Light weight concrete
One of the main advantages of conventional concrete is the self weight of concrete. Density of normal concrete is of the order of 2200 to 2600. This self weight will make it to some extend an uneconomical structural material.
- Self weight of light weight concrete varies from 300 to 1850 kg/m3.
- It helps reduce the dead load, increase the progress of building and lowers the hauling and handling cost.
- The weight of building on foundation is an important factor in the design , particularly in case of weak soil and tall structures. In framed structure , the beam and column have to carry load of wall and floor. If these wall and floor are made of light weight concrete it will result in considerable economy.
- Light weight concrete have low thermal conductivity.( In extreme climatic condition where air condition is to installed the use of light weight concrete with low thermal conductivity is advantageous from the point of thermal comfort and low power consumption.
- Only method for making concrete light by inclusion of air. This is achieved by a) replacing original mineral aggregate by light weight aggregate, b) By introducing gas or air bubble in mortar c) By omitting sand fraction from concrete. This is called no – fine concrete.
- Light weight aggregate include pumice, saw dust rice husk, thermocole beads, formed slag. Etc
- Light weight concrete aggregate exhibit high fire resistance.
- Structural lightweight aggregate’s cellular structure provides internal curing through water entrainment which is especially beneficial for high-performance concrete
9. lightweight aggregate has better thermal properties, better fire ratings, reduced shrinkage, excellent freezing and thawing durability, improved contact between aggregate and cement matrix, less micro-cracking as a result of better elastic compatibility, more blast resistant, and has better shock and sound absorption, High-Performance lightweight aggregate concrete also has less cracking, improved skid resistance and is readily placed by the concrete pumping method
- Aerated concrete is made by introducing air or gas into a slurry composed of Portland cement.
- No fine concrete is made up of only coarse aggregate , cement and water.These type of concrete is used for load bearing cast in situ external walls for building. They are also used for temporary structures because of low initial cost and can be reused as aggregate.
High density concrete
- The density of high density concrete varies from 3360 kg/m3 to 3840 kg/m3.They can however be produced with density upto 5820 kg/m3 using iron as both fine and coarse aggregate.
2. Heavyweight concrete uses heavy natural aggregates such as barites or magnetite or manufactured aggregates such as iron or lead shot. The density achieved will depend on the type of aggregate used. Typically using barites the density will be in the region of 3,500kg/m3, which is 45% greater than that of normal concrete, while with magnetite the density will be 3,900kg/m3, or 60% greater than normal concrete. Very heavy concretes can be achieved with iron or lead shot as aggregate, is 5,900kg/m3 and 8,900kg/m3 respectively.
- They are mainly used in the construction of radiation shields (medical or nuclear). Offshore, heavyweight concrete is used for ballasting for pipelines and similar structures
- The ideal property of normal and high density concrete are high modulus of elasticity , low thermal expansion , and creep deformation
- Because of high density of concrete there will be tendency for segregation. To avoid this pre placed aggregate method of concreting is adopted.
- High Modulus of Elasticity, Low thermal Expansion ,Low elasticity and creep deformation are ideal properties.
- The high density. Concrete is used in construction of radiation shields. They are effective and economic construction material for permanent shielding purpose.
- Most of the aggregate specific gravity is more than 3.5
Mass concrete is defined in ACI as “any volume of concrete with dimensions large enough to require that measures be taken to cope with generation of heat from hydration of the cement and attendant volume change to minimize cracking.” The design of mass concrete structures is generally based on durability, economy, and thermal action, with strength often being a secondary, rather than a primary, concern. The one characteristic that distinguishes mass concrete from other concrete work is thermal behavior. Because the cement-water reaction is exothermic by nature, the temperature rise within a large concrete mass, where the heat is not quickly dissipated, can be quite high. Significant tensile stresses and strains may result from the restrained volume change associated with a decline in temperature as heat of hydration is dissipated. Measures should be taken where cracking due to thermal behavior may cause a loss of structural integrity and monolithic action, excessive seepage and shortening of the service life of the structure, or be aesthetically objectionable. Many of the principles in mass concrete practice can also be applied to general concrete work, whereby economic and other benefits may be realized. Mass concreting practices were developed largely from concrete dam construction, where temperature-related cracking was first identified. Temperature-related cracking has also been experienced in other thick-section concrete structures, including mat foundations, pile caps, bridge piers, thick walls, and tunnel linings
Ready-mix concrete has cement, aggregates, water and other ingredients, which are weigh-batched at a centrally located plant. This is then delivered to the construction site in truck mounted transit mixers and can be used straight away without any further treatment. This results in a precise mixture, allowing specialty concrete mixtures to be developed and implemented on construction sites. Ready-mix concrete is sometimes preferred over on-site concrete mixing because of the precision of the mixture and reduced worksite confusion. However, using a pre-determined concrete mixture reduces flexibility, both in the supply chain and in the actual components of the concrete. Ready Mixed Concrete, or RMC as it is popularly called, refers to concrete that is specifically manufactured for delivery to the customer’s construction site in a freshly mixed and plastic or unhardened state. Concrete itself is a mixture of Portland cement, water and aggregates comprising sand and gravel or crushed stone. In traditional work sites, each of these materials is procured separately and mixed in specified proportions at site to make concrete. Ready Mixed Concrete is bought and sold by volume – usually expressed in cubic meters. Ready Mixed Concrete is manufactured under computer-controlled operations and transported and placed at site using sophisticated equipment and methods. RMC assures its customers numerous benefits.
