2nd Term - New Material And Technology

oWhy Use RMC ?  The quality of concrete will be superior than site mixed concrete. (depend on the controls & checks exercised at site and at RMC producer‟s plant).  Prevention of wastage of materials on site due to poor storage conditions and repeated shifting of the mixer location.  Using RMC can cause less congestion and better housekeeping on the site resulting in efficient working environment.

 The quality of concrete will be superior than site mixed concrete. (depend on the controls & checks exercised at site and at RMC producer‟s plant).  Prevention of wastage of materials on site due to poor storage conditions and repeated shifting of the mixer location.  Using RMC can cause less congestion and better housekeeping on the site resulting in efficient working environment.  The modern RMC plants have an automatic arrangement to measure surface moisture on aggregates.  This helps in controlling the w/c ratio which results in correct strength and durability.  RMC plants have proper facilities to store and accurately batch concrete admixtures (chemical & mineral).  This accuracy is useful to improve properties of concrete both in plastic and hardened stage.  RMC plants have superior mixers than the rotating drum mixers generally used for mixing concrete materials at site.  RMC plants have efficient batching and mixing facilities which improve both quality and speed of concrete production.  Temperature control of concrete in extreme weather conditions can be exercised in a much better manner than done at site.  Introduction of RMC improves the rate of supply of concrete in the formwork and thereby automatically improves quality of formwork, layout of rft. steel and its‟ detailing and safety/strength of scaffolding and staging. What are the checks needed at site prior to receipt of RMC ?  Reinforcement layout for proper concrete placement without segregation.  Adequacy of formwork to take the hydrostatic pressure and adequacy of loading on propping system to match the speed of placing.  Openings and chutes provided, at predetermined locations between reinforcement bars to lower the placing hose (if pumped concrete is planned) to avoid segregation of concrete. 1

   

Adequacy of manpower and equipment for placing, compacting, finishing and curing of concrete. Proper approach for transit mixers free from all encumbrances e.g. water logging, material stacking etc. Proper platform to receive concrete. Proper precautions required to be taken to ensure that concrete from the transit mixer is unloaded at the fastest possible speed and does not take more than 30 minutes.  If pumping is proposed, the location of the pump should be approachable.  Proper co-ordination b/w the RMC supply and placing & compacting gangs.  Proper signaling or communication at site is necessary.  Workability of concrete within accepted limits.  Adequacy of cohesiveness of concrete for pumpability.  Ensure that water or chemical admixtures are not added during transportation by RMC‟s unauthorized persons and without the knowledge of the site in charge of the consumer.  Temp. of concrete at the time of receipt at site (if specified).  Continuous and steady supply at site and speedy unloading of the transit mixers.  Monitor speed and progress of placing to avoid formation cold joints.  Monitor proper placement without segregation.  Monitor placement of concrete at the closest possible point to its final location.  Arrange for curing as soon as finishing is completed. This is especially in case of slabs, pathways and roads in hot/warm weather. RMC in India – Constraints  Mechanized approach, modifications in contract documents and specifications required by CPWD, PWD which are for in-situ concrete.  High capital cost of batching plant.  Costlier than site mixed concrete by 10%.  Sales tax component 4-10% on sale of RMC by government.  Lack of bulk cement handling units- 50 kg sacks are to be replaced by jumbo bags. Advantages of RMC  Good quality control hence better durability and strength  Speedy construction through mechanized operations  Lower labour and supervising costs  Disorderly storage of concrete ingredients at site can be avoided  Minimisation of material wastage  Clean environment  Shortage of water and interruptions with power supply at site are eliminated  Large volume production possible  Can be pumped horizontally and vertically  HPC ensuring durability & strength up to 60 MPa can be produced

Corrosion of steel: The increased volume of rust exerts thrust on cover concrete resulting in cracks, spalling or delamination of concrete.  Concrete loses its integrity in this kind of situations.  The cross section of reinforcement progressively reduces and the structure is sure to collapse. Corrosion control  Proper mix design  Use of right quality and quantity of cement for different exposure conditions.  Materials such as fly ash, blast furnace slag, silica fume etc. are required to be used as admixtures or in the form of blended cement in addition to lowest w/c ratio to make concrete dense. 2

