Current Developments in Concrete Construction. S A Reddi Retd.) Gammon India Ltd.

Transcription

1 Current Developments in Concrete Construction S A Reddi Dy.. MD (Retd( Retd.) Gammon India Ltd.

2 Mass Concrete Pours

3 Typical Structures With Mass Concrete Pours

4 General Overview Definitions & Standards, Thermal Cracking Temperature Rise Temperature & Stress Prediction Factors Affecting Temperature Rise Cement, Aggregate, Ambient Temp, SCMs, Placement Techniques Post-Placement Techniques Embedded Pipe Cooling Formwork Insulation Thermal Expansion Reinforcement

5 Mass Concrete Defined Any volume of concrete with dimensions large enough to require that measures be taken to cope with the generation of heat from hydration of the cement and attendant volume change to minimize cracking. (ACI Manual of Concrete Practice)

6 Mass Concrete Principles When dimensions are > 1m Cement Hydration is an exothermic process, leads to a rise in temp at the core of large pours; If the surface temperature deviates greatly from that of the core, thermal cracking will develop Codes require a temp differential of less than 20 deg C - surface to the core temperature rise should be considered Apply to large dams, Mass foundations, bridge piers, thick slabs, nuclear plants, structural columns, etc

7 Thermal Cracking Cement hydration produces a rise in internal temperature. The outer surface cools faster than the core of the section. By thermal expansion/contraction, the temp differential induces tensile stresses at the surface Stresses > Tensile Strength => Thermal Cracking

8 Temperature Rise Temperature rise varies by many parameters: Cement composition, fineness, and content Aggregate content and CTE (Coeff. of Thermal Expansion) Section geometry Placement & ambient temperatures (Mass) Most temp rise occurs in first 1-3 days after placement. For thick sections, cooling to ambient temps may take years.

9 For thick sections, cooling to ambient temps may take years.

10 Factors Affecting Temp. Rise Cement with a lower fineness will slow hydration, and reduce temperature rise Mass Concrete mixes should contain as low cement content as possible to achieve the desired strength This lowers the heat of hydration and temp rise Coarse Aggregate should be have an MSA of 150mm if possible. A higher coarse aggregate content (70-85%) can be used to lower the cement content, reducing temp rise

11 Factors Affecting Temp. Rise Fly Ash, and Slag can greatly reduce heat of hydration Pozzolans such as FA (class F is best > slower hydration) and Slag will produce between 15-50% of the heat of Ordinary Portland Cement. Highly reactive SCMs such as Silica Fume and Metakaolin do not substantially lower the heat of hydration Lower cement content + pozzolans greatly reduces temperature rise!

12 Factors Affecting Temp. Rise Placement Temperature Pouring at lower temperatures will reduce the thermal stresses in the section. Slows hydration > lowers heat of hydration, lowers temperature differential between the core and the outer surface. Lower ambient temps produce less temperature rise! Lower volume:surface ratio produce less temp rise! W/C has a large effect on temperature rise. Most Mass Concrete mixes have a 0-50mm slump.

13 Placement Techniques Cool aggregate by flushing with cold water to reduce placement temperatures. Replacing mix water with flaked/crushed ice can greatly reduce the temperature of the mix. Cooling the aggregate and use ice in mix water can reduce placement temperatures up to 12 C For heat transfer calculations, all components are changed to water equivalents. Core sample from a large column with thermal cracks.

14 Placement Techniques Flushing the mix is Liquid Nitrogen can reduce temps.

15 Post-Placement Techniques Two schools of thought exist on post-placement techniques to reduce thermal cracking: 1) Cool the core of the concrete to reduce the temperature differential. 2) Insulate the outer surface to reduce the temperature differential.

16 Post-Cooling Post-Cooling utilizes cold water flowing through pipes embedded into the concrete This helps to transfer heat from the core, and reduce the temperature differential.

