Presentation and analysis of Beams test readings. Illustrate your essay with specific examples.

The laboratory tests for the beam samples of different proportion of Metakaolin mix was carried out during 12th July to 18th July in the City Campus Concrete Laboratory room. The objective of this project was to understand the effect of different proportion of Metakaolin mix on the bend strength of the concentrate mixs. The samples have been taken out from the water tank after curing for 20 days. The samples were tested after five months of curing due to some unforeseen technical problems. The detailed background of the project has been reported in Table 1-3.

Results for 20 % Metakaolin:

Two different beam samples of 700 mm length, 100 mm width, and 150 mm height have been produced by compaction mechanisms by using a concrete mix containing 20 % Metakaolin. The composition of the mix has been reported in Table 2. The weight of each beam was approximately 25.65 kg. The 20% Meakeoline sample was containing approximately 0.137091 wt % glass fibre. The two beams, containing 20 % Metakaolin, are named as Beam 1 and Beam 2. Each of the beams have under gone bending test where the load has been applied from the top of the samples. The testing results for Beam 1 and Beam 2 are discussed below.

Beam 1:

The result for Beam 1 (20 % Metakaolin) is reported in Table 4.
The laboratory test result and load-deflection plot for Beam 1 are reported in Table 4 and Fig 1, respectively. A hair-line crack in the middle of Beam 1 was observed corresponding to the load of 28kN. The length and width of the crack were increased with further loading, and at a load of 38kN it was easily detectable. Another hair-line crack, located at the left side of the middle crack, was initiated at 40kN. Similar to the first crack the second crack has also propagated with further loading. Rapid propagations of the cracks took place after the loading step of 78kN. Finally the beam failed at 90kN with a permanent deformation of 5.97mm (permanent bending strain of 5.97/150 = 0.04).

Beam 2:

The result for Beam 2 (20 % Metakaolin) is reported in Table 5.
The laboratory test result and load-deflection plot for Beam 2 are reported in Table 6 and Fig 2, respectively. Two hair-line cracks near the middle of Beam 2 were observed corresponding to the load of 32kN and 34KN. The length and width of the cracks were further increased with further loading. At 44kN of load both of the cracks were easily detectable. Rapid propagations of the cracks were observed after the loading step of 78kN. The beam was resisted the loading and did not fail unto 97.3kN. As the testing machine has the capacity of 97.3kN therefore the un-failed beam was needed to unload. Surprisingly, the beam has shown some deflection while unloading to 0kN and which can also be seen from Fig 2. A possible explanation to this phenomenon is the beam was very near to its rupture point and therefore it shows a high deflection during the unloading. The permanent deformation of the beam at the end of experiment was 6.87mm (permanent bending strain of 6.87/150 = 0.046).
Overall behaviour of 20% Metakaolin: From both Beam1 and Beam 2 the overall behaviour of the concretes containing 20% Metakaolin can be understood as Beam 1 and Beam 2 have shown similar behaviours. The deflection vs loading curves (which can easily convert to the stress strain curve) for both of the beams were very similar for both cases till 70kN. Both Fig 1 and Fig 2 show plastic regions after 70kN of loading. For Beam 1 at the plastic region rapid deflection took place soon after 78kN for and the beam failed at 90kN due to the rapid propagation of cracks. On the other hand beam 2 resisted till the final loading of 97.3kN and Fig 2 indicates the cracks did not propagate as fast as Beam 1. This indicates the anisotropy in the brittle cement concretes plays a major role on the propagation of crack at plastic region and failure strength of the material. Finally, it can be concluded that for 20% Metakaolin
i) The first crack can be expected at a load range of 28-34kN.
ii) Plastic region was started at 70kN load.
iii) Rapid propagations of the cracks were observed after the loading step of 78kN.
iv) Extent of propagation of the cracks at the plastic region and the failure strength of the beams are very much dependent on the constitutional anisotropy of the material.
Results for 10 % Metakaolin:
Two different beam samples of 700 mm length, 100 mm width, and 150 mm height have been produced by compaction mechanisms by using a concrete mix containing 10 % Metakaolin. The composition of the mix has been reported in Table 2. The weight of each beam was approximately 24.49 kg. The 10% Meakeoline sample was containing approximately 0.16 wt % glass fibre. The two beams, containing 10 % Metakaolin, are named as Beam 3 and Beam 4. Each of the beams have under gone bending test where the load has been applied from the top of the samples. The testing results for Beam 3 and Beam 4 are discussed below.

