The three test samples show an increase in mass periodically; this is due to the fact that there is more cement. Cement has a higher density than water causing the mass to increase because the availability of water drops as it reacts with cement.
This increased strength however has lowered the workability of the concrete; it is less fluid which means it will set faster. Concrete may also crack more vigorously then with less concrete due to the increased speed at which it will set.
All the failings in compression were normal and complied with BS EN12390-3:2009.
Conclusion
If more cement is being added then hydration will occur at a faster rate and there will be more chemical reactions making the concrete stronger. This is only true however up until all the water has been used, after this point extra cement will not make it stronger. The concrete will not become stronger and if the cement levels are increased even more, than the extra cement will just stop the concrete from curing altogether. In conclusion the data shows that cement does make concrete stronger in compression. This however is an uncompleted test and requires more test samples to show that this is only true up to a certain level. Simscience.org indicates that cement levels should only vary between 7-15% of the total mass of the concrete.
Data from other groups
Group 1 – Conducting Same Test
Table 2
Conclusion
Group 1 carried out a similar investigation with a slight change in cement levels. The cubes were mixed and tested using the same methods as above. Table 2 shows a rise in compressive strength with increased cement levels. The data does not match exactly but the trend is the same, this therefore shows that it supports the conclusion raised from the previous test.
Group 3 – Changing Levels of Water
Table 3
Conclusion
For this experiment to support the argument raised in this report then it would be expected that the compressive strength of the concrete would drop with the increase in the water. This however is not clear from the table (Table 3). The first result seems to be an outlier, whereas the other results support the theory. This might lead to the impression that the first cube was not created correctly, which is also supported by the drastic mass difference compared to the other cubes. Test sample one can therefore be disregarded.
Group 4 – Changing Levels of Water
Conclusion
This is the same experiment as conducted by group 3 but with slight changes in water levels so it is expected it will show similar results. The trend is similar to group 3 but the data is far apart from each other. This leads to questioning the integrity of this investigation. For the 40% water to cement ration there is a difference of 26.2 (N/mm^2) in strength.
Compressive Strength of an Older Cube
For comparison reasons a cube that had longer time to cure was tested for compressive strength. The old cube had a standard 4,2,1 mix which is the same mix as the first test sample in the main investigation. The mass was 2.39kg, that’s only 0.01kg difference from the test sample cube. From this we can deduct that all factors had been maintained fairly apart from the age of the cube. From this experiment we expect to see a rise in compressive strength for the older cube. Concrete gains most of its strength in the first 28 days, roughly 98%, however this figure continues rising throughout the life of the concrete. The cube failed at 393kN, giving it a compressive strength of 39.3N/mm^2, making 6.1 units higher. The experiment therefore supported the theory that concrete strength increases with time.
Compressive Strength of a Cylinder
To compare how the shape effects the compressive strength of concrete a cylinder was tested until destruction . The cylinder will be compared to the “old” cube as they are both a 4,2,1 mix and have had similar times to cure. Due to the shape of the cylinder, 2:1 ratio compared to the 1:1 ratio of the cube, it should be able to take a higher load than the concrete but this does not make it stronger in compression. The cylinder failed at a load of 586kN, giving a strength of 33.1N/mm^2. This agrees with BS EN12390-3:2009 and therefore supports the idea that a more slender shape has a lower compressive strength.
Tensile Strength of Beam
Due to the properties of concrete, it has a much higher compressive strength than tensile strength. In compression the components of concrete act together to spread the load uniformly. Whereas when a tensile force acts on concrete it is concentrated meaning a small area has to handle the load. This leads to tensile cracks which cause the concrete to tear apart. To investigate this theory a beam with dimensions of 500x100x100 mm was put under tensile stress until destruction as described in BS EN 12390-5:2009.
From the table we can deduce that as the load was increasing the downward displacement was also increasing. As the force increases the area in which it is acting is decreasing, this eventually causes a tensile crack at 4.4 N/mm^2. This is a considerable drop from compressive strength, roughly 10-15% of the compressive strength. This is an undesirable property of concrete but can be battled by introducing steel rods within the concrete which will increase the strength considerably.
How Aggregate Effects Concrete Strength
The type of aggregate and the amount used is also an important factor in determining the strength of concrete. If large amounts of aggregate are used then this would make the concrete not as strong but it will be more economically viable. This is due to the fact that aggregate is a large proportion of concrete, over half, and it’s cheaper to its equivalent volume of cement. Simscience.org indicates that aggregate levels can be varied between 60-80% of the total mass of the concrete.
Appendices
Simscience.org suggests that cement water and aggregate should vary between these amounts:
Picture showing the test samples from main investigation.
Picture showing the cylinder that was tested for compressive strength.
Picture showing the beam that was tested for tensile strength.
Refrences
British Standard Institution, 2000. BS EN12390-4:2000. Testing hardened concrete. Specification for testing machines.
British Standard Institution, 2009. BS EN12390-2:2009. Testing hardened concrete. Making and curing specimens for strength tests.
British Standard Institution, 2009. BS EN12390-3:2009. Testing hardened concrete. Compressive strength of test specimens.
British Standard Institution, 2009. BS EN12390-5:2009. Testing hardened concrete. Flexural strength of test specimens.
Portland Cement Association ()
Richard Rogerson, Sandberg ()
Open source ()
