Durable Concrete, Cement, Aggregate and Mix Design

sustainable concrete and mix design

Durable Concrete, Cement, Aggregate and Mix Design

অনেক ভুল ধারণা এবং ভুল প্র্যাক্টিস প্রচলিত আছে এই কনক্রীট, সিমেন্ট, খোয়া, মিক্স অনুপাত ইত্যাদি নিয়ে । এমনকি সিভিল ইঞ্জিনিয়ারদের অনেকে বিষয়গুলি বুঝেন না । এই শর্ট কোর্সটি তাদের অনেকটাই কাজে আসবে বলে আশা করি। সাধারন মানুষও কিছুটা বুঝতে পারবে বলে মনে হয়। মিক্স ডিজাইনটা সহজভাবে উপস্থাপন করা হয়েছে । বাংলাদেশের উপযোগী করে কিভাবে Volumetric batching করে মানসম্পন্ন কনক্রীট বানানো যায় তার একটা গাইডলাইন দেওয়া আছে । এই কোর্সটি করার পর আপনারা বুঝতে পারবেন এই কোর্সটি করা আপনাদের জন্য কত জরুরী ছিল ।

Concrete mix design involves a process of preparation in which a mix of ingredients creates the required strength and durability for the concrete structure. Because every ingredient in the mix consists of different properties, it’s not an easy task to create a great concrete mix.

There are many misconceptions and wrong practices are going on related to concrete, cement, aggregate, mix ratio etc. Even many of the civil engineers do not understand the issues. I hope this short course will help them a lot. Ordinary people also seem to understand something. The Concrete mix design is simply presented. There is a guideline on how to make quality concrete by volumetric batching suitable for Bangladesh. After doing this course, you will understand how important it was for you to do this course

কি আছে এই শর্ট কোর্সে?

  • সিমেন্টের উপাদানসমূহ, বাংলাদেশে সিমেন্টের প্রকারভেদ, কোন সিমেন্ট ভাল?
  • বালি, খোয়া, পাথর (aggregate) কেমন হওয়া উচিত?
  • মানসম্পন্ন কনক্রীট কিভাবে বানাবেন?
  • পানি কিভাবে কন্ট্রোল করবেন?
  • মিক্স রেশিও কি হবে?
  • মিক্স ডিজাইন কিভাবে করবেন?
  • টেকসই কনক্রীট – বিভিন্ন পরিবেশ ও পরিস্থিতির জন‍্য উপযোগী কনক্রীট কিভাবে বানাবেন?
  • কনক্রীট কিভাবে শত বছর ভাল থাকে?
  • বাংলাদেশের জন্য উপযোগী কনক্রিট বানানোর গাইডলাইন – যা প্রত্যেক সিভিল ইঞ্জিনিয়ারের জানা উচিত

Prerequisite / Eligibility

  • BSc, MSc or PhD in Civil Engineering
  • Level 4 or 4th year student of bachelor’s in civil engineering

Course Teacher

Professor Dr. Jahangir Alam, Department of Civil Engineering, BUET, Dhaka, Bangladesh

Brief Biography of Course Teacher

Education

  • PhD in Geotechnical Earthquake Engineering, the University of Tokyo, Japan, 2005
  • MSc in Civil and Geotechnical Engineering, BUET, Dhaka, Bangladesh, 2002
  • BSc in Civil Engineering, BUET, Dhaka, Bangladesh, 1998

Biography

Professor Dr. Engr. Md. Jahangir Alam is faculty member at the Department of Civil Engineering, BUET, Dhaka-1000, since 1999. He completed his PhD from University of Tokyo in Geotechnical Earthquake Engineering in 2005 as a Monbu-Kagaku-sho Scholar. He was research fellow in Ecole Centrale Paris in 2008. Professor Jahangir did his BSc in Civil Engg with major in Structure and MSc in Civil and Geotechnical Engineering from BUET.

Professor Jahangir has multidisciplinary research interests and has publications in international journals and conferences. His current research topics are “Risk Sensitive Land Use Planning of Mega City”, Physical and Numerical Modeling of Liquefaction Hazard, Mitigation against Seismic Liquefaction, Cyclic Behavior of Non-plastic silt, Reinforced Earth, Earthquake Resistant Foundation in Soft Soil, Climate Resilient Concrete, Climate Resilient Road.

Professor Jahangir was involved in many consultancy projects where he designed/checked high rise RCC buildings, Communication towers, Jetty, Shore Pile, Embankment, Container Terminal, Ground Improvement, Bridge Foundation etc. He supervised many MSc and PhD students. He was involved in pile load testing, pile integrity testing, concrete mix design and development of laboratory and field-testing equipment.

Professor Jahangir actively involved in National and International professional bodies. He is life member and was Treasurer of Bangladesh Society for Geotechnical Engineering (BSGE), which is Bangladesh Chapter of ISSMGE. He is life fellow of Institute of Engineers Bangladesh (IEB). He is life member of Bangladesh Earthquake Society (BES). He was Treasurer of Bangladesh Earthquake Society (BES).

Certificate of Attendance

Certificate of attendance will be awarded after completion of all video lessons and quizzes

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Course Preview

Detail Course Outline

Binding materials

  • Binding Materials: Cement, Lime, Fly Ash, Slag
  • Manufacture of Portland Cement
  • Chemistry of Cement
  • Fly Ash and Pozzolans
  • Effect of fly ash in concrete: Workability, Bleeding, Segregation, Setting time, Heat of hydration, Early age strength, Long term strength, Permeability, Chloride resistance, Sulfate resistance, Resistance to carbonation
  • Available Cement Types: BDS EN 197-1:2003, CEM I, CEM II/A-M (S-V-L), CEM II/B-M (S-V-L).
  • Blast Furnace Slag / Steel Slag, Effect of slag in concrete

Aggregates

  • Granular materiel: sand, gravel, crushed stone (stone chips), crushed brick(chips), blast furnace slag, shingles
  • Calculation: Fineness Modulus, Gradation, Combined grading
  • Harmful materials in aggregate: Organic materials, Silt, clay and other fine particles, Salt
  • Specific gravity and density of aggregate
  • Absorption Capacity and Moisture Content
  • Examples
  • Limitations of brick chips
  • Topics of aggregate for road pavement

Concrete as Construction Material

  • What is Concrete?
  • What are the requirements for a successful concrete structure?
  • How strong can concrete be and How long can concrete last?
  • Concrete Mix Ratio
  • Properties of Fresh Concrete
  • Workability
  • Effect of grading of aggregate
  • Slump Test
  • Stress Concentration
  • w/c ratio, degree of compaction, strength and workability are related. How?
  • Effect of aging
  • Admixtures
  • Curing of Concrete
  • Maturity Rule of Concrete
  • Durability of concrete
  • Hydration and porosity of concrete
  • Testing of Concrete: Effect of platen restraint, Failure Modes, Merits and Demerits of Cylinder and Cube Test, Non-destructive test of concrete (NDT), Calibration of rebound hammer, Quality Control of Concrete by Testing.
  • Examples

