L Box Test on Self Compacting Concrete
The self-compacting concrete test is a method based on a Japanese design for testing underwater concrete, and was described by Peterson. The test is used to evaluate the flow of the concrete and how much it is blocked by reinforcement. The apparatus used in the test consists of an ‘L’ shaped rectangular box, with a movable gate separating the vertical and horizontal sections. Vertical lengths of reinforcement bar are placed in front of the gate. Concrete is then poured into the vertical section, and the gate is lifted to allow the concrete to flow into the horizontal section. The height of the concrete at the end of the horizontal section is measured as a proportion of the height remaining in the vertical section. This indicates the slope of the concrete when at rest, and is an indication of the passing ability or the degree to which the concrete can pass through the bars.
The horizontal section of the box can be marked at 200mm and 400mm from the gate, and the time taken for the concrete to reach these points can be measured. These times are known as the T20 and T40 times, and are used to indicate the filling ability of the concrete. The size of the reinforcement bar used in the test can vary, and they are spaced at different intervals in accordance with normal reinforcement considerations. A spacing of 3 times the maximum aggregate size may be appropriate. The spacing of the bar can be adjusted to make the test more or less severe in terms of passing ability of the concrete.
Assessment of test:
The SCC filling and passing ability test is commonly used in laboratories and on site to evaluate the stability of concrete and detect any potential segregation issues visually. In addition, the concrete can be inspected by sawing horizontal sections to identify any segregation that may have occurred. However, due to the lack of standardization regarding materials, dimensions, and reinforcing bar arrangement, it is challenging to compare results across tests. It is also unclear how the apparatus walls may impact concrete flow, although it does mimic the formwork used on construction sites. The test requires two operators if measuring time, and some degree of operator error is inevitable.
Equipment for L Box Test
There are three items: an L-shaped box made of stiff, non-absorbing material, a trowel, and a scoop. Additionally, a stopwatch may be assumed to be present.
Fig.:L Box test Apparatus
Procedure of L Box Test
To perform a concrete test, it is necessary to use approximately 14 liters of concrete, which should be sampled normally. The apparatus should be placed on firm ground, and it is important to make sure that the sliding gate can open freely and then close it. Next, 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, and then left to stand for 1 minute.
After the concrete has stood for 1 minute, the sliding gate should be lifted to allow the concrete to flow out into the horizontal section. At the same time, the stopwatch should be started, and the time for the concrete to reach the concrete 200 and 400 marks should be recorded. When the concrete stops flowing, the distances ‘H1’ and ‘H2’ should be measured. It is then necessary to calculate the blocking ratio by dividing H2 by H1.
It is important to perform the entire process within 5 minutes. This involves filling the apparatus with the concrete, allowing it to stand for 1 minute, and then lifting the sliding gate and starting the stopwatch to record the time and distances. By following these steps, it is possible to obtain accurate results for the concrete test.
Interpretation of the result:
The “blocking ratio” is a test value that indicates how freely concrete flows. If the concrete has the same fluidity as water, it will rest in a horizontal position, resulting in a ratio of H2/H1=1. The closer this ratio is to unity, the smoother the flow of concrete. In this regard, the European Union’s research team recommends a minimum acceptable value of 0.8 for the blocking ratio. Although T20 and T40 time can provide some indication of how easily concrete flows, no appropriate values have been universally accepted for this purpose. It is possible to visually detect apparent obstructions of coarse aggregate behind the reinforcement bars.