OBJECTIVE To study the workability of concrete mix and the strength of each cube that poured with the concrete mix.
EXPERIMENT OUTCOMES 1. Able to determine the workability of concrete b y slump test according to BS 1881: Part P art 102:
1983 2. Able to determine compressive strength on cubes according to BS 1881: Part 116: 1983
THEORY Concrete samples should be taken directly from the ready-mix ready-mix trucks’ discharge system and shall be taken out of the middle portion of the load. In any case, do not obtain samples until after all of the water has been added add ed to the load also do not obtain samples from the very ver y first or last portions of the batch. The elapsed time between obtaining the sample and initiating the testing should not exceed 5 minutes for slump and air content tests and 15 minutes for strength tests. It is best to transport the concrete sample to the place where the tests will be made and it should be combined and remixed with a shovel or scoop in order to offset any segregation that may have occurred during movement. All tools including shovels, shovels, scoops, tamping rod, cylinder molds, slump cone, and wheelbarrows should be rinsed with clean water and drained thoroughly thorou ghly just prior to each test. All tests should be performed on a level, rigid, horizontal surface surface free from vibration and as near as possible to the location where they will be stored during the first 24 hours. Out of many test applied to the concrete, this is the most 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. In our experiment, mould with size 10cm x 10cm x10cm is used. The concrete are pour in six mould.
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.
EQUIPMENTS 1. Concrete cubes mould
2. Slump-test apparatus
3.
Weighting machine
4. Curing tank
5. Concrete compression machine
MATERIALS (Needed for 6 moulds) 1. Cement – Cement – 7 7 kg
2. Sand – Sand – 14 14 kg
3. Aggregate – Aggregate – 28kg 28kg
PROCEDURE Slump Test
1. Before commencing the test, the plat base and an d inner surface of the mould is wiped with a damp cloth. 2. The slump cone is placed centrally on the plate with the small opening facing upward.
3. The slump cone is filled with concrete in 3 layers, each layer tamped lightly 25 times with steel rod in definite pattern, working from outside into the middle. 4. Top up the cone till it overflows. o verflows. Level off the surface with the steel rod using a rolling action 5. Cleaned off the plate base from any of concrete excess. 6. The cone is slowly raised vertically by the handles and allowed to fall in 5 to 10 seconds. 7. Turn the cone upside down and place the rod across the up-turned cone. 8. The slump is measured by using the ruler (difference in level between the height of the mould and that of the highest point of the subsided concrete) 9. All slumps must be measured and recorded (if the sample slumping still persist due to shear/collapse the test should be repeated).If the slump still persist, measure the height and types of the slump.
Cube test
1. 10 mm x 100 mm x 100 mm standard cube mold is to be used for the concrete mix 2. Make sure the apparatus and associated equipment are clean before test and free from hardened concrete and superfluous water 3. Assemble the cube mold correctly and ensure all nuts are tightened. 4. Apply a light coat of proprietary mold oil on the internal faces of the mold 5. Place the mold on level firm ground 6. Fill with sampled concrete to a layer of about 50 mm thick 7. Compact the layer of concrete thoroughly b y tamping the whole surface area with the tamping rod not less than 35 tamps 8. Repeat steps 6 & 7 until the mold is all filled 9. Remove the surplus concrete after the mold is fully filled and t rowel the top surface flush with the mold 10. Mark the cube surface with an identification number (1, 2, 3, 4, 5, and 6) with a nail or match stick 11. Cover the cube surface with a piece of damp cloth or polythene sheeting and keep the cube in a place free from vibration for about 24 hours to allow initial set 12. Strip off the mold pieces in about 24 2 4 hours after the respective pour is cast. Press the concrete surface with the thumb to see any an y denting to ensure the concrete con crete is sufficiently hardened, or otherwise de-molding has to be delayed for one more day and this occurrence should be stated clearly in the Test Report. 13. Mark the test cube a reference number with waterproof marker pen on the molded side
Place the cube and submerge in a clean water bath b ath or preferably a thermostatically controlled curing tank until it is delivered to the accredited a ccredited laboratory for testing. th
th
th
Testing for cube test (Testing should be carried out on the 7 and 21 OR 28 day)
1. Remove specified test specimen from the curing tank. Wipe to a surface dry condition and remove any loose sand grains for test surfaces 2. Determine the unit weight of the specimen by b y carefully weighing, and measuring the dimensions of, each cubical specimen. 3. Place specimen below the center of the upper bearing block of the testing machine ensuring the most flat and clean of the mold 4. Check the gauge and start the machine. Record the total maximum load as indicated by the testing machine.
