ADVANCE CONCRETE TECHNOLOGY NOTES (LAB MANUAL/PRACTICAL)

Posted by : Saurabh Gupta Saturday, December 14, 2013

Slump Test

Objective: To determine the consistency of concrete mix of given proportions.

Scope and Significance

Unsupported fresh concrete flows to the sides and a sinking in height takes place. This

vertical settlement is known as slump.

In this test fresh concrete is filled into a mould of

specified shape and dimensions, and the settlement or slump is measured when supporting mould

is removed. Slump increases as water-content is increased. For different works different slump

values have been recommended.

The slump is a measure indicating the consistency or workability of cement concrete. It

gives an idea of water content needed for concrete to be used for different works. A concrete is

said to be workable if it can be easily mixed, placed, compacted and finished. A workable

concrete should not shown any segregation or bleeding. Segregation is said to occur when

coarse aggregate tries to separate out from the finer material and a concentration of coarse

aggregate at one place occurs. This results in large voids, less durability and strength. Bleeding

of concrete is said to occur when excess water comes up at the surface of concrete. This causes

small pores through the mass of concrete and is undesirable.

By this test we can determine the water content to give specified slump value. In this test

water content is varied and in each case slump value is measured till we arrive at water content

giving the required slump value.

This test is not a true guide to workability. For example, a harsh mix cannot be said to

have same workability as one with a large proportion of sand even though they may have the

same slump.

Apparatus

Iron pan to mix concrete, slump cone, spatula, trowels, tamping rod and graduated

cylinder.

Slump test apparatus

Procedure

Four mixes are to be prepared with water-cement ratio (by mass) of 0.50, 0.60, 0.70 and

0.80, respectively, and for each mix take 10 kg of coarse aggregates, 5kg of sand and 2.5kg

of cement with each mix proceed as follows

1) Mix the dry constituents thoroughly to get a uniform colour and then add water

Place the mixed concrete in the cleaned slump cone mould in 4 layers, each

approximately ¼ of the height of the mould. Tamp each layer 25 times with

tamping rod distributing the strokes in a uniform manner over the cross-section of

the mould.

For the second and subsequent layers the tamping rod should

penetrate in to the underlying layer.

Strike off the top with a trowel or tamping rod so that the mould is exactly filled.

Remove the cone immediately, raising it slowly and carefully in the vertical

direction.

As soon as the concrete settlement comes to a stop, measure the subsidence of

concrete in mm which will give the slump.

Note: Slump test is adopted in the laboratory or during the progress of work in the field for

determining consistency of concrete where nominal maximum size of aggregate does not exceed

40mm

Any slump specimen which collapses or shears off laterally gives incorrect results and if

this occurs the test is repeated, only the true slump should be measured.

Observations & Calculations

Compaction Factor Test

Objective: To determine the workability of concrete mix of given proportions by the compaction

factor test.

Scope and Significance

Compaction factor test is adopted to determine the workability of concrete, where nominal

size of aggregate does not exceed 40mm, and is primarily used in laboratory.

It is based upon

the definition, that workability is that property of the concrete which determines the amount of

work required to produce full compaction. The test consists essentially of applying a standard

amount of work to standard quantity of concrete and measuring the resulting compaction. To

find the workability of freshly prepared concrete, the test is carried out as per specification of IS:

1199-

1959. Workability gives an idea of the capability of being worked, i.e., idea to control the

quantity of water in cement concrete mix to get uniform strength.

It is more sensitive and precise than slump test and is particularly useful for concrete mixes

of low workability. The compaction factor (C.F.) test is able to indicate small variations in

workability over a wide range.

Apparatus

Compaction factor apparatus, trowels, Graduated cylinder, balance, tamping rod and iron

buckets

Essential dimension of the compacting factor apparatus for use with

aggregate not exceeding 40 mm Nominal Maximum Size

Procedure

Keep the compaction factor apparatus on a level ground and apply grease on the inner

surface of the hoppers and cylinder.

Fasten the flap doors.

Weigh the empty cylinder accurately and note down the mass as W kg.

Fix the cylinder on the base with fly nuts and bolts in such a way that the central

points of hoppers and cylinder lie on one vertical line. Cover the cylinder with a

plate.

Four mixes are to be prepared with water-cement ratio (by mass) 0.50, 0.60, 0.70, and

0.80, respectively. For each mix take 9 kg of aggregate, 4.5 kg sand 2.25 kg of

cement. With each mix proceed as follows:

Mix sand and cement day, until a mixture of uniform colour is obtained. Now

mix the coarse aggregate and cement-sand mixture until coarse aggregate is

uniformly distributed throughout the batch.

Add the required amount of water to the above mixture and mix it thoroughly

until concrete appears to be homogeneous.

Fill the freshly mixed concrete in upper hopper gently and crefully with hand scoop

without compacting.

After two minutes, release the trap door so that the concrete may fall into the lower

hopper brining the concrete into standard compaction.

Immediately after the concrete has come to rest, open the trap door of lower hopper

and allow the concrete to fall into the cylinder bringing the concrete into standard

compaction.

