The construction of the brick pillars and casting of the mortar cubes was carried out on 13/10/2011, with the demoulding of the cubes happening the next day on 14/10/11. The mortar cubes required 7 days to achieve their working strength; therefore testing took place on 21/10/11, which meant that the brick pillars were 8 days old.
- Test Materials & Equipment
2.1 Bricks
The bricks shall each be defined as a ‘masonry unit made from clay or other argillaceous materials with or without sand, fuel or other additives fired at a sufficiently high temperature to achieve a ceramic bond’ (BS EN 771-1:2010). In this instance the bricks are known to be clay, though the age or date of delivery to the laboratory of the specimens are unknown. The bricks used in this experiment had average dimensions of 214.1mm x 102.5mm x 64mm (see Table 1) with one faced edge and 2 faced ends. The bricks also had a triangular prismoidal recess on one side (also know as a frog).
Table 1 – Properties of 10 randomly selected bricks
2.2 Mortar constituents
The mortar is to be mixed in the lab in a clean cement mixer using sand, cement and tap water. The sand and cement have been kept in a sealed container under a workbench, though these containers are not airtight and the possibility of contamination is high. The age or date of delivery to the laboratory of these is unknown. Prior to mixing an unknown quantity of silver sand had been added to the batch of sand, resulting in drier than normal sand in the mortar mix.
2.3 Water
All water used is tap water and is decanted using a measuring tube with a 2L capacity. Water is assumed to be free of salt minerals and any chemical compounds.
2.4 Cement Mixer
The cement mixer is a standard, electric powered horizontal rotary mixer.
2.5 Tools
2 different sizes of trowels were used, along with 10mm and 15mm gauges to standardize mortar thicknesses. A 2L measuring tube was used to decant water into the mixer. A tamping bar was used to mix the wet mortar in the cube moulds. A steel wool brush was used to clean the cement mixer out after use and after completion; raw aggregate was introduced into the mixer to dislodge any surplus mortar, which was then cleared using the brush. A wheelbarrow was used to dispose of any waste mortar in the mixing stage and the failed pillars and cube in the testing stage.
2.6 Hydraulic Crushing Machines
There were 2 machines used in this experiment, the first is an ADR-Auto 2000 by ELE International that can apply a maximum compressive force of 2000kN. This machine was digital and adjusted the load automatically without user input (whilst crushing was in progress). The group used this machine to test the cubes and not the pillars due to the size of the chamber. The user is required to input the size of the cube, rate of load application and type of medium being tested before pressing run, after which the machine applies the load at the prescribed rate and stops as soon as the load starts to drop off (meaning that the cube had failed).
The second machine was manufactured by Avery-Denison and was of a similar design to the ADR-Auto 2000 but was much larger. The Avery-Denison crushing machine can apply a maximum compressive force of 3000kN. The loading rate was inputted using an analogue knob and the adjustments for variations in the rate of load application where manual also, with one knob for loading and one knob for unloading.
- Experimental Procedure
3.1 Mixing the Mortar
The mortar used to bond the clay bricks together consists of Sand, Cement and water in the ratio of 6:1 sand to cement. Water used in the mix is gauged by the workability of the mortar as it is being mixed. Depending on the native moisture content of the sand used, different amounts of water will be required to obtain the desired workability. In this particular case 4.9L of water was required.
The constituent materials are measured out in accurately sized containers, one measuring 6L and the other 1L. Three and a half quantities of both sand and cement are required to provide enough mortar for 3, 3 course brick pillars, totalling 21L of sand, 3.5L of cement. With 4.9L of water this gives a ratio of 6:1:1.4 sand, cement and water respectively.
The mixing procedure involves placing the sand and cement in to a cement mixer and allowing the two to become evenly mixed. Water is then added slowly and evenly pausing periodically to ensure mixing is even. As the water content in the mortar approaches the required amount care should be taken not to add too much as at this stage a small amount of water can render the mix too wet and unusable.
3.2 Mortar Cube Construction
Mortar cubes are required so that the compressive strength of the mortar can be attained.
Three cubes are to be made so that an average strength of the mortar can be obtained. As with the brick pillars, the process for making the cubes is also important; ensuring that the cube is homogenous is essential, as any voids within the cube will affect its compressive strength.
