lecture 7 - aggregates

8/12/2019 Lecture 7 - Aggregates

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Lecture No. 6

Aggregates



Natural Rock in the form of aggregate particles

make up between 70 to 80% of the volume of

normal concrete.

Aggregates have two prime functions in

concrete:Providing concrete with a rigid skeletal

structure.

Reducing the void space to be filled by cementpaste.



• Rocks can be divided into three broad groups:

Igneous rocks – formed when molten rock material (magma)

crystallized as solid rock either on the surface as lava or withinthe Earth’s crust as intrusion.

Sedimentary rocks – formed by the accumulation of fragments of

pre-existing rocks resulting from processes of erosion, organicdebris such as shell fragments or plant material.

Metamorphic rocks – formed from pre-existing rocks of any type,sedimentary or igneous, which have been subjected to longperiods of increased temperature and or pressure within thecrust.



• Aggregates materials need to be essentially chemically inertand physically strong and stable.



• The distribution of the sizes of aggregates particles is calledgrading. Mainly described in terms of cummulativepercentage by mass of aggregate passing particular sieve

sizes.

• According BS 882 – The 5mm sieve seperates the coarse andfine aggregates (passing 5mm sieve is defined as fine

aggregates).



• Fine Aggregates are divided into 3 categories

• Coarse , Medium and Fine



• Grading of fine and coarse aggregates are importantproperties of aggregates for concrete because of theirinfluence on packing and thus voidage which will in turn

influence the water demand and cement content of the mix.

Refer to exercise



The above figure shows the effect of changes in sand grading at

constant workability. Grading limits have usually been derivedto maintain consistency with available materials.



Shape and Texture

Round and smooth particles in fine aggregates will aidworkability of concrete and lead to lower voidage and lower

water demand.

Angular and rough particles of coarse aggregate will bond betterto cement paste and often lead to higher strength. (Flakiness

and Elongation Index).

Water Absorption

Water absorption of an aggregate is related to its particleporosity. Typical values of water absorption for naturalaggregate lies in the range of 0.5 to 5% by mass.

Porosity is calculated as relative density (OD) x absorption

Influence on workability of a concrete mix , Strength , Free

Water/cement ratio



SSD Conditions



Relative Density

Relative density(specific gravity) of an aggregate is the ratio of

mass per cubic meter of the aggregate to mass of volume ofwater



Bulk Density , void content and voids ratio

Bulk density of dry or damp, compacted or uncompactedaggregate can be used in design of concrete mix, conversionfrom bulk volume to weight, purchase of stock.

Bulk Density is less for uncompacted than for compactedaggregate and for damp compared with dry or saturatedaggregate. This phenomenon is called bulking



Aggregate Strength

The strength of concrete cannot exceed strength of aggregate

Stress concentrations are generated when the concrete stressesare shared by the aggregate – cement paste.

Test done – 10% fines and Aggregate Impact Value



Drying Shrinkage

Usually due to the movement of cement paste – Mostaggregates restrain concrete shrinkage because they are lesselastic than cement paste.

Concrete with higher aggregate contents shrink substantiallyless.

Aggregates giving a concrete drying shrinkage of 0.075 % or lessis suitable for all concrete purposes.

Thermal PropertiesThe thermal coefficient of expansion of aggregates ranges from 2

to 16 micro-stains/oC in normal temperatures

Expansion coefficient of cement paste is higher than that ofaggregate.



Fines

Fines are defined in BS as material passing 75microns test sieve.

Material may be fine sand, silt, dust or clay.Excessive fines specially clay increase water/demand and reduce

strength.



Impurities

Chlorides occur in marine aggregates and some coastal andestuarine deposits.

Processing removes the majority chloride but traces do remain.

High level of chlorides can accelerate the set of fresh concreteand can lead to damp patches and efflorescence on hardenedconcrete.

Risk of Corrosion of steel reinforcement



Sulphates

High levels of sulphates can interfere with cement hydrations inconcrete known as thaumasite attack. The reaction isexpansive and leads to cracking and spalling of concrete.

BS EN 12620 limits the sulphate content of natural aggregates to0.2%.

Shell

High levels of shell in aggregate can reduce the workability offresh concrete and make the concrete difficult to finish.

BS 882 limits the shell content of 10mm aggregate to 20% and

coarser aggregates to 10%.

lecture 7 - aggregates

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