Sometimes it is essential to determine the proportions of various constituents of concrete. This can be done easily in the field if the concrete is fresh and analysis is done within five minutes of discharge of the concrete mix from the mixer or agitator. If it is not possible to start the test within five minutes after discharge of concrete from the mixer, the concrete is taken from the charge of the mixer within five minutes and placed in an air tight container till the commencement of the analysis.
The analysis of fresh concrete should be commenced within two hours from the time of addition of water to the solid ingredients. This analysis can be done with the help of Dunagon Buoyency apparatus. Before commencement of analysis of the fresh concrete samples of coarse and fine aggregates used in concrete should also be taken to determine their specific gravities, water absorption and for sieve analysis etc.
Apparatus:
ADVERTISEMENTS:
One form of the apparatus consists of the following parts:
A. Balance:
A semi-automatic balance capable of weighing upto 5 kg to an accuracy of 0.5 gram. The balance is provided with two counter poises to obtain equilibrium, one for when an empty bucket is being weighed in air and the other, when the bucket is being weighed whilst immersed in water. If the sample is to be weighed in air and covered with water at site before transporting it to a laboratory for analysis, then the balance capable of weighing upto 5 kg to an accuracy of 1 gram should be available at site.
B. At least eight seamless bucket shaped containers of corrosion resisting metal, such as stainless steel, chromium or nickel plated steel sheet 2.0 mm thick, each 200 ± 5 mm diameter at top (internal) and 180± 5 mm deep having smooth sloping side and a rounded bottom to prevent trapping of air when it is immersed.
ADVERTISEMENTS:
All the containers should be of the same weight in air correct up to ± 0.25 gram and each should be clearly marked with the necessary correction figure to allow for the difference between its loss in weight when immersed in water and weight of second counterpoise Fig. 7.18.
C. Tank:
The tank should be large enough to receive the bucket-shaped container easily and made of corrosion resisting metal such as stainless steel or chromium or nickel plated steel sheet 2.0mm thick. The internal diameter of the tank should be of the order of 500 mm and 300 mm deep as shown in Fig. 7.19. The tank is provided with an overflow spout in such a position that the rim of the bucket hung from the balance should remain completely immersed when the tank is full.
ADVERTISEMENTS:
The spout should preferably be 5 mm below the tap of the tank. The tank should be connected by a tap and flexible pipe to a subsidiary tank, so that the water level in the main tank can be raised or lowered without disturbance by altering the level of the subsidiary tank. The tap should preferably be at a distance of 20 mm from the top of the tank, so that in the lowest position of the subsidiary tank when the tap is open the level of water is below the lip of the bucket hanging or the balance.
A baffle plate of corrosion less metal as above (approximately 1.7 mm thick and 70 mm deep and 100 mm long) should be provided inside the main tank opposite the inlet of the tap extending from the tap of the tank to a position 50 mm below the centre of connection of the tap as shown in Fig. 7.19.
D. Subsidiary Tank:
ADVERTISEMENTS:
The subsidiary tank should be of suitable size and made of corrosion resisting metal as above. The bottom of this tank is connected to the top of the main tank by a flexible pipe. The position of the subsidiary tank should be movable, so that it can be raised or lowered from the position of the tap on the main tank as necessity arises.
E. Sieves:
Two nesting sieves 450 mm diameter are used. One I.S. sieve 4.75 mm (100 mm in depth) and the other I.S. sieve 150 micron (300 mm in depth) supported in the sieve frame by an adequately rigid spider of brass as shown in Fig. 7.20. The plan of spider is shown in Fig. 7.21.
F. Funnel:
ADVERTISEMENTS:
A funnel approximately 500 mm in diameter at top, 150 mm diameter at the bottom and 250 mm deep made of 1.6 mm thick corrosion resistant metal as shown in Fig. 7.22. is provided to facilitate the washing of materials from the large sieves into the buckets without loss of any material.
G. Hose:
A hose fitted with nozzle capable of giving a fairly fine and strong spray of water should be provided.
H. Stirring Rod:
A 15 mm diameter and 300 mm long mild steel rod is used for stirring purposes.
I. Weight Box:
A standard weight box of capacity 5 kg should be provided.
Samples:
Samples from each of the various sizes of aggregates in the same nominal proportions as used in the concrete are taken. The weight of the samples should be approximately 3.5 kg. The weight of sample of concrete should be about 4 kg if the maximum nominal size of aggregate used does not exceed 19mm, otherwise 8 kg weight of concrete sample should be taken and analysed in two parts, each of 4 kg. Before carrying out the actual analysis, the determination of specific gravities of aggregate and cement is essential.
