Component materials required for cement making - Limestone, shale, slate, clay, chalk - Lime 60%), silica 20%), alumina 10%) Others : Iron oxide, magnesium oxide, sulphur trioxide, alkalies, carbon-dioxide

 

Manufacturing process - Wet and dw methods - In both methods raw materials are homogenized by casting, grinding and blending -

Approximately 80% of the ground materials pass through #200 sieve - Primary and Secondary crushers; wet and dw grinding mills

 

-  Wet process: Mix containing homogenized constituents and 30 - 40 % of water is heated to 1510⁰ C in a revolving (slightly) inclined kiln - Oxide of silica, calcium and aluminum combine to form cement clinkers - Mixed with calcium sulphate (gypsum) to reduce the rate of setting and crushed into powder in ball mills before storing in silos or bags

 

-  Dry process: The homogenized mix is fed into the kiln and burned in a dry state Other steps are the same as for the wet process - Considerable savings in fuel consumption, but workplace is dustier

 

Constituents of cement: 75% is composed of calcium silicates; rest is made up of A1203, Fe203 and CaS04

Di-calcium silicate (C2S) - 2CaO.Si02 (15-40%)

Tri-calcium silicate (C3S) - 3CaO.Si02 (35-65%)

Tri-calcium aluminate (C3A) - 3CaO.A1203 (0-15%)

Tetra-calcium alumino-ferrite (C4AF) - 4CaO.A1203. Fe203 (6 -20%)

Calcium sulphate (CaS04) - (2%)

Types of cement (CSA)

Type 10 - Standard Portland cement - Used for general purposes; air entrained

                 (50% C3S•,       C2S; 11 ⁰/0C3A; 8% C4AF;       passing 45 um sieve)

Type 20 - Modified Portland cement - Used when sulphate resistance and/or generation of moderate heat of hydration are required; air entrained (42% C3S; C2S•, 5% QA; 13 ⁰ 0 C4AF; passing 45 um sieve)

Type 30 - High early strength Portland cement - Used for early strength and cold weather operations; air entrained (60% C3S; 13% C2S; 9% C3A; 8% C4AF;

Type 40 - Low heat Portland cement - Used where low heat of hydration is required; air entrained (26% C3S; C2S•, 5 ⁰ 0 QA; 12 ⁰ 0 C4AF;

Type 50 - High sulphate-resistant concrete - Used where sulphate concentration is very high; also used for marine and sewer structures; air entrained (40% C3S; 40 % 3.5 QA; 9% C4AF; passing 45 um sieve)

2(3CaO.Si02) + 6H20 = 3CaO.2Si02.3H20 + (Tricalcium silicate) (Tobermerite gel)

2(2CaO.Si02) + 4H20 =

         (Dicalcium silicate)          (Tobermerite gel)

3CaO.A1203+ 12H20+  3CaO. A1203.

      (Tricalcium aluminate)                      (Tetra-calcium aluminate hydrate)

4CaO.A1203,Fe203 + 1 OH20 + = 6CaO. A1203. Fe203.12H20

       (Tetra-calcium alumino-ferrite)                       (Calcium alumino-ferrite hydrate)

3CaO A1203+1 OH20+ CaS04.2H20 = 3CaO.A1203.CaS04 12H20

        (Tricalcium aluminate)                    (Calcium sulphoaluminate hydrate)

•           C3S hardens rapidly: responsible for early strength
•              C2S hardens slowly and responsible for strength gain beyond one week
•              Heat of hydration: Hydration is always accompanied by release of heat - C3A liberates the most heat      - C2S liberates the least

 

Properties of Aggregates, water and admixtures

- Aggregates make up up 59-75% of concrete volume; paste constitutes 25-40% of concrete volume. Volume of cement occupies 25-45% of the paste and water makes up to 55-75%. It also contains air, which varies from 2-8% by volume

Strength of concrete is dependent on the strength of aggregate particles and the strength of hardened paste

