L-box test - Test on Workability of Self Compacting Concrete

  

In this article, we explained how to measure the flowability of the concrete and to assess the extent of blocking by the reinforcement.

 

Theory and Scope of L- Box Test

The L-Box test measures the filling (flowing) and passing capacity of self-compacting concrete to flow through tight openings, including gaps between reinforcing bars and other barriers without splitting or blocking. There are two differences; Two bar tests and three bar tests. The three-bar test simulates high-density reinforcement.

The new concrete is initially placed on the vertical side of the box. The sliding gate is opened between the vertical or horizontal parts of the box, allowing the concrete to flow into the horizontal area of ​​the box through the gaps or gaps between the vertical and soft reinforcement bars. . The points 200 mm (T200) and 400 mm (T400) are recorded at the time of reaching the concrete in the horizontal part of the box.

After the concrete rests on the machine, the height of the concrete at the end of the horizontal section, H2, and at the beginning of the horizontal section, H1 are measured. The filling capacity or flow of self-compacting concrete is described in terms of the passing rate, H2 / H1.

 

Apparatus required for L- Box Test

·       L-box of a stiff non-absorbent material

·       Ruler

·       Container or bucket

·       Trowel

·       Scoop and Stopwatch.

 

Description of Apparatus

This machine, consisting of a rectangular box made of steel, plastic or plywood, shall have an 'L' shaped, smooth, flat and hard construction with surfaces that are not easily hit or stained by cement paste. The vertical and horizontal sections are separated by a movable gate, the front of which is fitted with longitudinal reinforcement bars. The horizontal part of the box is marked as 200 mm and 400 mm from the gate. The vertical hopper may be removable for easy cleaning. With the gate closed, the size of the vertical hopper should be (12.6 to 12.8) liters.

Devices holding reinforcement bars shall have two soft bars with a diameter of 12 mm with a spacing of 59 mm for the two-bar test and three soft bars with a spacing of 41 mm for the three-bar test. These encounters are interchangeable. Locate the bars in the L-box so that they are perpendicular and across the width of the box. The general structure of the L-box is illustrated in the figure below and the dimensions can withstand ±1 mm. Instead of the steel mould, the mould and end grain built into the 12mm coated formwork plywood can be sealed.

Ruler graduated from 0−300 mm in intervals of 1.0 mm.

Container of capacity of more than 14 litres to hold the concrete sample.

 

Details of L-box test

 

The procedure of L- Box Test

Two operators are required if times are measured, and operator error is inevitable.

Step 1: Collect a representative sample of concrete of about 17 litres to perform the test using a normal sampling procedure.

Step 2:  Set the apparatus on a level, firm horizontal base; ensure that the sliding gate can open freely.

Step 3: Moisten the inside surfaces of the apparatus and remove any surplus water.

Step 4: Close the sliding gate between the vertical and horizontal sections. Pour the concrete sample from the container into the hopper of the L-box to fill the vertical section of the apparatus; allow it to stand for 60 ± 10 seconds.

Step 5: Lift the sliding gate and start the stopwatch simultaneously.

Step 6: Allow the concrete to flow out into the horizontal section of the box and record T200 and T400, the times taken for the concrete to reach the 200 and 400 mm marks, respectively.

Step 7:  When the concrete has stopped flowing, measure the depth of the concrete at the end of the horizontal portion of the L-box at three positions equally spaced across the width of the box. These three measurements are used to calculate the mean depth of concrete as H2 mm.

Step 8: The procedure of Step 7 is used to calculate the depth of concrete immediately behind the sliding gate as H1 mm.

Step 9: Calculate the passing ability, PA as PA = H2/H1 to the nearest 0.01.

 

The real test has to be performed within 5 minutes.

 

Observations and Calculations

Type test: two bar or three bar test
Height of concrete behind the sliding gate	H1mm
Height of concrete at the end of the horizontal portion	H2mm
Passing ability ratio,	PA = H2/H1
The temperature of the concrete at the time of test,	°C
Time is taken in completing the test,	minutes
Passing ability, PA = __________________

 

DO’s AND DONT’s

1. Apparatus should be set on a level, firm horizontal base to avoid jamming of the sliding gate.

2. The inside surfaces of the apparatus should be just moistened with no surplus water to avoid change in mixed proportions.

