Civil Engineering Department
The word “concrete” originates from the Latin verb “concretus”, “concretus”, which means to grow together.
Concrete Technology
y g o l o n h c e T e t e r c n o C
Objectives
• To explain the basic concepts of concrete • To explain briefly the properties of freshly mixed concrete • To explain briefly the properties of hardened concrete • To explain briefly mix design of concrete
What is Concrete? Concrete? • Concrete is one of the most commonly used building materials. • In the United States almost twice as much concrete is used as all other building materials combined. • It is a mixture of cement, water, fine aggregates, coarse aggregates and in some cases, admixtures. • Cement and water form a paste that hardens and bonds the aggregates together. together. • Concrete is a versatile material that can easily be mixed to meet a variety of special needs and formed to virtually any shape. • Concrete is often looked upon as “man made rock”. • Demand for concrete with higher strength and better quality, coupled with larger and faster mixer trucks, led to the emergence of the ready-mix concrete industry in the post-World War II period.
Contd….. Cement
→ Powder
Cement + Water →Cement Paste Cement Paste + Fine Aggregate (FA) → Mortar Mortar + Coarse Aggregate (CA) → Concrete Portland
cement, water, water, sand, and coarse aggregate are proportioned and mixed to produce concrete suited to the particular job for which it is intended. When paste and aggregates are mixed together the particles of aggregates get coated with the paste. Because of the chemical reaction of the cement and water the paste hardens and binds the aggregates together to form a rocklike mass called “Concrete” Definition of Cement us ed to bind different aggregates (coarse Cement is a binding material that is used and fine) in the presence of moisture (water) through a chemical process known as hydration. Cement contains limestone, clay , cement rock and iron ore blended and heated to 1200 to 1500 C°. powder. The resulting product "clinker" is then ground to the consistency of powder. Gypsum is added to control setting time.
Contd….. Fine Aggregate •Normally called sand, this component can be natural sand or crushed stone,
and represents particles smaller than 4.75mm •Generally accounts for 30%-35% of the mixture. Coarse Aggregate
•May be either gravel or crushed stone. • Makes up 40%-45% of the mixture, comprised of particles greater than 4.75mm. Chemical Admixtures
•Materials added to alter the properties of concrete including: Air
entrainment Set accelerators Set retarders Water reducers • Air entraining admixtures add microscopic air bubbles to the concrete, enhancing its resistance to freeze/thaw cycles and makes the concrete easier to finish.
Contd….. •Set accelerators speed the set-time of the mixture, enabling finishing operations to begin sooner, useful during cold weather pours.
•Set retarders have the opposite effect, slowing the set and enabling delivery to distant sites and finishing during hot weather.
•Water reducers are used to reduce the amount of water required to produce a given slump. They also provide a ball bearing effect, making the concrete easier to finish, and produce better cement hydration. By reducing the amount of water required, cement amounts can be reduced because concrete strength is directly related to the water/cement ratio. Yield It
is the volume of fresh concrete produced from known quantities of component materials. Generally expressed in cubic yards or cubic meters.
Advantages of concrete •
Concrete has many environmental advantages, including durability, longevity, heat storage capability, and chemical inertness.
•
Ability to be cast
•
Economical
•
Durable
•
Fire resistant
•
Energy efficient.
•
Low maintenance.
•
On-site fabrication
•
Needs little or no finish or final treatments. Chemically inert concrete doesn't require paint to achieve a given colour; natural -mineral pigments and colouring agents can be added at the mixing to provide a rainbow of options.
•
Can be reused or recycled. Concrete can be reused with bituminous asphalt as road base materials, can be recycled and reused by crushing into aggregates for new concrete or as fill material for road beds or site works.
Disadvantages of concrete •
Low tensile strength
•
Low ductility
•
Volume instability
•
Low strength to weight ratio
Progress in Concrete Technology
Lightweight Concrete
High-Strength Concrete
High Workability or Flowing Concrete
Shrinkage Compensating Concrete
Fiber-Reinforced Concrete
Concrete Containing polymers
Heavyweight Concrete
Mass Concrete
Roller-Compacted Concrete
Quality of Concrete Aggregate Requirement • Aggregates should not contain deleterious materials such as chloride, sulfate, etc. that will cause deterioration of concrete. • The size, shape and particle size distribution (gradation), of the aggregates should be such that the least amount of paste is required to fill the spaces between aggregate particles. Paste Requirement
• Quality of paste greatly affects the quality of the concrete • Paste should be sufficient enough to completely coat each particle of aggregate and to fill spaces between aggregate particles completely. • The quality of hardened concrete is determined by the amount of water used in relation to the amount of cement (W/C ratio of the paste).
