Become Concrete Master - Concrete Mix Designing

[Dnyan Deshmukh]

Ground Granulated Blast Furnace Slag (GGBFS)

Granulated slag is a non-metallic product consisting essentially of glass containing silicates and aluminizes of lime and other bases. It is a product developed simultaneously with iron in blast furnace of pig iron furnace. Granulated slag is obtained by further processing the molten slag by rapidly chilling or quenching it with water or steam and air. Ground granulated blast furnace slag confirming IS 12089 can be used as a mineral admixture as part replacement to cement.

The following are the requirements of GGBFS (as per IS 12089).

The proportion of lumps exceeding 50mm size shall not be more than 5 % by mass.

The chemical constituents when tested as per IS 4032 should comply

a. Manganese oxide % Max 5.5
b. Megnesium oxide % Max 7.0
c. Sulphide sulphur % Max 2.0

The percentage of major oxides in the GGBFS determined in accordance to IS 4032 shall be least of the following.

CaO + MgO + 1/3 Al2O3
--------------------------------------- ≥ 1.0
SiO2 + 2/3 Al2O3​

CaO + MgO + Al2O3
--------------------------------------- ≥ 1.0
SiO2
​

However slag containing more than 2.5 % of manganese oxide (MnO), the slag should satisfy

CaO + CaS+ ½ MgO + Al2O3
---------------------------------------- ≥ 1.5
SiO2 + MnO ​

Insoluble residue 5 % Max

​

Glass content 85 % (Min)

Note: In general, GGBFS is used at 50 % cement replacement. However, IS 455 clause 4.1, specifies that 70% cement contents in the mix can be replaced with GGBFS.

 

[Dnyan Deshmukh]

Micro silica (or) Silica fume

Silica fume is a by-product resulting from the reduction of high-purity quartz with coal or coke and wood chips in an electric arc furnace during the production of silicon metal or ferrosilicon alloys. The silica fume, which condenses from the gases escaping from the furnaces, has a very high content of amorphous silicon dioxide and consists of very fine spherical particles. The SiO2 content of the silica fume is roughly related to the manufacture of silicon alloys as follows:



Using silica fume in concrete
Silica fume was initially viewed as a cement replacement material; and in some areas it is still used as such. In general applications, part of the cement may be replaced by a much smaller quantity of silica fume. For example, one part of silica fume can replace 3 to 4 parts of cement (mass to mass) without loss of strength, provided the water content remains constant. The reader is cautioned that replacement of cement by silica fume may not affect hardened concrete properties other than strength to the same degree.

Silica fume addition usually increases water demand. If it is desired to maintain the same water- to-cementitious materials ratio (by mass), water-reducing admixtures or HRWRA or both should be used to obtain the required workability. In order to maintain the same apparent degree of workability, a somewhat higher slump will normally be required for silica fume concrete because of the increased cohesion.

 

[Dnyan Deshmukh]

PHYSICAL PROPERTIES AND CHEMICAL COMPOSITION OF SILICA FUME

Color
Most silica fumes range from light to dark gray in color. Because SiO2 is colorless, the color is determined by the non silica components, which typically include carbon and iron oxide. In general, the higher the carbon content, the darker the color of the silica fume. The carbon content of silica fume is affected by many factors relating to the manufacturing process such as: wood chip composition, wood chip use versus coal use, furnace temperature, furnace exhaust temperature, and the type of product (metal alloy) being produced. The degree of compaction may also affect the color.

Density
The specific gravity of silica fume is approximately 2.2, as compared (about 3100 kg/m3) for normal portland cement. However, the density of some silica fumes may exceed 2200 kg/m3. Variations in density are attributed to the non silica components of the various silica fumes.

Bulk density

As-produced silica fume
The bulk density of as produced silica fume collected from silicon metal and ferrosilicon alloy production usually ranges from 130 to 430 kg/m3, although it is most common to see values near the middle of this range.

Slurried silica fume
Slurried silica fume will typically have a bulk density of about 1320 to 1440 kg/cum. The nominal silica fume content of most slurry is approximately 50 percent by mass. The actual silica fume content may vary depending upon the particular source and whether chemical admixtures have been added to the slurry.

