6. Concrete Tests (Hardened Concrete) –Destructive tests
Concrete Compression test in UTM (Universal testing machine)
Cube 150mmx150mmx150mm for maximum aggregate size >20mm
When maximum aggregate size < 20mm 100mmx100mmx100mm
When cylinder is used dimensions are diameter 15 cm and height 30 cm (2:1)
were cured and tested after 7 and 28 days
In Great Britain, Germany and Europe cubes are used. In the United States of America, France,
Australia, and Canada cylinders are used as standard specimens
IMPORTANT- 1. Results obtained from cylinder are found to be more precise than results from cubical moulds
2. Strength of concrete measured using cylindrical moulds is approximately 0.8 times than that obtained
from cubical moulds
Question for Home: Why Cube has more compressive strength than cylinder ?
7. Tensile strength of concrete
Unreinforced concrete specimen: Size of specimen 70cmx15cmx15cm
Done by casting a small beam of above dimensions
For aggregate size < 20 mm; 50cmx15cmx15cm
8. Tensile strength of concrete
Split test or Brazilian test
In this test, cylinder of 15 cm diameter and 30 cm height is loaded along
its length in compression.
Tension is created along the diameter and cylinder splits into two halves
Tensile strength = 2P/pixDxL
10. Not a Liquid - an Unstable mixture
Will segregate if handled improperly
Deposit in Formwork (methods)
Direct From the Truck
Bucket
Pump
Consolidate Mix
Apply Finish (if Required)
Concrete Placement
20. Vibration of concrete
• The purpose of compaction of concrete is to achieve
highest possible density of concrete by removing the
entrapped air
Types of vibrators
Internal vibrators
External vibrators
Vibrating tables
27. Cohesive Non
Cohesive
Segregation
Workability & Consistency
Cohesiveness of concrete
Is a measure of its ability to resist the segregation of its
constituents during handling, placing and compaction
29. Cohesiveness of concrete
Factors affecting the cohesiveness of concrete:
Water
- The higher the water content the greater is the risk of
segregation and bleeding
- High water content retards the stiffening of concrete allowing
sedimentation of heavier particles
Cold weather can also retard setting and promote bleeding
Dry mix can be friable and also prone to segregation
Grading
- Mixes that are deficient in fine particles tend to segregate more
readily during handling and bleed after finishing (entrained air
can help)
- Excessive fines make concrete “sticky” and very cohesive
30. Setting time
Can be determined by the test method described in ASTM C
403 by means of penetration resistance measurements
Made on mortar sieved from the concrete mixture
Initial and final time of setting corresponds to a penetration
resistance equals 500 psi (3.4 MPa) and 4000 psi (27.6 MPa)
Typical values range
Initial set: 2 to 6 hours
Final set: 4 to 12 hours
Factors affecting setting time of concrete:
W/C ratio
Cement type and content
Temperature
admixtures
32. Segregation and Bleeding
Segregation
(concentration of heavy particles)
Bleeding
(concentration of water)
33. Bleeding and settlement
Bleeding is the development of a layer of water at the top or surface
of freshly placed concrete. Water concentration at bottom of
aggregate and the reinforcing steel
Bleeding is normal and should not diminish the quality of properly
placed, finished and cured concrete. It occurs on the surface of
specimen due to the compacting process.
Some bleeding is helpful to control plastic cracking
36. Fresh Concrete – Slump Loss
It is defined as the loss of fluidity in fresh concrete with
time.
Due to: hydration of cement, loosing of water
Can be delayed or retarded by admixture
37. Fresh Concrete – Curing of Concrete
Curing is a process to control the loss of moisture from
concrete after placed and finished
Provides adequate time for the hydration of cement
particles therefore assures adequate strength
development and durability of the concrete
Curing period depends on:
The properties required of the concrete
The purpose for which it is to be used
And the ambient conditions (T°, RH..)
38. Fresh Concrete – Curing of Concrete
Curing may be done in three ways:
1. By preventing an excessive loss of moisture from the
concrete for some period of time
Leaving formwork in place
Covering concrete with an impermeable membrane
after formwork have been removed
Or by a combination of such methods
2. By continuously wetting the surface
Ponding or spraying the surface with water
3. By raising the temperature on concrete (steam curing) to
accelerate the rate of strength gain at early age
39. Curing methods and materials
Methods of curing concrete fall broadly into three
categories:
1. Those which prevent moisture loss by continuously
wetting the surface of the concrete (spraying water,
saturated wet coverings…)
2. Those which minimize moisture loss from the
concrete by covering it with a relative impermeable
membrane (plastic sheets, curing compound..)