Advantages of Ready mix Concrete over Site mix Concrete
- A centralised concrete batching plant can serve a wide area.
- The plants are located in areas zoned for industrial use, and yet the delivery trucks can service residential districts or inner cities.
- Better quality concrete is produced.
- Elimination of storage space for basic materials at site.
- Elimination of procurement / hiring of plant and machinery
- Wastage of basic materials is avoided.
- Labor associated with production of concrete is eliminated.
- Time required is greatly reduced.
- Noise and dust pollution at site is reduced.
Disadvantages of Ready-Mix Concrete
- The materials are batched at a central plant, and the mixing begins at that plant, so the traveling time from the plant to the site is critical over longer distances. Some sites are just too far away, though this is usually a commercial rather than technical issue.
- Access roads and site access have to be able to carry the weight of the truck and load. Concrete is approx. 2.5tonne per m². This problem can be overcome by utilizing so-called ‘minimix’ companies, using smaller 4m³ capacity mixers able to access more restricted sites.
- Concrete’s limited time span between mixing and going-off means that ready-mix should be placed within 2 hours of batching at the plant. Concrete is still usable after this point but may not conform to relevant specifications.
Concrete is porous. The porosity is due to air voids , water voids or due to inherent property of gel structures. On account of porosity strength of concrete is reduced , reduction of porosity result in increase in strength of concrete. The impregnation of monomer and subsequent polymerization is the latest technique adopted to reduce inherent porosity of concrete and increase strength and other properties of concrete
There are mainly 4 types of polymer concrete
1. Polymer impregnated concrete
2. Polymer cement concrete
3. Polymer concrete
4. Partially impregnated and surface coated polymer concrete.
Polymer impregnated concrete
It is a precast conventional concrete cured and dried in oven or by dielectric heating from which the air in the open cell is removed by vacuum. Then a low viscosity monomer is diffused through the open cell and polymerized by using radiation, application of heat or by chemical initiation.
Mainly the following type of monomers are used
Mainly the following type of monomers are used
2. t- butyl styrene
3. Other thermoplastic monomer
4. The amount of monomer that can be loaded into a concrete specimen is limited by the amount of water and air that has occupied the total void space.
5. PIC require cast in situ structures
Polymer cement concrete
Polymer cement concrete is made by mixing cement, aggregate, water and monomer. Such plastic mixture is cast in moulds , cured dried and polymerized. The monomer that are used in PCC are
1. Polyster- styrene
4. Vinyldene chloride
PCC produced in this way have been disappointing. In many cases material poorer than ordinary concrete is obtained.This is because organic material are incompatable with aqueous systems and some times interfere with the alkaline cement hydration process. Russians developed a superior polymer by incorporation of furfuryl alcohol and aniline hydrochloride in the wet mix. This material is dense and non shrinking and to have high corrosion resistance, low permeability and high resistance to vibration and axial extension .PCC can be cast in situ for field application.
Polymer concrete is an aggregate bound with a polymer binder instead of Portland cement as in conventional concrete. The main technique in producing PC is to minimize void volume in the aggregate mass so as to reduce the quantity of polymer needed for binding the aggregate. This is achieved by properly grading and mixing the aggregate to attain maximum density and minimum voids
It is defined as a mortar conveyed through a hose and pneumatically projected at high velocity on to a surface. There are mainly two different methods namely wet mix and dry mix process. In wet mix process the material is conveyed after mixing with water.
Pre packed concrete
In constructions where the reinforcement is very complicated or where certain arrangements like pipe, opening or other arrangements are incorporated this type of concreting is adopted. One of the methods is concrete process in which mortar is made in a high speed double drum and grouting is done by pouring on prepacked aggregate. This is mainly adopted for pavement slabs
Concrete poured into a framework that is fitted with a vacuum mat to remove water not required for setting of the cement; in this framework, concrete attains its 28-day strength in 10 days and has a 25% higher crushing strength. The elastic and shrinkage deformations are considerably greater than for normal-weight concrete.
Pumped concrete must be designed to that it can be easily conveyed by pressure through a rigid pipe of flexible hose for discharge directly into the desired area. Pozzocrete use can greatly improve concrete flow characteristics making it much easier to pump, while enhancing the quality of the concrete and controlling costs.