Properties of Aggregates

Compressive strength: Should be higher than concrete strength of 40-120

Voids: Represent the amount of air space between the aggregate particles

Course aggregates contain 30-50% of voids and fine aggregate 35-40%

Moisture content represents the amount of water in aggregates: absorbed and surface moisture - Course aggregates contain very little absorbed water while fine aggregates contain 3-5 ⁰ 0 of absorbed water and 4-5% surface moisture

Gradation: Grading refers to a process that determines the particle size distribution of a representative sample of an aggregate - Measured in term of fineness modulus - Sieve sizes for course aggregates are: 3/4" 1/2", 3/8", #4 and #8

Sieve sizes for fine aggregates are #4, #8 , #16, #30, #50 and #100

Durability of concrete: Determined by abrasion resistance and toughness

Chemical reactivity: determined by the alkali-aggregate reaction

 

Properties of Water

Any drinkable water can be used for concrete making - Water containing more than 2000 ppm of dissolved salts should be tested for its effect on concrete

- Chloride ions not more than 1000 ppm - Sulphate ions not more than 3000 ppm

Bicarbonate ions not more than 400 ppm

Need and types

Admixture are materials that are added to plastic concrete to change one or more properties of fresh or hardened concrete.

To fresh concrete: Added to influence its workability, setting times and heat of hydration

To hardened concrete : Added to influence the concrete's durability and strength

Types: Chemical admixtures and mineral admixtures

Chemical: Accelerators, retarders, water-reducing and air-entraining

Mineral : Strength and durability

 

CONSTITUENT

Chemical admixtures

-  Accelerating admixtures: Compounds added to cement to decrease its setting time and to improve the early strength developments - Used in cold-weather concreting - A 25 ⁰ 0 of strength gain observed at the end of three days - CaC12 (less than 2% by weight of cement); Not recommended for cold weather concreting; Triethanolamine; Sodium thiocyanate; Acetyl alcohol; Esters of carbonic and boric acids; Silicones - Problems: Increased heat of hydration, also leads to corrosion of steel

-  Retarding admixtures: Added to concrete to increase its setting times - Used in hot weather applications - Sodium/calcium triethanolamine salts of hydrogenated adipic or gluconic acid - Problem: early strength of concrete reduced - Water-reducing admixtures and super plasticizers water used in concrete mixes - High range water reducers reduce the water required for mixing by 12% or greater - Added to improve the consistency/workability of concrete and increase the strength - Water reducers: Lignosulphates, hydroxylated carboxylic acids, carbohydrates - Superplasticizers: Suphonated melamine/naphtalene formaldehyde condensates

-  Air-entraining admixtures: Allows dispersal of microscopic air bubbles (diameters ranging from 20 to 2000 um) throughout the concrete - Decreases the freeze-thaw degradation

-  Foaming agents: Vinsol resin; Sulphonated lignin compounds; Petroleum acid compounds; Alkyd benzene compounds

Mineral Admixtures:

-                     Used in concrete to replace part of cement or sand - When used to replace sand called as supplementary cementing materials - Added in large quantities compared to chemical admixtures.

-                     Pozzolans: Raw and calcined natural materials such as cherts, shale, tuff and pumice - Siliceous or siliceous and aluminous materials which by themselves possess no cementing property, but in fine pulverized form and in the presence of water can react with lime in cement to form concrete

-  Fly ash: By-product of coal from electrical power plants - Finer than cement Consists of complex compounds of silica, ferric oxide and alumina - Increases the strength of concrete and decreases the heat of hydration - Reduces alkali aggregate reaction.