3. Those which keep the surface moist and, at the
same time, raise the temperature of the concrete,
thereby increasing the rate of strength gain
41. Curing methods and materials
Ponding and Immersion
Flat surfaces, pavements, floors
Earth, sand or mortar dikes around the perimeter of the
concrete to retain a pond of water
Keep uniform temperature of the concrete
DT between water and concrete < 11°C (prevent thermal
cracks)
Water free of substances that will stain or discolor the
surface
Used only for small jobs
43. Curing methods and materials
Fogging minimizes moisture
loss during and after placing
and finishing
44. Curing methods and materials
Wet coverings
Fabric coverings saturated with cotton mats, rugs or other
moisture-retaining fabrics are commonly used for curing
Treated burlaps that reflect light and resistant to rot and fire are
available (ASTM C 171, AASHTO M182)
Must be free of any substance harmful to concrete or causes
discoloration
Should be placed immediately concrete has sufficiently hardened
/ including the edges of slab
Avoid wetting and drying / may cause crazing (cracks)
Avoid wet coverings of earth, sand or sawdust to prevent
possible discoloration of the concrete surface
45. Curing methods and materials
Impervious paper
Consists of two sheets of kraft paper cemented together by
bituminous adhesive with fiber reinforcement (ASTM C 71)
Efficient means of curing horizontal surfaces and other elements
with simple shapes
Advantage: no need for periodic addition of water
Applied immediately after concrete has hardened and wetted
46. Curing methods and materials
Plastic sheets
Consist of polyethylene film – ASTM C 171
Effective moisture retarder
Easy to apply to complex as well as simple shapes
May be clear or colored
Polyethylene film may also be placed over wet burlap to retain
the water in the wet covering material
Disadvantage: can cause patchy discoloration
47. Curing methods and materials
Curing compounds
Liquids formulated from wax emulsions, chlorinated rubbers,
synthetic and natural resins, and PVA emulsions
Sprayed directly onto concrete surfaces to form a relatively
impermeable membrane
Retards the loss of moisture from the surface
Should be able to maintain the
RH of the concrete surface above
80% for seven days to sustain
cement hydration
48. Curing methods and materials
Steam curing
Advantageous where early strength gain is needed
Or where additional heat is required to accomplish
hydration as in cold weather
Two methods of steam curing are used:
Live steam at atmospheric pressure (cast-in-place
structures and large pre-cast concrete units
High-pressure steam in autoclaves (for small
manufactured units)
49. Curing methods and materials
Live steam at atmospheric pressure
Widely used in the pre-cast concrete industry
Cost justified by higher turnover of formwork & productivity
Generally done in an enclosure to minimize moisture and
heat losses
A typical steam-curing cycle consists of
1. Initial delay prior to steaming 3 to 5 hours
2. Temperature increase period 2.5 hours (20°C /h)
3. Constant temperature period 6 to 12 hours (to achieve f’c)
4. Temperature decrease period 2 hours
50. Selecting a Method of Curing
Factors which affect the selection of a curing method include:
Type of member to be cured (slab, column, wall…)
Specified finish for the concrete element (will the final
“bond” be affected by the curing compound)
Whether the curing process will influence the appearance
of the concrete
The construction schedule of the project (will work need to
continue in the area during curing ?)
The cost and availability of materials
Weather conditions, exposure and location
51. Fresh Concrete – Plastic Shrinkage
Plastic shrinkage
Result of improper curing
Surface layer shrinks; inside concrete is plastic
53. Plastic Shrinkage Cracks
Usually associated with hot weather concreting or any time
when ambient conditions produce rapid evaporation
Occurs when water is lost from concrete during plastic state
(water evaporation > bleeding water)
by evaporation (bleeding, humidity, wind, T°C)
by suction of underlying dry concrete or soil
56. Plastic Shrinkage cracks
Special precautions in hot weather concreting
1. Moisten concrete aggregate that are dry and absorptive
2. Keep concrete temperature low by cooling aggregate and
mixing water
3. Dampen the subgrade and fog forms prior to placing
concrete
4. Erect temporary windbreaks to reduce wind velocity over the
concrete surface
5. Erect temporary sunshades to reduce concrete surface
temperatures
6. Protect the concrete with temporary coverings such as
polyethylene during delay between placing and finishing
7. Fog the slag immediately after placing and before finishing
8. Add plastic fibres