 The improvement in the microstructure of hydrated cement paste is ultimately responsible for protecting the steel reinforcement from corrosion. Why does rft. in concrete corrode ?  A properly designed and constructed concrete is initially water-tight and the rft. steel within it is well protected by a physical barrier of concrete cover which has low permeability and high density.  Concrete also gives steel within it a chemical protection.  Steel will not corrode as long as concrete around it is impervious and does not allow moisture or chlorides to penetrate within the cover area.  Steel corrosion will not occur as long as concrete surrounding it is alkaline in nature having a high pH value.  Depending upon the quality of design and construction, there will be an initial period in which no corrosion will occur as the external moisture or chloride is unable to reach the steel causing corrosion.  This initial period will depend on the environment in which the structure is constructed. Corrosion process  Accelerating factors  Wetting & drying cycles  Heating & cooling cycles  Loading & unloading cycles  Cyclic loading  Leaching of lime  Additions & alterations done on the structures  Isolated cracks  Voids  Entrapped air & large capillary pores get interconnected  External moisture & chlorides find their way to rft. steel and corrosion starts.  Corrosion process continues till such time large cracks develop and spalling of concrete occurs. Carbonation of concrete  Concrete when produced is highly alkaline having a pH value b/w 12.5 and 13.5.  Alkaline environment around the steel passivates corrosion process.  Due to CO2 and humidity present in the environment the exposed surface of concrete loses its alkalinity due to formation of carbonic acid.  This formation gradually penetrates into the concrete mass and is called carbonation of concrete.  When carbonation takes place beyond the concrete cover given to rft. steel, the environment around the steel loses it alkalinity – dropped to less than pH value 9.  The mitigation of corrosion no longer takes place due to chemical protection.  Lower grades of concrete have shown much deeper carbonation than higher grades of concrete, for a similar period of time.  Grades lower than M20 can carbonate beyond 25 – 35 mm within a matter of 20 years.  If quality of concrete in the cover region is poor, corrosion can take place much faster.  Slender sections like canopies, parapets, balcony slabs projecting on the building exterior show greater evidence of deterioration than other structural members. Steps necessary for repairs  Visual examination of the cracked or spalled concrete surface.  To determine the extent of corrosion and depth of carbonation in the affected areas.  To clearly identify the areas which need repairs.  To remove the defective concrete and corrosion products from embedded steel.  To clean the concrete and steel surface completely free from rust and other materials.  To examine the reduction of steel rft. diameter and arrange for full or part replacement of corroded steel.  To apply corrosion resistant barrier film or coating on the rft. to inhibit chances of future corrosion. 3

 To apply bond coat on the old surface over which repairs have to be carried out.  To apply a strong, passive carbonation resistant concrete cover of proper generics and reinstate the structural member to its original shape / form.  To apply a seal coat on the entire repaired surface to guard against future ingress of moisture and other harmful chemicals.  To paint the repaired surface as an added precautionary measure and for aesthetics. Measures to control the corrosion of steel reinforcement?  Metallurgical methods  Corrosion inhibitors  Coatings to reinforcement  Cathodic protection  Coatings to concrete  Design and detailing Metallurgical methods  Mechanical properties and corrosion resistance property of steel can be improved by altering its structure through metallurgical processes.  Rapid quenching of the hot bars by serious of water jets, keeping the hot steel bars for a short time in a water bath etc. Corrosion inhibitors  Using corrosion inhibiting chemicals such as nitrites, phosphates, benzoates etc.  Most widely used admixture is based on calcium nitrite - it is added to the concrete during mixing of concrete - typical dosage is 10-30 litres per m3 of concrete. Coatings to reinforcement  The object of coating to steel bar is to provide a durable barrier to aggressive materials.  Coating should be robust to withstand fabrication of reinforcement cage, and pouring of concrete and compaction.  Simple cement slurry coating is a cheap method for temporary protection. Fusion bonded epoxy coating  Effective method of coating rebars  Plants are designed to coat the straight bars in a continuous process.  Electrostatically charged epoxy powder particles are deposited evenly on the surface of the bar.  Costing thickness may vary from 130 to 300 microns, greenish in colour. Galvanised reinforcement  Dipping the steel bars in molten zinc  The zinc surface reacts with calcium hydroxide in the concrete to form a passive layer and prevents corrosion. Cathodic protection  One of the effective and extensively used methods for prevention of corrosion in concrete structures in more advanced countries.  Due to high cost and long term monitoring required for this method, it is not very much used in India.  Comprises of application of impressed current to an electrode laid on the concrete above steel reinforcement. This electrode serves as anode and the steel reinforcement which is connected to the negative terminal of a DC source acts as a cathode.  In this process the external anode is subjected to corrode and the cathodic reinforcement is protected against corrosion and hence the name “Cathodic protection”. Realkalisation & Desalination process Realkalisation : This brings back the lost alkalinity of concrete to sufficiently high level to reform and maintain the passive layer on the steel. Desalination : The chloride ions are removed from concrete, particularly from the vicinity of the steel reinforcement by certain electrical method to establish the passive layer on the steel. 4

Coatings to concrete  Epoxy coatings which does not allow the concrete to breathe should not be used for coating concrete members.  Epoxy based coating material is not resistant to UV rays when exposed to sunlight.  Whereas the coating material based on acrylic polymer is resistant to UV rays and retains the breathing property of concrete.

