Concrete is one of the widely used building materials on planet earth. Concrete has a reliable reputation, durable and useful material. Surprisingly, people always find it challenging to take samples correctly and manage the concrete cube testing as well as the certification process. Comprehensive strength of concrete cube test provides an idea about all the characteristics of concrete.
By carrying out the concrete cube test, concrete can be certified as compliant allowing clients to have confidence in its quality. The safety and reliability of concrete is the primary reason for the test.
You need the following to carry out a proper concrete cube test for your concrete:
• A wheelbarrow
• Slump Cone
• Cube moulds
• Slump tray
• Concrete thermometer
• Mound releasing agent or oil and brush
• Damp cloth
• Concrete curing tank
• Trowel or concrete float
• A steel tape measure
• A pen or marker for identifying samples
• Tamping rod (16mm diameter) or vibrating poker
• Concrete Pour Card
• Cube tag or sticker
After having all you need well set, the next stage is to embark on the test.
Preparation: Site Testing
If you are to decide to take a sample on the casting area I suggest avoid taking a sample from the first or last section of the pour; it will not be an accurate representation of the batch. The concrete is usually sampled after the 1st meter of concrete has been poured to ensure an excellent sample is taken. Pick a few samples from the pour to have the best representative sample.
And if you want to take the sample from the first truck delivery of concrete which was what we usually do, that’s fine just make sure to roll the drum of the truck mixer before you take a concrete sample for concrete cube testing. The representative is then used in making the cubes. The sample must be an excellent cohesive mix, and it may require some mixing once taken from the concrete batch to be suitable for a slump test and cubes
Prior to pouring the concrete into the moulds, coat the mixture in a mould release agent. It makes it easy to remove the concrete once you’ve finished testing. Afterward, you can scoop the concrete into the mould in three 50mm layers and compact each using a compacting rod. After successfully compacting the layers, remember to tap the mould with a hammer to remove trapped air.
You will then have to level off the mixture, vibrate it efficiently until all surface bubbles are removed after which labeling is done for easy identification.
Cover the cubes with a damp clothing, and a plastic sheet and store them in a dry, cool place in an environment with temperature ranging from 20 to 25 degrees, it is better if you have a curing room with air conditioning. The concrete cubes are removed from the moulds between 16 to 72 hours. They are placed in a curing tank operating at a temperature between 20 to 22 degrees. Provide a moist environment that allows the cubes to hydrate properly. Ensure the cubes are fully submerged in the curing tank at all times and record the tank water temperature every day.
The cube Test
The cubes are tested at 7 & 28 days. Usually, 2 cubes are tested at 7 days and 2 cubes at 28 days. Nonetheless, this may vary depending on the requirements, hence check the design first. The cubes are removed from the curing tank, dried and grit removed. The cubes are tested using a calibrated compression machine or a universal testing machine by internally competent personnel or by a certified test laboratory. The compression machine exerts a progressive force onto the cube until the cube fails, indicating the maximum strength of the concrete.
The following are six common quality tests on concrete before and after the completion of casting a site.
1. Slump test
You need to do a slump test before any cube is tested. It is vital that you do this to examine the suitability of the concrete cube. After the slump test, you are sure your concrete is usable. The designer does design the slump specific mix to aid in placement.
After conducting slump tests the samples are taken through a slump test. Notably, this ensures that the batched concrete is in compliance with the mix design before releasing it from the batching plant.
Upon arrival on site, a sample of fresh concrete shall undergo a slump test again checked with a calibrated thermometer. Two or three cubes or cylinders of samples from the site shall be taken for compressive strength tests.
2. Compressive strength test
Four samples of cubes are taken for concrete compressive strength test, depending on the specification. Two cubes out of the four are tested for 7 days and the other two cubes are tested for 28 days if one cube passes and the other one fails. If the consultant is not delighted with the results and he wishes to test the extra two, it shall be tested in 42 days. It is always advisable to save extra samples.
3. Water Permeability test
A water permeability test finds the durability of concrete. Three cubes are taken from fresh concrete and tested according to the German Standard DIN 1048 at 28 days. This kind of test shall be taken from substructures concrete elements like foundations, concrete water tank, retaining wall, etc.
4. Rapid Chloride Ion Penetration Test
Similar to water permeability test, this also shows the durability of concrete. Three cubes shall be taken from fresh concrete delivered on site and tested at 28 days.
5. Water Absorption Test
Another test that determines the durability of concrete is water absorption test. The three cube samples are taken from the delivered fresh concrete and kept in the curing tank for 28 days or after 24 hours. The concrete sample is remolded and sent directly to the approved third-party laboratory to ensure its curing.
6. Initial Surface Absorption Test
Three cubes samples are taken from the fresh concrete delivered on site. It is then cured and placed inside the curing tank for 28 days before testing.
The water permeability test, Rapid Chloride Ion Penetration test, water absorption test, and the initial surface absorption test are tests that aim at finding out the durability of concrete.
Importantly, identify each of the cubes uniquely and record where they have come from. Majorly, most companies have a process of labeling or tracking the cubes as such it appropriate to ask first and record properly.
Concrete cube testing is governed British Standards Institute or the client’s in-house specifications. These standards represent all aspects involving the process of carrying out tests, from the equipment to the method of testing.
In conclusion, concrete cube testing is vital in identifying the intensity of your concrete, or whether or not it is suitable for use. Much emphasis is placed on the significance of concrete cube testing in the construction industry. Consequently, the test complies with design soon after construction but also can save time and costly investigations later if things go wrong.
Quality Engineer Tips:
• Ensure that you inform the consultant the first time the concrete arrives on site.
• Always check the concrete delivery note or concrete ticket.
• Take concrete cube samples as required.
• Concrete technician must be experienced enough to do the initial tests on site.
• Read international standards about concrete.