Moulds are lightly wiped with release agent before being filled with mortar. Filling the mould by a third then agitating the mortar to remove any air bubbles with a tamping bar ensures that the mortar is consolidated. Another third is then added and tamped in the same way followed by a final third. The top of the cube is scraped flat with a trowel to remove any excess and then covered with a plastic card to prevent moisture evaporation.
After 24 hours the cubes are of sufficient strength for them to be removed from the moulds. They are then placed in a climate-controlled chamber to cure for at least 7 days before testing. The atmosphere in the climate-controlled chamber is kept at a constant temperature with a humidity of 100% to prevent moisture loss during the curing process.
Figure 1 – Mortar cube dimensions
3.3 Brick Pillar Construction
Three brick pillars are required for testing, each constructed differently to assess the most efficient method of construction. All three pillars will be made of 3 courses consisting of 2 bricks per course alternately arranged.
- Pillar 1 will have the bricks placed with the frogs facing downwards, bonded together with a mortar bed thickness of 10 mm.
- Pillar 2 will have the bricks placed with frogs facing upwards, bonded together with a mortar bed thickness of 10 mm.
- Pillar 3 will have the bricks place with the frogs facing upwards, bonded together with a mortar bed thickness of 15mm.
The procedure for building the brick pillars is as follows (see figure 2 for sketch):
- Apply a layer of mortar to one long face of a brick; use another brick to then hold the mortar in place whilst the bricks are placed on to a level surface.
- Moving the bricks in a backward-forward motion relative to each other will allow the bricks to be moved to achieve the required joint thickness. Gauges cut to the 10 and 15mm thickness of the joints are used to ensure that the joints are the correct thickness and even all round.
- The frogs are then filled with mortar to form a bed for the second course of bricks. The pillar is then rotated through 90° and the process repeated for the second layer.
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Once all three courses are laid a topping of mortar is applied to the pillar. Care should be taken to make this topping as flat and parallel to the base as possible. The topping provides a surface for the testing machine to apply load to the pillar, an uneven topping will result in the applied forces being distributed unevenly through the pillar during testing.
Figure 2 – Brick pillar dimensions
3.4 Mortar Cube Testing
The first step of the both experiments is to prepare the machines by making sure they are clean and free of debris. This entails brushing the plates with a wire brush to remove any fines or particles that may create a point load and result in an incorrect reading.
Once the machine is prepared the mortar cube is placed on the plate, ensuring the corners are square to the guide lines and the face of the cube that was not in contact with the sides of the mould is facing the user (See figure 3).
Figure 3 – Mortar cube prior to crushing
The guard on the front of the chamber must be shut to ensure the safety of anyone standing near the machine. This is mainly to avoid any particles entering the eyes of observers as the cube is being crushed and shatters. The functionality of the machine being used in this test is detailed in section 2.6, once the load rate and medium have been inputted the machine crushes the cube to it’s failure point and then retracts automatically.
The cube being tested will then be visually cracked, allowing for analysis of the mode of failure.
3.5 Brick Pillar Testing
The differences between the machine functionality and deign in this test and the machine used in the last test is detailed in section 2.3.
As before, the machine should be thoroughly cleaned before testing each pillar to ensure there is no debris in the machine. Once the pillar has failed, the load must be manually reduced and the pillar removed from the chamber. 2 out of the 3 pillars tested disintegrated as soon as they were removed from the chamber so the cracking and failure paths had to be observed whilst in the machine for these 2 (see figures 4 and 5).