Concrete mix design

  • Mean compressive strength?
  • Quality Control of Concrete by Testing
  • Mix Design as per ACI 211.1-91
  • Maximum permissible water-cement ratio
  • Recommended slump for various types of construction
  • Mix Design Example: Properties of fine aggregate, coarse aggregate, cement and water reducing admixture
  • ACI recommendation: w/c ratio, Dry rodded bulk volume, density of fresh concrete, mixing water content, Properties of aggregates,
  • Mix Design Steps: Selection of slump value, Selection of maximum size of coarse aggregate, Estimation of mixing water content and air content, Selection of w/c ratio, Calculation of cement content and admixture, Estimation of coarse aggregate content, Calculation of fine aggregate content, Adjustment and First Lab Trial Mix, Revision of mix proportion based on First Lab Trial Mix result, Final trial mix to confirm strength and slump, Adjustment for casting in site, Conversion to volumetric ratio.

Climate Resilient Concrete

  • Defining Climate Resilient Concrete
  • Concrete for coastal shore protection
  • Enemy of RCC
  • Durability of Concrete
  • Durability issues: Permeability, Chloride attack, Sulfate attack, Rebar corrosion, Carbonation, Acid attack, Effect of sea water, Cracking of concrete, Clear cover, Rebar corrosion, Alkali silica reaction (CC+RCC), Chloride attack, Salt weathering, Curing, Brick chips
  • Effect of w/c ratio on cement paste
  • Permeability of concrete
  • High strength vs normal strength concrete
  • Sulphate Exposure Limits
  • Cracking of concrete and durability
  • How brick chips destroying environment

Climate Resilient Concrete Manual for Volumetric Batching

  • Challenges of making climate resilient concrete
  • Defining parameter marks
  • Defining exposure class
  • Best Practices for Clear cover
  • Variations depend on exposure classes: w/c ratio and compressive strength, Concrete, Mix ratio.
  • Assumed Properties of Ingredients
  • How to maintain volumetric mix ratio

Course Content

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Course Content

Mix design of concrete for climate resilient development




Course Teacher:

Professor Jahangir Alam

Department of Civil Engineering, BUET, Dhaka



Organized by

Engineering Short Courses Online (ESCO) by ourprofessors.com



Course content


•          cement and its types

•          aggregate and its quality

•          climate resilient concrete and durable concrete

•          mix design of concrete

•          Question-Answer



Webinar on

Mix Design of Concrete for Climate Resilient Development


এই ওয়েবিনার (অনলাইন সেমিনার) করলে আপনার কি লাভ হবে?

• মিক্স ডিজাইন কিভাবে সহজে করতে হয় সেটা জানতে পারবেন

• কনক্রীট সংক্রান্ত সকল ভুল ধারনার অবসান

• কনস্ট্রাকশন সাইটে সহজে মান নিয়ন্ত্রণ

• পরিবেশ বান্ধব কনক্রীট কিভাবে বানাতে হয় জানতে পারবেন

• বালি, খোয়া, পাথর, সিমেন্ট সম্পর্কে সকল ভুল ধারনার অবসান

2
Documents of Durable Concrete, Cement, Aggregate and Mix Design

Binding Materials

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Binding Materials, Part-1

BINDING MATERIALS




Binding Materials

·        Cment

·        Lime

·        Fly Ash

·        Slag



Cement

·        Most common hydraulic cement is Portland Cement

·        Hydraulic?

·        Portland Cement = Joseph Aspdin patented in 1924, appearance is similar to portland stone

·        Portland stone = In UK, found in portland, high quality limestone used for construction

·        Use of cement: UK => USA => Worldwide



Natural and Artificial Cement

Natural cement:

·        Natural stones containing 25-40% clay + carbonate of lime => Burning and crushing to powder

·        Example: Roman cement in England = powdered calcined limestone


Calcination: to alter composition or physical state by heating to a specific temperature for a specific length of time

·        1500 deg C for limestone

·        Limestone = CaCO3, Lime = CaO

·        Limestone  Lime (Calcination)


Artificial Cement: Designed and manufactured to satisfy the needs



Manufacture of Portland Cement

Fine gray powder of raw materials

–        Calcium oxide (CaO), Silica (SiO2), Alumina (Al2O3), Iron oxide (Fe2O3)

–        Plus small quantity of Gypsum (CaSO4.2H2O)

–        Calcareous materials = compounds of Ca & Mg,

–        Argillaceous materials = SiO2, Al2O3, Fe2O3


Basic steps of manufacturing

–        Blending of raw materials (limestone & clay)

–        Heated in a kiln to 1400 to 1600 C where C3S, C2S, C3A, C4AF are chemically formed=>clinker

–        Grinding clinker to fine powder + gypsum + Fly Ash



Chemistry of Cement

Chemical compounds formed in the cement kiln


Name--------------------------------------Chemical formula----------------Shorthand notation

Tricalcium silicate (alite)               -----------------3CaO.SiO2-----------------------------C3S

Dicalcium silicate (belite)----------------2CaO.SiO2-----------------------------C2S

Tricalcium aluminate---------------------3CaO.Al2O3---------------------------C3A

Tetracalcium aluminoferrite ---------4CaO.Al2O3.Fe2O3-------------------C4AF

*Shorthand notation routinely used by cement chemists using abbreviations for the oxides: CaO = C; SiO2 = S; Al2O3 = A; Fe2O3 = F; SO3 = S; and H2O = H.


Tricalcium silicate + Water--->Calcium silicate hydrate+Calcium hydroxide + heat

2 Ca3SiO5 + 7 H2O ---> 3 CaO.2SiO2.4H2O + 3 Ca(OH)2 + 173.6kJ


Dicalcium silicate + Water--->Calcium silicate hydrate + Calcium hydroxide +heat

2 Ca2SiO4 + 5 H2O---> 3 CaO.2SiO2.4H2O + Ca(OH)2 + 58.6 kJ


•            Due to fast hydration of C3A

•            flash set occur

•            High temp and heat generation

•            Gypsum slows down hydration of C3A



Fly Ash and Pozzolans

Pozzolan: (ASTM C618)

–        A siliceous and/or aluminous material, which in itself possesses little or no binding properties, but which will, in finely divided form and in the presence of moisture, chemically react with Ca(OH)2 at ordinary temp to form compounds possessing binding properties

–        Natural pozzolans are volcanic ashes, calcined clay, rice hull ash etc.

