Concrete strength,distress in concrete,cracking

1. PROPERTIES OF
CONCRETE
Dr.S.P.SANGEETHA
VICE PRINCIPAL(ACADEMICS)
AARUPADAI VEEDU INSTITUTE OF
TECHNOLOGY

2. CONTENTS
Quality assurance for concrete construction as
built concrete properties, strength,
permeability, thermal properties and cracking

3. Concrete Properties
◼ Versatile
◼ Pliable when mixed
◼ Strong & Durable
◼ Does not Rust or Rot
◼ Does Not Need a Coating
◼ Resists Fire

4. CONSTITUENT MATERIALS AND PROPERTIES
Setting, Hydration and Hardening
- When cement is mixed with sufficient water, it loses its plasticity and
slowly forms into a hard rock-type material; this whole process is
called setting.
- Initial set: Initially the paste loses its fluidity and within a few hours
a noticeable hardening occurs - Measured by Vicat’s apparatus
- Final set: Further to building up of hydration products is the
commencement of hardening process that is responsible for strength
of concrete - Measured by Vicat’s apparatus
- Gypsum retards the setting process

5. HYDRATION OF CEMENT
2(3CaO.SiO2) + 6H2O = 3CaO.2SiO2.3H2O + 3Ca(OH)2
(Tricalcium silicate) (Tobermerite gel)
2(2CaO.SiO2) + 4H2O = 3CaO.2SiO2.3H2O+Ca(OH)2
(Dicalcium silicate) (Tobermerite gel)
3CaO.Al2O3 + 12H2O + Ca(OH)2 = 3CaO.Al2O3. Ca(OH)2.12H2O
(Tricalcium aluminate) (Tetra-calcium aluminate hydrate)
4CaO.Al2O3..Fe2O3 + 10H2O + 2Ca(OH)2 = 6CaO.Al2O3. Fe2O3.12H2O
(Tetra-calcium alumino-ferrite) (Calcium alumino-ferrite hydrate)
3CaO.Al2O3+10H2O+ CaSO4.2H2O = 3CaO.Al2O3.CaSO4.12H2O
(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

6. CONSTITUENT MATERIALS AND PROPERTIES
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
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% of absorbed water and 4-5%
surface moisture

7. CONSTITUENT MATERIALS AND PROPERTIES
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

8. CONSTITUENT MATERIALS AND PROPERTIES
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

9. MAKING AND TESTING OF CONCRETE (Cont’d)
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 concrete
strength, after proper mix design - If not properly carried out,
affects its strength, water tightness and durability
Methods of curing:
1. Ponding or immersion
2. spraying or fogging
3. wet coverings (with burlap, cotton mats or tugs)
4. Impervious paper (two sheets of Kraft paper cemented
together by bituminous adhesive with fiber reinforcements)
5. Plastic sheets (Polyethyelene films 0.10 mm thick)
6. membrane-forming curing compound; Steam curing

10. Top of Slab being protected during cold weather

11. 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

12. Sample collected
Slump Measured
Cone Removed and Concrete
Allowed to ‘Slump’
Slump Cone Filled

13. CONCRETE STRENGTH
FRESH CONCRETE
• SLUMP TEST
• CONSISTANCY TEST
• COMPACTION FACTOR TEST
HARDENED CONCRETE
COMPRESSIVE STRENGTH
TENSILE STRENGTH
FLEXURAL STRENGTH

14. QUALITY ASSURANCES FOR
CONCRETE CONSTRUCTION
Quality is a measure of the degree of
excellence and is indeed related to
fulfillment enjoyed by the user. In concrete
construction, even if rigid quality is not
followed, the material performs for a short
while without loss of strength.