-  Silica fume: By-product of electric arc furnaces - Size less than 0.1 um -

Consists of non-crystalline silica - Increases the compressive strength by 40-60 ⁰ 0

 

 

MAKING AND TESTING OF CONCRETE

Mixing, placing, finishing and curing of concrete

Mixing: Involves weighing out all the ingredients for a batch of concrete and mixing them together - A six-bag batch contains six bags of cement per batch - Handmixing (tools used) - Mixing with stationary or paving mixer - Mixing with truck mixers - Rated capacities of mixers vary

Pumping and placing: Concrete is conveyed to the construction site in wheel barrows, carts, belt conveyors, cranes or chutes or pumped (high-rise building)

Pumps have capacities to pump concrete up to 1400 feet and at 170 cu.yds. per hour Concrete should be placed as near as possible to its final position - Placed in horizontal layers of uniform thickness (6" to 20") and consolidated before placing the next layer

Finishing: The concrete must be leveled and surface made smooth/flat Smooth finish; Float/trowel finish; Broom finish: Exposed aggregate finish

 

Curing of concrete : Process of maintaining enough moisture in concrete to maintain the rate of hydration during its early stages - The most important single step in developing concltte strength, after proper mix design - If not properly carried out, affects its strength, water tightness and durability - Methods of curing: Ponding or immersion; spraying or fogging ; wet coverings (with burlap, cotton mats or tugs); Impervious paper (two sheets of Kraft paper cemented together by bituminous adhesive with fiber reinforcements); Plastic sheets (Polyethyelene films 0.10 mm thick); membrane-forming curing compound; Steam curing

Properties of Fresh Concrete: Concrete should be such that it can be transported, placed, compacted and finished without harmful segregation - The mix should maintain its uniformity and not bleed excessively; these two are collectively called as workability - Bleeding is movement and appearance of water at the surface of freshly placed concrete, due to settlement of heavier particles

Concrete Curing

• Must be kept Moist

•  Moisture Needed for:

  Hydration

  (Development of Strength)

  

 

Specified by 28 Day Compressive Strength

Measured in pounds of compressive strength per square inch (psi) or Newtons square metre

 

Primarily Determined By:

Amount of Cement

Water-CementRatio

Other influencing factors:

Admixture(s)

Aggregate Selection & Gradation

 

Strength Ranges: 2000 - 22,000+psi

If a low water cement ratio is desirable for quality concrete, why would one ever want to add excess water?

Concrete with high W C ratio is easier to place.

Workability, with desired qualities, often accomplished with admixtures

 

2.  Modulus of Elasticity

Durability of Concrete: Dependent on alkali aggregate reaction, freeze-thaw degradation and sulphate attack

-  Alkali-aggregate Itaction - Certain aggregates react with the alkali of Portland cement (released during hydration), in the presence of water, producing swelling - Form map-like cracks - Use low alkali cement to prevent this effect Use of fly ash minimizes

-          Freeze-thaw process: Water st01tcl in voids of concltte expands as a result of freezing - Generates stresses that tend to crack the concrete after a number of cycles  Air entrainment improves resistance to freezing-thaw cracking

-          Sulphate attack: Sulphates in soil and seawater react with aluminates in cement to produce compounds that incitase in volume - Leads to cracking - Use low alumina cement - Fly ash reduces sulphate attack

-          Carbonation of concrete: Carbon-di-oxide from the air penetrates the concltte and reacts with Ca(OH)2 to form carbonates; this increases shrinkage during drying ( thus promoting crack development) and lowers the alkalinity of concltte, which leads to corrosion of steel reinforcement.

-          Creep and Shrinkage: Creep is the time dependent increase in strain and deformation due to an applied constant load - Reversible creep and irreversible

creep - Shrinkage is made up of plastic shrinkage and drying shrinkage - Plastic shrinkage occurs when the concrete is plastic and is dependent on type of cement, w/c ratio, quantity and size of aggregates, mix consistency etc. - Drying shrinkage occurs when water is lost from cement gel - Smaller than 1500 x 10-06 (strain)

 CONCRETE MIX DESIGN

Objective : To determine the proportion of ingredients that would produce a workable concrete mix that is durable, and of required strength, and at a minimum cost

Principles of Mix Design

•                    Workable mix
•                    Use as little cement as possible
•                    Use as little water as possible
•                    Gravel and sand to be proportioned to achieve a dense mix
•                    Maximum size of aggregates should be as large as possible, to minimize surface area of aggregates

Methods of Mix Design

•            Volumetric method (arbitrary)
•            Proportioning from field data method
•            Proportioning by trial mixtures method
•            Mass proportioning method
•            Absolute volume method (CSA approved method)