–        Artificial pozzolans are bi-product from industry, fly ash (PFA = Palverized Fly/Fuel Ash)


• Fly Ash:

–        Fine residue resulting from the burning of powdered coal

–        Main constituent is silica

Hydration of cement produce Ca(OH)2 ; silica, alumina and iron in fly ash react with Ca(OH)2 to form binding material



Source of Fly Ash in Bangladesh

·        Most of the fly ash or slag used in the cement is imported from other countries.

·        Local Thermal Power Plants in Bangladesh produce around 52000 MT of fly ash every year, which is largely disposed of in dry embankments.

·        Qualitative analysis of fly ash sourced from Barapukuria Power Plant indicates it can be classed as Class F grade fly ash (Tammim et al 2013).



Effect of fly ash in concrete


Workability: Workability increases, water demand reduces

Bleeding, Segregation: less

Setting time: increases

Heat of hydration : reduce

Early age strength: reduce

Long term strength : increase

Permeability: Reduces significantly

Chloride resistance: increase

Sulfate resistance: increase

Resistance to carbonation: decrease



Available Cement Types


BDS EN 197-1:2003, CEM I/ 52.5N

·        ASTM C-150, Type – I

·        Clinker : 95-100%

·        Gypsum : 0-5%


BDS EN 197-1:2003, CEM II/A-M (S-V-L), 42.5N

·        ASTM C-595

·        Clinker : 80-94%

·        S-V-L : 6-20%

·        Gypsum : 0-5%


BDS EN 197-1:2003, CEM II/B-M (S-V-L), 42.5N

·        ASTM C-595

·        Clinker : 65-79%

·        S-V-L : 21-35%

·        Gypsum : 0-5%



Blast Furnace Slag

• Blast furnace slag is a nonmetallic by-product produced in the process.

• It consists primarily of silicates, aluminosilicates, and calcium-alumina-silicates.

• GGBFS = Ground Granulated Blast Furnace Slag

• Cement manufacturers import slag 



Types of blast furnace slag

• Different forms of slag product are produced depending on the method used to cool the molten slag.

• These products include

–        air-cooled blast furnace slag (ACBFS)**** [used as Aggregate]

–        expanded or foamed slag,

–        pelletized slag, and

–        granulated blast furnace slag *****

• GGBFS has cementitious properties, which make a suitable partial replacement for or additive to Portland cement



Steel Slag

• Steel slag, a by-product of steel making, is produced during the separation of the molten steel from impurities in steel-making furnaces.

• The slag occurs as a molten liquid melt and is a complex solution of silicates and oxides that solidifies upon cooling.

• Mostly used as Aggregate



Typical mechanical properties of steel slag

Los Angeles Abrasion (ASTM C131), %:   20 - 25

Sodium Sulfate Soundness Loss (ASTM C88), %: <12

Angle of Internal Friction:            40° - 50°

Hardness (measured by Moh's scale of mineral hardness)*:         6 – 7

California Bearing Ratio (CBR), % top size 19 mm (3/4 inch)**:    up to 300

* Hardness of dolomite measured on same scale is 3 to 4.

** Typical CBR value for crushed limestone is 100%.



BSRM Steel Slag Properties

• CBR = 20%

• Unit weight = 1660 kg/m3

• LAV = 40%

• Specific Gravity = 2.53

• Absorption Capacity = 1.70%



Effect of slag in concrete

Slag

• Slag (also called ground granulated blast furnace slag) is a hydraulic cementitous material produced during the reduction of iron ore to iron in a blast furnace.

• Molten slag is tapped from a blast furnace, rapidly quenched with water ("granulated"), dried and ground to a fine powder.

• The rapid quenching "freezes" the molten slag in a glassy state, which gives the product its cementitious properties.

• Chemically it is similar to, but less reactive than, Portland cement



Effects of Slag

Workability:       Increase

Setting time:     Increase

Compressive strength: Increase

Permeability:    Reduce

Heat of hydration:          Less

Resistance to Chloride attack:    Increase

Resistance to sulfate attack:       Increase

ASR:      Less

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Aggregates

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Aggregates, Part-1

Aggregates




Aggregates

·        Granular material= sand, gravel, crushed stone (stone chips), crushed brick (brick chips), blast furnace slag, shingles

·        60-75% volume of concrete

·        Affect workability of fresh concrete, and properties of hardened concrete



Some Index Properties

·        C….. Uniformly-graded soil

·        D …. Well-graded soil (Cu>4 for gravel, Cu>6 for sand, Cc=1-3)

·        E …. Gap-graded soil

·        D10, D30, D60 = ??

·        D10 = Effective size, D50 = Mean diameter

·        Coefficient of Uniformity,Cu = D_60/D_10

·        Coefficient of Curvature or Coefficient of Gradation ,Cc = ((D_60 )^2)/D_(10D_50 )



Homework – 1:

1. Draw gradation curve of three types of sand having FM = 2.50

2. Prove that minimum FM of sand is 0.00 and maximum FM = 5.00



Significance of aggregate grading (smooth, gap, poor, uniform)

·        Need to know >>> well graded agg or smooth grading, gap grading, uniform grading, poor grading

·        Smooth grading => minimum void => less cement requirement to fill void => cement paste can coat all particles

·        Poor grading => Harsh concrete mix

·        Excessive fine aggregate => more surface area => more cement

·        Maximum size of aggregate: larger => less cement requirement / But too many larger agg => more void unfilled by fine agg => more cement paste requirement


Who dictate nominal max size of agg?

·        Size and shape of concrete member

·        Clear spacing between reinforcing bars

·        Clear cover



Harmful materials in aggregate

·        Organic materials

·        Silt, clay and other fine particles

·        Salt


Salt Harmful effects:

·        Interfere in hydration process of concrete

·        Coatings prevents good bonding

·        Make unsound (volume expansion after hardening)



Specific gravity and density of aggregate

Specific gravity?

Four moisture conditions of aggregate

1.      Damp or Wet or moist (more than 100%)

2.      Saturated surface dry (SSD) (100%)

3.      Air-dry (less than 100%)

4.      Oven-dry or bone dry (0%)



Specific gravity of aggregate

·        Capillary pores are interconnected and extended upto surface

·        Absolute specific gravity: All pores are excluded from V

·        Apparent specific gravity

o  Impermeable pores are included

o  Permeable pores are excluded from V

·        Bulk specific gravity (OD weight is used): All pores are included in V

·        Bulk specific gravity SSD (SSD weight is used)



Bulk density

·        Loose or compacted (rodding, jigging)

·        Batching by volume

·        Bulk density = Mass/volume (volume includes all kind of voids; permeable, impermeable, inter-particle)

·        Factors affecting the bulk density

o  Moisture content, FA+CA

o  Grading, specific gravity, surface texture, shape and angularity

·        Rodded bulk density = 1200 – 1760 kg/m3 (75 – 110 lb/ft3)

·        Bulk density = Unit weight



Absorption Capacity and Moisture Content

·        Total Moisture Content

·        Surface Moisture = Free Moisture

·        Absorption Capacity

·        Total Moisture = Absorbed water + Surface water

·        Absorption Capacity = (SSD-OD)/OD X 100



Examples explaining use of specific gravity and moisture content


Example 9: Calculation of volume of a batch of concrete

The following masses of materials are used to produce a batch of concrete. What is the volume of the concrete if the air content is 3%? (Air content is the volume of air expressed as a percentage of the concrete volume.)



