CIE 4008 Construction and Materials technology : Predict Properties of
2. Demonstrate an application of scientific principles and demonstrate the capacity to research & investigate.
3. Develop numerical and manipulative skills.
4. Demonstrate effective and professional presentation of a report
Answer:
Aim
The aim of this experiment is to attest the models used to predict properties of unreinforced concrete.
Introduction:
Concrete is a very important material in civil engineering field. The basic constituents of concrete are cement, aggregates (coarse aggregates such as crushed stone or gravel and fine aggregates such as sand) and water. Other constituents including admixtures, reinforcement, polymers, fibres and pigments can be added so as to modify properties of concrete (Hanson Heidelberg Cement Group, (n.d.)). The mix design and procedure used to prepare concrete are also very important as they influence the strength of concrete (Camp, 2016). Concrete is a composite material that is very good in compression and weak in tension. This means that concrete can withstand a substantial amount of compressive forces but can easily break when subjected to tensile loads. It is for this reason that reinforcement is usually added to concrete so as to increase its resistance against tensile forces.
The main reason why concrete is good in compression and weak in tension is because of its structure. As stated before, concrete is made up of concrete, aggregates and water. The aggregates are glued together with the cement paste. There is a zone called interfacial transition zone that creates the aggregates’ interface. This is the weakest link of the concrete structure. When the concrete is subjected to compression, the zone is used to transfer stresses between consecutive aggregates. This does not subject the zone to a substantial amount of force. But when the concrete is subjected to tension, the aggregates are pulled away from each other. Since the cement paste holding the aggregates together is weaker than the aggregates, the concrete starts breaking or failing at very low stresses. In most cases, the compression force of concrete is 10 to 15 times greater than its tension force (Haring, 2017).
There are several tools used to predict properties of concrete batch. The specific tool used is determined by the property being predicted. The key concrete batch properties that are usually predicted are strength, durability and workability. The most common tool used to predict workability of concrete is slump test using lump cone. Workability is a very important parameter of concrete as it determines the ease or difficulty of handling, transporting and placing of concrete with minimal loss of its homogeneity (The Constructor, 2015). Concrete has high compressive strength and low tensile strength. Compressive strength of concrete is influenced by various factors such as concrete mix design and methods used to mix, compact and cure the concrete (Gupta, 2011). Tools used to predict strength of concrete batch include compressive concrete tests and concrete tensile tests using universal testing machine. Durability is tested so as to determine the ability of concrete to withstand thawing/freezing attack, weathering, abrasion, corrosion or chemical attack. There are various tools used to predict durability of concrete batch, including water permeability, water absorption and chloride ion penetration tests (Mades, 2015). Other new tools that can also be used to predict compressive strength of concrete batch are mathematical relationship, genetic programming and artificial neural network (Chopra, Sharma and Kumar, 2015; Abd, 2014).
Batch testing is very important as it helps in ensuring that the right quantity of concrete constituents are used in making the concrete (Jamal, 2014). This plays a key role in increasing accuracy of determining or predicting the properties of concrete thus establishing whether or not the concrete prepared has the capacity to support or withstand the deign loads of the structure being built within the conditions where the structure is built.
Apparatus
This experiment was performed using various apparatuses. The following apparatuses were used to prepare the concrete batches, cure the cubes and test them: bucket, trowel, cube molds, tamping bar, polythene sheet, marker, oil/grease, damp cloth, steel rule/tape measure, concrete curing tank, Denison universal testing machine (shown in Figure 1 below), and slump cone.
Figure 1: Denison universal test machine
Procedure
The following quantities were measured: cement = 510 kg, fine aggregate = 852.5kg, coarse aggregate = 697.5kg, and water = 229.5 litres. These quantities were used to prepare 1m3 of concrete batch. After mixing the fresh concrete, a portion of it was used to perform slump test so as to determine the workability of the concrete. The concrete was poured into the foundation then the batch was sampled and used to prepare 3 cubes and 1 prism/beam for compression and tension testing. The 4 specimens were cured in the concrete curing tank for 28 days. After 28 days, the specimens were removed from the tank for testing. The 3 cubes were subjected to compression testing so as to determine their compressive strength while the prism/beam was subjected to tension testing so as to determine its tensile strength. In all these tests, values of deflection at different load were recorded.
Results
The recorded slump from the slump test performed on the fresh concrete was 100mm.