Advantages of reducing the W/C ratio of the paste • Increase compressive strength and flexural strength. • Lower permeability, thus increased water-tightness and lower absorption. • Increased resistance to weathering. • Better bond between successive layers and between concrete and reinforcement. • Less volume change from wetting and drying • Reduced shrinkage cracking tendencies. Uses of Admixtures to Improve the Quality of Concrete
• Adjusting setting time or hardening in abnormal conditions of concreting such as cold and hot. • Reducing water demand • Increasing workability in case of a very low w/c ratio • Intentionally entrained air in concrete
Mixing Requirements • All the ingredients of concrete should be mixed properly so as to get a homogeneous mixture. • The sequence of charging ingredients into the mixer plays an important part in the uniformity of the finished product. • Size of the batch in relation to the size of the mixer. • The elapsed time between batching and mixing. • Design, configuration, and condition of the mixer drum and blades. Consolidation Requirements • Consolidation of the freshly placed concrete removes voids in concrete and improves the quality of concrete on account of increased density. • For a highly workable concrete the consolidation should be done by hand rodding. • For stiffer mixtures having low w/c ratio the consolidation should be done by vibration.
Portland Cement • Portland cement was named for the Isle of Portland, a peninsula in the English Channel where it was first produced in the 1800's. • Since that time, a number of developments and improvements have been made in the production process and cement properties. • The production process for portland cement first involves grinding limestone or chalk and alumina and silica from shale or clay. • The raw materials are proportioned, mixed, and then burned in large rotary kilns at approximately 1500°C until partially fused into marblesized masses known as clinker. • After the clinker cools, gypsum is added, and both materials are ground into a fine powder which is portland cement. • Three types of portland cement are used for construction purposes
Type I
Type II
Type III
Contd…… Type I
cement is the general purpose and most common type. Unless an alternative is specified, Type I is usually used. Type II It releases less heat during hardening. It is more suitable for projects involving large masses of concrete , heavy retaining
walls, or for suspension bridges. Type III It produces concrete that gains strength very rapidly. It is very finely ground and sets rapidly, making it useful for cold weather jobs.
Water • Good water is essential for quality concrete. • It should be good enough to drink--free of trash, organic matter and excessive chemicals and/or minerals. • The strength and other properties of concrete are highly dependent on the amount of water and the water-cement ratio. Aggregates
• Aggregates occupy 60 to 80 percent of the volume of concrete. • Sand, gravel and crushed stone are the primary aggregates used. • All aggregates must be essentially free of silt and/or organic matter. Admixtures
• Admixtures are ingredients other than portland cement, water, and aggregates. • Admixtures are added to the concrete mixture immediately before or during mixing.
Air Entraining agents • These are the most commonly used admixtures for agricultural concrete. • produce microscopic air bubbles throughout the concrete. • Entrained air bubbles: – improve the durability of concrete exposed to moisture and freeze/thaw action. – Improve resistance to scaling from deicers and corrosive agents such as manure or silage. Retarding admixtures
• are used to slow the rate of concrete hardening. • They are useful for concrete that is placed during hot weather. Accelerating admixtures
• such as calcium chloride, are used to increase the rate of hardening-usually during cold weather.
Water-to-Cement Ratio • The ratio of water-to-cement, or w/c, is the single most important parameter with regards to concrete quality and used to determine the water used in the mix. • Theoretically, about 0.22 to 0.25 is required for complete hydration – Practically, the useful limit is around 0.33 • Reducing the water for a given amount of cement will move the cement particles closer together, which in turn densifies the hydrated cement paste – This increases strength and reduces permeability • It also makes the concrete more difficult to work • In combination, the w/c and degree of hydration control many of the properties of the hardened concrete .
