U Box Test Method for Self Compacting Concrete
The U Box test, also known as the box-shaped test, is a method developed by the Technology Research Centre of the Taisei Corporation in Japan to assess the filing ability of self-compacting concrete. This test is carried out using a vessel that is divided into two compartments by a middle wall, with an opening and a sliding gate between them. The gate is reinforced with bars that have a nominal diameter of 134 mm and are spaced 50 mm apart, resulting in a clear spacing of 35 mm between the bars.
To conduct the U Box test, the left-hand compartment of the vessel is filled with approximately 20 liters of concrete. Then, the gate is lifted, allowing the concrete to flow upwards into the other compartment. The height of the concrete in both compartments is then measured to determine the filing ability of the self-compacting concrete.
Assessment of U Box test
A direct assessment of filling ability can be achieved through a simple test, but constructing the necessary equipment may prove challenging. The test evaluates the concrete’s performance in meeting its primary function, while also taking into account an unquantifiable need for passing ability. It’s worth noting that the 35 mm gap between the reinforcement sections may be too narrow, and there’s ongoing debate about what constitutes an acceptable filling height below 30 cm.
Equipment for U Box Test
- U box of a stiff non absorbing material
- Scoop
- Trowel
- Stopwatch
Fig: U box test Apparatus
Procedure for U Box Test on Self Compacting Concrete
To conduct a concrete test, approximately 20 liters of concrete are required, which should be sampled normally. The apparatus should be placed on firm ground, and the sliding gate must be checked to ensure that it can move smoothly. After closing the gate, the inside surface of the apparatus should be moistened, and any excess water should be removed. The vertical section of the apparatus should be filled with the concrete sample, which should then be left to stand for one minute.
Next, the sliding gate should be lifted, allowing the concrete to flow out into the other compartment. Once the concrete has come to rest, the height of the concrete in the compartment that was filled should be measured at two points, and the mean (H1) should be calculated. Additionally, the height of the concrete in the other equipment (H2) should also be measured. By calculating the difference between H1 and H2, the filling height can be determined.
It is important to note that the entire test must be completed within five minutes.
Interpretation of the result
When concrete has the ability to flow as easily as water, it will naturally settle in a horizontal position when at rest. This means that the difference between the two heights, H1 and H2, will be zero. The test value that measures this difference is known as the “filling height”.
The filling height is a crucial indicator of the flow and passing ability of the concrete. The closer the filling height is to zero, the better the concrete is able to flow and pass through tight spaces. In other words, a lower filling height indicates that the concrete has a higher level of fluidity and can easily navigate through small openings and obstacles.
Therefore, in the world of construction and engineering, the filling height is an important parameter to consider when evaluating the quality and performance of concrete. A low filling height suggests that the concrete is of high quality and has excellent flow and passing ability, which can lead to better overall construction results.