TMT      

Mild steel ribbed bars for better bond with concrete. High strength deformed bars (H.S.D.Bars). Cold twisted deformed bars (C.T.D Bars). Thermo–processed rebars – commonly referred to as T.M.T. bars. High-strength rebars using micro-alloys. Ribbed bars with corrosion resistance through additions of Cr and Cu in the steel used for thermoprocessed rebars T.M.T.42 C.R.S bars of TISCO. Mild steel ribbed bars  Mild steel ribbed bars were introduced around 1960.  For preventing „slip‟ and improving the mechanical bonding between steel rebars and cement concrete.  Rolling mills in different countries followed different pattern of ribs.  All standards specified only the bond strength & testing procedures. High Strength Deformed Bars ( H.S.D. BARS)  Introduction of H.S.D bars reduced quantity of steel used in RCC.  Steel bars having strength 400 – 500 N/mm2 with adequate ductility. Cold Twisted Deformed Bars (C.T.D bars )  C.T.D. bars were introduced during 1970s.  Cold working increases steel strength – hence reduction of quantity of steel used in R.C.C. structures.  Yield strength of around 400 N/mm2.  Higher strength C.T.D. bars did not gain acceptance since elongation values dropped to 12% while the strength increased.  Other drawbacks – surface stress and visible cracks due to twisting which led to higher corrosion rate and durability problems.  When cold twisting is done, the bar is subjected to very severe mechanical stress as the material is deformed while in the plastic stage and any significant invisible defect would cause the bar to fracture during the twisting itself.  This resulted in the protective scale layer to fall off during twisting.  Therefore, most of the European countries gave up the use of C.T.D bars within a few years.  However, in India C.T.D bars were commonly used upto 1992 under the brand name “Torsteel”. T.M.T. Bars  Today‟s requirements – low cost deformed bars with min. guaranteed yield strength 500 N/mm 2 and having adequate ductility.  Requirements of high seismic zones – 60% of land in India falls under high seismic zones (Zones III, IV & V).  In the mid-eighties, rapid quenching technology known as “Thermex” was developed & patented.  Tamm, an international authority on steel rft. was the inventor of the quenching system.  Thermex technology adopt online quenching and self-tempering of steel, with a guarantee for elongation of above 16% in Fe500 & Fe600 grade.  The quenching and self-tempering process is known as “Thermo Mechanical Treatment”.