The designer must be aware of the need to improve the grade and maintain uniformity of the various materials used in the pumped mix in order to achieve greater homogeneity of the total mix. Three mix proportioning methods frequently used to produce pump able concrete are :
Maximum Density of Combined Materials
Maximum Density – Least Voids
Minimum Voids – Minimum Area
Mixes must be designed with several factors in mind:
1. Pumped concrete must be more fluid with enough fine material and water to fill internal voids.
2. Since the surface area and void content of fine material below 300 microns control the liquid under pressure, there must be a high quantity of fine material in a normal mix. Generally speaking, the finer the material, the greater the control.
3. Coarse aggregate grading should be continuous, and often the sand content must be increased by up to five percent at the expense of the coarser aggregate so as to balance the 500 micron fraction against the finer solids.
Unfortunately, adding extra water and fine aggregate leads to a weaker concrete. The usual remedies for this are either to increase the cement content, which is costly, or to use chemical admixtures, which can also be costly and may lead to segregation in marginal mixes. There is another and far more effective alternative:
There are many advantages to including POZZOCRETE in concrete mixes to be pumped. Among them are :
1. Particle Size. Pozzocrete meets IS 3812 Specification with 66% passing the 325 (45-micron) sieve and these fine particles are ideal for void filling. Just a small deficiency in the mix fines can often prevent successful pumping.
2. Particle Shape. Microscopic examination shows most Pozzocrete particles are spherical and act like miniature ball bearings aiding the movement of the concrete by reducing frictional losses in the pump and pining. Studies have shown that Pozzocrete can be twice as effective as cement in improving workability and, therefore, improve pumping characteristics.
his chemical reaction combines the Pozzocrete particles with the calcium hydroxide liberated through the hydration of cement to form additional cementitious compounds which increase concrete strength.
Excess water in pumped mixes resulting in over six inch slumps will often cause material segregation and result in line blockage. As in conventionally placed mixes, pumped concrete mixes with excessive water also contribute to lower strength, increased bleeding and shrinkage. The use of Pozzocrete in pumped or conventionally placed mixes can reduce the water requirement by 2% to 10% for any given slump.
Sand/Coarse Aggregate Ratio:
In pumped mixes, the inclusion of liberal quantities of coarse aggregate can be very beneficial because it reduces the total aggregate surface area, thereby increasing the effectiveness of the available cementitious paste. This approach is in keeping with the “minimum voids, minimum area” proportioning method. As aggregate size increases, so does the optimum quantity of coarse aggregate. Unfortunately, this process is frequently reversed in pump mixes, and sand would be substituted for coarse aggregate to make pumping easier. When that happens, there is a need to increase costly cementitious material to compensate for strength loss. However, if Pozzocrete is utilized, its unique workability and pump ability properties permit a better balance of sand to coarse aggregate resulting in a more economical, pump able concrete.
Shotcrete is a process where concrete is projected or "shot" under pressure using a feeder or "gun" onto a surface to form structural shapes including walls, floors, and roofs. The surface can be wood, steel, polystyrene, or any other surface that concrete can be projected onto. The surface can be trowelled smooth while the concrete is still wet.
Shotcrete has high strength, durability, low permeability, excellent bond and limitless shape possibilities. These properties allow shotcrete to be used in most cases as a structural material. Although the hardened properties of shotcrete are similar to conventional cast-in-place concrete, the nature of the placement process provides additional benefits, such as excellent bond with most substrates and instant or rapid capabilities, particularly on complex forms or shapes. In addition to building homes, shotcrete can also be used to build pools
Methods of Application
Wet Mix – All ingredients, including water, are thoroughly mixed and introduced into the delivery equipment. Wet material is pumped to the nozzle where compressed air is added to provide high velocity for placement and consolidation of the material onto the receiving surface.
Dry Mix – Pre-blended dry or damp materials are placed into the delivery equipment. Compressed air conveys material through a hose at high velocity to the nozzle, where water is added. Material is consolidated on the receiving surface by the high-impact velocity.
The properties of both wet and dry process shotcrete can be further enhanced through the addition of many different additives or admixtures such as:
Silica Fume – Provides reduced permeability, increased compressive and flexural strength, increased resistance to alkali and chemical attack, improved resistance to water washout, reduced rebound levels and allows for thicker single pass applications.
Air-Entraining Admixtures – Improve pumpability and adhesion in wet-process shotcrete and freeze-thaw durability in both wet and dry processes.
Fibers – Control cracking, increase toughness values and improve impact resistance and energy absorption.
Accelerators – Improve placement characteristics in adverse conditions, allow for thicker single pass applications, increase production capabilities and reduce the occurrence of fallouts on structures subjected to vibration.