Example 12: Calculation of mixing water and water-cementitious material ratio

In SI units:

What is the mixing water content and water-cementitious material ratio for the following 1-m3 batch of concrete?


Material--Batch mass, kg

Cement : 267

Fly ash: 89

Wet sand (absorption 1.0%, total moisture content 6.1%): 943

Wet gravel (absorption 0.7%, total moisture content 1.3%): 1092

Water (added through batching system): 146


For sand:

Total moisture content =

(943- W_OD)/W_OD X 100 = 6.1%

943 – WOD = 0.061 WOD

WOD = 943/1.061 = 889 kg


Surface moisture content of sand             =             6.1 – 1.0 =      5.1%

Surface moisture content of gravel          =             1.3 – 0.7 =       0.6%

Free moisture on sand                                  =     0.051 x 889 =  45.3 kg

Free moisture on gravel               =   0.006 x 1078 =    6.5 kg

Total free moisture on aggregate             =        45.3 + 6.5 = 197.8 kg

                                                                                                               or     198 kg

Water-cementitious material ratio           = 198/ (267 + 89) =    0.55



Limitations of brick chips as base or subbase

·        Breakage of particles during vehicle dynamic load

·        Low abrasion resistance

·        Fine sand and brick chips mix is a gap graded aggregate

·        Fine sand and brick chips mix in subbase is a sand matrix where brick chips are suspended and isolated particle – this kind


Solution:

use of locally available fine sand, silt and clay soils with chemical stabilization



Topics of aggregate for road pavement

·        Flakiness index

·        Elongation index

·        10% fines value

·        LAV

·        CAV

·        Significance of these indexes

·        Absorption capacity

·        Agg for base and subbase

·        FA and CA mix proportion for subbase

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Aggregates, Part-4
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Aggregates, Part-9

Concrete as Construction Material

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Concrete as Construction Material, Part-1

Concrete as Construction Material




Concrete

·        Concrete is a Masonry work; consider the coarse aggregate particles as brick or masonry unit; C+FA = mortar

·        Concrete is a matrix of inert materials; hydrated cement is the binder: inert materials are FA+CA



Importance of concrete technology

Cooking biriani = making concrete



What are the requirements for a successful concrete structure?

·        strength

·        durability

·        economy


How strong can concrete be?: 50,000 psi !

How long can concrete last?: 1,000 years !



Concrete Mix Ratio which destroying our infrastructures

Mix Ratio (volumetric)

C : FA : CA

1 : 1.5 : 3

1 :  2  : 4

1 :  3  : 6



Properties of Fresh Concrete

·        Workability

o  Consistence (Degree of wetness)

o  Factors: Water, agg, agg/c

o  Measurement of workability (e.g. Slump test)

·        Segregation: Opposite of Cohesion

·        Bleeding Or Water gain



Workability

·        Amount of useful internal work necessary to produce full compaction

·        Workability is inverse of energy required

·        Energy/work is required to overcome internal friction between the individual particles



Factor Affecting Workability of Fresh Concrete

·        Water content *****

o  Aggregate

o  Agg type

·        Grading ***

·        Agg/cement ratio: Inverse relation

·        Admixture

·        Fineness of cement


Advise

·        Pls read pp. 63-64 of Concrete Technology (by Neville and Brooks)

·        Effect of grading on workability is well described here



Effect of grading of aggregate on workability

Four interacting factors

1.      Surface area,

2.      Agg/cement,

3.      Segregation potential,

4.      Amount of fines


Surface Area

·        Smaller particles need more cement paste and water

·        But fine particle (less than 150 micron) act as lubricant


Agg/cement ratio

·        Well graded agg and cement paste exactly necessary to coat the particles  harsh and unworkable mix

·        Excess cement increase workability

·        Excess mortar improves workability



Effect of grading of aggregate on workability

Segregation Potential

·        Well graded agg lead to a dense concrete

·        But small particles can segregate in dry state creating voids in agg

·        What is the relation with workability?

·        Segregation leads to less workable mix


Amount of fine particles in aggregate

·        Less than 300 micron

·        These are very fine sand

·        Some amount of fine particles are necessary for workability of concrete



Strength of Concrete

Factors affecting strength of concrete

·        w/c ratio, degree of compaction

·        Agg/cement ratio

·        Agg properties

o  Shape of agg

o  Size and grading of agg

·        Age


Primary factor is

POROSITY: The relative volume of pores or voids in the cement paste

Then Flaws and Discontinuities

Stress concentration is the mechanism of strength reduction



Admixtures

Admixtures of ConMix

·        MegaFlow R is a retarding, water reducing concrete admixture. Conforms to Type B & D of ASTM C494-2004

·        MegaFlow P4 is a water reducing and retarding plasticising admixture. Conforms to Type D of ASTM C494-2004.

·        MegaFlow P401 is a water reducing, plasticising and retarding admixture. Conforms to Type D of ASTM C494-2004.

·        MegaFlow SP4 is a high range water reducing and set retarding concrete admixture. Conforms to Type G of ASTM C494-2004.

·        MegaFlow SP401 is an advanced superplasticiser, higher grade than SP4. Conforms to Type G of ASTM C494-2004

·        MegaFlow SP102 is a high early strength and high range water reducing superplasticiser.

·        MegaFlow SP103 is a high range water reducing and accelerating, high performance superplasticiser.

·        MegaAir is an air entraining and plasticising concrete admixture. Conforms to ASTM C260.

·        MegaFlow MP is a normal setting and air entraining plasticiser. Conforms to BS:4887.

·        MegaAdd WL1 is a waterproofing admixture for concrete and mortar.

·        MegaAdd CI is a corrosion inhibiting admixture.

·        MegaAdd SAL is a liquid, shotcrete accelerating admixture.



Conplast® SP430 (FOSROC)

Uses

·        To produce pumpable concrete

·        To produce high strength, high grade concrete M30 & above by substantial reduction in water resulting in low permeability and high early strength.

·        To produce high workability concrete requiring little or no vibration during placing.

·        Conplast SP430 has been specially formulated to give high water reductions upto 25% without loss of workability or to produce high quality concrete of reduced permeability.

·        As a guide, the rate of addition is generally in the range of 0.5 - 2.0 litres/100 kg cement.



Available Admixture Brands in Bangladesh

·        Baral Chemicals

·        BASF

·        CONMIX

·        FairMate

·        FOSROC



Curing of Concrete

·        What is meant by curing of concrete?