Properties of fresh concrete Workability
• Workability is the most important property of freshly mixed concrete. • It is desirable that freshly mixed concrete be relatively easy to transport, place, compact and finish without harmful segregation. • A concrete mix satisfying these conditions is said to be workable.
• There is no single test method that can simultaneously measure all the properties involved in workability. • It is determined to a large extent by measuring the “consistency” of the mix. • Strongly associated with the slump test
Slump Cone Test
• Slump Test is related with the ease with which concrete flows during placement
10 cm
30 cm
20 cm
The slump cone is filled in 3 layers. Every layer is evenly rodded 25 times.
Measure the slump by determining the vertical difference between the top of the mold and the displaced original center
Contd….
Factors Affecting Workability • • • • • • •
Method and duration of transportation Quantity and characteristics of cementing materials Aggregate grading, shape and surface texture Quantity and characteristics of chemical admixtures Amount of water Amount of entrained air Fluidity, stability, pumpability, compactability and finishability These properties are affected by: Design of mix Adjustment to the mix constituents Environmental Conditions: Ambient and concrete temperature Relative humidity and the air speed Degree of agitation and friability of aggregrates Elapsed time since mixing
Consistency •
Consistency is the fluidity or degree of wetness of concrete.
•
It is generally dependent on the shear resistance of the mass.
•
It is a major factor in indicating the workability of freshly mixed concrete. Test methods for measuring consistency are : Flow test → measures the amount of flow
Kelly-Ball test → measures the amount of penetration Slump test (Most widely used test!)
Segregation •
Segregation refers to a separation of the components of fresh concrete, resulting in a non-uniform mix.
The primary causes of segregation are differences in specific gravity and size of constituents
of concrete. Some of the factors affecting segregation:
– Larger maximum particle size (25mm) and proportion of the larger particles. – Improper mixing, improper placing and improper consolidation. – High specific gravity of coarse aggregate. – Decrease in the amount of fine particles. – Particle shape and texture. – Water/cement ratio.
Bleeding •
Bleeding is the tendency of water to rise to the surface of freshly placed concrete. It is caused by the inability of solid constituents of the mix hold all of the mixing water as they settle down. • A special case of segregation.
•
• •
Undesirable effects of bleeding are: With the movement of water towards the top, the top portion becomes weak and porous (high w/c). Thus the resistance of concrete to freezing-thawing decreases. Water rising to the surface carry fine particles of cement which weaken the top portion and form laitance. This portion is not resistant to abrasion. Water may accumulate under the coarse agg. and reinforcement. These large voids under the particles may lead to weak zones and reduce the bond between paste and agg. or paste and reinforcement. It is decreased by:
Increasing the fineness of cement
Increasing the rate of hydration (C3S, C3A and alkalies)
Adding pozzolans
Reducing water content
Mixing of concrete
The aim of mixing is to blend all of the ingredients of the concrete to form a uniform mass and to coat the surface of aggregates with cement paste.
Ready-Mix concrete: In this type ingredients are introduced into a mixer truck and mixed during transportation to the site. •
Wet – Water added before transportation
•
Dry – Water added at site
Mixing at the site •
Hand mixed
•
Mixer mixed
Mixing time should be sufficient to produce a uniform concrete.
The time of mixing depends on the type of mixer and also to some properties of fresh concrete.
