The Construction aggregate can be classified as, (i) Normal weight aggregates, (ii) Light weight aggregates and (iii) Heary weight aggregates Light.

UNIT I CONCRETE MAKING MATERIALS

AGGREGATE CLASSIFICATION The Construction aggregate can be classified as,

(i) Normal weight aggregates,(ii) Light weight aggregates and(iii) Heary weight aggregates

Light weight aggregate and heavy weight aggregate will be discussed later briefly in this site. The properties of normal weight aggregates will only be discussed now.

Normal weight aggregates can be further classified as natural aggregates and artificial aggregates.

The Natural Aggregates are, Sand, Gravel, Crushed Rock such as Granite,Quartzite, Basalt, Sandstone.

The Artificial Aggregates are, Broken Brick, Air-cooled Slag, Sintered fly ash, Bloated clay.

Aggregates can also be classified on the basis of the size of the aggregates as coarse aggregate and fine aggregate.

SPECIFICATION OF A CONCRETE MIX

The object of concrete mix design is to find the proportions in which the concrete materials - cement, water, fine aggregate and coarse aggregate - should be combined in order to provide the specified strength, workability and durability and possibly meet other requirements, as listed in standards such as the British

PROPERTIES

Concrete has relatively high compressive strength, but significantly lower tensile strength, and as such is usually reinforced with materials that are strong in tension

The elasticity of concrete is relatively constant at low stress levels but starts decreasing at higher stress levels as matrix cracking develops

Concrete has a very low coefficient of thermal expansion, and as it matures concrete shrinks.

concrete structures will crack to some extent, due to shrinkage and tension

Concrete which is subjected to long-duration forces is prone to creep.

CEMENT AND GRADE OF CEMENT

CEMENT

a cement is a binder, a substance that sets and hardens independently, and can bind other materials together.

The word "cement" traces to the Romans, who used the term opus caementicium to describe masonry resembling modern concrete that was made from crushed rock with burnt lime as binder.

The volcanic ash and pulverized brick additives that were added to the burnt lime to obtain a hydraulic binder were later referred to as cementum, cimentum, cäment and cement.

GRADE OF CEMENT

GRADES Grade refers to the strength of cement at

28 days, when tested as per IS: 4031-1988. If the 28 days strength is less than 33 N/mm2, it is called 33 Grade Cement. If the 28 days strength is lies between 33 to 43 N/mm2, it is called 43 Grade Cement. If the 28 days strength is lies between 43 to 53 N/mm2, it is called 53 Grade Cement.

HYDRATION OF CEMENT

Cement hydration is a series of irreversible chemical reactions between cement and water.

During hydration, the cement-water paste sets and hardens, “gluing” the aggregate together in a solid mass

CHEMICAL ADMIXTURES

Chemical admixtures are materials in the form of powder or fluids that are added to the concrete to give it certain characteristics not obtainable with plain concrete mixes.

In normal use, admixture dosages are less than 5% by mass of cement and are added to the concrete at the time of batching/mixing

common types of admixtures are as follows.

Accelerators speed up the hydration (hardening) of the concrete. Typical materials used are CaCl2, Ca(NO3)2 and NaNO3. However, use of chlorides may cause corrosion in steel reinforcing and is prohibited in some countries, so that nitrates may be favored.

Corrosion inhibitors are used to minimize the corrosion of steel and steel bars in concrete.

MINERAL ADMIXTURES

There are inorganic materials that also have pozzolanic or latent hydraulic properties. These very fine-grained materials are added to the concrete mix to improve the properties of concrete (mineral admixtures),or as a replacement for Portland cement (blended cements).

1) FLY ASH : A by-product of coal-fired electric generating plants

, it is used to partially replace Portland cement (by up to 60% by mass).

The properties of fly ash depend on the type of coal burnt. In general, siliceous fly ash is pozzolanic, while calcareous fly ash has latent hydraulic properties

MINERAL ADMIXTURES

2)GROUND GRANULATED BLAST FURNACE SLAG A by-product of steel production is used to

partially replace Portland cement (by up to 80% by mass). It has latent hydraulic properties

3) Silica fume: # A by-product of the production of silicon

and ferrosilicon alloys. Silica fume is similar to fly ash, but has a particle size 100 times smaller.

. # Silica fume is used to increase strength and durability of concrete, but generally requires the use of superplasticizers for workability.[18]

SPECIFIED GRADINGS

Aggregate grading is the term given to the percentages of the different size fractions, after sieving, that go to make up the whole material.

To obtain the different size fractions for weighing, the sample of aggregate is sieved on the appropriate sieve sizes for the particular material, and the retained aggregate amounts weighed.