·        Why is curing important?

·        Durability of concrete is dependent on length of curing. How?

·        Types of curing

o  Normal curing: Spraying, ponding, covering with wet materials, membrane curing

o  Steam curing


·        Disadvantage of membrane curing?

·        Effect of curing temperature on strength

·        What is maturity rule? What is the limitation of maturity rule?

·        Durability, permeability and curing; how are they related?



Durability of concrete

To make durable concrete

1. w/c ratio: w/c ratio as low as possible

2. Compaction: proper compaction makes concrete durable

3. Curing: inadequate curing keep pores in concrete

4. Clear Cover: more than carbonation depth within lifespan

5. Type of Cement: other than CEM-I

6. Aggregate: Avoid brick chips and local sand, use well graded stone chips


Questions:

Durability and permeability of concrete are related. Explain – how?

Explain the three main factors of durability of concrete.



Hydration and porosity of concrete

·        Cement paste contains an interconnected system of pores, when partially hydrated

o  >> lower strength, higher permeability >> vulnerable to chemical, freezing-thawing

·        Pore system become segmented/isolated when degree of hydration is sufficiently high



Durability of concrete – an example

Sulphate attack

·        Symptoms: Whitish appearance, cracking and spalling of concrete

·        Mechanism: formation of calcium sulphate and calcium sulpho-aluminate (from C3A + sulphate), products volume is greater than reactants, resulting expansion and disruption of concrete

·        Damage extent depends on concentration of sulphate and permeability of concrete

·        Remedy:

·        Use of blast furnace slag cement and Portland-pozzolan cement

·        Compaction, curing, clear cover >> durable concrete



Testing of Concrete

Compressive strength

Tensile strength (1/10 of comp strength)

Flexure test

Splitting test

Tensile splitting strength = 2P/(π Ld)



Merits and Demerits of Cylinder

Less end restraint and more uniform distribution of stress over the cross section

Cylinder strength is closer to true uniaxial compressive strength of concrete than the cube strength

Casting and testing in same direction



Non-destructive test of concrete (NDT)

·        Schmidt hammer / rebound hammer / impact hammer test

1.      Rebound number >> comp strength

2.      10-12 readings are necessary for one spot

3.      Plunger must be normal to surface of concrete

·        Penetration resistance

·        Pull out test

·        Ultrasonic pulse velocity test



Quality Control of Concrete by Testing

·        The ACI code specifies that a pair of cylinders must be tested for each 150 yd3 of concrete or for 5000 ft2 of surface area actually placed, but not less than once a day.

·        To ensure adequate concrete strength in spite of such scatter, the ACI code stipulates that concrete quality is satisfactory if

(1) no individual strength test result (the average of a pair of cylinder tests) falls below the required fc’ by more than 500 psi when fc’ is 5000 psi or less or by more than 0.10 fc’ when fc’ is more than 5000 psi, and

(2) every arithmetic average of any three (pair) consecutive strength tests equals or exceed fc’



Quality Control of Concrete – Example

Example – 1: design compressive strength of concrete of a structure is 4000 psi. the test results of concrete are as follows:

Day 1: (2500 psi, 4500psi), (3000 psi, 4680 psi), (4200 psi, 4520 psi)

Day 2: (3500 psi, 4050 psi), (3800 psi, 3900 psi)

Day 3: (4100 psi, 4360 psi)

Day 4: (4500 psi, 4150 psi)

Are these results satisfactory?

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Concrete as Construction Material, Part-2
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Concrete as Construction Material, Part-3
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Concrete as Construction Material, Part-4
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Concrete as Construction Material, Part-5
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Concrete as Construction Material, Part-6
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Concrete as Construction Material, Part-7
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Quality Control of Concrete by Testing, Part-1
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Quality Control of Concrete by Testing, Part-2

Concrete mix design

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Concrete mix design, Part-1

Concrete mix design (ACI method)




Concrete Mix Design

·        Mean compressive strength?

·        Design compressive strength of concrete

o  Or Specified Design strength (US)

o  Or Specified Characteristic strength (UK)

o  Minimum requirement of strength from structural analysis and design

·        Concrete mix design aims at a mean strength

·        Mean strength > Design strength

o  So that most of the concrete strength become stronger than design strength

·        Design Engineer tells you Design Strength necessary for the construction

·        You need to find Target Mean Strength = ? To design the mix

·        You need to use standard deviation to find the mean strength

·        Standard deviation comes from previous experience of mix design and/or test results of concrete



Quality Control of Concrete by Testing

o  The ACI code specifies that a pair of cylinders must be tested for each 150 yd3 of concrete or for 5000 ft2 of surface area actually placed, but not less than once a day.

o  To ensure adequate concrete strength in spite of such scatter, the ACI code stipulates that concrete quality is satisfactory if

(1) no individual strength test result (the average of a pair of cylinder tests) falls below the required fc’ by more than 500 psi when fc’ is 5000 psi or less or by more than 0.10 fc’ when fc’ is more than 5000 psi, and

(2) every arithmetic average of any three (pair) consecutive strength tests equals or exceed fc’



Mix Design as per ACI 211.1-91

o  ACI = American Concrete Institute

o  ACI 211.1-91 is part of ACI Manual published by American Concrete Institute.

o  Standard practice for selecting proportions for normal, heavyweight and mass concrete is described here.

o  Guide for selecting proportions for high-strength concrete is described in ACI 211.4R-93.

o  Mean target strength upto 6000 psi (41 MPa) is considered as normal strength concrete and mean target strength above 6000 psi is considered as high-strength concrete.

o  Smaller size aggregates have been shown to provide higher strength potential.

o  Exposure condition of concrete limits the w/c ratio to ensure durability of concrete in adverse environment.

o  Slump value of concrete should be selected based on scope of compaction during casting, for example, to make flowing concrete slump should be in the range of 150 – 200 mm, concrete for cast in situ piles should have slump value in the range of 150-200 mm.



Mix Design Example

o  ACI method

o  Composite cement

o  Ready mixed concrete will be cast by pumping

o  Admixture (Super Plasticizer) will be used to make flowing concrete

o  Concrete will remain under sea water (i.e. offshore structure)

o  Design strength of concrete is 35 MPa

o  All necessary data are given in the following tables



Step 1: Selection of slump value

To make flowing concrete, slump = 100 – 150 mm



Step 2: Selection of maximum size of coarse aggregate

(a) Nominal maximum size of coarse aggregate should be the largest possible which is economically available.

(b) Maximum size of coarse aggregate should be less than

o  One-fifth of the narrowest dimension of the structure

o  One-third of the depth of slab

o  Three-fourth of minimum clear spacing between bars

o  Clear cover

Here, we have no information about size of structure and maximum size of available aggregate is 20 mm. So, maximum size of CA = 20 mm



Step 3: Estimation of mixing water content and air content

Super plasticizer Megaflow230 will be used to increase workability. Megaflow230 can reduce mixing water content upto 20% depending on its dose.