DATA ANALYSIS SLUMP TEST Slump = The height of the cone – cone – The The height of the concrete cone after the slump = 30.0cm – 30.0cm – 17.0cm 17.0cm = 13cm
CUBE TEST
Specimen size : 100mm x 100mm No. of cubes : 6 Cubes ( three for 7 days and three for 28 days compression ) Area for each cube :
= 0.1m x0.1m 2
= 0.01m Volume for each cube :
0.1m x 0.1m x 0.1m -3
2
= 1 x 10 m Ref. Mark 1 2 3 4 5 6 Density
Age (Days)
Mass (Kg)
7 7 7 28 28 28
2.36 2.46 2.48 2.44 2.46 2.46
Max load (KN) 202.8 202.8 140.1 193.9 290.0 230.0
Density each cube = Cube 1 : =
2
= 2360 kg/m Cube 2 : =
2
= 2460 kg/m
Density 2 (kg/m ) 2360 2460 2480 2440 2460 2460
Compressive 2 strength (KN/m ) 20280 20280 14010 19390 29000 23000
Cube 3 : =
2
= 2480 kg/m Cube 4 : =
2
= 2440 kg/m Cube 5 : =
2
= 2460 kg/m Cube 6 : =
2
= 2460 kg/m Compressive strength
Compressive strength of each cube = Cube 1 : =
= 20280 kN Cube 2 : =
= 20280 kN Cube 3 : =
= 14010 kN Cube 4 : =
= 19390 kN Cube 5 : =
= 29000 kN Cube 6 : =
= 23000 kN
RESULT 1. From the graph, we can observe the difference of compressive strength. Mould 1, 2, and 3 are curing for 7 days and give lower compressive strength compare with the 28 days curing. These means that the longer the age of curing, the better value of compressive strength will be. 2. Mean compressive strength for 7 days
=
= 18190
Mean compressive strength for 28 days = 19390
+29000
+23000
= 23796.67
Weight of water Weight of cement
= 1.5 kg = 3.0 kg
Water-cement ratio
=
= 0.5 From the water cement ratio, cube test for 7 days shows 24 35.5
.
Convert units from
7 days test 24
= 24000
28 days test 35.5
= 35500
to
and for 28 days indicates
Comparison between mean compressive strength and actual result. Days
Mean compressive strength (
)
7 28
18190 23796.67
Actual compressive strength (
24000 35500
)
DISCUSSION The standard slump test is very simple and can be performed with great accuracy if the ASTM guidelines are followed. The two things that contribute most to an invalid test test are improper rodding and carelessness in removing removing the mold. A lack of rodding may cause poor compaction and over rodding rodding could bring about segregation. Both lead to invalid results. Removing the cone is also very critical and a sudden twist or jerk can greatly reduce the accuracy of the results. It is also very important that the base on which the slump test is being taken is level and free from even the slightest slightest vibration. Movement will very likely indicate a greater slump than is actually being used. Finally, do not use a piece of rebar or any steel rod with with a square end to tamp the sample. This may force the large aggregate to the bottom of the cone and cause the test to be invalid. As with any type of quality assurance testing, the people who perform the tests should be well aware of the proper procedures to be followed and that if specific standards are not met the test results may prove to be invalid. This will consequently result in a loss of valuable time and may result in the need for more expensive testing. Function why cube test is carried out because we want to determine the characteristic strength of the concrete. In this experiment, we already compress two concrete cube. First test is after seven day concrete have been harden and second test is after 28 day concrete have been harden. From this two experiment, we found that concrete in 28 day more strength because the result show the load that can be patch by the concrete is 23796.67kN compare to concrete 7 day is only 18190kN Another function why cube test is carried out is to measure the workability of concrete. The workability of the fresh concrete shall be judged by its suitability for the condition of handling and placing so that after a fter compaction, it surround all, reinforcement, tendons and du cts and completely fills the formwork. Our cube strength onl y 23796.67kN this is because in our mixture of concrete, volume of water is higher so that our strength of concrete is lower. To get higher strength of concrete, volume of water must reduced. Problem of concrete we need to face is segregation and bleeding. Another problem is shrinkage, expansion, cracking, tension cracking and so on.
CONCLUSION Pattern for slump test
Pattern for cube test
From both test, we can conclude that the longer the age of curing, the higher the compressive strength. There are huge different between the mean compressive strength and actual compressive strength due to lower workability of the concrete. The lower of workability of the concrete can we proof by the slump test which the difference between the height of cone and the concrete after the cone released is 13cm which mean it have lower workability. This two test is important to determine the strength and the workability of conc rete before pour to mould to make structure of building.
