lime and cement block 3

8/12/2019 Lime and Cement Block 3

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Initial Set, Final Set & Hardening

Initial set - provides an estimate of when (in hours &

minutes) the concrete has reached the point where it

has stiffened to such an extent that it can no longer

be (vibrated) without damaging the concrete. This is

roughly equivalent to a penetration resistance (using

a weighted needle) of 3.4 MPa.

Under hot weatherconditions, the time to initial

set will be shorter than under

normal temperatures, which

will affect the construction

crews ability to consolidate &

finish the in-place concrete.

Under cold weather

conditions the time to initial

set is extended.

Concrete Penetrometer

for determination of settingtime of the mortar fraction

of fresh concrete. Spring

loading device graduated

from 2 to 150 lbs.

Stainless steel needle

points of 650, 325, 160,

65, 32, and 16 mm area.


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Final set - is roughly equivalent to a penetration resistance (weighted

test needle) of 27.6 MPa at which point the concrete has reached acompressive strength of around 0.6 MPa and it could carry some

measurable loads.

Initial Set, Final Set & Hardening

Soon after mixing, if insufficient sulfate is available (in solution in the

pore fluid) from the added gypsum to slow the hydration of C3A, the

resulting rapid hydration of the C3A produces AFm phase (e.g.

monosulfate) in the form of hydroxy-AFm.

These crystals link up the particles (cement, aggregate, hydrates) in the

mix & quickly cause the mix to become unworkable. This is termed a flash set. The rapid hydration of the C3A results in

the evolution of much heat. Concrete strengths where flash setting has

occurred may be lower than normal.

Flash set v.s. False set


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Conversely, if an excess of sulfate is present in the form of hemihydrate

(possibly caused by high mill temp.), the hemihydrate dissolves & then re-precipitates as gypsum.

Again, the growth of gypsum crystals cause a rapid set due to the linking

of the particles in the mix. This is referred to as false setting.

However, unlike flash set, there is no rapid evolution of heat because

C3A hydration is inhibited by the sulfate.

Another difference between false set & flash set is that a false set may

be reversed if continued mixing can break up the gypsum crystals

bonding the solids. The resulting gypsum fragments then slowly dissolve &

the concrete should set normally & show fairly normal strength growth.

High mill temperatures can decompose the gypsum (CaSO4.2H2O) i.e. dihydrate

form, to hemihydrate (CaSO4~0.5H2O), also known as plaster of Paris.

Reminder, solubilities: hemihydrate > gypsum > natural anhydrite

Flash set v.s. False set

Particle size analysis laser diffraction

Cement ground to a finer particle size will

react more quickly with water than the same

cement milled more coarsely.

Fineness is measured as the specific

surface area in (m2 kg-1).

Typical range: (350-450 m2 kg-1)

rapid-hardening (high early strength) cementswill be at the higher end of this range.

Blaine fineness apparatus

(air permeability)

Clinker Grinding & Gypsum Addition


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The control of fineness is critical to cement performance.

Particles that are too fine (< 2m) lead to the cement curing

exothermically, setting too fast & cracking.

Large particles (> 32m) may not be fully hydrated, reducing

the strength of the final product.

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Hydration of cement and water/cement ratio

The water/cement (w/c) ratio is a crucial parameter in cement hydration

because it directly affects porosity & permeability. The w/c ratio influencesconcrete strength & resistance to water ingress, frost attack, leaching &

other detrimental chemical & physical processes.

w/c ratio is defined as: (mass of water)/(mass of cement)

Most concrete has a w/c ratio between 0.3 and 0.7.

Below a w/c ratio = 0.44 there will always be some residual un-hydrated

cement if no water additional to the original mix water was available during

curing.

Above w/c = 0.44, there is sufficient water for complete hydration & also

enough space in which the hydration products can form, so in principle, thecement can hydrate fully.

Compaction is a process which

expels entrapped air from freshly

placed concrete & packs the

aggregate particles together so as to

increase the density of concrete.

Compaction increases significantly

the ultimate strength of concrete &

enhances the bond with

reinforcement. When first placed in the form,

normal concretes, excluding those

with very low or very high slumps, will

contain between 5% & 20% by

volume of entrapped air.

Figure shows that the strength of

concrete containing 10% entrapped

air (air voids) may be as little as 50%

of a fully compacted concrete.