17 Post-Cooling Example of embedded pipe grid Area cooled by one pipe -Pipe spacing is judged by heat transfer principles. Must be spaced in a manner which achieved the desired temperature differential

18 Post-Cooling Instrumentation is used to monitor temperature, to determine flowrates through cooling pipes Steel pipes are the most effective at extracting heat from the core

19 Insulation After Placement Method to reduce the temp gradient By limiting the heat loss from surface, the difference in temperature between the surface and the core is minimized Removing formwork too soon can cause thermal shock to the surface, and extensive cracking will occur Metal formwork is very conductive to heat, so additional insulation needed to limit heat loss 25mm wood ~500mm of addl concrete

20 Expansion Reinforcement Expansion Reinforcement can be used to lessen thermal cracking. Must be designed in addition to loads placed onto the structure. Expansion reinforcement distributes thermal stresses to minimize crack widths. Expansion reinforcement is impractical for very large pours.

21 Expansion Reinforcement General procedure for design. Determine maximum temperature gradient Determine section restraint characteristics r) Determine physical properties of the concrete: CTE, E, Tensile Strength Determine the allowable crack width: Must consider durability and permeability Determine area of steel required to limit cracking

22 Large Volume Pours Notable Indian Cases Foundation raft for 500 MW Fast Breeder Nuclear Reactor at Kalpakkam near Chennai : Maximum single pour was 5500 m 3 Many rafts for chimneys, storage silos with single pour of more than 2000 m 3

23 Large Volume Pours Special Attention Required Concrete Mix proportion Mineral Admixtures Concrete supply Casting sequence Cold joints Plastic settlement Heat of hydration Early age thermal cracking

24 Large Volume Pours :Benefits Savings in cost and time resulting from the reduction in the number of joints Joints expensive to form, requiring stop ends, careful preparation of the concrete surface when new concrete is cast Construction joints Delays construction Labor, material cost in making the joints Practical difficulties in forming joints Difficulties in preparing concrete surface

25 Large Volume Pours Formation of Sound Construction Joints Impossible Typically in heavily reinforced foundations to core shaft tower blocks, formation of sound construction joints is virtually impossible because of very congested reinforcements Similar conditions are often found in large sections such as bridge decks, where the position of void formers, stressing cables, ducts etc render the installation of construction joints extremely difficult

26 Large Volume Pours Concrete Mix Proportioning Requirements for strength and workability Thermal characteristics minimize risk of thermal cracking Delayed stiffening time to avoid cold joints Heat evolution Thermal expansion coefficient Tensile strain capacity

27 Large Volume Pours Composite Cements Combinations of OPC with either PFA or GGBS hydrate at slower rate than OPC and produce a lower temperature rise. This applies to both PPC, PSC and blends of OPC and PFA,GGBS produced in a batching plant The level of PFA in such blends is up to 40%; up to 75% if GGBS is used.

28 Large Volume Pours Measuring Temperature Rise in Conc Cast an insulated One m 3 block using suitable formwork available at site and measure the temp. rise at the centre using thermo couple A simple plywood form lined with 50mm thick expanded polystyrene is adequate to provide comparative results

29 Cost Reduction By Use Of Fly Ash Improvement In Properties Of Concrete

30 Fly ash / Fly Ash In Concrete Pulverized Fuel Ash (Fly Ash) or Fly Ash is a material produced in thermal power stations as coal is burnt. It is pozzolanic, combines with free lime in cement to form calcium silicate, aluminium hydrates; categorised as Mineral Admixture in IS 456 : 2000 Fly Ash conforming to Grade 1 of IS 3812 may be used as part replacement of OPC provided uniform blending is ensured;

31 Mineral Admixtures Fly ash India produces about 100 million tonnes, disposal is a problem; can be partly used as mineral admixtures in concrete of all grades Special applications High Strength Concrete Roller Compacted concrete Embankment fills Reinforced earth fills Flyash bricks

32 Mineral Admixtures Processed Fly Ash Power station ash does not always meet the requirement; only some ESP streams qualify; remaining ash needs processing before use As per EN 450 fly ash may before its use be subject to processing by classification or selection to increase its fineness and to improve other properties

