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2. Concrete.pptx
1. In-Situ Concrete, Formwork
& Reinforcement - II
BE-1111 Building Design and Construction II
Level-1, Semester-2
Dr. Thilini Jayawickrama
2. Slump test
• Measure wetness or stiffness of concrete - workability
• Indirect measurement of consistency
Method
• Slump cone
• Fill properly and tamp
• Remove the cone
• The remaining concrete called “slump”
• Shear or collapse slumps are undesirable
• Advise to change the proportion
4. Slump test contd. …
• The slump can be controlled by a change in any one or all of
the following:
– Grade of aggregates,
– Proportion of aggregates, or
– Moisture content.
5. Cube test
• Test for compressive strength
• Steel/cast iron molds in cubical shape
• Tamping rod
• Suitable identification number for each sample
• Soak in water for curing
• Test on a compression testing machine
• For 3, 7, 28 days
6. Cube test contd. …
Fill molds and tamping with rod
Trowel
Test for compression with machine
7. Classification of Concrete
By Strength
• Grade (N/mm2)
• In general at 28 days
By composition
• Normal weight concrete
• Lightweight concrete
Special types of concrete
• Hot weather concrete
• Self-compacted concrete
• Pumped concrete
8. Lightweight concrete
• Normal concrete has a considerable self-weight (density 2200-2600
kg/m3)
• Increase the size of building elements – uneconomical
• Reduce the self-weight and increase the efficiency
– Low-density concrete (non-structural purposes) (density 300-800
kg/m3)
– Structural lightweight concrete (density 1350-1900 kg/m3)
• The compressive strength is not as great as ordinary concrete
• Methods of production
– By replacing usual aggregate with lightweight aggregate
– By Introducing gas or air bubbles into the mortar
– By omitting the fine aggregate fraction - no-fines concrete
9. Lightweight concrete contd. …
Advantages
• Economical
• Better fire resistant
• Lower thermal conductivity - thermal comfort
• Use of by-products/waste is environmentally beneficial
Disadvantages
• Higher moisture movement
• Higher initial drying shrinkage (5-40%)
12. Cellular concrete
• A type of lightweight concrete
• This is produced by introducing gases or air bubbles in the concrete
• Also known as Gas concrete, Aerated concrete, Foam concrete
• The methods of manufacturing cellular concrete includes;
– mixing foaming agents
– adding expansion agent to the concrete mix to generate gas (chemical
reaction ), and form a gas-bubble structure within the concrete
• Cellular concrete has the following characteristics;
– Very low density and strength
– High fire resistance
• This can be used for in-situ concrete as well as pre-cast concrete
• Insulation, partition walls, prefabricated elements
14. No-fines concrete
• A type of lightweight concrete
• This is produced by omitting the fine aggregate fraction from the
concrete
• A mix consist of coarse aggregate particles, each surrounded by a
coating of cement paste
• It has the following characteristics;
– Low shrinkage
– High water absorption
– Not easy to cut or nail
– Very high skid resistance
• This can be used for Block work, infill panels or partitions, Road
work or road surfaces (to improve the water drainage)
16. Cyclopean/plum concrete
• Inclusion of large stone into a concrete mass
• Method of placing
– Placing a layer of normal concrete
– Spreading large stones (plums)
– Place another layer of concrete
17. Hot weather concrete
• Recommended temperature for concrete mixing is 27+/- 20C
• Not possible to hold construction during hot weather
Problems with concrete in hot weather
Fresh concrete
• Quick settlement creates problems in pouring, compaction, and
finishing
Hardened concrete
• Likelihood of shrinkage cracks due to difference in temperature
between a concrete element and rest of the building or within the
same section
• Reduced durability, probability of corrosion of reinforcement bars
due to cracks
• Colour difference of the outer concrete surface due to different
hydration rates
18. Hot weather concrete contd. …
• Use cement types with slower rates of dehydration
• Use cold water
• Use low heat cement
• Storing aggregate in a shaded area
• Continuous spraying of water into aggregate
19. Self-compacted concrete
• This type of concrete is independent of workers’ performance
in compaction
• Self-compaction by means of self-weight of concrete
• Compact into every corner of formwork
Benefits
• Shorter construction period
• Assuring compaction in confined parts where vibrating
compaction is difficult
• Eliminate noise during compaction
20. Pumped concrete
• Pump a Ready- mixed concrete
• Pre-planning is required
– Pump volume
– Pressure capability
– Pipeline diameter
– Horizontal and vertical distance to be pumped
• Quality is important to ensure uniformity
• Precautions for hot/cold extreme weather conditions
• Workability of concrete affects pumping (Slumps 50 –
150mm)
21. Pumped concrete contd. …
Coarse aggregate
• Maximum size should be considered (limited to 1/3 of the
smallest inside diameter of the pipeline)
• Content to be reduced
Fine aggregate
• Together with cement and water, fine aggregate provides the
mortar (fluid) to convey coarse aggregate in suspension
Water
• Affects workability
Admixtures
• Those which are increasing workability will improve the ability
to pump
22. Concrete properties
Workability and Consistency
• Consistency is the fluidity or wetness indicating workability
• Fresh concrete should be easy to mix, transport, place,
compact and finish
• Workability is often considered to be a measure of the work
needed to compact the wet concrete.