Beam 3:

The result for Beam 3 (10 % Metakaolin) is reported in Table 6.
The laboratory test result and load-deflection plot for Beam 3 are reported in Table 6 and Fig 3, respectively. A hair-line crack in the middle of Beam 3 was observed corresponding to the load of 26kN. The length and width of the crack were increased with further loading and at a load of 34kN it was easily detectable. Two more hair-line cracks, located at the left side of the middle crack, was initiated at 36kN. Similar to the first crack the second and third cracks also propagated with further loading. A minor rapid deflection of the beam was visualised at 62kN load. Rapid deflection of the beam due to rapid propagations of the cracks took place after the loading step of 68kN. Finally the beam failed at 86kN of loading with a permanent deformation of 5.24mm (permanent bending strain of 5.24/150 = 0.035).

Beam 4:

The result for Beam 4 (10 % Metakaolin) is reported in Table 7.
The laboratory test result and load-deflection plot for Beam 4 are reported in Table 7 and Fig 4, respectively. Beam 4 exhibited a very high number of cracks. Total number of cracks till failure for Beam 4 was five, whereas that for other beams was within three. The first hair-line crack was found at the middle of the beam at a load of 18kN. The second and third cracks have appeared at 24KN of loading. The second crack and third crack were at the right and left of the middle crack, respectively. All of the three cracks propagated further with further loading and were visible after a load of 40kN. Another hair-line crack appeared at 36kN load but it did not propagated much during the loading-unloading cycle. The fifth crack was appeared very near to the middle crack at a load of 62kN. A rapid rate of deflection was observed after 66kN of load and, finally, the beam failed at 81.4kN load with a permanent deformation of 4.796mm (permanent bending strain of 4.8/150 = 0.032).

Overall behaviour of 10% Metakaolin:From both Beam3 and Beam 4 the overall behaviour of the concretes containing 10% Metakaolin is difficult to understand as Beam 3 and Beam 4 have not shown similar behaviour. The deflection vs loading curves for both of the beams were totally different at the primary loading steps. At the primary stage of loading Beam 3 had exhibited a higher permanent deflection (deformation) than that of Beam 4. For example: it was 0.08mm (or permanent strain of 0.08/150 = 0.0005) for Beam 3 at 18kN, where as, was 0.62mm (or permanent strain of 0.62/150 = 0.0041) for Beam 4 at 18kN. Therefore at 18kN beam 4 possessed 7.75 times permanent deformation than beam 3. On the contrary to that, the failure strength was 8% lower for Beam 4 than Beam 3. These again indicate that a deviation in the physical properties, present due to constitutional anisotropy, in different samples of similar compositions. As a result, extremely different results were obtained for beam 3 and beam 4 and, therefore, it is difficult to generalise the overall behaviour of 10% Metakaolin. But it can be concluded that beams of 10% Metakaolin have lesser bending strength than that of 20% Metakaolin as,

a) First crack initiates at lower load
b) Fails at a lower load
Results for 5 % Metakaolin:
Two different beam samples of 700 mm length, 100 mm width, and 150 mm height have been produced by compaction mechanisms by using a concrete mix containing 5 % Metakaolin. The composition of the mix has been reported in Table 2. The weight of each beam was approximately 24.53 kg. The 5% Meakeoline sample was containing approximately 0.17 wt % glass fibre. The two beams, containing 5 % Metakaolin, are named as Beam 5 and Beam 6. Each of the beams have under gone bending test where the load has been applied from the top of the samples. The testing results for Beam 5 and Beam 6 are discussed below.

Beam 5:

The result for Beam 5 (5 % Metakaolin) is reported in Table 8.
The laboratory test result and load-deflection plot for Beam 5 are reported in Table 8 and Fig 5, respectively. A hair-line crack in the middle of Beam 5 was observed corresponding to the load of 22kN. The length and width of the crack were increased with further loading. Another hair-line crack, located at the left side of the middle crack, was initiated at 30kN. The third crack has appeared at 64kN. The third crack has connected the first and second cracks. The interesting characteristic of beam 5 was continuous yield till failure which is very unlikely behaviour of a concrete mix. The material was failed at a load of 91.6 and no rapid deflection was found prior to fracture. The beam had a permanent deformation of 5.06mm (permanent bending strain of 5.208/150 = 0.035) at the time of failure.

