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