Undermixing → non-homogeneity Overmixing → danger of water loss, brekage of aggregate particles
Ready Mix Concrete
Mixing at site
Methods of transporting concrete
• Wheelbarrows • Hoists
• Trucks • Chutes • Pumps • Pipelines
Trucks • Trucks range from 1.8 mini trucks to 7.5 metres capacity • When using trucks we may need to specify – Size of truck i.e 6 wheeler or 8 wheeler – A cubic metre concrete weighs 2.5t – A 7.5 metre truck will carry 18.75t of concrete – This will need to be taken account of when driving over structures – Always be aware that after trucks discharge concrete they actually get higher Chutes A convenient method to transport concrete
• Items that can be used to make a chute – Roof sheeting – Plywood
Pumps and pipelines • Concrete pumps come in two types, line and boom pumps Concrete pumps
• When ordering pumps consideration – A pump’s footprint (a 42m boom pump may need up to 12m in width to operate properly) – Generally boom pumps will take up 2 lanes of road traffic and traffic management plans will need to be put in place – Boom pumps can reach approximately 15 floors after this concrete line pumps can only be used
Placing of Concrete • Placed vertically and near its final position • If it needs to be moved it needs to be done with shovels • Dropping of concrete should be restricted to 1.8m to avoid segregation • Sequence should be planned to avoid cold joints on large slabs Compacting
• Compaction is required to achieve – Maximum Strength – Watertight Concrete – Fill in Sharp Corners – Good bond to reinforcement – Good surface appearance
Inadequate Compaction
• This has a severe effect on concrete strength
Consolidating concrete
Inadequate consolidation can result in:
– Honeycomb – Excessive amount of entrapped air voids (bug holes) – Sand streaks – Placement lines (Cold joints)
Vibration of concrete
The process of compacting concrete consists essentially of the elimination of entrapped air. This can be achieved by:
–
Tamping or rodding the concrete
–
Use of vibrators
Internal vibrator: The poker is immersed into concrete to compact it. The poker is easily removed from point to point.
External vibrators: External vibrators clamp direct to the formwork requiring strong, rigid forms. Vibrator
d
1½ R
Radius of Action
Internal Vibrators • To aid in the removal of trapped air the vibrator head should be rapidly plunged into the mix and slowly moved up and down.
The actual completion of vibration is judged by the appearance of the concrete surface which must be neither rough nor contain excess cement paste.
Contd….. Adapted from ACI 309
Diameter Recommended Approximate Rate of of head, frequency, radius of placement, (vib./min.) action, (mm) (m3/h) (mm)
20-40
30-60
50-90
9000-15,000
8500-12,500
8000-12,000
80-150
130-250
180-360
Application
0.8-4
Plastic and flowing concrete in thin members. Also used for lab test specimens.
2.3-8
Plastic concrete in thin walls, columns, beams, precast piles, thin slabs, and along construction joints.
4.6-15
Stiff plastic concrete (less than 80-mm slump) in general construction .
External Vibrators •
Form vibrators
•
Vibrating tables (Lab)
•
Surface vibrators
– Vibratory screeds – Plate vibrators – Vibratory roller screeds – Vibratory hand floats or trowels
•
External vibrators are rigidly clamped to the formwork so that both the form & concrete are subjected to vibration.
•
A considerable amount of work is needed to vibrate forms.
•
Forms must be strong and tied enough to prevent distortion and leakage of the grout.
Contd…..
Vibrating Table: used
for small amounts of concrete (laboratory and some precast elements)
Systematic Vibration
CORRECT Vertical
penetration a few inches into previous lift (which should not yet be rigid) of systematic regular intervals will give adequate consolidation
INCORRECT
Haphazard
random penetration of the vibrator at all angles and spacings without sufficient depth will not assure intimate combination of the two layers
Curing of concrete •
Properties of concrete can improve with age as long as conditions are favorable for the continued hydration of cement.
•
These improvements are rapid at early ages and continues slowly for an indefinite period of time.
•
Concrete that has been specified, batched, mixed, placed, and finished "letterperfect" can still be a failure if improperly or inadequately cured.
•
Curing is the procedures used for promoting the hydration of cement and consists of a control of temperature and the moisture movement from and into the concrete.
•
Curing is usually the last step in a concrete project and, unfortunately, is often neglected even by professionals.
•
The primary objective of curing is to keep concrete saturated or as nearly saturated as possible.
•
Hydration reactions can take place in only saturated water filled capillaries.
•
A seven-day (or longer) curing time is recommended.
Curing Methods 1.
Methods which supply additional water to the surface of concrete during early hardening stages.
–
Using wet covers
–
Sprinkling
–
Ponding or immersion
Keep water on the concrete during
the curing period.
Contd…..
2. Methods that prevent loss of moisture from concrete by sealing the surface.
–
Water proof plastics
–
Use liquid membrane-forming compounds
–
Forms left in place
Such methods provide some
cooling through evaporation, which is beneficial in hot weather.
Contd….. 3. Methods that accelerate strength gain by supplying heat & moisture to the concrete.
–
By using live steam (steam curing)
–
Heating coils.