Figure 4 – Pillar failure in situ Figure 5 – pillar failure when removed
- Experimental Results
4.1 Mortar Cubes
The results of the mortar cube crushing tests were as follows (see Figure 6 for visual comparison):
- Cube 1 failed under a compressive load of 59.5 kN
- Cube 2 failed under a compressive load of 57.4 kN
- Cube 3 failed under a compressive load of 61.1 kN
As the dimensions of the cubes are uniform (100mm x 100mm), their compressive strengths can be calculated using the following formula:
(Failure Load (kN) x 1000) / (100mm x 100mm) = Strength (N/mm2)
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Compressive strength of cube 1 is equal to 5.95 N/mm2
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Compressive strength of cube 2 is equal to 5.74 N/mm2
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Compressive strength of cube 3 is equal to 6.11 N/mm2
(Refer to Figure 7 for visual comparison)
BS EN 772-1:2011 requires the calculation of the coefficient of variation of the compressive strengths of the cubes to the nearest 0.1%. This can be calculated using the following formula:
(Standard deviation / Average) x 100 = Coefficient of variation (%)
The standard deviation of the results was equal to 0.186 (to 3 sig. figures)
The average of the results was equal to 5.93 N/mm2
This gave the coefficient of variation to be 3.1% (to the nearest 0.1%)
4.2 Brick Pillars
The results of the brick pillar crushing tests were as follows (see Figure 6 for visual comparison):
- Brick Pillar 1 (10mm mortar joints, frogs down) failed under a compressive load of 129 kN
- Brick Pillar 2 (10mm mortar joints, frogs up) failed under a compressive load of 179 kN
- Brick Pillar 3 (15mm mortar joints, frogs up) failed under a compressive load of 185 kN
The dimensions of the surface that came into contact with the plate of the crushing machine where uniform in pillars 1 and 2 but slightly higher in pillar 3 due to the extra 5mm. Their compressive strengths can be calculated using the following formula:
(Failure Load (kN) x 1000) / [Length x Width (mm)] = Strength (N/mm2)
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Compressive strength of pillar 1 is equal to [(129 x 1000) / (214.1 x 215)] = 2.80 N/mm2
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Compressive strength of pillar 2 is equal to [(179 x 1000) / (214.1 x 215)] = 3.89 N/mm2
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Compressive strength of pillar 3 is equal to [(185 x 1000) / (214.1 x 220)] = 3.93 N/mm2
(Refer to Figure 7 for visual comparison)
Figure 6 – Failure loads
Figure 7 – Compressive Strengths
- Analysis & Discussion
5.1 Mortar Cube Analysis
The mortar cubes behaved as expected i.e there was little difference in the compressive strengths of the three specimens as they were all of uniform size and came from the same mix. This was evident from the low coefficient of variance across the 3 cubes (3.1%).
5.2 Brick Pillar Analysis
The expected result of the testing of the pillars was that the pillars with the bricks that had the frogs facing upwards were to be stronger than the pillar with the frogs facing downwards (Pillar 1), and that the pillar with the 10mm mortar joint (Pillar 2) was to be the stronger of the 2 frog-upwards pillars. In fact, under testing the pillar with the 15mm mortar joints and the frogs facing upwards (Pillar 3) was found to have the highest compressive strength (albeit by a margin of 0.04 N/mm2 over Pillar 2).
As load is applied to the pillar, localised forces within the mortar begin to build. In the initial stages of loading the mortar settles to redistribute the load evenly through the pillar. This can be seen as the reading output of the machine takes a little time to stabilise.
As further load is applied the whole pillar begins to support the loading by transferring load through the bricks and mortar. Failure begins to occur in the bricks in the form of cracks, as the bricks are brittle in nature. At this point the resistance of the pillar reduces but does not indicate total failure.
This is a similar process to mortar settling but within the bricks, forces are redistributed around the cracks and the pillar can take further strain. Eventually numerous cracks within the pillar cause total structural failure as the pillars ability to redistribute load is overcome.
There are several possible reasons for the discrepancy between the expected and recorded results:
5.2.1 Materials
The mortar cube strength experiment showed that there was negligible variance between the samples of mortar, however there was a large variance between the strengths of the 3 pillars. It should be noted that the surface areas of all three pillars were not equal, though the variation that this created was so negligible it can be discounted. As the mortar in the three cubes was almost identical, differences in strength of the mortar across the 3 pillars should not be considered as a contributing factor to the differences in compressive strength of the 3 pillars.
As Table 1 shows, the strengths of the 10 bricks that were tested in compression varied by up to 24.8%. This may be a factor in the difference in compressive strength in the pillars but it must also be noted that it is unknown whether the bricks used in the pillars came from the same batch as those tested.
