JOMO KENYATTA UNIVERSITY OF
AGRICULTURE AND TECHNOLOGY
Civil, Construction & Environmental Engineering Department.
A PRACTICAL REPORT ON COMPRESSIVE STRENGTH OF CONCRETE
2nd YEAR FIRST SEMESTER.
ECE 2206.
CIVIL ENGINEERING MATERIALS 1.
SUBMITTION DATE:19/12/2015.
PREPARED BY ODHIAMBO BRIAN ODHIAMBO.
A.AIMS.
1.To finish compressive strength test on cylindrical and cubical concrete specimens(C30)(already prepared).
2.To observe behavior of the concrete specimen under compression.
3.To dertemine Compressive strength for concrete specimens.(cubes and cylinders
).
B.INTRODUCTION.
One of the principle concerns of an engineer is the analysis of materials used in structural applications. The term structure refers to any design that utilizes materials that support loads and keeps deformation within acceptable limits .The compressive strength is a measure of the concrete's ability to resist loads which tend to crush it. Many tests are used to evaluate the hardened concrete properties, either in the laboratory or in the field. Some of these tests are destructive, while others are non-destructive. The most common test performed on hardened concrete is the compressive strength test. Compressive strength is one of the main structural design requirements to ensure that the structure will be able to carry the intended load. The Compressive strength is measured by breaking a cylindrical or cubical concrete specimen of known dimensions in a compression testing machine .The compressive strength is calculated from the failure load divided by the cross-sectional area resisting the load and reported in MPa(SI) unit however the UTM directly displays the compressive strength of concrete.
Relevance of the practical to our course:
Compressive strength test results are primarily used to:
1.Dertemine that the concrete mixture as delivered meets the requirements of the strength specified in the job specification
2.Quality control and acceptance of concrete-by maintaining standards use worldwide.
3.Estimating the concrete strength in a structure for the purpose of scheduling construction operations such as form removal of formwork.
As it was early explained during the units lecture sessions the strength of concrete is dependent on a number of factors namely water cement ratio, type of cement, type of aggregate among others.
In this laboratory practical we prepared a mix design of class 30 using pre-dertemined proportions of our mix ingredients (BRE CONCRETE DESIGN METHOD)and carried out compressive strength test on our 150mm by 150mm and 100mm by 200mm cubic and cylindrical concrete specimens respectively .The compressive strength tests were done 3,7 and 14,21,28 days after casting.
C.APPARATUS.
Preparation of specimens
1.Portland cement.
2.Weighing balance.
3.Mixing trough.
4.Cylindrical and cubical steel moulds.
5.3Spades.
6.Curing Bath.
7.Source of water supply.
8.3Trowels
9.Tamping rods.
Compressive Test.
1.Universal Testing Machine
2.Large Absorbent Cloth.
3.Weighing Balance.
4.Steel plates(2).
5.Pair of Gloves.
6.150mm by 150mm cube specimens.
7.Cylindrical specimen 100mm by 200mm.
APPARATUS SET UP:
D.PROCEDURE.
1.PREPARATION OF CONCRETE BLOCKS.
1.We measured accurately 28.708kg of coarse aggregate and spread it on a dry clean metallic mixing trough.
2.15.311Kg of fine aggregate already measured was spread on the coarse aggregate in the mixing trough.
3.13.588Kg of cement already measured was also spread on the ingredients already present in the mixing trough.
4.The ingredients in the trough were thoroughly mixed using the standard procedure for hand-mixing until a uniform colour mixture attained .
5.5.70Kg of Water already measured was added gradually while mixing was continued until uniform coloured mixture was achieved.
6.Using trowels the steel moulds were filled with the concrete mix in layers tamping 25times before another layer was added. The surfaces were levelled using a trowel and the top surface of the specimen marked for identification.
7.The moulds were left in the laboratory for 3days.After which the demoulding was done and the block placed inside a temperature controlled water bath for curing.
NB.As the major concern of this experiment was more of compressive strength than mix proportion the following values were used for quantity evaluation:
Water cement ratio:0.42.
Density of Fine Aggregate:1600 kg/m3.
Density of Coarse Aggregate:1500 kg/m3.
Density of cement:1420 kg/m3.
Max aggregate size:20mm kg/m3.
15% allowance for wastage and shrinkage.
Volume of a bag of cement:0.035M3.
1:1:2-( C30 cement type 42.5).
2. COMPRESSIVE STRENGTH TEST.
1. All required apparatus/equipments were confirmed.
2.4 Specimens i.e 1cylinder and 1cube were removed from the curing bath.
3.Wore hand gloves,Excess water was wiped out and the specimen left to dry out for approximately 5mins.
4.The mass of the 2 specimens was each measured using a weighing balance and recorded.
5.The cube specimens were centered in the universal testing machine after wiping the upper and lower plates of the UTM however for the cylindrical specimen steel plates were place above the specimen which lay in a horizontal position each at a time.
6.The UTM was then set to compression ,the value for the load rating and the operation of machine were done through assistance by the laboratory technician on duty.
7.The load was continuously applied at a constant rate without no shock.
8.Immediately the specimen start to break the UTM automatically switched off.
10.The machine was cleaned by cleaning the creaked concrete from the machine.
11. The recorded value was then matched by the display once more and then the machine was switched off.
12.The procedure was repeated for all the specimens for all the days we carried the compressive strength i.e day 3,7 and 14,21 and 28 days as directed by the laboratory technician.
E.DATA TABLE & DATA ANALYIS.