Beam 6:

The result for Beam 6 (5 % Metakaolin) is reported in Table 9.
The laboratory test result and load-deflection plot for Beam 6 are reported in Table 9 and Fig 6, respectively. Beam 6 shows a high level of rigidness till 44kN where practically no deflection took place till that load. But an unexpected amount of deflection occurred in Beam 6 at 46kN due to formation of two major cracks during loading of 46kN. The cracks are then propagated further with further loading. At 60kN another crack appeared. The third crack did not propagate much as it had an angle of 30° with the horizontal part of the sample. Rapid propagations of the cracks were observed after the loading step of 78kN and the beam fails at 84.1kN with a permanent deformation of 5.088mm (permanent bending strain of 5.088/150 = 0.034).
Overall behaviour of 5% Metakaolin: From both Beam 5 and Beam 6 the overall behaviour of the concretes containing 5% Metakaolin is difficult to understand as both of the beams have not shown similar behaviours. The deflection vs loading curves for both of the beams were totally different from just 2kN of loading. From Fig 5 it can be seen that Beam 5 has shown some of the features of the load-deflection paths exhibited by Beam 1-3, although, the rapid deflection part was partly absent. On the other hand, Beam 6 has exhibited a strange kind of bending characteristic. The probable reason behind its initial rigidness is the concentration of hard materials at the top part (as the load has been applied from the top). If we carefully study the constituents of 5% Metakaolin then we can se the presence of 0.17 wt % of glass fibre. Glass fibre is one of such reinforcing components which have highest strength to weight ratio. The result indicates improper mixing led to a high concentration of glass fibre at the top of the sample. A sudden and rapid propagation of cracks at 46kN indicates that the brittle part of the top failed at this load and the material continues to behave in a usual manner. Therefore, proper care at the time of mixing of components is needed to avoid constitutional anisotropy. Also a well planned and proper sampling (curing, compaction etc.) is a pre-requirement to achieve high accuracy level. Unfortunately for this project the later was not achieved due to some problem with the testing machinery. However, both inhomoginity of the constituents in the concrete or improper sampling may lead to anisotropy in the physical properties of the materials. Due to the extremely different results for beam 5 and beam 6 it is difficult to generalise the overall behaviour of 5% Metakaolin. But it can be concluded that beams of 5% Metakaolin have lesser bending strength than that of higher grades of Metakaolin (20%-10%) as,
a) First crack initiates at lower load
b) Fails at a lower load

Results for 0 % Metakaolin:

Two different beam samples of 700 mm length, 100 mm width, and 150 mm height have been produced by compaction mechanisms by using a concrete mix containing 0 % Metakaolin. The composition of the mix has been reported in Table 2. The weight of each beam was approximately 24.56 kg. The 0% Meakeoline sample was containing approximately 0.18 wt % glass fibre. The two beams, containing 0 % Metakaolin, are named as Beam 7 and Beam 8. Each of the beams have under gone bending test where the load has been applied from the top of the samples. The testing results for Beam 7 and Beam 8 are discussed below.

Beam 7:

The result for Beam 7 (0 % Metakaolin) is reported in Table 10.
The laboratory test result and load-deflection plot for Beam 7 are reported in Table 10 and Fig 7, respectively. A hair-line crack in the middle of Beam 7 was observed corresponding to the load of 20kN. The length and width of the crack were increased with further loading and at a load of 32kN it was easily detectable. Another two hair-line crack, located at the left side of the middle crack, was initiated at 44kN. Similar to the first crack the second and third cracks have also propagated with further loading. Rapid propagations of the cracks took place after the loading step of 76KN. Finally the beam failed at 86.9kNof loading with a permanent deformation of 6.363mm (permanent bending strain of 6.363/150 = 0.042).