15. QUALITY MANAGEMENT
SYSTEM
 1) Quality assurance plan(QAP)
 2) Quality control process(QC)
 3) Quality Audit(QA)

16. QUALITY ASSURANCE PLAN
 - Organizational Set-up
 - Responsibilities of personnel
 - Coordinating personnel
 - Quality control measure
 - Control norms and limit
 - Acceptance/rejection criteria
 - Inspection program
 - Sampling, testing and documentation
 - Material specification and qualification
 - Corrective measure for noncompliance
 - Resolution of disputed/difficulties
 - Preparation of maintenance record

17. Quality Control Plan
 It is a system of procedures and standards by
which the contractor, the product manufacture
and the engineer monitor the properties of the
product.
 Generally the contracting agency is responsible
for the QC process
 A contractor responsible for quality control
incurs a cost for it, which is less than the
uncontrolled cost for correcting the defective
workmanship or replacing the defective material

18. PERMEABILITY
The rates at which liquids and
gases can move in the concrete
are determined by its
permeability.

19. FACTORS AFFECTING PERMEABILITY
a)Varying and continuing hydration of the specimen.
b) Incomplete and variable initial saturation.
c) Lack of absolute water cleanliness.
d) Chemical reaction of specimen with the test fluid
e) Effect of dissolved gases where high pressure air is used
to pressurize the water.
f) Silting due to movement of fines.
g) Micro structural collapse and macroscopic instability
when very high flow pressures are used.
h) Lack of attainment of steady state condition.

20. Quality Audit
 This is the system of tracing and
documentation of quality assurance and
quality control program.
 It is the responsibility of the process owner.
 Both design and construction processes comes
under this process.
 The concept of QA encompasses the project
as a whole.
 Each element of the project comes under the
preview of quality audit.

21. THERMAL PROPERTIES
Thermal properties are important in
structures in which temperature differentials
occur including those due to solar radiation
during casting and the inherent heat of
hydration.
 Thermal conductivity
 Thermal diffusivity
 Specific heat
 Coefficient of thermal expansion

22. THERMAL CONDUCTIVITY
 This measures the ability of material to
conduct heat.Thermal conductivity is
measured in joules per second per square
meter of area of body when the
temperature deference is 10C per meter
thickness of the body.
 The conductivity of concrete depends on
type of aggregate, moisture content,
density and temperature of concrete.

23. Thermal Diffusivity
 Diffusivity represents the rate at which
temperature changes within the concrete
mass.
 Conductivity
 Diffusivity = ---------------
C X P
 Where C is the specified heat and P is the
density of concrete

24. Specific Heat
 It is defined as the quantity of heat
required to raise the temperature
of a unit mass of a material by one
degree centigrade.T
 he common range of values for
concrete is between 840 and 1170
j/kg per 0C.

25. Coefficient of thermal
expansion
 Coefficient of thermal expansion is
defined as the change in length per degree
change of temperature.
 In concrete it depends upon the mix
proportions.Type of aggregate and
content of aggregate influences the
coefficients of thermal expansion of
concrete

26. Improperly consolidated Concrete

28. DEGRADATION MECHANISMS IN
CONCRETE STRUCTURES
1. Freeze-thaw damage (physical effects, weathering).
2. Alkali-aggregate reactions (chemical effects).
3. Sulphate attack(chemical effects).
4. Microbiological induced attack(chemical effects).
5. Corrosion of reinforcing steel embedded in concrete (chemical effects).
a)carbonation of concrete
b) chloride induced.
6. Abrasion (physical effects).
7. Mechanical loads(physical effects).

29. EFFECT OF FREEZING AND THAWING:
 The most severe climate attack on concrete occurs
when concrete containing moisture is subjected to
cycles of freezing and thawing.
 The capillary pores in the cement are of such a size
that water in them will freeze when the ambient
temperatures is below 00C.

30. Poor design details leads to
1. Leakage through joints
2. Inadequate drainage
3. Inefficient drainage slope
4. Unanticipated shear stresses in piers, column
and abutments etc..,
5. Incompatibility of materials of sections.
6. Neglect in design
7. Errors in design
8. Errors in earlier repairs
9. Overloading
10. External influences such as(a) Earthquake
(b) Wind(c) Fire(e) Cyclone(f) Flash floors etc..,

31. Cracking
• Plastic shrinkage cracks
• Water from fresh concrete can be lost by evaporation,
absorption of sub grade, formwork and in hydration
process. When the loss of water from the surface of
concrete is faster than the migration of water from interior
to the surface dries up. This creates moisture gradient
which results n surface cracking while concrete is still in
plastic condition. The magnitude of plastic shrinkage and
plastic shrinkage cracks are depending upon ambient
temperature, relative humidity and wind velocity.
• Rate of evaporation of water in excess of 1 kg/m2 per hour
is considered critical. In such a situation the following
measures could be taken to reduce or eliminate plastic
shrinkage cracks

32. THANK YOU