Densified (compacted) silica fume
Densification from an initial bulk density of 200 kg/m3 to a densified value of 500 kg/cum has been reported. The bulk density of commercially available densified silica fume ranges from approximately 480 to 640 kg/cum. Beyond 720 kg/cum level, it may become increasingly difficult to disperse densified silica fume particles within concrete.

Fineness, particle shape, and oversize material

Silica fume consists primarily of very fine smooth spherical glassy particles with a surface area of approximately 20,000 m2 /kg when measured by the nitrogen-adsorption One of the most common tests conducted upon silica fume is the residue (oversize) on the 45-µ (No. 325) sieve. The extreme fineness of silica fume ranges between 13,000-30,000 m2/kg, measured by nitrogen adsorption test.

One of the most common tests conducted upon silica fume is the residue (oversize) on the 45-µ (No. 325) sieve. In this test a sample of silica fume is washed through a 45-µ sieve, and the mass and composition (wood, quartz, carbon, coal, rust, and relatively large silica fume agglomerates) of the oversize particles are reported. The amount of oversize material is strongly influenced by the silica-fume collection system; and the amount of oversize material may vary considerably from one system to another.

The Canadian Standard, “Supplementary Cementing Materials” (Canadian Standards Association 1986), limits the maximum amount retained on the 45 µ sieve to 10 percent.

To know the effect of micro silica on fresh concrete properties Click Here

 

[Dnyan Deshmukh]

We just finished the 1st part of Materials used for producing concrete.

Now we will move to next part i.e.- Testing of Materials.

If you have any query question regarding any post in this thread you can post it in Help thread by Clicking Here!

 

[Dnyan Deshmukh]

Testing of Materials

Cement

We already seen the chemical and physical properties of cement in previous section.
You can refer to how to check cement Quality by Clicking Here!

Fineness:
IS 4031 Part 1 – Determination of fineness by dry sieving.
Sieve about 100 g of cement on a 90 micron sieve. The amount retained should be not more than 5% by weight for Rapid hardening cement and 10% by weight for other cement types. Usually this requirement is met. If not a better test using Blaine’s apparatus will be necessary to find out specific surface in m2 / kg

Standard consistency:
IS 4031 part 4 – Determination of standard consistency of cement paste.
The standard consistency of the cement paste is “ as that consistency which will not permit the vicat plunger to penetrate to a point 5 to 7 mm from bottom of mould. In general the consistency ranges between 27 % to 34 % .

Setting Time:
IS 4031 part5 – Determination of setting time of cement
The cement paste made with 85 % of standard consistency water, the initial setting time should not be less than 30 minutes and final setting time should not be more than 10 hours. However the limits of setting time for different types of cement should confirm to the respective Indian standards.

Loss on Ignition:
IS 4032 – Method of Chemical Analysis of Hydraulic cement
This shows the amount of moisture and carbon dioxide absorbed by the cement. If cement is stored for a longer time (say three months) or if deterioration is suspected, check the cement for loss on ignition. This test can be done in an outside laboratory. The value should not be more than four or five percent by weight of cement.

Insoluble Residue:
IS 4032 – Method of Chemical Analysis of Hydraulic cement
Insoluble residue is found by dissolving cement in hydrochloric acid. For ordinary Portland Cement this should not be more than 2 to 4 % by weight of cement. For pozzolana cement this may amount to 15% to 20 %

Test for Soundness:
IS 4031 part 3 – Determination of soundness for hydraulic cement.
Le Chatelier test reveals the soundness caused by excess amount of uncombined(free) lime. Autoclave tests reveal the soundness caused by excess amount of percales crystals(MgO)It is not necessary to perform tests for soundness at site.

Compressive strength of Mortar cubes:
IS 4031 part 6 – Determination of compressive strength of Hydraulic cement other than masonry cement.-
This is the average compressive strength of three mortar cubes of size 70.7 x 70.7 x 70.7 mm prepared using one part of cement with three parts of standard sand confirming to IS 650 with ( P/4 + 3 .0) percent (of combined mass of cement and sand) water.

Where P = Standard consistency of the cement in %

 

[Dnyan Deshmukh]

Problems of setting and hardening due to cement.

