Concrete Slump Test To Check Workability - Civil Engineering - Civil Engineering

Sunday, 17 February 2019

Concrete Slump Test To Check Workability - Civil Engineering

Concrete Slump Test To Check Workability


Concrete slump test are done to check workability of concrete, all about slump test are described below.

Video Of Concrete Slump Test



STATEMENT OF PROBLEM


i) For M150, 220 litres of total aggregates is required to be used per bag of cement. Slump desired is 50 - 100 mm.

Assume fine aggregate to coarse aggregate ratio as 1:2 using zone II sand.

Keeping F.A /C.A ratio same, calculate slump when w/c ratio is 0.6, 0.68, 0.75 and state how the slump change with increase in water.

ii) Keeping W/C ratio at 0.68, FA:CA = 1:2, observe slumps, when total aggregate is progressively reduced from 220 litres per bag to 200 litres per bag and 180 litres per bag. State how slump,cohesiveness and harshness changes with change in C/A ratio.


iii) Keeping W/C ratio at 0.68, (C/A as 50 kg/220 litres ) observe slump, cohesiveness and harshness, when FA : CA ratio changes from 1:1, 1:2, 1:3.

State how slump, cohesiveness and harshness changes with the change in FA:CA ratio.

THEORY 


Concrete slump test, conduct of which will be described in following paragraphs, gives a measure of workability of the mix in terms of slump observed after the subsidence of a concrete mix (Fig. 18).


One can get a fairly good idea of cohesiveness by gently tapping the platform on which cone stands. A good cohesive mix, subsides further without the coarse aggregate tending to fall out of the mix during tapping.

Harshness can be detected by troweling the mix to obtain a smooth surface. Harsh concrete is normally under sanded and does not give smooth finish to the surface even with troweling under pressure. Adequately sanded mix gives smooth finish with normal trowelingOver sanded mix gives smooth finish with even light troweling.

Limitation Of Slump Test


Because of its simplicity in performing the experiment and its sensitivity to changes in moisture content of the successive intended identical mixes, it is widely used in the field for judging the workability.


This test has limitations. Slump observed strictly, speaking has no relation to useful internal work required for full compaction.

Also a large variation can be obtained with the same concrete. 

Three types of slumps are obtained :

a) True slump

b) Shear slump

c) Collapse slump
Shear & Collapsible Slump
Shear & Collapsible Slump


Collapse slump is normally obtained with lean harsh or very wet mixes. It is difficult to measure slump when shear slump is obtained. Generally concrete giving shear or collapse slump are considered unsatisfactory for placing. Fig. 17 shows different types of slumps obtained. Rich mixes normally behave better than lean dry and very wet mixes.

Choice Of Slump


The slump observed during test is required to be compared to -some standard value of slumps considered desirable for various types of placing and vibrating condition.



Basically higher slump is chosen when vibration is done manually, sections are small are heavily reinforced. Also greater the nominal size of aggregate, more slump is preferred.

Based on the experience and experiments, various authorities such as Concrete Association of India, P.C.A. Illinois, have suggested values of slumps and are reproduced in part I. Out of all the data given, the one given by McIntosh is more comprehensive.


The table gives the range of slumps to be aimed at. As a rule lower limits of the ranges are preferable. But in case of difficult placing conditions, it is better to aim at the higher limit of the range.

Use Of Slump Test


The slump test of concrete is very useful on site to keep check on day-to-day, hour-to-hour variations in the materials fed into the mixes. Increase in slump may indicate unexpected increase in the moisture content of aggregate. Or it may indicate change in gradient of aggregate e.g. deficiency of sand, or change in shape of the aggregate.


Too-high or too-low slump gives immediate warning to the operator and enables him to remedy the situation.



For dry mixes with very low Water/Cement ratios, the slump test gives zero slumps and is not useful for high strength concrete.This test should not be used to compare workability of mixes of different proportions or types of aggregate as the results might be misleading.