  This process is known as "grading", or, more scientifically

put you are determining the particle size distribution of the material. 

The necessary sieve sizes for a particular material will be found in the appropriate specification the material is supplied to. 

SPECIFIED GRADINGS

TESTING AGGREGATES

The reverse process to performing a grading on a material is a supplier blending appropriate amounts of single size aggregates to create the correct blend of aggregate to satisfy the "mix" specified. 

The Client/Engineer will in due course perform a grading on supplied material to ensure it meets the specification. 

FLAKINESS

Flakey is the term applied to aggregate or chippings that are flat and thin with respect to their length or width, 

Aggregate particles are said to be flakey when their thickness is less than 0.6 of their mean size. 

The flakiness index is found by expressing the weight of the flakey aggregate as a percentage of the aggregate tested. 

This is done by grading the size fractions, obtained from a normal grading aggregate, in special sieves for testing flakiness. These sieves have elongated rather than square apertures and will allow aggregate particles to pass that have a dimension less than the normal specified size, i.e. 0.6 of the normal size. 

FLAKINESS This grading process is normally performed by hand because flakey

chippings tend to 'lie' on the sieve surface rather than fall through the apertue. 

There are a number of material and aggregate specifications that have a maximum amount of flakey material allowed, e.g. surface dressing chippings. 

Flakey aggregate has less strength than cubical aggregate, and does not create the dense matrix that well graded cubicle aggregate is able to do, and it will provide less texture when used in surface dressing. 

E.g. Granular sub-base with a high proportion of flakey aggregate tends to segregate and be difficult to compact, although performing a normal aggregate grading test will show it conforms to specification. 

Flakey chippings do not create the surface texture that a cubicle or angular chipping is able to produce. 

FLAKINESS

WELL GRADED

Well graded means that within a material that is well graded there is a good distribution of all the aggregate sizes from largest to smallest, coarse aggregate to ‘‘dust’’. 

With a well graded material all the different size aggregate particles will position themselves within the total matrix in such a way to produce a tightly knit layer of maximum possible density, when compacted correctly. 

A well graded material is better able to carry and spread load imposed on it than a poorly graded material. 

A well graded material will possess good stability, with good distribution of load / stress spreading out uniformly through the material to the road pavement layer below. 

CONCRETE SLUMP TEST

The concrete slump test is an emprical test that measures the workability of fresh concrete. More specifically, it measures the consistency of the concrete in that specific batch. It is also used to determine consistency between individual batches. The test is popular due to the simplicity of apparatus used and simple procedure. Unfortunately, the simplicity of the test often allows a wide variability in the manner that the test is performed

CONCRETE SLUMP TEST

Principle The slump test result is a

measure of the behaviour of a self-compacted inverted cone of concrete under the action of gravity. It is a measure of the concrete's workability or the dampness of concreteApparatus

Apparatus Slump cone (Abrams cone), scale for

measurement.

Interpretation of results The slumped concrete takes various shapes, and according

to the profile of slumped concrete, the slump is termed as true slump, shear slump or collapse slump.

If a shear or collapse slump is achieved, a fresh sample should be taken and the test repeated.

A collapse slump is an indication of too wet a mix. Only a true slump is of any use in the test.

A collapse slump will generally mean that the mix is too wet or that it is a high workability mix, for which slump test is not appropriate.

 Very dry mixes; having slump 0 - 25 mm are used in road making, low workability mixes; having slump 10 - 40 mm are used for foundations with light reinforcement, medium workability mixes; 50 - 90 for normal reinforced concrete placed with vibration, high workability concrete; > 100 mm.

COMPRESSIVE STRENGTH TESTS

1. To check the potential strength of the concrete under controlled conditions against the desired strength; and

2. To establish a strength-age relationship for the concrete under job conditions as a control for construction operations or the opening of the work

CASTING CYLINDERS

In making the test specimens, place molds on a level, firm foundation in a sheltered place where they can remain undisturbed and protected from direct sunlight and from temperatures below 15°C (60°F) for at least 24 hours

If metal molds are used, lightly oil the inside before placing concrete

in them. If a suitable casting site is not available in the immediate proximity of the work, transport

the concrete and cast specimens to a location where they are kept undisturbed for the initial 24-

hour period. If sampling has caused segregation, re-mix by hand shoveling prior to casting the test

cylinder.

FLEXURAL STRENGTH TESTS

These routine tests are usually made only on paving jobs and are tested at the job site.

Rehabilitation projects requiring early openings may also utilize flexural tests.

As with all testing practices, the value of the standard flexural test results depend entirely on uniform adherence to the standard procedure outlined below. Tests are normally made at the standard ages of 7 and 28 days. High early strength concrete may require additional beams to verify opening strengths.