5

Thermex process  In this process the heat energy of the rolled bar, after the finishing stand of the rolling mills is used.  In the normal process the heat of rolled bars (950 0C - 10000C) is wasted and bars are allowed to cool in the cooling bed to the ambient temperature.  The rolled bar leaving the rolling mill is guided through specially designed thermex pipes wherein the surface temperature of 9500C - 10000C of the bar is brought down in a very short time, approximately 1 sec, on account of drastic, intense and uniform cooling by high pressure water.  The system is installed between the last stand of the rolling mill and cooling bed and always produced rebars with a concentric tempered “martensite” periphery and “pearlite ferrite core”.  The thermex system permits production of rebars of different yield strengths by varying the thickness of the hardened periphery by changing the quenching rate.  Objectives  Low cost, high strength rebars with high ductility  High yield strength of 500 N/mm2 or more.  An elongation of around 20%.  Excellent weldability  Rebars that can be safely used in high earthquake-prone areas.  By this process, while the surface temperature of the steel bar is drastically brought down, the temperature of the core is not affected to a large extent.  The pre-determined cooling of the bar by high pressure water transforms the peripheral structure of the steel bar to “martensite” and would need to be annealed to make the bars fit for use as reinforcing bars.  As the rolled bars at around 950 0C - 10000C passes at its normal speed through the Thermex process/system, the periphery / surface is subjected to rapid cooling while the core remains unaffected for a short time.  Annealing is achieved through the heat available at the core.  The peripheral / surface part of the bar has been transformed into “martensite” begins to gain temperature from the core as soon as the bar leaves the Thermex system.  The thermal exchange in the steel bar is continued till it reaches the equalising temperature.  The difference in the peripheral and core temperature equalises finally at around 6000C.  During slow cooling on the cooling bed, the core transforms into ferrite and pearlite.  The result of this process is that the temporal martensite (25 – 35 % of area) is in the hardened periphery and the soft core which is fine-grained for ferrite-pearlite (65 – 75%) is in the middle of the steel bar.  The microstructure of thermo-processed bar, for martensite periphery and ferrite pearlite core is different from the microstructure of rolled steel which has not undergone thermo processing. High-strength rebars using micro-alloys  High-strength rolled bars of high strength using micro alloys such as Niobium (N b), Vanadium (V) and Titanium (Ti) in various proportions were also tried out in advanced countries.  These rebars have limited usage due to the use of costly micro-alloys in their production. Thermo-mechanical Treatment  Many rolling mills adopt quenching and tempering technologies engineered by them and produce steel which do not meet the requirements or which do not attain mechanical properties of T.M.T.bars produced by Thermex process.  Since the method of steel bar production is not detailed out in I.S. 1786-1985, many rolling mills produce bars under the T.M.T. brand name without any technology change in the mill set up and also have no quenching system.  They take shelter under the pretext that hot rolling mills heat the billets and ingots and so thermal work is involved. 6

 They sell these bars as T.M.T.bars.  Failures of structures in Ahmedabad due to the Bhuj earthquake of Jan. 2001 is due to the use of the T.M.T.bars produced by mills which do not adopt Thermex Technology. Corrosion Resistance of T.M.T.Steel bars  Resistance to corrosion of rebars depends on the chemical composition and also on processing parameters.  By reducing the carbon content in rebars, weldability improves, while strength is reduced.  To improve the strength of rebars with reduced carbon content, the manufacturing process has to be modified as :• Cold twisting • Addition of Micro-alloys in production process • T.M.T. rebars ( online quenching and tempering)  In the first two processes, micro-cells are formed on the surface which may accelerate corrosion.  In the production of T.M.T.bars, strength of bars is carefully controlled by the water pressure for specific diameter and length of quenching line.  In T.M.T. bars, the surface is tempered by the core heat of rebars giving a uniform stress-free tempered martensite rim with very few micro-cells.  This provides an optimum combination of strength, ductility, toughness and corrosion resistance to some extent.  Addition of some micro-alloying elements in the production of T.M.T.bars enhances the corrosion resistance of the bars. Online control of T.M.T. bars  The control variables in the online process of quenching and tempering are • Length of quenching line • Cooling water flow rate  This can be easily adjusted during the rolling process and have a strong effect on yield strength of the bars. Resistant to corrosion of T.M.T. 42 C.R.S. bars  When steel is alloyed with Phosphorous and chromium, the oxide layer formed is dense with minimum surface defects such as pores and cracks.  Hence ingress of oxygen, chlorides etc. through the oxide layer is reduced. Superplasticizers Superplasticizers can produce:  At the same w/c ratio much more workable concrete than the plain ones.  For the same workability, it permits the use of lower w/c ratio.  As a consequence of increased strength with lower w/c ratio, it also permits a reduction of cement content. Classification of Superplasticizers Following are a few polymers which are commonly used as base for superplasticizers  Sulphonated melamine-formaldehyde condensates (SMF)  Sulphonated naphthalene-formaldehyde condensates (SNF)  Modified lignosulphonates (MLS)  Other types [ India manufacture and use this four types] New generation Superplasticizers  Acrylic polymer based (AP)  Copolymer of carboxylic acrylic acid with acrylic ester (CAE)  Cross linked acrylic polymer (CLAP)  Poly Carboxylate ester(PCE) 7

