PROPERTIES OF FRESH & HARDENED CONCRETE
BY THE END OF THIS LESSON YOU WILL BE ABLE TO:
Define workability with the respect to cohesion, mobility, compactability & flow characteristics
List the major factors which influence workability & surface finish characteristics, & state their effects
QUALITY CONTROL OF CONCRETE
Quality control may, in this instance, be defined as an operation designed to set and maintain standards of quality in the production of concrete and concrete products.
QUALITY CAN BE ACHIEVED
In the first instance by:
Careful selection of materialsMix designTrial mixes & adjustments where necessary
QUALITY CAN BE ACHIEVED
Afterwards it may be maintained by:
Ensuring proper storage facilities for materials Monitoring the quality of incoming materials Plant maintenance Continuously sampling & testing the concrete
produced The operation of an efficient fault diagnosis &
complaints procedures.
MATERIALS
Materials It is important to select the right materials for
the proposed concrete works, which are used in the correct combinations.
This is a requirement to ensure the properties of fresh & hardened concrete meet the required specifications for the job.
BATCHING
Batching Once the carefully selected materials are
purchased, it is necessary they are batched to the right quantities.
Recognised methods of batching
By weight By volume
Batching by weight is the preferred method, as it is more accurate than volume batching
MIXING
Mixing
In general terms, mixing is said to commence once all of the constitutes, including the mixing water, are fed into the mixer.
The cycle should continue for as long as it takes the material to become one consistency in colour & workability
No advantages are gained from over-mixing – may cause issues through additional heat transferred into the mix
The type & capacity of mixer will be chosen to suit the job requirements
TRANSPORTING
Transporting – All too often the importance of proper transportation of concrete from the point of mixing to the point of placing is overlooked.
Many forms of transport include: Wheelbarrows Dumpers Mixer Trucks Hoists Cranes & Skips Tipper Lorries Concrete Pumps
FORMWORK
Formwork The choice of materials & the proper erection of
formwork is extremely important, both in terms of economy & the finished appearance of a concrete structure
The design & erection should be considered with ease of stripping in mind & attention to detail given to the following areas.
FORMWORK
The formwork should be:- Accurately positioned, plumb, & level Be adequately propped & supported to deal with all
imposed loads, for example – Fluid concrete Men Equipment
Joints should be leak proof A release agent should be applied to all surfaces
in direct contact with the concrete Design & erection should be consistent with easy
stripping
REINFORCEMENT
Generally, reinforcement for concrete comes in one of two forms - Steel bars Fabricated steel mesh
The steel is usually - Mild steel – smooth round bars High yield steel – deformed bars
The condition of the steel should be – Free from oil, grease or other contaminations There should be an absence of loose, flaky rust
or millscale
PLACING
Concrete should be placed as near as possible to where it will remain
If this is done with just enough surcharge or overfill to allow for compaction, then the tasks of compacting & levelling are relatively easy.
COMPACTING
Compacting is the operation of removing all entrapped air from a concrete mix.
It may be done by simple hand tamping or ramming, or by mechanical equipment
METHODS OF COMPACTION
Vibrating beamsFor floors, efficient up to a depth of 30mm also
levels the floor
Immersion type poker vibratorsFor deeper floor sections, walls & beams
External clamp vibrators Often used for concrete sections which are
heavily congested with steel reinforcement & would otherwise be inaccessible.
FINISHING
Examples of finishes Surfaces left as cast from formwork, which are
many & varied
Finishes produced by removing the initial surface by tooling or sandblasting
Surfaces produced by trowelling, floating or tamping the concrete
Applied toppings, such as Cement & sand screeds Tiles
CURING
Curing is the process of preventing the loss of moisture from the concrete whilst maintaining a satisfactory temperature.
PROPERTIES OF FRESH CONCRETE
Fresh concrete may be defined as –
Concrete in the initial period after the addition of the water & the completion of the mixing cycle
It will, under normal conditions, remain in this state for approximately 2 hours.