The Table below shows the data values at different test dates of concrete specimens C30.
SPECIMEN
3DAYS
7 DAYS
14 DAYS
21DAYS
1.CUBE
18.874MPa
24.176 MPa
27.257 MPa
29.761 MPa
Mass
7.915kg
8.013 kg
8.061 kg
8.118
Volume
0.003375m3
0.003375m3
0.003375m3
0.003375m3
Density
2345 kg/m3.
2374 kg/m3.
2388 kg/m3.
2405 kg/m3.
3.CYLINDER
15.134 MPa
20.034 MPa
22.045
23.431 MPa
Mass
3.563 kg
3.596kg
3.607 kg
3.621kg
Volume
0.00157 m3
0.00157 m3
0.00157 m3
0.00157 m3
Density
2268 kg/m3.
2290 kg/m3.
2297 kg/m3.
2306 kg/m3.
Ratio of strength of cylinder:strength of cube
1:1.24
1:1.21
1:1.24
1:1.27
These values were read directly from the UTM and therefore there use of the formula of compressive strength=maximum load/AREA was avoided hence eliminating errors introduced by rounding off if the latter was to be considered.
Volume of cube:L3
Volume of cylinder:ΠD2/4×H
Density=mass/volume.
D=diameter ,L=Length ,H-Height.
A graph of compressive strength against days;
GRAPH1
A GRAPH OF LOAD AGAINST TIME GENERATED BY UTM DURING CRUSHING OF CONCRETE.
GRAPH 2
F.DISCUSSION.
It was observed that the strength we obtained at 7 days was higher than the minimum strength that is expected that is 20MPa.This proper strength can be attributed to proper hydration of the cement 32.5 used in mix design process,reduction in voids by proper compaction while doing the mix design proper curing of the concrete blocks.Values for the cylindrical specimen are observed to be lower than the cubical specimens from our values it can be seen that the values for compressive strength of cylindrical specimen is about 82% that of cubic specimens this is slightly above the common theoretical percentage of 80%.
The density of concrete for the cube specimens were also found to within the acceptable range of 2300-2400kg/m3.Through consultation we were able to get a reasonable explanation for the low values of the cylindrical specimen however we were not able to prove its validity.The low compressive strength for cylinders is related to its long span(height) the long span cause unequal distribution of loading hence cracking is experienced earlier compared to cube specimens which due to their equal dimensions have equal distribution of loading hence cracking is experienced a later stage. Cube specimens were observed to equally crack on the four faces however little damage was done to the surface in contact with the plates of the UTM. This nature of fracture is acceptable in the Compressive strength test, for the cylinders the cross-sectional area was also observed to have fractures while little damage was done to the area where the load was applied ,however if the specimen was small enough to be able to fit in the UTM machine a conical nature of fracture was to be expected without which the values for the test specimen could have been disregarded. In general, large specimens have less variability and better representation of the actual strength of the concrete than small specimens. Therefore, the 0.15-m by 0.30-m size is the most suitable specimen size for determining the compressive strength. However, some agencies use 0.10-m diameter by 0.20-m high specimens. The advantages of using smaller specimens are the ease of handling, less possibility of accidental damage, less concrete needed, the ability to use a low-capacity testing machine, and less space needed for curing and storage. Because of the strength variability of small specimens, more specimens should be tested for smaller specimens than are tested for standard-sized specimens. The following explanation can be used to describe the reason of concrete failure The interface between the hardened cement paste and aggregate particles is typically the weakest location within the concrete material. When concrete is stressed beyond the elastic range, micro-cracks develop at the cement paste–aggregate interface and continuously grow until failure.
fig 2.showing nature of breaking of concrete.
From graph 1 it can be observed that there is an increase in compressive strength of concrete as days increase this can be attributed to progressive hydration of cement compounds .Effects of Compounds like C3S making up 50% of cement and contributing to strength of cement for the first 4weeks and hence concrete strength can be attributed to the progressive increase in concrete strength for the first 28days.In our practical however we were not able to dertemine the strength of concrete at 28days due to time limitation.
In Graph 2 there is continous increase in magnitude of load until the specimen breaks ,after which the graph curves and there is a decrease in load until the UTM automatically switches off
G.SOURCES OF ERRORS.
1.Lack of proper compaction leading to low density and low strength of concrete.
2.Unsuitable curing conditions.
3.Too much wastage during hand-mixing.
H.PRECAUTIONS.
1.All masses should be measured acurrately and weighing balance checked for errors
2.Ensure specimen is centred inside the UTM before starting the compression test.
I.CONCLUSION.
Compression test on each sample was done until failure(breaking)of the specimen the following values were obtained for the compressive strength of concrete.
Concrete cubes:
18.874MPa(3days)
24.176 MPa(7days)
27.257 MPa(14days)
29.761 MPa(21days)
Concrete cylinders:
15.134 MPa(3days)
20.034 MPa(7days)
22.045(14days)
23.431 MPa(21days)
Concrete specimens were also observed to crack on surfaces where no loading was applied.The was no observable change in the concrete structure only until the specimen started cracking.
K.REFERENCES.
1.Civil materials lecture notes by Mr Gathimba.
2.Properties of concrete by A.M Neville.
3.Materials for civil and construction Engineers-Mamlouk.J.
4.Advanced Concrete Technology 3rd Edition by John Newman and Ban Seng.
COMPRESSIVE STREGNTH AGAINST DAYS
DAYS
COMPRESSIVE STRENGTH(MPa)