Curing Requirements •
Curing of a placed and finished concrete is done for maintaining a satisfactory moisture and temperature in concrete for some defined period after placing and finishing, to allow the complete hydration of the cement.
Advantages of curing •
More strength and abrasion resistance
•
More watertigtness
•
Less volumetric changes; more volume stability.
•
More resistance to freezing and thawing and deicer salts.
•
More durability
Effect of curing on Strength
Hot Weather Concrete • •
Rapid hydration early setting rapid loss of workability Extra problems due to – Low humidity – Wind, excessive evaporation – Direct sunlight Solutions – Windbreaks – Cooled Concrete Ingredients – Water ponding (cooling due to evaporation) – Reflective coatings/coverings
Cold Weather Concrete •
Keep concrete temperature above 5 °C to minimize danger of freezing
Solutions
– Heated enclosures, insulation – Rely on heat of hydration for larger sections – Heated ingredients concrete hot when placed – High early strength cement
Uniformity of concrete Concrete uniformity is checked by
conducting tests on fresh and hardened concretes. Slump, unit weight, air content
tests Strength tests
Due to heteregeneous nature of concrete, there will always be some
variations. These variations are grouped as:
– Within-Batch Variations : inadequate mixing, non-homogeneous nature – Batch-to-Batch Variations : type of materials used, changes in gradation of aggregates, changes in moisture content of aggregates
Properties of Hardened concrete
The principal properties of hardened concrete which are of practical importance can be listed as: 1. Strength 2. Permeability & durability 3.Shrinkage & creep deformations 4.Response to temperature variations 5.Control of cracking out of these compressive strength is the most important property of concrete. Because
•
Concrete is used for compressive loads
•
Compressive strength is easily obtained
•
It is a good measure of all the other properties.
Strength Development and Strength Measurement • • • •
Aggregates “glued” together by cement paste to form concrete Cement hydration is a chemical reaction which requires water Strength gain reflects degree of hydration Strength gain depends on – Type of cement – Temperature history – temperature and time – Curing – Admixtures Factors Affecting Strength
• Effect of materials and mix proportions • Production methods • Testing parameters
Strength of concrete
The strength of a concrete specimen prepared, cured and tested under specified conditions at a given age depends on:
w/c ratio
Degree of compaction
Contd…..
Compressive strength Compressive Strength is determined by loading properly prepared
and cured cubic, cylindrical or prismatic specimens under compression. Cubic: 15x15x15 cm
Cubic specimens are crushed after rotating them 90° to decrease the amount of friction caused by the rough finishing. Cylinder: h/D=2 with h=15
To decrease the amount of friction, capping of the rough casting surface is performed.
Tensile ensile Strength Tensile Strength can be obtained either by direct methods or indirect
methods.
Direct methods suffer suffer from a number of o f difficulties related to holding the specimen properly in the testing machine without introducing stress concentration and to the application of load without eccentricity.
Direct Tensile strength
Split Tensile Strength
Due
to applied compression load a fairly uniform tensile stress is induced over nearly 2/3 of the diameter of the cylinder perpendicular to the direction of load application.
σst =
2P πDl
Splitting Tensile Strength
Contd….. P: applied compressive load D: diameter of specimen l: length of specimen
• The advantage of the splitting test over the direct tensile test is the same molds are used for compressive compressive & tensile strength strength determination. • The test is simple to perform and gives uniform results than other tension tests.
Flexural Strength
The flexural tensile strength strength at failure or the modulus of rupture is determined by loading a prismatic concrete beam specimen.
The results obtained are useful because concrete is subjected to flexural loads more often than it is subjected to tensile loads.
Factors Affecting the Strength of Concrete
1. Factors depended on the test type: – Size of specimen – Size of specimen in relation with size of agg. – Support condition af specimen – Moisture condition of specimen – Type of loading adopted – Rate of loading – Type of test machine
2. Factors independent of test type: – – – – – –
Type of cement Type of agg. Degree of compaction Mix proportions Type of curing Type of stress situation
Permeability of concrete
Permeability is important because: 1.
The penetration of some aggresive solution may result in leaching out of Ca(OH)2 which adversely affects the durability of concrete.
2.