If we use the dose of Megaflow230 = 1000 ml per 100 kg cement and assume that it would reduce water content by 15%,

So, Mixing water = 210.5 x 0.85 = 179 kg per 1 m3 fresh concrete



All the ingredients as calculated above are mixed in mixer machine. From visual observation it seems that more water is necessary to get required slump. 0.3 kg more water is added and concrete is mixed again. Freshly mixed concrete is taken out of mixer machine and slump test is performed.

Slump value = 100 mm found from slump test.

To measure density of fresh concrete, 3 empty cylinders are weighed, filled with concrete, compacted, leveled and cleaned outside of mold. Then filled up molds are weighed. Density of concrete is calculated as follows.


Measured density of fresh concrete = 2390 kg/m3

Mass of water added = 3.20 + 0.30 = 3.50 kg

Mass of ingredients mixed

= 3.50 + 9.42 + 14.26 + 19.92 + 0.079

= 47.179 kg



Step 9: Revision of mix proportion based on First Lab Trial Mix result

ACI suggests that if the slump of the trial mix does not satisfy the requirement, increase or decrease the re-estimated water content by 2 kg/m3 for each increase or decrease of 10 mm slump.


To get target mean strength or same durability, we have to keep the w/c ratio constant as calculated in step 4.



Step 10: Final trial mix to confirm strength and slump

Consider 25% loss during handling,

Volume of fresh concrete needed for final trial mix

= 1.25*0.0141 = 0.0176 m3


This is very small amount of mix which may lead to significant errors due to errors in measurement and loss in mixing. For this reason, it is wise to make at least 0.025 m3 concrete for final trial mix.



Step 11: Adjustment for casting in site

From table 9,

Moisture content in FA = 5%

Moisture content in CA = 1%

FA (OD) = 635 kg,

CA (OD) = 1005 kg


Adjusted mixing water

= 198 – 23.2 – 3.1 = 171.7 kg = 172 kg

Adjusted FA = 644 + 23.2 = 667 kg

Adjusted CA = 1012 + 3.1 = 1015 kg



Step 12: Conversion to volumetric ratio

Note that generally

1 bag cement

= 50 kg cement mass

= 1.25 cft compacted bulk volume of cement

= 1.60 cft loose bulk volume of cement



However, among practicing engineers in Bangladesh, 50 kg cement = 1.25 cft bulk volume of cement is known and used for calculating volumetric ratio.

Volumetric ratios like 1:1.5:3 or 1:2:4 which are very familiar to practicing engineers in Bangladesh have no rational basis. This kind of ratios should not be used any more.

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Concrete mix design, Part-4
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Concrete mix design, Part-6
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Concrete mix design, Part-7
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Concrete mix design (Excel sheet)

Climate Resilient Concrete

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Climate Resilient Concrete, Part-1

Climate Resilient Concrete




Defining Climate Resilient Concrete

The infrastructure which

1.      Is durable

o  so that less cement will be used in its life-cycle but initial cement consumption may be higher than traditional practices

o  Because 1 ton of clinker production emits approximately 1 ton of green house gas


2.      Uses recycled material

o  So that earth become free from hazardous materials

o  Fly Ash and Slag are two important recycled material for cement

3.      Uses local material

o  So that green house gas emission by transportation will be less



Enemy of RCC

Porosity >>>>>>> Rebar Corrosion

Result is total disintegration of concrete



Reasons

Poor quality of concrete?

Water ingress from roof to entire structure?

Harmful materials in ingredients of concrete?



Durability issues

o  Permeability

o  Chloride attack

o  Sulfate attack

o  Rebar corrosion

o  Carbonation

o  Acid attack

o  Effect of sea water

o  Cracking of concrete

o  Clear cover



How to Make Durable Concrete

1. w/c ratio: w/c ratio as low as possible

2. Compaction: proper compaction makes concrete durable

3. Curing: inadequate curing keep pores in concrete

4. Clear Cover: more than carbonation depth within lifespan

5. Type of Cement: other than CEM-I

6. Aggregate: Avoid brick chips and local sand, use well

           graded aggregate


Questions:

Durability and permeability of concrete are related. Explain – how?



High strength vs normal strength concrete

1.      Use high strength concrete

o  Durable

o  Small dimension

2.      Use stone chips for all concrete exposed to weather

o  Fly over

o  Bridge

o  Footing, pile, grade beam, roof,

o  RCC Road pavement

o  Concrete in coastal area

o  Roof of building and water tanks



Concrete and the Passivating Layer

o  Although steel’s natural tendency is to undergo corrosion reactions, the alkaline environment of concrete (pH of 12 to 13) provides steel with corrosion protection.

o  At the high pH, a thin oxide layer forms on the steel and prevents metal atoms from dissolving. This passive film does not actually stop corrosion; it reduces the corrosion rate to an insignificant level.

o  For steel in concrete, the passive corrosion rate is typically 0.1 µm per year. Without the passive film, the steel would corrode at rates at least 1,000 times higher (ACI 222 - 2001).



Carbonation

o  Cement paste contains 25-50 % wt calcium hydroxide [Ca(OH)2], which mean that the pH of the fresh cement paste is at least 12.5.

o  The pH of a fully carbonated concrete is about 7. 

o  The concrete will carbonate if CO2 from air or from water enters the concrete according to:

Ca(OH)2 + CO2    ->        CaCO3 + H2O

o  The rate of carbonation depends on porosity & moisture content of the concrete.

o  Optimal conditions for carbonation occur at a RH of 50% (range 40-90%).

o  Normal carbonation results in a decrease of the porosity making the carbonated paste stronger. Carbonation is therefore an advantage in non-reinforced concrete.

o  However, it is a disadvantage in reinforced concrete, as pH of carbonated concrete drops to about 7; a value below the passivation threshold of steel.