Read Also:

⇒  7 Cement Test To Check Its Quality

⇒ Laboratory Compaction Test of Soil

⇒ Core Cutter Method For Determination of Field Density

⇒ Field Measurement of Shear Strength of Cohesive Soil By Vane Shear Test

APPARATUS 


⇰ Mould in the form of a frustrum of a cone (Fig.18).

⇰ Tamping rod, 16 mm dia. and 0.6 meter long, rounded at one end ⇰ Trough

⇰ Trowel

⇰ G. I. Plain sheets

⇰ Steel Scale

PROCEDURE


Procedure of Slump test are described below

A) How To Conduct A Slump Test


a) The internal surface of the mould is throughly cleaned and freed from superfluous moisture and set concrete, if any, before commencing the test.



b) The mould (see fig.18) is placed on a smooth, horizontal, rigid and non-absorbent surface such as carefully leveled metal plate.
Fig.18 Slump Cone & Measurement of Slump

c) The mould is held firmly in place before the concrete is filled in.


d) Concrete under test is filled in the mould in four layers and each layer is approximately a quarter of the height of the mould. Each layer is tamped with 25 strokes of the round end of the tamping bar. The stroke should be distributed over the entire area of the mould.


e) After the top layer has been rodded, the concrete should be struck off level with a trowel or the tamping road so that the mould is exactly filled. All mortar which have leaked between the mould and the base plate is cleaned away.


f) The mould is immediately raised from the concrete slowly and carefully in a vertical direction. This allows the concrete to subside and the slump is measured immediately by determining the difference between the height of the mould and that of the highest point of the specimen being tested in mm.
Measured of slump
Measured of slump

B) To Verify The Effects Of FA/CA Ratio, Total Aggregate/Cement Ratio, Observe Effects Of w/c Ratio On Slump


a)

⇒ Take suitable proportion say 1: 2: 4 by volume or by weight.

⇒ Calculate ingredient for 3 kg batch of cement.

⇒ Make three trials by adding different quantities of water and measure slump and obtain workability, harshness and cohesiveness.

⇒ It will be found that more the amount of water used, greater is the slump.Conversely in field, sudden increase in slump indicates increase in water contents if other factors remain constant and water has to be changed accordingly.

b) 

Given constant w/c ratio and constant total aggregate cement ratio, observe effect of F.A/C.A ratio on slump.


⇒ Assume (suitable) proportions.

⇒ determine the ingredients for 3 kg batch of cement.

⇒ Make three trials by keeping w/c and total aggregate/cement ratio same, but by varying F.A/C.A ratio and observe slump and workability, harshness and cohesiveness.

⇒ It will be observed that deficiency in F.A tends to segregate the concrete and more percentage of F.A requires more water for obtaining desired slump.

⇒ In field, if slump test indicates segregating tendency, it means that the aggregate is deficient in fines. Or if water is required for desired slump, it means there is excess of fine aggregate.


c)

Given constant W/C ratio and F.A./C.A. ratio, observe the effect of A/C ratio on slump.


⇒ Select suitable proportion by volume or by weight.

⇒ Calculate ingredient for 3 kg batch of cement.

⇒ Keeping W/C constant and F.A./C.A. ratio constantmake three trials by varying total quantity of aggregate.

⇒ Measure slump in each case and observe workability.

⇒ It will be observed that smaller the A/C ratio, greater is the slump. In field, if the slump test starts indicating greater slump, check on batching has to be done to verify that proper quantities are added as per design.

PRECAUTIONS


a) Test shall be carried out at a place free from vibrations or shocks and within a period of two minutes after mixing if it is a field test. For laboratory test, reliable results corresponding to site conditions can be had if slump test is carried out 10 minutes after mixing.



b) If slump collapses or shears off laterally, test maybe repeated and if again same results are obtained, the fact should be recorded and slump measured.


c) For laboratory test, it is better to convert all quantities into weight, by multiplying volume by bulk densities. In absence of exact data, assume bulk densities from 1.5 to 1.6 for F. A. and 1.6 for C. A. 

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