False set:
Concrete may stiffen too early say with 5 to 20 minutes after mixing. This problem arises because of inadequate cooling during grinding of cement or because cement is fresh and hot from the mill. If false set occurs remix the concrete without adding water. Plasticity will return and concrete will set in the normal manner.(IS 4031 part 14 – Method of test for determination of false set).

I have seen false setting in some brands of cement as Today's market demand is more and supply is less, Manufacturer do send very fresh cement considering it will get cool down during transport and storage at customers godown.

Flash set:
If concrete stiffens too early and does not regain plasticity on re-mixing, flash set might have occurred. The cause may be ,
- Gypsum dosing have problem during final grinding of cement.
- Use of hot water ( 60 o C to 80 o C) for mixing.
- Presence of calcium chloride, especially in admixture.
- Contamination with high alumina cement or calcium chloride , possibly stored for emergency repairs.

Blended Cement:

Concrete with pozzolana cement and slag cement hardens at slower rate. Therefore, prolonged curing is necessary.

 

[Dnyan Deshmukh]

Storage of Cement

Cement shall be stored at the work site in a building or a shed which is dry, leak-proof and as moisture-proof as possible. The building or shed
for storage should have minimum number of windows and close fitting doors and these should be kept closed as far as possible.

Cement shall be stored and stacked in bags and shall be kept free from the possibility of any dampness or moisture coming in contact with them. Cement bags shall be stacked off the floor on wooden planks in such a way as to keep about 150 mm to 200 mm clear above the floor. The floor may comprise of lean cement concrete or two layers of dry bricks laid on well consolidated earth. A space of 600 mm minimum shall be left around between the exterior walls and the stacks.

In the stacks the cement bags shall be kept close together to reduce circulation of air as much as possible. Owing to pressure on the bottom layer of bags sometimes ‘warehouse pack’ is developed in these bags. This can be removed easily by rolling the bags when the cement is taken out for use. Lumped bags, if any should be removed and disposed off.



The height of stack shall not be more than 10 bags to prevent the possibility of lumping up under pressure. The width of the stack shall be not more than four bags length or 3 metres. In stacks more than 8 bags high, the cement bags shall be arranged alternately length-wise and cross-wise so as to tie the stacks together and minimize the danger of toppling over. Cement bags shall be stacked in a manner to facilitate their removal and use in the order in which they are received; a lable showing date of receipt of cement shall be put on each stack to know the age of cement.

For extra safety during the monsoon, or when it is expected to store for an unusually long period, the stack shall be completely enclosed by a waterproofing membrane such as polyethylene, which shall close on the top of the stack. Care shall be taken to see that the waterproofing membrane is not damaged any time during use.

 

[Dnyan Deshmukh]

Storage and Handling of aggregate

As per IS 4082 – Aggregates shall be stored at site on a hard dry and level patch of ground. If such a surface is not available, a platform of planks or old corrugated iron sheets, or a floor of bricks, or a thin layer of lean concrete shall be made so as to prevent contamination with clay, dust, vegetable and other foreign matter.

Store sand and coarse aggregates of different size fractions in separate stock piles, on firm ground or platform.

If aggregates are stored directly over firm ground, do not use the material in the bottom 10 cm portion( This will be a “Dead storage”).

Provide ample space between adjacent stock piles (about one meter).

Build up the stock pile in horizontal or gently sloping layers. Do not allow dumping of aggregates down the sloping sides of big stock piles.

Do not allow wheel loaders, trucks and bull dozers over the stock piles.

 

[Dnyan Deshmukh]

Selection of size of aggregate for concrete:

The nominal maximum size of coarse aggregate should be as large as possible within the limits specified but in no case greater than one-fourth of the maximum thickness of the member, provided that the concrete can be placed without difficulty so as to surround all reinforcement thoroughly and fill the corners of the form. For most work, 20 mm aggregate is suitable. Where there is no restriction to the flow of concrete into sections, 40 mm or larger size may be permitted. In concrete elements with thin sections, closely spaced reinforcement or less cover, consideration should be given to the use of 10mm nominal maximum size.

Plums above 160 mm and up to any reasonable size may be used in plain concrete work up to a maximum limit of 20 percent by volume of concrete when specifically permitted. The plums shall be distributed evenly and shall be no closer than 150mm from the surface.