Acid Attack (CC+RCC)

o  Concrete is susceptible to acid attack because of its alkaline nature.

o  The components of the cement paste break down during contact with acids.

o  Most pronounced is the dissolution of calcium hydroxide which occurs according to the following reaction:

           2 HX + Ca(OH)2 -> CaX2 + 2 H2O

           (X is the negative ion of the acid)

o  The decomposition of the concrete depends on the porosity of the cement paste, on the concentration of the acid, the solubility of the acid calcium salts (CaX2) and on the fluid transport through the concrete.

o  An acid attack is diagnosed primarily by two main features:

·        Absence of calcium hydroxide in the cement paste

·        Surface dissolution of cement paste exposing aggregates



Alkali silica reaction (CC+RCC)

o  Alkali silica reaction is a heterogeneous chemical reaction which takes place in aggregate particles between the alkaline pore solution of the cement paste and silica in the aggregate particles.

o  Hydroxyl ions penetrate the surface regions of the aggregate and break the silicon-oxygen bonds. Positive sodium, potassium and calcium ions in the pore liquid follow the hydroxyl ions so that electro neutrality is maintained. Water is imbibed into the reaction sites and eventually alkali-calcium silica gel is formed.

o  The reaction products occupy more space than the original silica so the surface reaction sites are put under pressure. The surface pressure is balanced by tensile stresses in the center of the aggregate particle and in the ambient cement paste.

o  At a certain point in time the tensile stresses may exceed the tensile strength and brittle cracks propagate. The cracks radiate from the interior of the aggregate out into the surrounding paste.

o  Alkali silica reaction is diagnosed primarily by four main features

·        Presence of alkali silica reactive aggregates

·        Crack pattern

·        Presence of alkali silica gel in cracks and/or voids

·        Ca(OH)2 depleted paste



Sulfate Attack (CC+RCC)

o  External sulfate attack is a chemical breakdown mechanism where sulfate ions from an external source attack components of the cement paste.

o  Such attack can occur when concrete is in contact with sulfate containing water / soil, e.g. seawater, swamp water, ground water or sewage water.

o  The often massive formation of gypsum and ettringite formed during the external sulfate attack may cause concrete to crack and scale.

o  However, both laboratory studies and examinations of field concrete show that external sulfate attack is often manifested, not by expansion or cracking, but by loss of cohesion and strength. 

o  The microscopic appearance of concrete suffering from external sulfate attack appears to be quite variable. Some diagnostic features such as

·        Surface parallel cracks

·        Presence of gypsum and ettringite

·        Depletion of calcium hydroxide, and

·        Decalcification of C-S-H



Chloride attack (RCC)

o  Primary action is corrosion of steel

o  Chloride ion destroy the passivating oxide film on steel surface

o  In the presence of H2O and O2, corrosion of steel occurs

o  The primary corrosion rate-controlling factors are the availability of oxygen, the electrical resistivity and relative humidity of the concrete, and the pH and temperature.

o  There is no corrosion in dry concrete (RH < 60%)

o  There is no corrosion in concrete which is fully immersed in water

o  Optimum RH = 70 – 80% for corrosion of steel inside concrete

o  Pore system in hardened concrete is the major influencing factor of corrosion; because electrochemical cell requires connection between anode and cathode through pore water

o  Electrical resistivity greatly influenced by it moisture content, ionic composition pore fluid and continuity of pore system

o  Two consequences of corrosion

·        Cracking, spalling, delamination of concrete due greater volume of product of corrosion reaction

·        Reduction of steel cross-section leading reduced capacity of RCC section



Concrete in Sea Water with alternating wetting and drying

o  Chloride attack - discussed

o  Sulfate attack - discussed

o  Salt weathering

o  Abrasion



Salt weathering

o  Alternating wetting and drying

o  During drying, pure water evaporates, salts are left behind in form of crystals, mainly sulfates

o  These crystals re-hydrate and grow upon subsequent wetting and thereby exert expansive force

o  High humidity and air-borne salt deposition on concrete also do the same



Cracked Concrete (Flexural Members) and durability

o  Relevant research by Hearn (1999) shows also that the micro-cracking due to early shrinkage contributes much more to the water infiltration than concentrated load-induced cracks.

o  If that is true, does it make a sense for durability design to limit the load-induced crack width?

o  We often design water retaining structures considering uncracked section

o  To limit the shrinkage crack smaller and distributed to whole structure maximum spacing of rebar should be limited to 150 mm (my opinion)

o  Check serviceability / deflection (crack will be controlled) (my opinion)

Ref: Hearn N (1999) Effect of shrinkage and load-induced cracking on water permeability of concrete. ACI Mater J 96(2):234–241



Cracking of concrete and durability

o  Although all concrete structures in service exhibits some crack; cracking can be controlled by appropriate structural design, detailing and construction procedure

o  Cracks wider than 0.2 – 0.4 mm are harmful ( Naville, 1995)

o  Prestressed concrete is crack free; if good quality concrete is used – it is best; but presstressing steel is more vulnerable to corrosion than other steel



Curing and Durability

o  Cement paste contains an interconnected system of pores, when partially hydrated

>> lower strength, higher permeability >> vulnerable to chemical, freezing-thawing

o  Pore system become segmented/isolated when degree of hydration is sufficiently high



Clear Cover and durability

o  More clear cover more durability of concrete; however, the concrete is porous, clear cover can not increase durability.

o  Excessive clear cover will lead to temp and shrinkage cracks

o  If cracking occurs, clear cover may be detrimental

o  Clear cover should not exceed 80 – 100 mm (Naville, 1995)

o  In thin sections, it is difficult to maintain large clear cover



Criticism on MM recommendation

No mix ratio (volumetric or mass) is specified

No w/c ratio is specified

Corrosion inhibitor used – controversial

Exposure class – it is good initiative



Wrong spec

w/c is specified – it is good and correct

Mix proportions are not compatible with w/c ratio

Mentioned compressive strength is not compatible with w/c ratio

CEM-I is suggested which is not good for durable concrete

No spec for fine aggregate

Maximum size of coarse aggregate is not specified

Stone chips is specified – it is good and required for durable concrete



Brick chips – post mortem

How brick chips destroying environment

o  Brick fields uses soil from agriculture land

o  Emit green house gases during burning

o  Emit green house gases during breaking

o  Utilize 3 – 5 times cement in a 100 years life of a structure. 1 ton cement emit 1 ton CO2

o  Life safety is at risk due to unsafe

o  Building, bridge, culvert etc



Concrete age > 15 yrs

o  The condition of marine concrete structures greater than 15 years old in the exposed coastal Upazillas were found to be severely deteriorating.

o  Half-cell potential testing of most of the concrete structures at this age suggest high-severe risk of reinforcement corrosion.

o  The visual observations on concrete cores extracted from these structures suggest workmanship issues related to use of poor graded aggregates, non-homogeneous concrete mix and voiding at the interface between deck slab concrete and wearing course layer.



Concrete age = 5 - 15 yrs

o  The visual observation of concrete core sample extracted from the culvert at Boalkhali road, Islamabad Union, Cox’s Bazaar, under the 5-15 year age category, suggest that the stone aggregates used in the concrete were poorly graded with high proportion of >25mm particle size aggregates.

o  The additional survey of buildings in this age category suggest that concrete with brick aggregates especially in exposed coastal Upazilla showed signs of early deterioration of concrete caused by salt scaling and corrosion of reinforcement.