Determination of Specific Gravity of Aggregate:
The specific gravities of the aggregates should be determined under identical conditions under which the analysis of the concrete is to be carried out. Temperature variation during the test should not be more than 2°C. Each sample of coarse and fine aggregate should be dried in a ventilated oven at a temperature of 100°C to 110°C for 24 hours, cooled and weighed. The weights of coarse and fine aggregate in gram should be denoted by Aa and As, as per I.S.I 199-1959.
Now each sample is placed in a clean bucket shaped container and filled with water upto within 25 mm of the lip. To remove any entrapped air from the sample, it is stirred for one minute and the bucket shaped container or bucket is hung in the water tank from the balance. The water level in the tank should then be raised steadily by raising the subsidiary tank till the water starts over-flowing through the spout. The weight of sample is taken at this situation in water. During weighing to get accurate results, maximum movement of the bucket should be limited upto 6.5 mm.
Now the sample is left under water for 20 minutes, then stirred, re-immersed, and reweighed. This procedure is repeated till the variation in weight between two consecutive weighing is less than 0.5 gram, but in no case the period of immersion should exceed 8 hours. The final weights of coarse and fine aggregate should be recorded as Ba and Bs respectively. Time required to get constant weights should also be noted.
Calculations:
Specific gravity of coarse aggregate = Aa/Aa – Ba
Specific gravity of fine aggregate = As/As – Bs
Note:
Average specific gravity for each of them should be calculated. The maximum time required for any sample to attain the constant weight, shall be taken as the time required for absorption of water by the aggregates as a whole.
Specific Gravity of the Cement Concrete:
For the purpose of this test, the specific gravity of Portland cement can be taken as 3.15. For other cements, the value of specific gravity should be determined with the help of specific gravity bottle using an inert liquid.
Procedure:
The specific gravity bottle is first weighed in air. Let its weight be W1 gram. Now the bottle is filled with water and wiped dry from outside and weighed. Let this weight of bottle filled with water be W2 gram.
Then the bottle is emptied and dried by rinsing it with alcohol. About 20 grams of cement is then placed in the bottle and weighed. Let this weight be W3 grams. Now kerosene oil or benzene is filled upto about half the capacity of the bottle and shaken thoroughly to expel all the air from the cement. When air bubbles cease coming out and cement is fully dispersed in the liquid, then the bottle is fully filled with the liquid and weighed. Let this weight be W4 gram. The bottle is emptied again and washed thoroughly with the liquid to clean it of all cement. Now it is filled with this liquid and weighed. Let this weight be W5 grams-
then sp. gr. of liquid = [(W5 – W1)/(W2 – W1)] = d1 (say)
weight of cement in bottle = (W3 – W1) gram
weight of an equal volume of liquid = [(W5 – W1) – (W4– W3)] grams
vol. of this weight of liquids = [(W5 – W1) – (W4– W3)/d1]
density of cement = (W3 – W1)d1/[(W5 – W1) – (W4– W3)/d1]gr/c.c
which is also equal to sp. gr. of cement.
Sieve Analysis:
Each of the samples used to determine the specific gravity of the aggregate should be used for its sieve analysis.
One sample of the coarse aggregate should be put on 4.75 mm I.S. sieve placed over I.S. 150 micron sieve and washed for two minutes under the spray of water, the aggregate being stirred during the washing. The material retained on I.S. sieve 4.75 mm is then washed into a clean bucket by means of the funnel, stirred, immersed in water and weighed. Let the weight be Da.
One sample of fine aggregate is then added to the material retained on I.S. 150 micron sieve and washed under spray of water for atleast ten minutes, continuing till the water is clear. The residue retained on the sieve is washed into a clean bucket, stirred, immersed and weighed. Let the weight be Ds. Care should be taken that no material is wasted while transferring the sample to the sieve and back to the bucket.
Correction factor then shall be calculated as follows:
Correction factor coarse aggregate Ca = Ba/Da
Correction factor fine aggregate Cs = Bs/Ds
The correction factor is used to make allowance for the amount of coarse aggregate passing I.S. sieve 4.75 mm and the amount of total aggregate passing 150 micron sieve. Ba and Bs are same as determined in the sp. gr. determination.
Analysis of Concrete:
1. The sample of concrete is put in a clean bucket and its weight is taken in air. Let the weight of concrete in air be Wkg.
2. Now the bucket is filled with water upto within 25 mm of the lip and the contents stirred thoroughly for one minute to remove any entrapped air.