• But it is also used to quantify the ease with which concrete
can be placed, although this depends on other properties
such as cohesiveness.
• The workability of the mix is determined by the slump test.
• It must retain free from segregation during these operations
23. Concrete properties contd. …
Segregation
• Segregation is the tendency for coarse aggregate to separate
from the sand-cement mortar
• This results in part of the batch having too little coarse
aggregate (likely to shrink more) and the remainder having
too much (poor resistance to abrasion)
• The method and equipment used to transport and handle the
concrete must not result in segregation of the concrete
materials
• When placing concrete, segregation can occur when mortar
tends to flow ahead of coarse materials
24. Concrete properties
Uniformity
• Degree of homogeneity or state of distribution within the
mix.
Testing uniformity
• Two samples are taken and concrete properties are
determined from each sample.
– First sample of concrete - when approximately 15% of the total
batch has been discharged
– Second sample of concrete - when approximately 85% of the
total batch has been discharged
• The properties of two samples are then compared to each
other and the property differences are determined.
25. Concrete properties contd. …
Bleeding
• Bleeding is the tendency of water to rise to the surface of
freshly placed concrete
• Develop a layer of water at the top or surface
• It is caused by the settlement of solid particles (cementing
materials and aggregate) and simultaneous upward migration
of water
• Inability of solid particles of the mixture to hold the water
amount as they settle down
• Bleeding is normal and it should not diminish the quality of
properly placed, finished, and cured concrete
26. Concrete properties contd. …
Bleeding contd. …
• Excessive bleeding increases the water-cement ratio near the
top surface
• Poor durability particularly if finishing operations take place
while bleed water is present
• Water rising to the top surface carries fine particles
• a weak top layer with poor durability
• not resistance to abrasion
• A water pocket or void can develop under a prematurely
finished concrete
28. Types of Cement
Ordinary Portland Cement (OPC)
• The most common type due to suitability for many types of works
• It has medium type setting and hardening time
Rapid Hardening Portland Cement
• Sets faster than OPC
• This type is used for quick construction where speed is very critical
Sulphate Resisting Portland Cement
• It has better resistance to Sulphate attack than OPC
• This type is suitable for marine structures and foundations which built in
soils having high risk of Sulphate attack
Low Heat Portland Cement
• Suitable for mass concrete
• Fineness is higher than OPC and has lesser early strength
29. Types of Cement contd. …
High Alumina Cement
• It has very good resistivity for Sulphate but weak in acids
• The water requirement for hydration is higher (about twice) than in
OPC
Supersulphated Cement
• This is suitable for mass concrete work due to its low heat
generation during the hydration
• This type has resistance to chemicals
Portland Blast Furnace Cement
• This also generates less heat than OPC and has high resistivity to
Sulphate
White Cement
• Metal oxides, primarily iron and manganese
• This is expensive than OPC but not strong as OPC
30. Admixtures
• Admixtures may be used to modify the properties of the concrete to make
it better serve its intended use or for better economic reasons.
Retarding admixtures
• Slow down the hydration of cement, lengthening set time
• Suitable for hot weather conditions
• Most retarders also act as water reducers (frequently called water-
reducing retarders)
Accelerating admixtures
• Shorten set time of concrete
• Allowing cold-weather pour
• Facilitate early removal of formwork, early surface finishes, and
sometimes early load application
• Drying shrinkages of concrete can be increase – care must be taken while
selecting type and proportion of accelerators
31. Admixtures contd. …
Super plasticizers
• Also known as plasticizers
• Includes water-reducing admixtures
• These are “high range water reducers”
• Allow large water reduction
• Gives great flowability without substantially slowing set time or increasing
air entrainment
• They can maintain a specific consistency and workability at a greatly
reduced amount of water
Water reducing admixtures
• Require less water to make a concrete of equal slump, or increase the
slump of concrete at the same water content
• They can have the side effect of changing initial set time
• Mostly used in hot weather concrete placing and for pumping
32. Admixtures contd. …
Air-entraining admixtures
• Entrain small air bubbles in the concrete
• While some loss of strength typically accompanies increase air in
concrete, it generally can be overcome by reducing the water-
cement ratio via improved workability (due to air-entraining agent
itself) or by using other appropriate admixtures
33. Quality assurance
• Concrete with high quality and to the required international
standard and project specification
• Calibrate laboratory devices periodically
• Obtain samples correctly according to the standard
specifications
• Perform and review tests based on the quality system
34. Formwork
• Molds to keep concrete mix in place
• Sufficiently water-tight to prevent leakage
• Properly braced or tied to maintain position and shape
• Design as not to damage previously place structures
• Consider method of placing concrete
• Consider loads
• Uniform surfaces for smooth finish
• Special design requirements (domes)
• Reuse where possible
35. Types of Formwork
By material
• Wood
• Steel
• Aluminum
• Fiberglass
• Plastic
By usage
• Temporary
• Left-in
36. Formwork – types for tall buildings
Flying (table forms)
• Typical span length
• Continual re-use
• Once the concrete gets strong enough, remove, clean, flown
with a crane to the next level to re-use
• Useful for high-rise buildings with typical floors
38. Formwork – types for tall buildings
Slip
• Continuously re-employed
• Using jacks, “slip” the section of formwork above the level
• Use in wall sections to higher levels as the concrete cures