For heavily reinforced concrete members as in the case of ribs of main beams, the nominal maximum size of the aggregate should usually be restricted to 5 mm less than the minimum clear distance between the main bars or 5 mm less than the minimum cover to the reinforcement whichever is less.

 

[Dnyan Deshmukh]

Procedure for sampling of Aggregates:

Aggregates may be sampled either

i) From moving conveyor belts or
ii) While loading / unloading or
iii) From stock piles.

As per IS 2430, For obtaining reliable conclusions, it is recommended that as far as possible aggregates be sampled when in motion, that is, from conveyors or during loading and unloading.

Sampling from stock pile is commonly used everywhere, if have good setup better to sample each truck at unloading.
Aggregate samples may be drawn for approximately every 200 cum in case sample is taken from conveyor belt or a stock pile.

The weight of the gross sample shall depend on the maximum nominal size of aggregates and shall be according to IS 2430 Table 2. In order to obtain this weight of gross sample, at least 10 increments of a suitable weight, each not less than 1 kg, shall be taken. Increments shall be
taken with the help of a suitable scoop at regular intervals.

Draw samples using scoops. Each scoop full of aggregate is called an “Increment”.

If the nominal size of coarse aggregate is more than 20 mm, use a scoop of 10 kg capacity.
If the nominal size of coarse aggregate is 20mm or less, use a scoop of 5 kg capacity.
For sand, use a scoop of 2 kg capacity.

 

[Dnyan Deshmukh]

Sampling From Stock Pile
samples from stock piles should be made up of the required number of increments taken equally from top third, at the mid point and at the bottom third of the volume of the pile. A board put vertically into the pile just above the sampling point helps in preventing further segregation. In sampling stock piles of the aggregates, the outer layer which may have become segregated, should be removed and the sample taken from the material beneath.

REDUCTION OF GROSS SAMPLE
Each gross sample shall be reduced separately. The process of mixing and reduction of each gross sample shall be repeated until the material required for each test, as specified in the relevant test method specification, IS : 2386 ( Parts 1 to 8 ) is obtained.

Reduce the gross sample to the required quantity, by quartering, as follows:

- Mix the aggregate and gradually build up a conical pile on a hard surface (tarpaulin can be used beneath).
- Flatten the cone by pressing down the top of the pile, Divide the flattened heap in to four quarters, by drawing two lines which cross at right angles at the center of the heap.
- Reject any two opposite quarters and return the rejected aggregates to the stock pile.
- Repeat it until the material is reduced to the required amount.
- Do not add or remove material in order to arrive at exact amount of reduced sample. Leave the excess amount without further quartering.

The minimum amount of sample required for sieve analysis is listed below;

 

[Dnyan Deshmukh]

Field tests for Deleterious Substances:
Aggregates intended for use in concrete should be free of deleterious substances such as dust, clay, silt, organic matter, coal, pyrites, gypsum and mica.
Only two tests need to be done at site laboratory, on a routine basis. namely;

Clay and silt in sand
Organic impurities in sand

Clay and silt in sand:
Clay and silt are the finest fraction in sand and pass through a 75 micron sieve. The maximum amount of clay and silt in aggregated should not exceed three percent by weight for natural sand, 15% by weight for crushed sand and 3 % by weight for crushed / uncrushed coarse aggregates.

Organic impurities in sand:
This test may be required once in a day, for large concreting works. For washed sand with a satisfactory record, this test may be done once a week.

Mica and talc in sand:
Presence of mica and talc in sand may reduce the compressive strength of concrete because of the increase in water demand for giving the required workability. Therefore, check for possible strength reduction and make suitable allowance in cement content.

Mica present in the aggregates is of two types:
Muscovite
Muscovite is potassium aluminium silicate (KAl2 (Si3Al)O10(OH2))which is colour less or has a silvery or pearly lustre.
Biotite
The second variety of mica boitite which is in black brown or dark green in colour, a complex silicate of potassium, magnesium, iron and aluminium K2(MgFe)16 – (SiAl)8 O20 (OH4).

The muscovite variety is now believed to be more harmful for concrete. To give and idea for their related influence if 1 to 2 percent muscovite mica brings down the strength of concrete by 15 percent and the same reduction may be expected in the presence of 10 to 15 percent of biotite mica in concrete sand.