Remove the excess concrete above the top of the cylinder by a pair of trowels, one in

each hand will blades horizontal slide them from the opposite edges of the mould

inward to the centre with a sawing motion.

10.

Clean the cylinder from all sides properly. Find the mass of partially compacted

concrete thus filled in the cylinder, say W kg.

11.

Refill the cylinder with the same sample of concrete in approximately 50mm layers,

vibrating each layer heavily so as to expel all the air and obtain full compaction of the

concrete.

12.

Struck off level the concrete and weigh and cylinder filled with fully compacted

concrete. Let the mass be W kg.

Observations, Calculations & Results
Mass of cylinder W1 = ………

Compressive strength of concrete:

Out of many test applied to the concrete, this is the utmost important which gives an idea about all the characteristics of concrete. By this single test one judge that whether Concreting has been done properly or not. For cube test two types of specimens either cubes of 15 cm X 15 cm X 15 cm or 10cm X 10 cm x 10 cm depending upon the size of aggregate are used. For most of the works cubical moulds of size 15 cm x 15cm x 15 cm are commonly used.

This concrete is poured in the mould and tempered properly so as not to have any voids. After 24 hours these moulds are removed and test specimens are put in water for curing. The top surface of these specimen should be made even and smooth. This is done by putting cement paste and spreading smoothly on whole area of specimen.

These specimens are tested by compression testing machine after 7 days curing or 28 days curing. Load should be applied gradually at the rate of 140 kg/cm2 per minute till the Specimens fails. Load at the failure divided by area of specimen gives the compressive strength of concrete.

Following are the procedure for Compressive strength test of Concrete Cubes

APPARATUS

Compression testing machine

PREPARATION OF CUBE SPECIMENS

The proportion and material for making these test specimens are from the same concrete used in the field.

SPECIMEN

6 cubes of 15 cm size Mix. M15 or above

MIXING

Mix the concrete either by hand or in a laboratory batch mixer

HAND MIXING

(i)Mix the cement and fine aggregate on a water tight none-absorbent platform until the mixture is thoroughly blended and is of uniform color

(ii)Add the coarse aggregate and mix with cement and fine aggregate until the coarse aggregate is uniformly distributed throughout the batch

(iii)Add water and mix it until the concrete appears to be homogeneous and of the desired consistency

SAMPLING

(i) Clean the mounds and apply oil

(ii) Fill the concrete in the molds in layers approximately 5cm thick

(iii) Compact each layer with not less than 35strokes per layer using a tamping rod (steel bar 16mm diameter and 60cm long, bullet pointed at lower end)

(iv) Level the top surface and smoothen it with a trowel

CURING

The test specimens are stored in moist air for 24hours and after this period the specimens are marked and removed from the molds and kept submerged in clear fresh water until taken out prior to test.

PRECAUTIONS

The water for curing should be tested every 7days and the temperature of water must be at 27+-2oC.

PROCEDURE

(I) Remove the specimen from water after specified curing time and wipe out excess water from the surface.

(II) Take the dimension of the specimen to the nearest 0.2m

(III) Clean the bearing surface of the testing machine

(IV) Place the specimen in the machine in such a manner that the load shall be applied to the opposite sides of the cube cast.

(V) Align the specimen centrally on the base plate of the machine.

(VI) Rotate the movable portion gently by hand so that it touches the top surface of the specimen.

(VII) Apply the load gradually without shock and continuously at the rate of 140kg/cm2/minute till the specimen fails

(VIII) Record the maximum load and note any unusual features in the type of failure.

NOTE

Minimum three specimens should be tested at each selected age. If strength of any specimen varies by more than 15 per cent of average strength, results of such specimen should be rejected. Average of there specimens gives the crushing strength of concrete. The strength requirements of concrete.

CALCULATIONS

Size of the cube =15cm x15cm x15cm

Area of the specimen (calculated from the mean size of the specimen )=225cm²

Characteristic compressive strength(f ck)at 7 days =

Expected maximum load =fck x area x f.s

Range to be selected is …………………..

Similar calculation should be done for 28 day compressive strength

Maximum load applied =……….tones = ………….N

Compressive strength = (Load in N/ Area in mm²⁾=……………N/mm²

=……………………….N/mm²

REPORT

a) Identification mark

b) Date of test

c) Age of specimen

d) Curing conditions, including date of manufacture of specimen

f) Appearance of fractured faces of concrete and the type of fracture if they are unusual

RESULT

Average compressive strength of the concrete cube = ………….N/ mm² (at 7 days)

Average compressive strength of the concrete cube =………. N/mm² (at 28 days)

Percentage strength of concrete at various ages:

The strength of concrete increases with age. Table shows the strength of concrete at different ages in comparison with the strength at 28 days after casting

Age	Strength per cent
1 day	16%
3 days	40%
7 days	65%
14 days	90%
28 days	99%

Compressive strength of different grades of concrete at 7 and 28 days

Grade of Concrete	Minimum compressive strength N/mm² at 7 days	Specified characteristic compressive strength (N/mm²) at 28 days
M15	10	15
M20	13.5	20
M25	17	25
M30	20	30
M35	23.5	35
M40	27	40
M45	30	45