining of an exca­vated tunnel support is cracked due to high flexural stresses, the fibers can accommodate the tensile forces and act as an excellent yielding support. This interaction between sprayed concrete and fibers, therefore also increases the mechanical capacity of the lining. The reinforcement can then be reduced or light reinforcement can be omitted completely. The result is quicker and cheaper tunnel excavation supports.

FIRE PROTECTION

                                 

Steel fiber make concrete very much more fire resistant. The fibers are added to the concrete mix during its production. If a fire breaks out, e.g. in a tunnel, the synthetic fibers melt within the concrete and this creates a capil­lary system through which the water vapor pressure can be relieved. Concrete spalling is prevented or very significantly reduced, as are any necessary repairs, whilst increasing the durability, stability and safety of the structure.

PRECAST CONCRETE

The use of fibers in precast concrete results in lighter and more economic units because the possible reduction in steel reinforcement saves weight and reduces production time. The homogeneous distribution of the fibers throughout the concrete cross-section also gives high impact resistance right to the edges and corners. This allows secure installation on site without damage and with the use of synthetic fibers there is no hidden risk of injury to workers during production or installation.

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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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