3. Any delays in opening the box would likely reduce the blocking ratio because of any segregation.

4. The real test has to be performed within five minutes.

 

Conclusion

The L-box test measures both the passing ability and the filling ability of SCC because the extent to which concrete flows down the horizontal portion of the box depends on the yield stress (filling ability) of the concrete and the extent of blocking caused by the row of bars. It enables visualisation of the flow of the concrete in the test, especially any blocking behind the bars.

The stability, i.e., resistance to segregation can be assessed visually. A concrete sample with coarse aggregate particles that reach the far end of the horizontal part of the box exhibits good resistance to segregation. However, there is no evidence of what effect the wall of the apparatus and the consequent ‘wall effect’ might have on the concrete flow, but this arrangement does, to some extent, replicate what happens to concrete on site when it is confined within formwork. The passing ability ratio, H2/H1, for most tests should be 0.85 to 0.90.

The major advantage of the L-box test over the J-ring test is that the amount of mass available to push concrete through the bars is more representative of field conditions than in the J-ring test. The relationship between the test results and field performance is better established than for the J-ring test.

However, the test does not measure the passing ability sufficiently independent of filling ability, i.e., it does not distinguish between passing ability and filling ability. Therefore, the test is essentially a pass/fail test because it is not clear whether concrete with a low blocking ratio exhibits inadequate filling ability, passing ability, or both.

The term blocking ratio generally defined as the ratio of concrete height in the horizontal portion to that in the vertical portion of the box is a misnomer, because higher blocking ratios correspond to less blocking, greater filling ability, or both. A term such as passing ratio is more appropriate.

The L-box does reflect field conditions; however, the number of bars through which the concrete must pass is limited. The J-ring has more bars and would likely exhibit less variability from one test to another. The L-box, however, has an advantage over the J-ring in that a larger mass of concrete is available to push concrete through the bars. On visual inspection of the area around the rebar, an even distribution of aggregate indicates good passing ability.

The test can be used as a reference test for passing ability. However, if the L-box results are used to qualify an SCC mixture design, then an L-box with the same rebar spacing and dimensions should be used during production QC testing.

The test is not as simple as the slump flow test, the test apparatus is relatively bulky, difficult to clean, and thus, not well-suited for use in the field. The L-box test is preferred to the U-box test.

NATIONAL STANDARDS

1. IS 456-2000 (4th revision, reaffirmed 2011): Code of Practice for Plain and Reinforced Concrete.

2. IS 460(Parts 1 and 2)-1985(3rd revision, reaffirmed 2008): Specification for Test Sieves.

3. IS 2386 (Part 3) -1963 (reaffirmed 2011): Methods of Test for Aggregates for Concrete: Part 3: Specific Gravity, Density, Voids, Absorption and Bulking.

4. IS 3812 (Part 1)-2003 (2nd revision, reaffirmed 2007): Specification for Pulverized Fuel Ash: Part1: For Use as Pozzolana in Cement, Cement Mortar and Concrete.

5. IS 10262-2009 (1st revision): Concrete Mix Proportioning— Guidelines.

REFERENCES

1. ASTM C 1610/C 1610M-06; Standard Test Method for Static Segregation of Self-Consolidating Concrete Using Column Technique, ASTM International.

2. ASTM C 1611/C 1611M-05; Standard Test Method for Slump-Flow of Self-Consolidating Concrete, ASTM International.

3. ASTM C 1621/C1621M-06; Standard Test Method for Passing Ability of Self-Consolidating Concrete by J-Ring, ASTM International.

4. EFNARC (2005); European Guidelines for Self-Compacting Concrete.

5. EN 12350-1, Testing Fresh Concrete − Part 1: Sampling.

6. European Guidelines-2005; European Guidelines for Self-Compacting Concrete: Specification, Production and Use.

7. Gambhir, M. L., Concrete Technology, 4th Edition, McGraw-Hill Education (India), 2009

8. Gambhir, M. L. and Neha Jamwal, Building Materials: Products, Properties and Systems, McGrawHill Education (India), 2011.

9. ISO 3310-2; Test sieves − Technical Requirements and Testing − Part 2: Test Sieves of Perforated Metal.

10. ISO 5725:1994; Precision of Test Methods − Determination of Repeatability and Reproducibility for a Standard Test Method by Inter-laboratory Tests.