In R/C ingress of moisture of air into concrete causes corrosion of reinforcement and results in the volume expansion of steel bars, consequently causing cracks & spalling of concrete cover.
3.
The moisture penetration depends on permeability & if concrete becomes saturated it is more liable to frost-action.
In some structural members permeability itself is of importance, such as, dams, water retaining tanks. The permeability of concrete is controlled by capillary pores. The permeability depends mostly on w/c, age, degree of hydration.
In general the higher the strength of cement paste, the higher is the durability & the lower is the permeability.
Contd……
Failure modes of concrete
Normal
Abnormal
Durability • If acceptable materials are used, the properties of concrete, such as durability, freeze/thaw resistance, wear resistance, and strength depend on the cement mixture. • A mixture with a sufficiently low ratio of water to cement plus entrained air, if specified, is the most desirable. • These properties--and thus the desired concrete quality--can only be fully achieved through proper placement and finishing, followed by prompt and effective curing. • A durable concrete is the one which will withstand in a satisfactory degree, the effects of service conditions to which it will be subjected. Factors Affecting Durability: External → Environmental Internal → Permeability, Characteristics of ingredients, Air-Void
System.
Structure of “un-damaged” Concrete •
Macrostructure
– Aggregates (CA, FA) – Hydrated cement paste (hcp) – Entrapped air voids •
Microstructure
– Hydrated cement paste (Hydration products: C-S-H, ettringite, monosulfate; porosity: gel, capillary pores entrained/ entrapped air voids) – Transition zone (TZ)
Macrostructure
Microstructure
Structure of “damaged” Concrete Macrostructure Visible cracks in hcp
and aggregates due to volume changes (to understand cause of cracks, microstructure should be examined)
Microstructure Alkali-silica reaction: Reaction product forms at TZ and expands Frost action: Water freezes in capillary pores and expands Sulfate attack: reaction products form in hcp and expand
Leaching & Efflorescence When water penetrates into concrete, it dissolves the non-hydraulic
CH (and various salts, sulfates and carbonates of Na, K, Ca) Remember C-S-H and CH is produced upon hydration of C3S and C2S These salts are taken outside of concrete by water and leave a salt
deposit.
Sulphate Attack Ground water in clayey soils containing alkali sulfates may affect concrete. These solutions attack CH to produce gypsum. Later, gypsum and calcium alumina
sulfates together with water react to form “ettringite”. Formation of ettringite is hardened cement paste or concrete leads to volume
expansion thus cracking. Moreover, Magnesium sulfate may lead to the decomposition of the C-S-H gel. Seawater contains some amount of Na and Mg Sulfates. However, these sulfates
do not cause severe deleterious expansion/cracking because both gypsum and ettringite are soluble in solutions containing the Cl ion. However, problem with seawater is the frequent wetting/drying and corrosion of reinforcing steel in concrete. To reduce the sulfate attack 1. Use low w/c ratio→ reduced permeability & porosity 2. Use proper cement → reduced C3A and C3S 3. Use pozzolans → they use up some of the CH to produce C-S-H
Corrosion Electrochemical reactions in the steel rebars of a reinforced concrete
structure results in corrosion products which have larger volumes than original steel. Thus this volume expansion causes cracks in reinforced concrete. In fact, steel is protected by a thin film provided by concrete against corrosion. However, that shield is broken by CO 2 of air or the Cl- ions.
Freezing and Thawing Water when freezes expands in volume. This will cause internal
hydraulic pressure and cracks the concrete. Important for exterior concrete. If aggregates or concrete absorbs so much water that when the water
freezes and expands the concrete cannot accommodate the build up of internal pressure, pop –outs may occur.
To prevent the concrete from this distress airentraining admixtures are used to produce air-entrained concrete.
Concrete Mix Design
Cement Concrete Mix Design means, determination of the proportion of the concrete ingredients i.e. Cement, Water, Fine Aggregate, Coarse Aggregate which would produce concrete possessing specified properties such as workability, strength and durability with maximum overall economy.
Methods of Concrete Mix Design I.S.
