LECTURE 3FRESH CONCRETE

University of Technology, Jamaica

Presented by: Mr. Milton McIntyre

Sep. 2015

CONCRETE 2

Components of concrete Properties of fresh concrete Factors affecting the properties of fresh

concrete Mixing water for concrete Admixtures in concrete Questions

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Concrete is a composite building material Cement (11%) Fine Aggregate (26%) Coarse Aggregate (41%) Water (16%) Air (6%)

The strength performance of concrete is dependent on the properties of its constituents, especially cement.

ADVANTAGES FOR CONCRETE USE

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It is economical Concrete's long life and relatively low

maintenance requirements increases its economic benefits.

It is not likely to rot, corrode, or decay as other building materials.

It has the ability to be molded or cast into almost any desired shape.

It is a non-combustible material which makes it fire-safe and able to withstand high temperatures.

It is resistant to wind, water, rodents, and insects.

DISADVANTAGES FOR CONCRETE USE

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Concrete does have some limitations despite its numerous advantages. It has a relatively low tensile strength (compared to other building materials)Low ductilityLow strength-to-weight ratioSusceptible to cracking.

Concrete remains the material of choice for many applications regardless of these limitations.

STAGES OF CONCRETE

Properties of concrete at both stages are relevant to the construction professional.

First – mixing, fresh, plastic or fluidSecond – Setting and Hardened

Several variables affect the properties of fresh concrete and some parameters pertaining to fresh concrete affect the performance of hardened concrete.

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PROPERTIES OF FRESH CONCRETE

A proper mix should maintain its uniformity inside formwork and should not bleed excessively.

It should set within a reasonable amount of time.

It should hydrate in a manner that ensures adequate strength when the structure is put into service.

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PROPERTIES OF FRESH CONCRETE

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WORKABILITY9

Describe “that property of freshly mixed concrete or mortar that determines the ease with which it can be mixed, placed, consolidated, due to its consistency, the homogeneity with which it can be made into concrete, and the degree with which it can resist separation of materials”.

Workability affects the time and labour required for full compaction. Inadequate workability can be identified by the inability to satisfy mixing, transporting, compacting and finishing requirements.

TEST TO MEASURE WORKABILITY

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Slump (Consistency) Vebe time Compacting Factor Flow

SLUMP TEST

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DETERMINATION OF SLUMP

Sep. 1 , 2012Prepared by: Mrs. Barbara A. Cooke

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TYPES OF SLUMP13

SIGNIFICANCE

It is used to indicate the degree of wetness

Wetter mixes will be more workable than drier mixes

The slump test is suitable for slumps of medium to high workability (25 – 125mm ) 

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LIMITATION

The slump test is suitable for slumps of medium to high workability (25 – 125mm ) 

Not very useful to determine the variation in workability for stiff or wet mixes.

It is limited to concrete formed of aggregates of less than 38 mm

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COMPACTING FACTOR TEST

Measures the degree of compaction for a standard amount of work.

Compacting factor is a ratio of partially and fully compacted weights, which is always less than 1.

Used on dry mixes where slump test is not satisfactory

Unsuitable for concrete with compacting factor below 0.7 or above 0.98.

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FLOW TEST

Use to measure the ability of concrete to flow.

Used only on concrete with agg. largest size 20mm.

Cone filled in two layers with 10 light strokes of the tamper.

Cone removed gently after 30s. Lift table 15 times.

The spread of concrete measured as flow in mm

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FACTORS THAT INFLUENCE WORKABILITY

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Admixture Cement Water content Aggregate (size, shape, grading,

aggregate ratio, texture and absorption) Ambient Conditions (temperature,

humidity and wind velocity) Time

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IMPORTANCE OF WATER/CEMENT RATIO

The quality of hardened concrete is largely controlled by the amount of water used to produce concrete.

The advantages of reducing water content are as follows:

Increased compressive and flexural strength

Increased water-tightness Lower absorption

(permeability) Increased resistance to

weathering Improve bond - layers Improve bond – concrete

& rein. Less volume change

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Typical Water/cement relationship to strength

Sep. 1 , 2012 21Source: Concrete Scientific Principles.mht

Example

Water 190kgCement 350kgFine aggregate 740kg

Coarse aggregate 1120kg

w/c = 190/350 = 0.54 But

water content = 190kg

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Water may contain impurities that may be harmful to concrete if used. These are;

Salts Acids Algae Sugar Sulphates and Chlorides Carbonates and Bicarbonates

They affect strength, setting time, volume change, efflorescence, staining or corrosion in reinforcement.

MIXING WATER FOR MIXING WATER FOR CONCRETECONCRETE

Carbonates (CO32-) and Bicarbonates

(HCO3−)

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Sodium and Potassium affect setting time Sodium Carbonate may cause very rapid

setting while Sodium Bicarbonate may either accelerate or retard setting in excess of 1000ppm.