Concrete age = 1 - 5 yrs

o  The newer concrete structures (1-5 years age category) predominantly had stone aggregates in concrete, which provides better durability compared with brick aggregate concrete.

o  The inspection of new construction sites suggested that in the case of manual production of concrete workmanship issues related to use of poor graded aggregates, improper compaction of concrete, use of saline water for concrete mixing and lack of quality control testing were observed.



Chloride content in water

o  The comparison of salinity of local water samples obtained close to the road structures surveyed in each district suggest that the chloride content in ground water was observed to be low as compared with canal/river water in the exposed coastal Upazillas.

o  The chloride content of water sourced from interior coastal Upazillas were observed to be very low/negligible. As the water sampling was done during the rainy monsoon season (July-October), the chloride content of water is expected to be low compared to summer season.



Most important

o  In-situ concrete strength for most of the structural elements were found to be much lower than the design strength of 20 MPa

o  The deterioration process is rapidly accelerated in concrete structures containing brick aggregates especially in exposed coastal districts



Corrosion inhibitor???

The study on use of calcium nitrate corrosion inhibitor suggests that at recommended 3-4% dosage of corrosion inhibitor has accelerated the setting time of cement drastically.

However, with the use of set retarders, the accelerating effect can be counteracted.

The experimental trials at different dosages of corrosion inhibitor and set retarder suggested optimum combination at 3.5% corrosion inhibitor and 1.8% set retarder resulted in acceptable setting time results in cement samples.



Experimental finding on durability

o  The durability of brick aggregate concrete mixes was significantly poorer than the stone aggregate concrete mixes

o  In the case of concrete with blended cements, there was no relationship between strength and durability performance

o  The durability performance of concrete improved with increase in cement content of the concrete. However, in the case of 100% CEM I concrete mix no further improvement in durability performance was observed with increase in cement content from 450 kg/m3 to 550kg/m3.

o  Concrete mixes with fly ash addition showed better durability performance in comparison to slag based concrete mix.

o  In general, among the different cement types, 100% CEM I concrete mix showed poor durability performance as compared to blended cement based concrete mix.

o  Among all the concrete mixes tested in the experimental programme, concrete mix with 30% fly ash as cementitious addition and 550 kg/m3 cement content showed the best durability performance to resist chloride induced corrosion

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Climate Resilient Concrete, Part-2
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Climate Resilient Concrete, Part-3
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Climate Resilient Concrete, Part-4
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Climate Resilient Concrete, Part-5
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Climate Resilient Concrete, Part-6
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Brick chips post mortem

Climate Resilient Concrete Manual for Volumetric Batching

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Climate Resilient Concrete Manual, Part-1

Climate Resilient Concrete Manual for Volumetric Batching




Challenges of making climate resilient concrete

o  w/c ratio at site condition

o  Improper mix ratio

o  Cement type

o  Unsuitable aggregates



Exposure classes

o  Severe

o  Moderate

o  Low

o  normal

Note: Index for distance from shore line, outdoor/indoor, within soil/water, cyclic wetting/drying zone, brick chips used, sulfate/chloride content in water/soil, test results available/not available, section thickness, other threats to durability considered



Concrete for Severe Exposure

o  Maximum w/c = 0.40

o  Minimum compressive strength = 5000 psi (35 Mpa)

o  Slump = 50 – 100 mm except tremie concrete

o  Mix proportion = 1 : 1.2 : 2.1

o  Cement type = CEM-II

o  Fine Aggregate = Sylhet sand or eauivalent (FM>2.40)

o  Coarse Aggregate = 20 mm down well graded stone chips (can be achieved by mixing 20 mm and 12 mm mixed 50:50 ratio)

o  Special note: water reducing admixture need to be used if slump required is more than 100 mm



Concrete for Moderate Exposure

o  Maximum w/c = 0.45

o  Minimum compressive strength = 4500 psi (31 Mpa)

o  Slump = 50 – 100 mm except tremie concrete

o  Mix proportion= 1 : 1.5 : 2.3

o  Cement type = CEM-II

o  Fine Aggregate = Sylhet sand or eauivalent (FM>2.40)

o  Coarse Aggregate = 20 mm down well graded stone chips (can be achieved by mixing 20 mm and 12 mm mixed 50:50 ratio)

o  Special note: water reducing admixture need to be used if slump required is more than 100 mm



Concrete for Low Exposure

o  Maximum w/c = 0.50

o  Minimum compressive strength = 4000 psi (28 Mpa)

o  Slump = 50 – 100 mm except tremie concrete

o  Mix proportion= 1 : 1.7 : 2.6

o  Cement type = CEM-II

o  Fine Aggregate = local sand and Sylhet sand mixed in 25:75 ratio (FM>2.00)

o  Coarse Aggregate = 20 mm down well graded stone chips (can be achieved by mixing 20 mm and 12 mm mixed 50:50 ratio)

o  Special note: water reducing admixture need to be used if slump required is more than 100 mm



Concrete for None Exposure

Maximum w/c = 0.55

Minimum compressive strength = 3000 psi (21 Mpa)

Slump = 50 – 100 mm except tremie concrete

Mix proportion= 1 : 2 : 2.8

Cement type = CEM-II

Fine Aggregate = local sand and Sylhet sand mixed in 50:50 ratio (FM>1.70)

Coarse Aggregate = 20 mm down well graded stone chips (can be achieved by mixing 20 mm and 12 mm mixed 50:50 ratio) OR brick chips

Special note: water reducing admixture need to be used if slump required is more than 100 mm



How to maintain volumetric mix ratio

1.      Use known volume containers

2.      Assume 1 bag cement = 1.25 cft; never measure in containers

3.      Mixing water shall be determined by trial to get required slump value; never use specified w/c ratio

4.      Strictly follow specified mix ratio

5.      Use mixer machines

6.      Frequently measure slump value

7.      Use vibrator for compaction



Curing

o  Curing means ensuring the hydration of cement with water.

o  Curing must be started after final setting time (approximately 10-12 hours) of concrete.

o  After initial setting time, concrete shall be covered with polythene or any other material in hot weather with relative humidity less than 60%.

o  If relative humidity is more than 80%, no curing is required for concrete.

o  Preferable curing time is 28 days. Minimum curing period is 14 days.


Curing of slab, pile cap, footing and road pavement

o  Ponding is the best method for these concrete structures. After one hour of concrete casting, concrete surface shall be covered by polythene or Hessian at hot weather with relative humidity less than 60%.

o  After 12 hours of concrete casting, ponding shall be done by placing clay barrier at perimeter and pouring water on surface of concrete.


Curing of column, abutment, wing wall and beam

o  Curing of these structures may be done by covering with polythene sheets. Properly covered concrete does not need any water for curing.

o  Wet Hessian (Jute Textile) may be used for curing. Hessian shall be wetted twice daily.

2
Climate Resilient Concrete Manual, Part-2
3
Climate Resilient Concrete Manual, Part-3

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Durable Concrete, Cement, Aggregate and Mix Design
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