Loss of strength through

incomplete compaction

Fresh Concrete Workability


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Sufficient workability is necessary for virtually full compaction to be possible,

using a reasonable amount of work. Higher w/c ratio concrete mixes are more convenient when placing the

concrete, as it flows more easily. However, the consequences for the

hardened concrete are strongly negative.


Voids in hardened concrete are, in fact, either bubbles of entrapped air

or spaces left after excess water has been removed.

The volume of excess water depends solely on the w/c ratio of the mix.

Whereas the presence of air bubbles is governed by the grading of the

fine particles in the mix & by the fact that the bubbles are more easily

expelled from a wetter mix than a dry one.



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Water content of the mix adding more water increases inter-particle lubrication.

Finer particles require more water to wet their larger specific surface. Irregular shape & rougher texture of an angular aggregate demands more

water than a rounded aggregate.

Porosity or absorption of the aggregate mixing water will be removed from that

required for lubrication of the particles.

For a constant w/c ratio, workability increases as the aggregate/cement ratio is

reduced because the amount of water relative to the total surface of solids is

increased.

A high ratio of volumes of coarse to fine aggregate can result in segregation & in

a lower workability, so that the mix is harsh and not easily finished.

Too much fine aggregate leads to a higher workability, but such an over-sanded

mix makes less durable concrete. Air entrainment (admixture) - reduces the water requirement for a given

workability.

Factors affecting workability:

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Slump: true, shear & collapseWorkability tests

Cone height = 305mm, base dia. 203mm

placed on a smooth surface with smaller opening

(102mm dia.) at the top. Inside of container ismoistened & is filled with concrete in 3 layers.

Each layer is tamped 25 times with a standard

16mm dia. steel rod. Top surface is struck off by

means of screeding & rolling motion of the

tamping rod.

Cone is slowly lifted & unsupported concrete

will slump. The decrease in height of the centre

of the slumped concrete is called the slump

(measured to 5mm).

Shear slump indicates harsher mixes (harsh = high

vol. coarse to fine agg.) caused by lack of cohesion inthe mix.

In lean mixes (i.e. high aggregate/cement ratio) with

tendency to harshness, a true slump can easily

change to the shear type or even to collapse.

Slump test is unreliable for lean mixes.

Compacting factor apparatusWorkability tests

Compacting factor test:

Test consists of 2 hoppers (with hinged doors at the bottom) &

1 cylinder. Total height = 1.2m.

Upper hopper is gently filled with concrete. Door is released &

concrete falls into lower hopper. This hopper is smaller than the

upper one & is therefore filled to overflowing (this reduces the

influence of the personal factor in filling the top hopper).

Door of lower hopper is released & concrete falls into cylinder,

top surface is struck off. Net mass of concrete in the known volume of the cylinder is

determined.

Compacting factor = ratio of density actually achieved in the

test to the density of the same concrete fully compacted (i.e. 4

layers each tamped or vibrated).

Unlike the slump test, the variations in the workability of dry

concrete are reflected in a large change in the compacting

factor, i.e. The test is more sensitive at the low workability

end of the scale than at high workability .


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Workability tests

Degree ofworkability

Slump(mm)

CompactingFactor

Applications

Very Low 0-25 0.8 Vibrated concrete in roads or other

large sections

Low 25-50 0.87 Mass concrete foundations without

vibration. Simple reinforced sections

with vibration.

Medium 50-100 0.935 Normal reinforced work without

vibration & heavily reinforced sections

with vibration.

High 100-180 0.96 Sections with congested reinforcement.Not normally suitable for vibration.

Vebe apparatusWorkability tests

Vebe test

A standard slump cone is placed in a cylinder

240mm in dia. & 200mm high.

The slump cone is filled in the standard manner,

removed, & a disc-shaped rider (weighing 2.75kg) is

placed on top of the concrete.

Compaction is achieved using a vibrating table

(50Hz) & is assumed to be complete when the

transparent rider is totally covered with concrete & allcavities in the surface of the concrete have

disappeared. This is judged visually.

It is assumed that the input of energy required for

full compaction is a measure of workability of the mix,

& this is expressed in Vebe seconds.

This is a good test for very dry mixes. This is in contrast to the compacting factor test

where error may be introduced by the tendency of some dry mixes to stick in the hoppers.