The main factor which influences the rate of stiffening & early hardening of concrete is the hydration of the cement
WORKABILITY
The strict definition of workability is the amount of useful internal work necessary to produce full compaction, & refers to the concrete’s ability to overcome internal friction & to be self compacting.
WORKABILITY
Water is mixed with the other mix ingredients for two reasons
Facilitate hydration of the cement To make the concrete workable enough to allow
the tasks of transporting, placing, compacting & finishing to be carried out with relative ease.
WORKABILITY TERMS
Cohesion Often termed Stability, & is the tendency of a
mix to stick or hold together
Mobility Represents the ease with which concrete can
flow into & completely fill a mould
Compactablility Is the capacity of a mix to be fully compacted
with 99% of entrapped air being removed by vibration or hand tamping
FACTORS AFFECTING WORKABILITY
Water Content – An increase in water content raises the level of workability of a mix & maybe effected by Increasing the amount of mixing water An increase in the moisture content of
aggregates
Cement Content Workability increases as cement content
increases, all other factors remaining the same.
Cement Fineness As fineness increases, workability decreases.
FACTORS AFFECTING WORKABILITY
Aggregates Size
Workability increases as maximum aggregate size increases
Grading Workability increases as the grading becomes coarser
Shape Workability increases for round aggregates, but
decreases for angular Texture
Workability increases for smooth, decreases for rough – surface area
FACTORS AFFECTING WORKABILITY - SEGREGATION
Segregation
may be defined as the separation of the constituents of the concrete mix, so that distribution is no longer equal.
FACTORS AFFECTING WORKABILITY - SEGREGATION
Most common in wet mixes, when the cement paste segregates from the aggregate. Mainly occurs when too much free water is
added to the mix
Larger heavier particles of coarse aggregate separate downwards More likely to happen in dry or lean mixes
FACTORS AFFECTING WORKABILITY - SEGREGATION
Dropping from heightIt is preferable to avoid this practice
whenever possible, as it is likely to result in the displacement of reinforcement or damage formwork.
FACTORS AFFECTING WORKABILITY - SEGREGATION
Over-vibration May be true for poorly designed mixes, but
provided the mix is well designed & cohesive in nature, it will not generally suffer from segregation
Under-vibration Alternatively, the under vibration of concrete is
more likely to have far more serious complications, as 1% air equates to 5% strength loss in the hardened concrete.
BLEEDING
Also known as water gain, partial segregation or sedimentation
Water rises to the surface of a concrete floor slab soon after it has been laid & compacted, the concrete is said to bleed, & is indicative once again of a lack of cohesion in the mix.
BLEEDING
The result of any one or a combination of the above is a reduction in the specific surface of the mix
The bleed water is now surplus to requirements & has the same effect as adding too much water
The solid particles of the mix settle downwards & displace the bleed water upwards
Water becomes trapped under reinforcement & aggregate, once cured water evaporates and leaves voids – strength loss / potential weakness
PLASTIC CRACKING
Two forms of plastic cracking exist – Plastic settlement cracking Plastic shrinkage cracking
Both are in some way relating to bleeding
PLASTIC SETTLEMENT CRACKING
These occur as a direct result of bleeding & settlement
When bleeding takes place, the solid particles of the mix settlement downwards & the bleed water is free to rise to the surface
If this settlement is downwards in any way restricted by reinforcement, or uneven because of a change of depth in a continuous pour, then plastic cracking may occur.
PLASTIC SETTLEMENT CRACKING
Prevention Use air entrainment admixtures
Cure Re-vibrate, to close the cracks, provided that the
concrete is still fluid enough to be moved by poker or vibrating screed & not so stiff that a void is left when the poker is removed
PLASTIC SHRINKAGE CRACKS
Caused by rapid drying out or evaporation of water, cracking may happen in either plain or reinforcement concrete & is most common on hot or windy days
PLASTIC SHRINKAGE CRACKS
Contributory Factors Wind High Ambient Temperatures High Concrete Temperatures Low Ambient Temperatures High Workability
All lose water to the atmosphere by evaporation
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