concrete materials technology

8/7/2019 Concrete Materials Technology

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Concrete Materials Technology



Concrete

Properties required� A concrete structure needs to be designed specifically

for the conditions which it will experience in service

� Concrete constructed in aggressive environment willrequire different performance characteristics from that

laid in flooring or used in dams

� Achieves high compressive strength but low tensile

strength; suffers thermal movement, vapor permeable

� Affected by drying shrinkage and moisture

movements and is subject to creep



Concrete

Production� Manufactured in wide variety of plant and with

various types of materials

� Controls and tests are employed to obtain aconsistent product

� Aggregates with differences in shape and

texture will require different water demands

� To obtain a durable material the water content

must be controlled during manufacture



D

urability of Concrete� For a concrete to be durable, it should be of a

high quality

� Concrete suffers attack from atmosphericpollution, flowing water, frost & actions of sea

� Durability is related to the behaviour of concrete

over its life span; select materials bearing in

mind environment in which conc will be placed



Permeability of Concrete

Reinforcement Corrosion� A highly permeable concrete will allow water and

materials in solution to enter; these will affect

durability and reinforcement; more at risk from attackby frost than a dry concrete

� Reinforcement in permeable concrete prone to

corrosion in presence of water & oxygen; steel

expands and damaging stresses set-up within conc� Cover to reinf increased with the increasing severity

of environment & decreased with higher conc grades



I

mpermeable Concrete� Permeability is reduced by lowering the W/C ratio &

ensuring adequate compaction and curing

� Failure of concrete under the action of fire is due todifferential expansion between cooler concrete inside

and the surface which becomes hot

� This differential movement between the surface & the

concrete at depth leads to spalling of concrete

� The reinforcement acts as a heat conductant, spreads

heat and looses strength rapidly



Abrasion Resistance of Concrete� A pavement material must neither abrade too quickly

nor must it polish under traffic or reduced skidding

resistance will result� A small reduction in water cement ratio or a small

increase in curing time produces a large improvement

in the abrasion resistance of concrete



Mix Design Considerations

� Mix design is partly art and partly science, as with the

cooking of good food

� To minimize cost, cement content is kept as low asthe durability, strength & supervision quality will

permit

� Almost every property of concrete is adversely

affected by an increased water content; so limits areplaced on the water cement ratio



Concrete Mix

Target Mean StrengthChoose the water content for the required workability,

the maximum aggregate size, the aggregate shape, the

cement type or source and the admixture dosageFrom an estimate of the variability in the strength of the

mix due to variation in supervision, equipment and

materials and the specified characteristic strength,

select a Target Mean StrengthUsing previous experience, select a suitable w/c ratio

to achieve the Target Mean Strength



Concrete Mix

The Water Content� The water content is selected from Table and divided

by w/c ratio to get the cement content which must be

above the minimum for durability and below themaximum for avoidance of high temp. in concrete

which may cause cracking as they cool down

unequally or may produce slightly lower strength

� Cement content can be lowered by using differentadmixtures or different dosages or even a larger

nominal size of aggregate if the cover permits



Concrete Mix

The Density of Concrete� The density of concrete is estimated from the

expected water content and the relative density of

aggregates� The density minus the cement & water content = total

aggregate content; the relative proportions of sand

and coarse aggregates are then estimated from

experience; normally 1:2 & 1:1.5 for pumped conc.� The coarse aggregates subdivided roughly 1:2 for

10mm & 20mm sizes, 1:1.5:3 for 10mm, 20mm &

40mm sizes



Concrete Mix

The Density of Concrete� When the fine aggregate grading becomes finer, say

from Zone II to Zone III, the extra water demand

should be off-set by reducing the fine aggregatecontent by about 5 to 8% of the total aggregate

content and by increasing the coarse aggregate

content correspondingly



Cement ± Tricalcium Aluminate

� This hydrates quickly and produces large quantity of

heat, but does not possess any great resistance to

chemical attack and contributes very little to the finalstrength of the concrete

� Desirable to restrict tricalcium aluminate content to

between 5 & 8% in order to improve durability



Deterioration of Cement During Storage

� Moisture may come in contact with cement during

storage; bagged cement should be raised from the

ground and covered� Do not stack too high; as a result of compaction a

set can take place

� Bagged cement should normally be used within a

month



Aggregates - General

� Aggregates normally consists of gravel ± crushed or

uncrushed ± or crushed stone

� Natural sand & gravel found in most parts of India;can also be dredged from the sea

� Crushed rock is obtained from a quarry by drilling &

blasting; crushed sand is also manufactured

� Impurities are removed by washing / screening



Classification of Fine Aggregates

� Classified into 4 zones mainly on the basis of material

passing 600 microns. ZoneIV is the finest sand and

ZoneIis the coarsest

� By the selection of suitable mixed design proportions

it is possible to use a sand that falls into any of these

4 zones (Refer IS:38)



Moisture Content of Aggregates

� Aggregates may arrive at site with considerable

quantity of moisture; in case of sand this could be

very high & hence necessary allowance is made� A sample of 2 Kg. coarse aggregate or 0.5 Kg. sand

is placed in a metal tray & heated. Stir the material

until all moisture is removed

� Calculate %age of moisture and adjust the mixproportions accordingly

� Modern batching plants provided with moisture

meters & automatic corrections to the mix



Silt in Fine Aggregates

� Silt is material which passes through 75 micron sieve

� Silt content tested by weight or volume

� Permissible 3% by weight is equivalent to 8% byvolume

� Do not under-estimate importance of silt; it affects the

water demand of the concrete; increased water

demand is reflected in lower strength of concrete

� Silt also affects pumpabilty of concrete , may lead to

blockage of pipelines



Water for Mixing Concrete

� IS : 456 specifies the requirements

� Impurities in water may interfere with the proper

hydration of cement� Water shall be of drinking quality

� A water that does not taste brackish or saline is

normally suitable

� In large projects water of doubtful quality is treated

before use

� Use of sea water is not allowed



Admixtures ± Water Reducers

� Water content is reduced in order obtain impermeable

concrete leading to durability

� Water reducers are also used to increase workability� Super-plasticizers used to obtain a large increase in

workability

� High strength concrete achieved by reductions in

water content; super-plasticizers can reduce water content by about 30% and maintain workability; PC

type super-plasticizers reduces by about 40%



Concrete Site Work



Storage of Aggregates

� Materials delivered by trucks to stockpiles behind the

batching plant. Inspect the aggregates before

unloading to ensure they are of correct size and typeand are cleaned; with a little experience, any gross

violation is readily noticed

� To prevent contamination by mud carried by the

wheels, a hard access area is recommended, laid to afall away from the mixer; this will assist drainage of

excess water



Storage of Aggregates

� Allow sand to drain overnight prior to use

� Avoid inter-mixing of separate sizes by providing bin

walls, with vertical H steel sections and timber or concrete planks slotted into them

� Shade the stock piles with heat reflecting material to

keep the temperature down; water spraying has a

cooling effect as it evaporates



Storage of Cement

� Cement is normally stored in silos; do not operate

batching plant without storage silos of appropriate

capacity; bags are useful for buffer stock� Bags should be stacked not more than 6 to 8 high

on pallets, in a well ventilated, dry shed

� Silos are usually filled from bulk tankers; they

need to hold minimum 35 tonnes for continuity of concrete production



Storage of Water

� Provide water storage tanks to ensure continuity

of production; white painted tanks & pipes help

to reflect solar heat, keeping the water and

hence the concrete cool

� Water must be tested before use

� Mains water preferred

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