If present in large concentrations may reduce concrete strength.

Magnesium and Calcium Bicarbonates are not considered harmful to concrete in excess of 400ppm.

Inorganic salts (Manganese, tin, copper, and lead) can cause significant reduction in strength and large variations in setting time.

Chlorides and Sulphates25

Chlorides and Sulphates in high concentrations are the main cause of concrete deterioration and corrosion of embedded reinforcements.

Magnesium Sulphate or ‘Chloride has no harmful effect on concrete strength (up to 40,000ppm).

While Calcium Chloride can be used to accelerate both hardening and strength gain (30,000 – 40,000ppm).

Seawater26

Generally with low concentration of dissolved salts can be used to make concrete.

Its presence usually results in high early strength, but lower after 28days.

It may increase the risk of corrosion of embedded steel, but can be reduced by providing adequate cover, improving water-tightness, air-entraining and reducing the w/c ratio.

Seawater with sodium and potassium may react with alkali reactive aggregates.

Acids27

The pH (measure of H- in water) of neutral water is 7.0; values below 7.0 is considered acidic and values above 7.0 are considered alkaline.

Water containing Hydrochloric, sulphuric or other inorganic acids in excess of 10,000ppm have no effect on concrete strength.

However, water with pH value less than 3.0 may create handling problems.

Sugar28

Small amounts of sugar, as little as 0.03% usually retards the setting time of cement.

Algae

Water containing large amounts of algae can cause excessive reduction in strength either by affecting the hydration of cement or by influencing the air content in concrete.

Oils29

Mineral oils (petroleum) not mixed with animal or vegetable oils has less effect on strength than other oils.

However, it may reduce the concrete strength by 20% if in excess of 2% by weight of cement.

ADMIXTURES30

Concrete strength may be affected by the addition of admixtures.

Admixtures are materials other than cement, aggregate and water that are added to concrete either before or during its mixing to alter its properties, such as workability, air content, curing temperature range, set and hardening time, permeability or strength.

Why used Admixtures

The major reasons for using admixtures are: To reduce the cost of concrete construction. To achieve certain properties in concrete more

effectively than by other means. To maintain the quality of concrete during the stages

of mixing, transporting, placing, and curing in adverse weather conditions.

To overcome certain emergencies during concreting operations.

Admixtures should be used in conjunction with, and not as a substitute for, good concreting practice.

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TYPES OF ADMIXTURES

Air Entrainers Accelerators Bonding

agents Retarders Water

reducers Water repellers Super-

plasticizer

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Air Entrainers33

These are usually foaming agents eg. wood resins, synthetic detergents or gas generators (zinc or aluminum powder react with Ca(OH)2)

They form small stable discrete bubbles (air) uniformly dispersed in concrete.

They increase resistance to frost action, reduce permeability of concrete, improve workability and reduce bleeding and segregation.

Their use results in some reduction in concrete strength.

Accelerators34

Setting and hardening accelerators.They increase both setting and early strength development.They evolve high heat (used mostly for concrete in cold environment). Their use results in increased drying shrinkage, decreased resistance to sulfate attack and increase corrosion of reinforcement.

Setting accelerators.

High alkaline solutions

Used for reducing setting time of concrete.

Their use results in a reduction in concrete strength and should not be used where strength is important.

Bonding Agents35

These are usually organic polymer emulsions, such as polyvinyl acetate or styrene butadiene or acrylic

They are used to increase the bonding properties of concrete, especially for patching or remedial works.

Their use results in an increase in abrasion resistance and tensile strength but also cause a reduction in the compressive strength of concrete.

Retarders36

These are usually tartaric acids, ligno-sulphonic or hydroxylated-carboxylic acids with cellulose or starch.

They are used mainly in hot environments where high temperatures increase setting and hardening.

They delay the onset of setting and hardening. Their use results in increased bleeding with some

types and increased drying shrinkage with others. There effects is dependent on the amount used,

the cement type, mix proportions used, time of addition and ambient temperature.

Water Reducers37

These are usually from the same acids as retarders.

They are used to increase concrete strength by reducing water content or reduce cement content of concrete for a given w/c and the also improve workability,.

Their use results in shrinkage cracking due to a reduction in cement content.

Their effect is dependent on the amount used, cement type, aggregate type and grading, mix proportions and ambient temp.

Water Repellers38

These are metallic soaps or mineral and vegetable oil derivatives.

They are used to decrease permeability of concrete.

Super-plasticisers39

They used to produce flowing concrete (increased workability and reduce water content) without changing the mix composition or causing a strength reduction of the concrete.

Their use results in increase bleeding and segregation.

Reading Assignment40

Read-up on the importance of air content in concrete and air entrained concrete

Questions

Sep. 2015Presented by: Mr. Milton McIntyre

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