As an example 2 % mica content in aggregate reduces the compressive strength by 11% to 29 % at 7 days and 24 % to 30% at 28 days.

 

[Dnyan Deshmukh]

Soundness of Aggregates:
Concrete liable to be exposed to the action of frost coarse and fine aggregates shall pass sodium or magnesium sulphate accelerated soundness test as per IS 2386 part V. The permissible limits are as follows:

Fine aggregate
10 percent when tested with sodium sulphate (Na2SO4)
15 percent when tested with magnesium sulphate (MgSO4)

Coarse aggregate
12 percent when tested with sodium sulphate (Na2SO4)
18 percent when tested with magnesium sulphate (MgSO4)

Grading Requirements
Sand consists of material mostly between 4.75 mm and 150 micron size, coarse aggregate consists or material mostly retained on 4.75 mm sieve (say larger than 5 mm)

In some cases, natural aggregate deposit having gravel and sand in suitable proportion to give the desired overall grading is available. This type is called “pit run’’ aggregate or “all-in- aggregate”. This aggregate can be used without separating and recombining if it is within the limit specified for all-in-aggregate (IS 383). It is advisable to use naturally graded aggregates only for mass concrete works and foundations. It should not be used for structural concrete.
However aggregate (crushed or natural) can be washed, sieved, and blended to fall within the limits.These blended aggregates can be used for concrete.

Coarse aggregates are designated by “Nominal Maximum Size”. About 5 percent over size aggregate, exceeding the normal maximum size is allowable.
For normal reinforced concrete works “graded aggregates” are used. The grade coarse aggregates are preferred to single size coarse aggregates.

For fine aggregates (sand) the grading requirement are given for four grading zones. Zone I is the coarsest sand and Zone IV is the finest sand. Avoid zone IV sand for reinforced concrete and prestressed works as this requires special precautions.

It is useful to remember that sands can be classified on the basis of percentage passing 600 micron sieve.

15 – 34 % Zone I - Coarse sand
35 – 59 % Zone II - Medium sand
60 - 79 % Zone III - Fine sand
80 – 100 % Zone IV - Very fine sand.

This simple criterion using only 600 micron sieve, gives a quick method for judging the grading zone of sand.

Always ensure that the total aggregate (coarse and fine) after blending should fall within the limits (IS 383) for all-in-aggregate. The grading of total aggregates should fall preferably in the middle of the limits to get a cohesive and dense concrete mix.

 

[Dnyan Deshmukh]

 

[Dnyan Deshmukh]

 

[Dnyan Deshmukh]

Lab Tests on Aggregates
At Site lab following tests can be done
Specific Gravity
Water Absorption
Rodded Density
Dry Loose Bulk Density

Mechanical test like
Impact Value,
Crushing Value and,
Los Angle’s abrasion value can be done at site laboratory or at out side laboratory as per requirement.

Rodded density is checked for graded aggregates.

Site labs tests are generally done at weekly / monthly intervals.

Apart from above when aggregates are wet, moisture content need to be found out before using aggregates for producing concrete.

 

[Dnyan Deshmukh]

 

[Dnyan Deshmukh]

Testing of Water
Water which we can drink is good for concrete. Potable water should be used for concreting activities.

Water should be tested from third party lab for properties stated above in material specification.

It should be tested when source or season is changed.

It is advisable to check the water quality every month during monsoon.

 

[Dnyan Deshmukh]

Testing of Chemical Admixtures
In order to check the properties of admixture, we need to take the trials of concrete with each new received lot of admixture.

Trial should be check for
Effect on workability
Retention time
Any other visual defects observed in fresh concrete.
Setting time of concrete.
Compressive strength of concrete.

Result should be compare with previous batch of admixture or the master report made during selection of admixture.

It is advisable to keep a master sample on receiving each batch, this will help you to avoid disputes for change in admixture properties.
You can make comparative trials of old and new batch of admixture received.

It is good to get the admixture chemical properties tested from third party laboratory at least once in six months.
Its result should be compare with the manufacturer certificate and allowable tolerance as per IS 9103

 

[er_naveen]

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