Method British Method A.C.I. Method These Methods are based on two basic assumptions
Compressive Strength of Concrete is governed by its WaterCement Ratio Workability of Concrete is governed by its Water Content
I.S. Method of concrete mix design Data required for concrete mix design
Grade of Concrete
e.g.: RCC-M30-A20
Slump required in mm
e.g.: 25 – 75 mm
Degree of Site Control
e.g.: Good
Type of Exposure
e.g.: Moderate
Grade of Cement e.g.: OPC 43 Grade Workability (Clause 7.1, IS:456-2000) Placing Conditions
Blinding Concrete; Shallow Sections; Pavements using pavers Mass Concrete; Lightly reinforced sections in Slabs, Beams, Walls, Columns; Floors; Hand placed Pavements; Canal lining; Strip Footings Heavily reinforced sections in Slabs, Beams, Walls, Columns; Slip form work; Pumped Concrete.
Degree of Workability
Slump(mm)
Very Low
See 7.1.1
Low
25-75
Medium
50-100
Degree of Site Control (Table 8, IS:456-2000) Site control having proper storage of cement; weigh batching of all materials; Controlled addition of water,
Good
regular checking of all materials, aggregate grading and moisture content; and periodical checking of workability and strength. Site control having deviation from the above.
Fair
Approximate Quantity of Materials required for concrete mix design 1.
Cement : 200 Kg.
2.
Fine Aggregate : 240 Kg.
3.
Coarse Aggregate : 180 Kg. (20 mm) 180 Kg. (10 mm) 69
Type of Exposure (Table 3, IS:456-2000) Sl. No. i)
Environment
Mild
Exposure Conditions Concrete surfaces protected against weather or aggressive conditions, except those situated in coastal area.
Moderate
Concrete surfaces sheltered from severe rain or freezing whilst wet. Concrete exposed to condensation and rain. Concrete continuously under water. Concrete in contact or buried under non-aggressive soil/ground water. Concrete surfaces sheltered from saturated salt air in coastal area.
Severe
Concrete surfaces exposed to severe rain, alternate wetting and drying or occasional freezing whilst wet or severe condensation. Concrete completely immersed in sea water. Concrete exposed to coastal environment.
iv)
Very Severe
Concrete exposed to sea water spray, corrosive fumes or severe freezing conditions whilst wet. Concrete in contact with or buried under aggressive sub-soil/ground water.
v)
Extreme
ii)
iii)
Surface of members in tidal zone.
Steps Involved in concrete mix design Step I:- Determine the physical properties of
concrete ingredients. I. Cement (OPC 43 Grade) Sl.No.
Particulars of Test
1
Standard consistency (% by weight)
2
Setting Time in minutes a) Initial b) Final
3
Compressive Strength in N/sq.mm at the age of a) 3 days b) 7 days c) 28 days
4
Specific Gravity
5
Fineness in Sq.m/Kg
Specifications As per IS:8112-1976 30-32 30 Minimum 600 Maximum 23 Minimum 33 Minimum 43 Minimum 2.5-2.7 225 Minimum
II.
contd….
Fine Aggregate Sieve Analysis Sieve Size
Specifications for Zone –II (passing) As per IS:383-1970
10.0 mm
100
4.75 mm
90-100
2.36 mm
75-100
1.18 mm
55-90
600 micron
35-59
300 micron
8-30
150 micron
0-10
III. 20.0mm Coarse aggregate Sieve Analysis Sieve Size
Specifications As per IS:383-1970 Graded
Single Sized
40.00mm
100
100
20.00mm
95-100
85-100
10.00mm
25-55
0-20
IV.
Sl.No.
V.
contd….
Mechanical properties Particulars of Test
Specifications As per IS: 383-1970
1
Crushing Value in %
30 Maximum For wearing surfaces 45 Maximum For other concrete 30 Maximum For wearing surfaces 45 Maximum For other concrete
2
Impact Value in %
3
Los Angeles Abrasion Value in %
30 Maximum For wearing surfaces 50 Maximum For other concrete
10.0mm Coarse aggregate Sieve Analysis Sieve Size
Specifications As per IS:383-1970 Graded
12.50mm 10.00mm
–
4.75mm 2.36mm
–
– –
Single Sized 100 85-100 0-20 0-5
contd….