33 Processed Fly Ash Factory Dirk India,a German Company, has installed an Unit in Nashik,, with capacity 1000 TPD, (three classification plants) There are a few agencies supplying fly ash, but they merely collect fly ash from power stations and supply in bags; the product may or may not conform to IS Specification

34 Use of PFA From Power Station Electrostatic Precipitators in power station discharge usable PFA in separate streams Collected and fed into processing units for classification as fly ash suitable for use with concrete

35 Typical Fly Ash Processing Unit Raw Fly Ash fed into a classifier The coarser particles move down in the classifier, discarded Fine particles collected through a cyclone, shown on right

36 Use of Fly Ash Ready Mix Concrete Plants All ready mix concrete plants have started using varying percentages of cement replacement by fly ash, with or without the knowledge of the Purchasers Indiscriminate, uncontrolled use of non processed fly ash by some RMC plants have resulted in quality deficiencies RMC plants should declare percentage replacement while supplying concrete

37 Mineral Admixtures Blended cements Vs site mixing As per IS 456 there is no distinction on technical grounds between blended cement and site blending. Either method may be selected. Advantages of site blending in concrete o Increased flexibility in proportioning o Cost-effective; savings up to 20 % compared to blended cement o Effective monitoring of quality and percentage of Mineral Admixtures

38 Mineral Admixtures Blended cements Vs site mixing No manufactured blended cement will be able to satisfy all specifications or uses. Proportions are set by the manufacturers and data on percentage blended not available to the users Because of limited choice of proportions in standard blended cements, their properties may not lead to optimum concrete properties for different purposes.

39 Mineral Admixtures Advantages of Site Mixing Vary proportions to achieve optimum concrete properties & economical Savings in transport vs moving Fly Ash to factory for blending & delivering PPC Suitable where different percentages of Fly ash are specified; consistent strength achieved with low standard deviation 2 MPa with site blending Suppliers charge same price for OPC, PPC; much cheaper if OPC + Fly Ash is used

40 Benefits of Fly Ash in Concrete Improved Workability Low bleeding Low heat of hydration Improved durability Sulphate resistance Reduced risk of Alkali Silica Reaction Reduced Construction Costs

41 Fly Ash Status in India Uncertain quality, needs processing Separate processing facility required Electricity Board s s attitude towards disposal Codes, Specifications vs Reality Govt. policies, attitudes, excise duty R&D : Processing Requirements Role of Cement Producers Role of End Users

42 Perception About Fly Ash Fly ash should not be treated as waste product, but as useful by-product from Thermal Power Station Fly Ash is often an essential ingredient for durability of concrete As part replacement of cement, Fly Ash assists in conserving non-renewable energy sources

43 Fly Ash What is the Problem? Power stations priority : High Thermal efficiency, emissions secondary They produce Fly ash,, a waste material having variable carbon content, color, fineness and density, impurities Potential users require Fly Ash of uniform quality and fineness, free of impurities Fly Ash Processing absent in India, except at Nashik and one or two other centres

44 Fly Ash Property Variations Quality of Coal Coal milling operations Boiler loading Supplementary fuel used Operational influences

45 Fly Ash From Power Stations Fit and operate ESPs ESPs arranged in series Finer particles progressively removed Use grading facility of ESPs Separately store raw Fly Ash output Classify raw Fly Ash for compliance Offer classified Fly Ash for sale

46 Process Plant for Fly Ash Transport fly ash from PS Transfer to raw material silos Feed fly ash to production units Process by air classification units Transfer to Fly Ash storage silos Dispatch through tankers

47 Concrete Made with Fly Ash Impact On Strength Concrete made with fly ash will be slightly lower in strength upto 28 days than OPC concrete Equal strength at 28 days, substantially higher strength within a year s s time In concrete without flyash,, free lime remains intact and over time would be susceptible to the effects of weathering and loss of strength, durability.