Beam 8:

The result for Beam 8 (0 % Metakaolin) is reported in Table 11.
The laboratory test result and load-deflection plot for Beam 8 are reported in Table 11 and Fig 8, respectively. A hair-line crack in the middle of Beam 8 was observed corresponding to the load of 18kN. The length and width of the crack were increased with further loading and at a load of 26kN it was easily detectable. Two more hair-line cracks, located at the left side of the middle crack, was initiated at 36kN. Similar to the first crack the second and third cracks also propagated with further loading. Rapid propagations of the cracks took place at 80KN and the beam failed at 87.2kN without any prior caution. Beam 8 has, therefore, exhibited a complete brittle failure at 87.2kN load with a permanent deformation of 4.37mm (permanent bending strain of 4.37/150 = 0.029).

Overall behaviour of 0% Metakaolin: From both Beam 7 and Beam 8 the overall behaviour of the concretes containing 0% Metakaolin is difficult to understand as both of the beams have not shown similar behaviours. The deflection vs loading curves for both of the beams were totally different from just 4kN of loading. From Fig 7 it can be seen that Beam 7 has followed a usual load-deflection path similar to most of the other beams and undergone a rapid deflection prior to failure. On the contrary, Beam 8 has undergone a complete brittle failure very near to its elastic limit. Beam 8 had a permanent deflection of 0.03mm (or permanent strain of 0.03/150 = 0.0002) at 84KN. On the other hand, Beam 7 had a permanent deflection of 0.04 mm (or permanent strain of 0.04/150 = 0.0003) at only 6kN of load. Such deviation in the test results indicates the possibility of inhomoginity in the mixing or improper treatments (curing, compaction etc.) during sample preparation. Both inhomoginity in the concrete or improper sampling may lead to anisotropy in the physical properties of the material. Due to the extremely different results for beam 7 and beam 8 it is difficult to generalise the overall behaviour of 0% Metakaolin. But it can be concluded that beams of 0% Metakaolin have lesser bending strength than that of other grades of Metakaolin (20%-5%) as,

a) First crack initiates at lower load
b) Fails at a lower load
Also a completely brittle failure of Beam 8 indicates a proper characteristic of a pure concrete.

Conclusions:

Effect of Metakaolin: The bending test of Beam 1-8 have shown a significant effect of Metakaolin on the bending characteristics of concretes. The effects are discussed below,
(a) Effect on maximum bending strength: From Fig 9a it can be seen that bending strength increases with increase in the presence of Metakaoline (By ignoring the result of Beam 6). The maximum deflection of the beams is also increases with the increase of Metakaoline and can be seen from Fig 9b.
(b) Effect on first crack initiation: The results of beam tests show that the load of first crack initiation increases with increase in the presence of Metakaoline. The load corresponding to the first crack initiation has a significant effect on bending strength as from that load the materials effectively and permanently weakens. Fig 10 represents a graphical representation of load of first crack initiation Vs % Metakaoline.

Fig 10: Effect of Metakaolin on the first crack initiation

Presence of Anisotropy in the material: The presence of anisotropy in the testing materials may give rise to some confusing and non related results. For example, it is very difficult to correlate the results between Beam 5 and Beam 6. Similar to that Beam 7 and Beam 8 exhibited completely different characteristics. The probable reasons for such deviations in physical properties are discussed below,
(c) Improper mixing: The constituents of the concrete beams are cement, Metakeoline, sand, C. Aggregate and glass Fibre. Water was used as binding materials. None of the constituents got fused during the mixing and the physical properties of individual constituents remained unaffected. In case of improper mixing, different areas possess different physical properties depending upon the local concentration of the constituents. Therefore the factors like bending strength or crack initiations are very much dependent on local conditions.
(d) Air entrapment: Entrapment air in the concrete decreases strength. Air entrapment may be detected by ultrasound measurements. An entrapped air can be a big factor during bending test as rapid propagation of cracks takes place through it.
(e) Improper compaction: Improper compaction may lead to different physical characteristics between two identical specimens. Improper compaction does not ensures the required density of the samples, and have higher amount of porosity or air entrapments in the samples. Presence of glass fibre: The cement mix used for this project was containing glass fibre as a mixing constituent. From the testing results it also been seen that the average loads vary with the presence of glass fibre in the mix. The effect of % glass fibre on the average load and the average load of first crack initiation is given in Table 12.
Finally, it can be concluded that the cement mixtures corresponding to 20% Metakaolin and 0.137 wt % Glass fibre was the hardest samples amongst all grades of cement mixtures. Therefore, Metakaolin can be used as partial replacement to cement to achieve improved physical properties such as bending strengths, point of first crack initiation.