Vebe test also has the advantage that the treatment of concrete during the test is

comparatively closely related to the method of placing in practice.


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Vebe apparatusCompacting factor

apparatus

Reminder:

Slump is insensitive at the low workability end (i.e. unreliable for lean mixes).

Vebe is less sensitive at the high workability end (good test for very dry mixes).

Compacting Factor test is more sensitive at the low workability end of the scale

than at high workability.

Flow table testWorkability tests

Test is more widespread for

flowing concrete made with

superplasticizing admixtures.

Concrete is compacted by light

tamping in a cone: 200mm high,

bottom dia. 200mm, top dia.

130mm.

Board is hinged along one side

& can be lifted up to a stop sothat the free edge rises 40mm.

After mould is removed, table

top is lifted & allowed to drop 15

times, each cycle taking approx.

4 sec.

The concrete spreads & the

max. spread in mm parallel to the

2 edges of the table is measured.

A value of 400 indicates a

medium workability, 500 = high

workability.


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Density of compacted

fresh concrete

Obtained by weighing thecompacted fresh concrete

in a standard container of

known volume & mass.

From the known density

(), the volume of the

concrete can be found from

mass of the ingredients.

When these are

expressed as quantities in

1 batch put into the mixer,we can calculate the yield

of concrete per batch.

V = volume of compacted concrete obtained

from one batch (yield).

Masses per batch of: W = water, C = cement,Af= fine aggregate, Ac= coarse aggregate,

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The correct quantities of cement,

aggregate, & water, possibly also of

admixture, are batched & mixed in a

concrete mixer.

This produces fresh concrete, which is

transported from the mixer to its final

location.

The fresh concrete is then placed in

the forms, & compacted so as to

achieve a dense mass which is allowed,

& helped to harden.

Note: the size of a mixer should be described by the volume of concrete

aftercompaction, as distinct from the volume of the unmixed ingredients in

a loose state, which is up to 50% greater than the compacted volume.

Ready-mixed concrete

If instead of being batched & mixed on site, concrete is delivered for

placing from a central plant, it is referred to as ready-mixed or pre-mixed

concrete. This type of concrete is used extensively as it offers numerous

advantages, including:

Close quality control of batching which reduces the variability of the

desired properties of hardened concrete. Use on congested sites or in highway construction where there is little

space for a mixing plant & aggregate stockpiles.

Use of agitator trucks to ensure care in transportation, thus preventing

segregation & maintaining workability.

Convenience when small quantities of concrete or intermittent placing is

required.


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350 m3/ h output Capacity

200 tons x 4 cement silo

30 m3 x 10 aggregates hopper

Central-mixed concrete

Concrete Batching Plant

Transit-mixed concrete:

materials batched at a central

plant but are mixed in the truck

either in transit or immediately

prior to discharging the

concrete on site.


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Agitating differs from mixing

solely by the speed of rotation

of the mixer;

agitating speed = 2 to 6 rpm

mixing speed = 4 to 14 rpm.

The time during which

cement & moist aggregate

are allowed to remain in

contact should be limited to 2

hours (BS 5328:1991).

Ready-mixed concrete

Placing & Compacting

The main objective is to deposit the concrete as close as possible to its final

position so that segregation is avoided & the concrete can be fully compacted.

Collision between concrete & formwork or reinforcement should be avoided.


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The concrete should be placed in uniform layers, not in large heaps orsloping layers.

The rates of placing & of compaction should be equal.

Each layer should be fully compacted before placing the next one, &

each subsequent layer should be placed whilst the underlying layer is still

plastic so that monolithic construction is achieved (long delays can result

in the formation of cold joints)


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For deep sections, a long

down pipe ensures accuracy

of location of the concrete &minimum segregation.


The purpose of compaction is to remove as much of the entrapped air as possible

so that the hardened concrete has a min. of voids & consequently is strong, durable

& of low permeabil ity.

This effort is mainly provided by the use of internal vibrators. Poker is moved

from place to place so concrete is vibrated every 0.5-1m. Other methods: External

vibrators (clamped to formwork which rests on an elastic support) & Vibrating tables.

Placing of poker (or immersion) vibrators


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Concrete Vibrator Poker

(70-200Hz).

High frequency external vibrators


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lime and cement block 3

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