 Multicarboxylatethers(MCE)  Combinations of above Retarders: Slows down the chemical process of hydration so that concrete remains plastic and workable for a longer time.  Used to overcome the accelerating effect of high temperature on setting properties of concrete in hot weather concreting.  Used in grouting oil wells : Cement grout is required to be in mobile condition for about 3 to 4 hours at high temp. (about 2000 C) without getting set.  In RMC practices, setting of concrete will have to be retarded so that concrete when finally placed and compacted is in perfect plastic state. Accelerators : Accelerating admixtures are added to concrete to increase the rate of early strength development in concrete to  Permit earlier removal of formwork  Reduce the required period of curing  Advance the time that a structure can be placed in service  Partially compensate for the retarding effect of low temperature during cold weather concreting  In the emergency repair work Air-entraining Admixture: 85% of concrete manufactured in America contains air entraining agent and it is considered a necessary „fifth ingredient‟ in concrete making.  Air entraining concrete is made by mixing a small quantity of air entraining agent or by using air entraining cement.  These air entraining agents incorporate air bubbles, which will act as flexible ball bearings and will modify the properties of plastic concrete regarding workability, segregation, bleeding and finishing quality of concrete. Damp-proofing and Waterproofing Admixtures  Waterproofing admixtures may be obtained in powder, paste or liquid form and may consist of pore filling or water repellant materials  The chief materials in the pore filling class are silicate of soda, aluminium and zinc sulphates and aluminium and calcium chloride (chemically active pore fillers)  Accelerate the setting time of concrete and thus render the concrete more impervious at early stage  The chemically inactive pore filling materials are chalk, fullers earth and talc and these are usually very finely ground.  These improve workability and facilitate the reduction of water for given workability and to make dense concrete which is basically impervious  Materials like soda, potash soaps, calcium soaps, resin, vegetable oils, fats, waxes and coal tar residues are added as water repelling materials Plasticizers  A high degree of workability is required in situations like deep beams, thin walls of water retaining structures with high percentage of steel reinforcement, column and beam junctions, tremie concreting, pumping of concrete, hot weather concreting, for concrete to be conveyed for considerable distance and in ready mixed concrete industries.  Conventional methods for obtaining high workability is by improving the gradation, or by the use of relatively higher percentage of fine aggregate or by increasing the cement content. Admixtures  Admixture is defined as a material, other than cement, water and aggregates, that is used as an ingredient of concrete and is added to the batch immediately before or during mixing.  Additive is a material which is added at the time of grinding cement clinker at the cement factory. 8

Pozzolanic or Mineral Admixtures Pozzolans in optimum proportions mixed with Portland cement improves qualities of concrete such as  Lower the heat of hydration and thermal shrinkage  Increase the watertightness  Reduce the alkali-aggregate reaction  Improve resistance to attack by sulphate soils and sea water  Improve workability  Lower costs  Mineral admixtures are added to concrete in relatively large amounts, generally in the range 20 to 70 % by mass of the total cementitious material.  Many industrial by-products have become the primary source of mineral admixtures in concrete due to economic and environmental considerations. Natural Pozzolans Artificial Pozzolans  Fly ash  Clay and Shales  Silica Fume  Opaline Cherts  Rice Husk Ash  Diatomaceous Earth  Surkhi  Volcanic Tuffs and Pumicites  Metakaoline 

Blast Furnace Slag

Fly Ash There are two ways that the fly ash can be used  One way is to intergrind certain percentage of fly ash with cement clinker at the factory to produce Portland Pozzolana cement (PPC)  Second way is to use the fly ash as an admixture at the time of making concrete at the site of work.  The second method gives freedom and flexibility to the user regarding the percentage addition of fly ash. ASTM broadly classify fly ash into two classes  Class F: Fly ash normally produced by burning anthracite or bituminous coal, usually has less than 5% CaO. Class F fly ash has pozzolanic properties only.  Class C : Fly ash normally produced by burning lignite or sub-bituminous coal. Some class C fly ash may have CaO content in excess of 10%. In addition to pozzolanic properties, class C fly ash also possess cementitious properties. Blast Furnace Slag: Nonmetallic product consisting essentially of silicates and aluminates of calcium and other bases.  The molten slag is rapidly chilled by quenching in water to form a glassy sand like granulated material.  The granulated material when further ground to less than 45 micron will have specific surface of about 400 to 600 m2/ kg (Blaine).  The chemical composition of Blast Furnace Slag (BFS) is similar to that of cement clinker. Applications of Mineral Admixtures

 Workability improvement: With fresh concrete mixtures that show a tendency to bleed or segregate, it is well known that incorporation of finely divided particles generally improves the workability by reducing the size and volume of voids.  

Durability to thermal cracking: The total heat of hydration produced by the pozzolanic reactions involving mineral admixtures is considered to be half as much as the average heat produced by the hydration of Portland cement. Durability to chemical attack: As the pozzolanic reaction involving mineral admixtures causes pore refinement that reduces the permeability of concrete, considerable improvement in the chemical durability of concrete containing mineral admixtures.

9