CSA Design based on Absolute Volume

1 Using the given data, select the maximum slump as per the task

2 Select the maximum size of aggregates

3 Estimate the mixing water and air content

4 Select the w/c ratio

5 Calculate the cement content

6 Estimate the weight of th•y rodded coarse aggregates

7 Estimate the fine aggregate content

8 Find the weights of field mix (containing moisture) per unit volume

9 Compute the field mix proportions

 

CONCEPT OF REINFORCING CONCRETE WITH STEEL REINFORCEMENT

• Why do you need steel reinforcement? 
• Properties of steel reinforcing bars
• Size, grade, identification marks, ribbed
• Bars, welded wire mesh
• Standard hooks, ties and stirrups
• Chairs and bolsters for supporting reinforcing bars in beams and slabs
• Continuity in beams and slabs
• One-way or two-way reinforced beams and slabs

 

Concrete Reinforcing

-     Concrete - No Useful Tensile Strength

-    Reinforcing Steel - Tensile Strength

Similar Coefficient of thermal expansion

               Chemical Compatibility

               Adhesion Of Concrete To Steel

Theory of Steel Location

"Place reinforcing steel where the concrete is in tension',

 

 

 

 

 

Reinforcing Steel

• Sizes

Eleven Standard Diameters

3, 4, 5, 6, 7, 8, 9, 10, 11, 14, 18

Number refers to 1/8ths of an inch

• Grades

40, 50, 60

Steel Yield Strength

(in thousands of psi)

 

 

 

 

Reinforcing Stirrups

• Position Beam Reinforcing

•            Resist Diagonal Forces / Resist Cracking

 

Reinforcing a Continuous Concrete Beam

•          Most Beams are not simple span beams
•          Location of Tension Forces Changes
•         Midspan - Bottom in Tension
•          At Beam Supports - Top in Tension

 

 

 

 

d                      Reinforcing Concrete Columns

 

d   #Vertical Bars

Carry Compressive & Tension Loads

Bar Configuration

Multi-story

 

 

#Ties - Small bars

•       Wrapped around the vertical bars
•       Help prevent buckling
•       Circular or Rectangular
•       Column Ties or
•       Column Spirals

     

    #Installation

 

Welded Wire Fabric (WWF)

   

#Type of Reinforcing

#Grid of "wires" spaced 2-12 inches apart

#  Specified by wire gauge and spacing

#  Typical Use - Horizontal Surfaces

# Comes in Mats or Rolls

# Advantage - Labor Savings

 

 

TYPE OF CONCRETE FOR STRUCTURAL USE

•  Mass concrete
•   Normal reinforced concrete - Beam behavior and cracking
•   Pre-stressed concrete
•   Mechanics of pre-stressing
•   Pre-tensioned and post-tensioned profile of pre-stressing bars
•   Casting of a concrete wall
•   Casting of a floor and roof framing system

 

 

 

 

Prestressing – Pretensioning

• Pretensioning

Prior to concrete placement Generally performed at a plant - WHY??

                                                                 

Cables positioned prior to concrete placement

Stressed after concrete placement (& curing)

Generally performed at the jobsite

                                   

1. In post tensioning, the concrete is not allowed to bond to the steel strands during curing.

 

 

                       

2. After the concrete has cured, the strands are tensioned with a hydraulic jack and anchored to the ends of the beam. If the strands are draped, as shown here, higher structural efficiency is possible than with straight strands.

 

 

 

Casting A Concrete Wall

• Layout, Install one side, anchor, & brace
• Coat w/ Form Release

     

 

 

- Install Form Ties

"Small diameter metal rods which hold the forms together

(generally remain in the wall)

 

 

 

 

 

• Install Embeds (if required)
• Install Bulkheads
• Inspect
• Erect second side
• Plumb & Brace
• Establish Pour Hgt.

 

                         

 

Elevated Framing Systems

• One-Way System

Spans across parallel lines of support furnished by walls and/or beams

 

•        Two-Way System

Spans supports running in both directions