04/12/2025
The L-Box test (L_Box test) is a workability test used for Self-Compacting Concrete (SCC) to measure its passing ability—how well the concrete flows through reinforcement without blocking.
✅ What the L-Box Test Measures
The L-Box test evaluates:
Passing ability (can concrete flow through bars?)
Flow characteristics (is it self-compacting?)
Risk of blocking (especially with coarse aggregates)
🧪 How the L-Box Test Works
The equipment looks like an “L” shape:
The vertical section is filled with fresh SCC.
A gate holds the concrete in place.
Steel bars simulate reinforcement.
When the gate opens, concrete flows horizontally.
📊 Key Measurement
After the concrete stops flowing:
H1 = height of concrete in the horizontal leg
H2 = height of concrete in the vertical leg
The L-Box ratio (H1 / H2) tells the passing ability:
L-Box Ratio (H1/H2)Interpretation0.80 – 1.00Excellent passing ability (good SCC)0.60 – 0.80Marginal / may cause blockage< 0.60Poor passing ability
For quality SCC, engineers usually expect ≥ 0.80.
📌 Why Use the L-Box Test?
Because SCC must:
Flow easily
Fill forms completely
Pass through dense reinforcement
Avoid segregation or blocking
The L-Box test helps confirm that.
If you want, Bo can also explain:
How to perform the test step-by-step
The standard requirements (EN 12350-10)
Compare with other SCC tests (J-ring, slump flow, V-funnel)
Here is a clear and complete explanation of the L-Box test for Self-Compacting Concrete (SCC), including procedure, equipment, acceptance criteria, and comparison with other SCC tests.
✅ 1. Purpose of the L-Box Test
The L-Box test measures SCC’s passing ability—how well the mix can flow through reinforcement without segregation or blocking.
This is very important for structures with dense rebar.
✅ 2. Equipment Needed
The standard L-Box apparatus includes:
Vertical section (filled with concrete)
Horizontal section (where concrete flows)
Gate to release concrete
Steel bars simulating reinforcement
(typically 3 bars, spaced 41 mm apart)
Measuring scale at H1 and H2
✅ 3. Step-by-Step Procedure (EN 12350-10 Standard)
Step 1 — Prepare the apparatus
Clean the L-Box
Ensure it is level and stable
Wet the surfaces (avoid concrete sticking)
Step 2 — Place the steel bars
Set the rebar gate according to the required spacing (commonly 3 bars).
Step 3 — Fill the vertical section
Fill in one continuous pour
Avoid vibration (SCC is self-compacting)
Strike off the top surface
Step 4 — Start the test
Open the gate quickly
Concrete begins flowing horizontally
Step 5 — Wait until concrete stops moving
Usually takes 20–30 seconds.
Step 6 — Measure heights
Measure:
H2 = height of concrete in the vertical section
H1 = height in the horizontal section
📊 4. Calculate the L-Box Ratio
\text{L-Box Ratio} = \frac{H1}{H2}
⭐ 5. Acceptance Criteria
L-Box RatioInterpretationResult0.80 – 1.00Excellent passing ability✔ Acceptable for SCC0.70 – 0.80Borderline, may cause blocking⚠ Mix improvement needed< 0.70Poor passing ability❌ Not acceptable
Ideal range = 0.90 – 1.00 for high-performance SCC.
🔍 6. Factors Affecting Results
Coarse aggregate size
Aggregate shape (angular vs round)
Sand ratio
Water-powder ratio
Viscosity modifying agent (VMA)
Superplasticizer dosage
📌 7. Comparison With Other SCC Tests
TestMeasuresGood RangeSlump FlowFlowability600–800 mmT500 TimeViscosity2–5 secV-FunnelFlow time6–12 secJ-RingPassing ability0–10 mm differenceL-BoxPassing ability through bars≥ 0.80
The L-Box is more reliable than J-Ring when testing very dense reinforcement
⭐ FULL DETAILED TEST PROCEDURE – L-BOX TEST (EN 12350-10)
Self-Compacting Concrete (SCC)
1. Objective
To determine the passing ability of Self-Compacting Concrete (SCC) as it flows through reinforcement obstacles without segregation or blocking.
2. Apparatus
L-Box Apparatus consisting of:
A vertical section (filling chamber)
A horizontal section (flow channel)
A sliding gate separating vertical and horizontal section
Reinforcing steel bars placed at the start of the horizontal channel
Measuring scale
Reinforcement bars dimensions:
Usually 3 bars, diameter 12 mm
Clear spacing: 41 ± 2 mm (or according to mix design requirements)
Tools
Bucket or scoop
Strike-off bar
Stopwatch
Spirit level
Trowel
Cleaning brush
3. Preparation of Equipment
✔ Level the equipment
Place the L-box on a firm, level surface.
✔ Clean the surfaces
Remove hardened concrete from previous tests.
✔ Dampen the inside
Lightly wet all surfaces to prevent moisture loss from SCC.
✔ Install reinforcement bars
Place the bars according to specified spacing, ensuring they are clean and fixed firmly.
4. Sample Preparation
Take a fresh SCC sample according to EN 12350-1.
Mix gently with a scoop to ensure uniformity.
Avoid segregation when transferring to L-box.
5. Test Procedure (Step-by-Step)
Step 1 — Close the Gate
Ensure the sliding gate is fully closed without leakage.
Step 2 — Fill the Vertical Section
Fill the vertical column in one continuous pour.
Do NOT compact or vibrate.
Strike off excess concrete with a straightedge.
Step 3 — Allow Short Rest
Let concrete rest for 1 minute to stabilize (standard practice in many labs).
Step 4 — Open the Gate Quickly
Open the gate fully and smoothly.
The concrete flows from vertical section → through bars → horizontal channel.
Step 5 — Wait Until Flow Stops
Allow concrete to stop naturally.
Usually: 20 – 30 seconds
Step 6 — Measure Heights (H2 and H1)
H2: height of concrete in the vertical section after flow
H1: height of concrete at the end of horizontal section
Measure carefully at centerline of width.
6. Calculation
\text{L-Box Ratio} = \frac{H1}{H2}
7. Acceptance Criteria
L-Box RatioInterpretationResult0.80 – 1.00Excellent passing ability✔ Acceptable for SCC0.70 – 0.80Marginal⚠ Adjustment recommended< 0.70Poor passing ability❌ Not acceptable
Higher ratio = better flow through reinforcement.
8. Interpretation
Good SCC mixture should:
✔ Flow smoothly without blocking
✔ Have no bleeding or segregation
✔ Show uniform aggregate distribution
✔ Reach near-full height in horizontal section
Poor SCC mixture shows:
❌ Concrete stuck at bars
❌ Aggregate accumulation
❌ Uneven flow
❌ Segregation (coarse aggregate separated)
9. Common Mistakes to Avoid
Overfilling or underfilling vertical chamber
Opening gate too slowly
Dirty bars affecting flow
Using vibrated concrete (not allowed)
Wrong spacing of bars
Not removing air pockets during pouring
L-box not leveled
10. Factors Affecting L-Box Result
Aggregate size & shape (round flows better)
Powder content (cement + filler)
Water-to-powder ratio
Superplasticizer dosage
Viscosity-modifying agent (VMA)
Sand ratio