The dimensions and mass of the specimens tested were as shown in Table 1 below
Table 1: Specimen dimensions and mass
|
Cube 1 |
Cube 2 |
Cube 3 |
Prism/beam | |
|
Mass (kg) |
2.20 |
2.21 |
2.22 |
11.86 |
|
Dimensions (mm) |
100x98x100 |
100x99x100 |
100x99x101 |
103x101x508 |
The values of failure loads or maximum compressive loads obtained for the 3 cubes subjected to compression test are provided in Table 2 below
Table 2: Failure loads for the 3 specimens
|
Cube 1 |
Cube 2 |
Cube 3 |
Prism/beam | |
|
Failure load/maximum compressive load (kN) |
306.4 |
322.5 |
350.7 |
11.6 |
|
Maximum deflection (mm) |
3.79 |
- |
- |
1.21 |
Analysis
The slump of 100mm is within expected limits of a concrete slump test thus the results of the experiment are acceptable. The load-deflection plots for concrete cube and beam are as shown in Figure 4 and 5 in the Appendix respectively. The load-deflection graph for the concrete cube (Figure 4 in the Appendix) shows that the compressive load increased until reaching the maximum value, which corresponded to maximum compressive strength of the concrete. Thereafter, the load decreased with gradual increase in deflection until the concrete cube failed. Also, the load-deflection graph for the concrete beam (Figure 5 in the Appendix) shows that the tensile load increased gradually until reaching a maximum value that corresponded to the maximum tensile strength of the concrete. After the maximum load, the load decreased abruptly with a small increase in deflection until the concrete beam failed. This abrupt decrease in tensile load shows that concrete is weak in tension. From the two graphs, the values of compressive load was very high compared with the values of tensile load. These load-deflection plots are converted into stress-strain plots by converting the load into stress and deflection into strain as follows:
Stress = Force (load) divided by cross-sectional area
Strain = change in length (deflection) divided by original length
Average cross-sectional area of the cubes = [(100x100) + (100x100) + (100x101)]/3 = 10,033.33 mm2 = 0.01m2
Average cross-sectional area of the beam = πr2 = π x 0.05152 = 0.00833m2
Original length of the cube = 100mm = 0.1m
Original length of the beam = 101mm = 0.101m
After conversion, the stress-strain plots for the concrete cube and beam are as shown in Figure 2 and 3 below
Figure 2: Stress-strain plot for concrete cube
From Figure 2 above, elastic modulus of the cube is calculated as follows:
Figure 3: Stress-strain plot for concrete beam
From Figure 3 above, the elastic modulus is calculated as follows:
Discussion
The plot in Figure 2 above shows that the behavior of concrete in compression is initially linear meaning that strain increases proportionally with increase in stress. However, the plot becomes non-linear and this is because of the microcracks’ coalescence at the interface between aggregates and cement paste. The concrete attains ultimate stress when the cracks form a large network. After attaining ultimate stress, the stress of concrete starts reducing non-linearly with increase in strain. The decrease in stress with increase in strain occurs slowly because concrete has the ability to withstand more compressive force before failure can occur.
On the other hand, the plot in Figure 3 depicts the behavior of concrete in tension. The initial part of the curve is not linear through it is assumed so. The plot is similar to that in Figure 2 but it changes after reaching the ultimate stress. Beyond ultimate stress, the stress of concrete drops drastically with an increase in strain because the concrete cannot withstand much tension.
The maximum stress in compression is approximately 30635 kN/m2 whereas the maximum stress in tension is approximately 1392.557 kN/m2. These values shows that concrete is very strong in compression yet very weak in tension (The Irish Concrete Society, 2015). From this analysis result, it is indeed true that concrete has the capacity to resist more compressive force than tension force. Because of the low tension –resisting ability, reinforcement is usually added to the concrete. These results are what were expected because concrete is known to be stronger in compression and weak in tension.
Conclusion:
From this experiment, it was found that concrete exhibits similar but distinct behavior when subjected to compressive and tensile forces. Generally, concrete has greater ability to withstand compressive force than tension force. The values of maximum stress in compression and tension were found to be 30635 kN/m2 and 1392.557 kN/m2 respectively. Thus the experiment verified that concrete is stronger in compression and weaker in tension.
References:
Abd, M. (2014) Compressive strength prediction of Portland cement concrete with age using a new model. HBRC Journal, 10(2), pp. 145-155.
Camp, C. (2016) Properties of Concrete [Online]. Available: https://www.ce.memphis.edu/1101/notes/concrete/section_3_properties.html [Accessed April 5, 2017].
Chopra, P., Sharma, R.K. and Kumar, M. (2015) Prediction of compressive strength of concrete using artificial neural network and genetic programming. Advances in Materials Science and Engineering, 2016 (2016), pp. 1-10.
Gupta, Y.P. (2011) A view of concrete technologies and required related research for materials of construction and their testing methods [Online]. Available: https://www.nbmcw.com/concrete/26930-concrete-technologies-and-required-related-research.html [Accessed April 6, 2017].
Hanson Heidelberg Cement Group (n.d.) Concrete constituents [Online]. Available: https://www.hanson.co.uk/en/technical-information/concrete-constituents [Accessed April 5, 2017].
Haring, B. (2017) Tension vs. Compression of Concrete [Online]. Available: https://www.hunker.com/12003167/tension-vs-compression-of-concrete [Accessed April 5, 2017].
Jamal, H. (2014) Batching, mixing, placing and compaction of concrete [Online]. Available: https://www.aboutcivil.org/batching-mixing-placing-compaction-of-concrete.html [Accessed April 5, 2017].
Mades, N. (2015) Top 6 important quality tests of concrete [Online]. Available: https://www.qualityengineersguide.com/top-6-important-quality-tests-of-concrete [Accessed April 5, 2017].
The Constructor (2015) Various Tests for Workability of Concrete at Construction Site and Recommended Values [Online]. Available: https://theconstructor.org/practical-guide/tests-workability-of-concrete-site-values/5150/ [Accessed April 5, 2017].
The Irish Concrete Society (2015) Concrete Properties, Testing & Standards [Online]. Available: https://www.concrete.ie/concrete_pro.asp [Accessed April 5, 2017].
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