Step II:-
Compute Target Mean Compressive Strength:
Fck = Where, Fck =
f ck + t * S
f ck
=
Characteristic Compressive Strength at 28 days in N/Sq.mm
S
=
Standard Deviation in N/Sq.mm
t
= =
Target Mean Compressive Strength at 28 days in N/Sq.mm
A Statistic, depending on accepted proportion of low results. 1.65 for 1 in 20 accepted proportion of low results
Assumed Standard Deviation (Table 8, IS:456-2000) Grade of Concrete
Assumed Standard Deviation (N/Sq.mm) Good Site Control
Fair Site Control
M10, M15
3.5
4.5
M20, M25
4.0
5.0
M30, M35 M,40,M45
5.0
6.0
contd….
Step III:- Select the Water-Cement ratio of trial mix from experience S.No.
Concrete Grade
Minimum expected W/C
1
M10
0.9
2
M15
0.7
3
M20
0.55
4
M25
0.50
5
M30
0.45
6
M35
0.40
7
M40
0.35
8
M45
0.30
Step IV:- Select the water content per cubic meter of concrete from table 2 of I.S: 10262-2009.
Maximum size of Aggregate (mm)
Water Content per cubic meter of concrete (Kg)
10
208
20
186
40
165
contd…. Approximate water content (Kg) per cubic meter of concrete (Table 32, SP:23-1982) Maximum Size of Aggregate (mm)
Slump (mm)
10
20
40
30-50
205
185
160
80-100
225
200
175
150-180
240
210
185
Volume of Coarse Aggregate per Unit Volume of Total Aggregate (Table 3, IS:10262-2009)
Maximum Size of Aggregate (mm)
Volume of Coarse Aggregate per Unit Volume of Total Aggregate
Zone IV
Zone III
Zone II
Zone I
10
0.50
0.48
0.46
0.44
20
0.66
0.64
0.62
0.60
40
0.75
0.73
0.71
0.69
contd….
Step V:- Compute the quantity of cement as follows. Water Cement = ------------W/C Ratio
Step VI:- Then we find the quantities of Fine & Coarse aggregate by absolute volume method. V = (W+C/Sc+(1/p) * (fa/Sfa)) * (1/1000) {for fine aggregate} and V = (W+C/Sc+(1/(1-p)) * (ca/Sca)) * (1/1000) {for coarse aggregate} Where V = Absolute volume of fresh concrete = 1 m 3 W = Mass of Water (Kg) per m3 of concrete C = Mass of Cement (Kg) per m 3 of concrete p = Percentage of fine aggregate. fa = Mass of fine aggregate ca = Mass of coarse aggregate Sc = Specific gravity of cement. Sfa = Specific gravity of fine aggregate. Sca = Specific gravity of coarse aggregate.
contd….
Step VII:- Make slump trials to find out the actual weight of water to get required slump. Make corrections to the water content & %FA, if required.
Step VIII:- Compute 2 more trial mixes with W/C ratios as 0.40 & 0.50, taking %FA as 34% and 38% respectively.
Step IX:- Cast atleast 3 cubes for each trial mix.
Step X:- Test the cubes for compressive strength at 28 days.
Step XI:- Draw a graph between compressive strength Vs C/W Ratio.
Step XII:- From the graph, find the W/C ratio for the required target mean compressive strength.
Step XIII:- Calculate the mix proportions corresponding to the W/C ratio, obtained from the graph.
Step XIV:- Check the cement content & W/C ratio against the limiting values given able-5 of I.S: 456-2000 for given type of exposure & type of Concrete.
contd…. Table-5 (IS:456-2000) Minimum Cement content Maximum Water-Cement ratio and Minimum Grade of Concrete for different exposures with normal weight of aggregate of 20mm nominal maximum size. Plain Concrete Sl. No .
Exposure
Reinforced Concrete
Minimum Cement Content kg/m3
Maximum Free Water Cement Ratio
Minimum Grade of Concrete
Minimum Cement Content kg/m3
Maximum Free Water Cement Ratio
Minimum Grade of Concrete
i)
Mild
220
0.60
-
300
0.55
M20
ii)
Moderate
240
0.60
M15
300
0.50
M25
iii)
Severe
250
0.50
M20
320
0.45
M30
iv)
Very Severe
260
0.45
M20
340
0.45
M35
v)
Extreme
280
0.40
M25
360
0.40
M40