5.2.2 Error in Pillar Construction
Although the pillars were constructed by inexperienced technicians (students with little or no experience of masonry construction), the pillars were inspected and construction was supervised by all group members and by an experienced lab technician. Detailed instruction was given on correct methods of construction of the pillars before work began and precision tools such as trowels and gauges were utilised to ensure correct thicknesses of mortar joints and beds. These measures did not, however, ensure uniform and standard construction as a different person built each pillar and due to the technician’s inexperience each course within the same pillar was constructed differently again. All 3 pillars were then cured in the same fashion and for the same length of time to ensure uniform physical condition of the pillars.
The point at where the load is applied to the pillar is important, as the capping mortar should be as flat and parallel to the base as possible. Should this not be the case and the capping mortar is not parallel to the base then the load will not be applied normally to the pillar and a portion of the normal load will be applied horizontally. If the surface is uneven or has high spots, point loads will be applied down through the pillar, giving an inaccurate representation of the pillar strength. After testing, there was a clearly visible contact pattern on the top of the pillar where the plate had been applying pressure. This contact pattern varied between the 3 pillars and was never centered, meaning that the loads were not being applied uniformly across the 3 pillars, introducing further error.
5.2.3 Human Error
Human error includes the poor construction of the pillars as mentioned in section 5.2.2 but also includes any error in the manual control of the rate of load application. This could be a major factor as a different operator with different techniques performed each test. The critical point at failure was the most sensitive and required the quick reaction of the operator, if the operators were not able to increase or reduce the load fast enough then artificial weakness or strength may be shown in the final reading.
5.2.4 Machinery Failure
Though unlikely, errors within the machinery may have contributed to the discrepancies between expected results and actual results. Possible errors could include calibration errors or errors in the measurement of the failure load. Calibration date or to what standard are not noted but can be assumed to be accurate.
5.3 Failure Mechanics
Theory tells us that the mechanics involved with the failure of a brick and mortar construction are;
- Applied vertical loading creates settlement within the cement, this movement of the cement flows along the path of least resistance (outwards through the joints).
- As more vertical load is transferred in to lateral forces within the cement, the bricks that are bonded together with the cement begin to take tensile loading.
- As the bricks are weaker in tensile strength than compressive strength the ultimate failure occurs when the bricks fail when they reach their tensile strength limit.
This can be seen in the observed tests where the final failure of the pillars is by a crack forming directly through the centre, displaying the expected characteristics.
Theory shows that the compressive strength of the pillar containing brick placed frogs down should be significantly less than that of the two pillars constructed with frogs up.
As previously discussed unevenly distributed forces are likely causes of this premature failure. Not all of the cement within the bed is placed under any compression and therefore no forces are being applied to the mortar squeezing it out. The loads that are applied are being transmitted through a smaller surface area of cement making the compression forces within the contact areas of this pillar much higher than the other two pillars.
Law of composites: Composites of 2 materials of different strengths shows that as the proportion of a higher strength material is increased so is the strength of the composite.
- Conclusion
The results from the brick pillar testing under compression shows that it was not as accurate as expected. We know this because the theory indicates that the brick pillar with 10mm mortar frogs up, should have the highest compressive strength, due to the higher brick to mortar ratio. However the 15mm mortar brick pillar with frogs up had the highest compressive strength in our test.
If the pillar compression experiment were to be repeated, it could be improved. Firstly there was human error in the production of the brick pillars, with there not being a consistent amount of mortar applied and the technique in which it was laid. Secondly if a template was available to aid the brick placement, the template would focus on the brick alignment in the pillar, and resultantly by doing this it would benefit during the compression testing as the pressure would be distributed a lot more evenly; therefore making the results more accurate. Finally the spacers and measuring rods were not of accurate dimensions therefore it affected the overall pillar, which again would affect the pillar during testing.
Due to a thicker mortar bed in the joint, the greater the expansion of the mortar, resulting in more lateral force. However in the frog down brick arrangement pillar, the smaller the surface area, the lower the overall compressive strength. This is because a smaller surface area is taking the same load.
Table 1 in section 2.1 (brick failure loads) and the results in section 4.1 (cube failure loads) shows that bricks can take on average nine times as much compressive load than mortar before failing.
- References
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BS EN 771-1:2003 - Specification for masonry units - Part 1: Clay masonry units.
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BS EN 772-1:2011 - Methods of test for masonry units - Part 1: Determination of compressive strength.
- Appendices