3. The sample is left immersed in water for a period of time not less than that required for absorption of water by the aggregate as determined above, but not longer than 8 hours in any case. After this period the concrete is again thoroughly stirred for one minute to remove air expelled from the aggregates.
4. The bucket is hung in the water tank from the balance with the water level in the tank below the top of the bucket. The bucket then is carefully filled with water up to the lip and the sample left to settle for five minutes.
5. The water level in the tank is now raised steadily by raising the subsidiary tank. When water begins to overflow from the spout in the main tank, the tap is closed and the weight of sample is taken in water. Let it be ws, during the process, care is taken that sample is not shaken and the maximum movement of bucket is not allowed more than 6.5 mm to avoid inaccuracy. Less cement particles will flow out of the bucket, if the water level is raised steadily. If the water in the tank becomes dis-coloured, it should be changed between weighing to avoid any change in the specific gravity of the aggregates.
6. Now the concrete of the bucket is washed on to the I.S. sieve 4.75 mm placed over 150 micron sieve. No particles should remain in the bucket. After this, the sample is washed in the sieve under spray of water at least for two minutes continuously till the coarse aggregate is clean. During washing process, the material is kept stirred.
7. The clean coarse aggregate retained on I.S. sieve 4.75 mm is then washed into a clean bucket with the help of funnel and spray of water. The spray is used to remove any small particles from the mesh. The washing should be repeated till the water becomes clear, before the aggregate is placed in the bucket.
8. The coarse aggregate in the bucket is covered with water and stirred thoroughly for one minute. The bucket is immersed in water in the tank and the aggregate weighed as before. Let the weight of coarse aggregate be Wa.
9. Fine aggregate retained on sieve 150 micron is also washed under spray. The clean fine aggregate also is washed clean into a clean bucket, stirred, immersed in water and weighed as before. Let the weight be Ws.
Calculations of Proportions:
1. The weight of coarse aggregate in the sample Wa = wa CaFa
2. The weight of fine aggregate in the sample Ws = wsCsFs
3. The weight of cement in the sample Wc = [w – (waCa + wsCs)] Fc
4. The weight of water in the sample Ww = W – (wa + ws + wc)
where,
W = the weight of concrete in air
w = the weight of concrete in water
Wc = the weight of coarse aggregate in water
Ws = the weight of fine aggregate in water
Ca = the correction factor for coarse aggregate
Cs = the correction factor for fine aggregate.
Determination of weight per cubic metre of concrete, yield, cement factory and air content of freshly mixed concrete are carried out as follows:
1. Weight per Cubic Metre of Concrete:
Following apparatus is needed to measure the weight of the freshly prepared concrete:
(a) Measure:
The measure should be strong enough with handles usually 4 mm to 5.5 m thick. Its dimension may be as follows:
(b) Tamping Bar:
It is a steel bar 38 cm long, having ramming face square. Its weight is 1.8 kg.
Procedure:
The prepared concrete to be tested is taken and filled in the measure selected from the above table and compacted fully either by hand or by vibration. It should be compacted in such a way that neither it results in segregation nor in excessive laitance. While compacting with hand, the number of strokes per layer required to produce the specified condition vary according to the consistency of concrete but in no case the number of strokes should be less than 60 per layer for 0.01 m3 measure or 120 strokes per layer for 0.02 m3 measure. The strokes should be distributed uniformly over the whole surface.
To remove the air from the concrete, the exterior surface should be tapped 1 to 15 times and then top surface struck off and finished smoothly. Care should be taken to see that measure remains full. The excess concrete from the exterior is wiped off and cleaned. Now the measure is weighed.
Calculations:
The weight of concrete per cubic metre is calculated by dividing the weight of fully compacted concrete of the measure by its volume. Let the weight of concrete be W kg, then-
where,
V = volume of concrete produced per batch
N = No. of 50 kg cement bags used per batch
wf = total weight of fine aggregate used per batch in kg
wc = total weight of coarse aggregate used per batch in kg
ww = total weight of water used per batch in kg
W = weight of concrete in kg/m3.
Yield per bag of cement
It is given as Y = V/N … (7.2)
where.
Y = yield of concrete per 50 kg bag of cement in m3
V = volume of concrete produced per batch in m3
N = number of 50 kg bags of cement used per batch.
Cement factor N1 = N/V … (7.3)
Air content A. Air content is given as –
A = [(T – W)/T] x 100 … (7.4)
where,
A = air content (% voids) in the concrete
T = theoretical weight of concrete in kg/m3
W = actual weight of concrete in kg/m3.