48 Strength gain properties of pfa Strength Development with Time

49 Concrete With Fly Ash Addition Improved Workability

50 Fly Ash Increases Conc Workability Fly ash produces more cementitious paste; has a lower unit weight; contributes roughly 30% more volume of cementitious material per kg of cement. The greater percentage of fly ash in the paste, the aggregates are better lubricated and the concrete flows better However, IS 456 limits Fly Ash content to not more than 35% for normal Concrete; can go upto 50% for Self Compacting Concrete

51 Fly Ash Reduces Water Demand Fly ash reduces water needed to produce a given slump. The spherical shape of fly ash particles and its dispersive ability provide water-reducing reducing characteristics similar to a water reducing admixture Water demand of a concrete mix with fly ash is reduced by 2% to 10%, depending on a number of factors including the amount used and class of fly ash.

52 Fly Ash Reduces Sand Content Fly ash reduces the sand needed in the mix to produce workability It creates more paste, and by dispersive action makes the paste more slippery Sand has a much greater surface area than larger aggregates & therefore requires more paste By reducing sand, the paste available can more efficiently coat the surface area of the aggregates

53 Fly Ash Improves Corrosion Protection By decreasing concrete permeability, fly ash can reduce the rate of ingress of water, corrosive chemicals and oxygen thus protecting steel reinforcement from corrosion and its subsequent expansive result Fly ash concrete is less permeable because fly ash reduces the amount of water needed to produce a given slump

54 Fly Ash Reduces Heat of Hydration in Concrete During hydration of cement, heat is generated quickly, causing concrete temperature to rise This rapid heat gain increases the chances of thermal cracking, leading to reduced concrete strength and durability Fly ash generates only % as much heat compared to cement at early ages. Replacing part of cement with fly ash reduces the damaging effects of thermal cracking

55 Use Of Fly Ash as per IS 456 IS provides for use of fly ash (pulverised fuel ash) conforming to Grade 1 of IS 3812 as part replacement of Ordinary Portland Cement provided uniform blending with cement is ensured Please note that fly ash may be mixed with OPC only for making concrete Some unscrupulous RMC plants / contractors mix fly ash with PPC not allowed in the code

56 Use Of Fly Ash as per IS 456 Grades Of Concrete IS 456, Table 2 specifies three groups of concrete: Ordinary Concrete M10, M15, M20 Standard Concrete M25 to M55 High Strength Concrete M60 to M80 Fly ash may be used for all groups and grades of concrete; percentage replacement may vary

57 Use Of Fly Ash as per IS 456 Percentage Replacement For Ordinary and Standard Concrete, fly ash may replace concrete by upto 35% based on trial mixes For High Strength Concrete M60 and above, percentage replacement may be increased For Self Compacting Concrete, percentage may go upto 50 For concrete roads, desirable to use fly ash upto 50% replacement

58 Use Of Fly Ash as per IS 456 Interpretation Of Fly Ash Quantities Table 5 specifies minimum cement content for different exposures ; building foundations fall under Severe Exposure, with minimum 320 kg cementitious material per cubic metre of conc If for example 25% fly ash replaces cement, mix will consist of 240kg cement + 80kg fly ash Fly ash % is based on total cementitious materials used and not on cement only

59 Economics of Concrete Site Mixed Fly Ash vs PPC Assume 25% replacement by fly ash and 320kg cementitious material. Cost of materials: OPC 240 kg at say Rs 300 per bag: Rs 1440 Fly Ash 80 kg at Rs 1000/t : Rs 80 Total cost cementitious material : Rs 1520 In case PPC is used instead of site

60 Use Of Fly Ash as per IS 456 Formwork Stripping Time The stripping time specified in clause 11.3 is applicable where OPC is used For other cements, stripping time may be suitably modified For PPC or where fly ash is blended at site, due to slow setting time in the early ages, stripping time needs to be increased by about 50%

61 Use Of Fly Ash as per IS 456 Curing Exposed surfaces of concrete shall be kept continuously in a damp or wet condition by ponding or by covering with a layer of sacking, canvas, hessian and constantly kept wet for at least 7 days in case of OPC At least 10 days where mineral admixtures or blended cements are used Approved curing compounds may be used in both cases; one time application

62 Portland Pozzolona Cement (Fly Ash Based) as per IS 1489 Part 1 The strength properties of PPC at 28 days are similar to OPC 33 Grade; 28 day strength of cement is specified as 33 mpa OPC is available in 3 strength grades 33, 43, 53 mpa at 28 days Thus PPC can be used only for lower grades of concrete such as M20, M30 etc For higher Grades only OPC + fly ash site mixed is suiteable

63 Portland Pozzolona Cement Strength Gradation Attempts By BIS For several years, the Bureau of Indian Standards have proposed 3 Grades of PPC, namely 33, 43, 53 similar to OPC Some cement manufacturers have agreed for the gradation Some others who do not have the capability to produce higher Grades of PPC have been opposing the move; the stalemate continues As on date (2009) PPC available only in

64 Portland Pozzolona Cement As per European (EN)Standards( EN 197 1: 2000 is the reference document In Europe PPC is available in 3 Grades viz 32.5, 42.5 and 52.5 ( the numbers indicate 28 day strength of cement The combination of fly ash may vary : Option 1 : 6 to 20% fly ash Option 2 : 21 to 35% fly ash

65 Current Status Of Use of PPC In India Until the relevant BIS Codes are revised, PPC can be used only for lower grades of Concrete For higher Grades only processed fly ash may be blended with OPC for manufacture of concrete

66 Batching Plant Separate Silo For Fly Ash

67 Use Of Fly Ash as per IS 456 Clause fly ash ( Pulverized fuel ash ) Fly ash conforming to Grade 1 of IS 3812 may be used as part replacement of Ordinary Portland Cement provided uniform blending with cement is ensured Uniform blending is achieved when concrete is prepared in a batching plant; separate cement silos are required for OPC and fly ash

68 Site Mixing Fly Ash Bandra Worli Sea Link Contract specified minimum 400 kg cementitious material per cubic metre of concrete Even for M60 Gr Concrete the contractors have designed a mix with site blending of fly ash in concrete, using only 320 kg OPC plus 80 kg fly ash; project completed successfully

69 Characteristic strength 60 Mpa Target strength 74 Mpa Cement 320 kg Micro silica 45 kg Fly ash 80 kg w/c ratio 0.31 Admixtures 10 kg Slump 120 mm 28 day strength 75 Mpa Bandra Worli Sea link

70 Self Compacting Concrete Cement Content Cement content should be minimized; powder content composed of mineral admixtures such as fly-ash, GGBS etc. SCC in Nuclear Power Projects in India, cement limited to 250 Kg/m 3 ; and fly- ash also 250 Kg/, m 3 for concretes up to 50 MPa For 80 MPa concrete, mix successfully designed with cement content not exceeding 350 Kg/m 3

71 Palais Royale Tower, Mumbai Brief Description Height : 320m Tallest in India (2008) Foundations : Raft, upto 3.5m thick, Grade M40 - Basements, Superstructure - Columns : SCC M80, M60 - Slabs, Beams : M60 Concrete - Pre-tensioned flat slabs at lower levels Basement + 80m height utilized for car parks, sports facilities, Building Management Systems (BMS) Precautions against Fire & Fire Fighting measures

72 Palais Royale Tower, Mumbai Materials for SCC OPC 53 Grade from selected factory Processed fly-ash from Nashik Silica Fume - imported PC based super plasticizer imported Coarse aggregates from local quarries Fine aggregates from Gujarat Water from local sources

73 Palais Royale Tower 80 MPa SCC Cement : kg Silica fume : 10 % Water : 135 litres Aggregates : Local PC based admix : % Viscosity Modifier : 0.2 % Concrete produced at site batch plant

74 Thank You