L-Box Test for Self-Compacting Concrete: Procedure, Blocking Ratio & Limits

Slump flow tells you a self-compacting mix can spread. The L-box test answers the harder question: can it get through the reinforcement? By forcing fresh concrete to flow horizontally past a row of vertical bars, the L-box directly measures passing ability — the property that decides whether SCC will fill a congested column or beam without honeycombing.

If your SCC is going into anything with dense rebar — columns, shear walls, beam-column joints — this is the test that proves the mix belongs there.

What “Passing Ability” Means

Passing ability is the capacity of fresh concrete to flow through tight openings — the gaps between reinforcing bars — without blocking and without the aggregate separating from the paste. A mix can have an excellent slump flow and still fail on site if the coarse aggregate arches between bars and jams, leaving voids behind the reinforcement.

The L-box quantifies this with the blocking ratio (also called the passing ability ratio):

PL = H₂ / H₁

where H₁ is the mean depth of concrete left in the vertical section and H₂ is the mean depth at the far end of the horizontal section, both in millimetres. A perfectly fluid material would level itself (PL = 1.0); a mix that blocks behind the bars leaves a steep slope and a low ratio.

Governing Standard

The L-box test is covered by EN 12350-10:2010 — Testing fresh concrete, Part 10: Self-compacting concrete — L box test. The method originated from a Japanese design for underwater concrete and was adopted into the EFNARC SCC guidelines.

Standards correction: Some older write-ups cite ASTM C1621 for the L-box. That is wrong — ASTM C1621 is the J-ring test. The L-box has no ASTM equivalent; the correct reference is EN 12350-10.

Apparatus

The L-box is an L-shaped steel box with rigid, smooth, rust-free surfaces:

Part	Typical dimension
Vertical section (hopper)	600 mm tall, 100 × 200 mm in section
Horizontal section	700 mm long, 200 mm wide, 150 mm tall
Sliding gate between sections	With vertical rebar obstacles mounted in front of it

The rebar arrangement is standardized in two versions:

• Two-bar test: two smooth steel bars, 12 mm diameter, giving a clear gap of (59 ± 1) mm between bars.
• Three-bar test: three smooth steel bars, 12 mm diameter, giving a clear gap of (41 ± 1) mm between bars.

The three-bar arrangement is the more severe test and simulates more congested reinforcement. Bars are mounted vertically and spaced evenly across the width of the box.

Test Procedure (EN 12350-10)

1. Set the L-box on firm, level ground with the gate closed. The inside surfaces should be damp (wipe with a moist cloth) but with no standing water.
2. Fill the vertical section with fresh SCC in one operation — no tamping, no rodding, no vibration. Let the concrete rest in the box for about 1 minute (this is the standard conditioning period).
3. Lift the gate smoothly in one motion and let the concrete flow through the bars into the horizontal section.
4. If you are recording flow times, note the time the leading edge takes to reach the 200 mm and 400 mm marks from the gate — these are the T20 and T40 times, an extra indication of filling ability.
5. When flow has completely stopped, measure the concrete depths: H₁ (mean depth in the vertical section, three readings averaged) and H₂ (mean depth at the far end of the horizontal section, three readings averaged).
6. Calculate the passing ability (blocking) ratio: PL = H₂ / H₁, reported to the nearest 0.01.

Blocking Ratio Limits: PA1 and PA2

EFNARC (2005) defines two passing-ability classes for the L-box:

Class	Requirement	Bar arrangement	Typical application
PA1	H₂/H₁ ≥ 0.80	2 rebars	Structures with moderate reinforcement spacing (gaps ~80–100 mm)
PA2	H₂/H₁ ≥ 0.80	3 rebars	Heavily reinforced structures (gaps ~60–80 mm)

Note that the ratio limit is the same (≥ 0.80); what changes is the severity of the obstacle. Passing PA2 means the mix went through the tighter 41 mm gaps — a stronger statement about real-world performance in congested members.

Interpreting the Results

Result	What it usually means	What to adjust
PL ≥ 0.80 (correct bar setup)	Passing ability acceptable	Confirm with slump flow and V-funnel before approval
PL 0.60–0.80	Partial blocking at the bars	Reduce coarse aggregate content or max size; check grading
PL < 0.60	Severe blocking	Mix not suitable — redesign: more paste, smaller aggregate, adjusted admixture
Aggregate piled behind bars, paste ran ahead	Segregation during flow	Increase paste viscosity (VMA), reduce water
T20/T40 times very long with good PL	Mix passes but is sluggish	Acceptable if within time — or trim viscosity for faster placement

Worked Example

A trial SCC mix for a heavily reinforced shear wall (minimum bar spacing 65 mm) is tested with the three-bar arrangement:

• Mean depth in vertical section: H₁ = 98 mm
• Mean depth at end of horizontal section: H₂ = 88 mm
• Flow times: T20 = 1.8 s, T40 = 3.2 s

Blocking ratio: PL = H₂ / H₁ = 88 / 98 = 0.898 → 0.90

Interpretation: 0.90 ≥ 0.80 with three rebars → the mix meets PA2, so its passing ability is acceptable for the congested wall.

L-Box vs J-Ring: Which Passing-Ability Test?

Both tests measure passing ability, but they do it differently:

Feature	L-box	J-ring
What is measured	Blocking ratio H₂/H₁ (a single number)	J-ring spread vs slump flow, and/or the blocking step between bars
Flow direction	Horizontal, through bars, into a channel	Radial, spreading outward through a ring of bars
Equipment	Bulky L-shaped box — mainly a lab test	Ring + slump cone — easy on site
Best for	Detailed mix development, congested members	Routine site quality control

They complement rather than replace each other. Develop the mix with the L-box in the lab; control it on site with the J-ring. For the companion tests, see the V-funnel test (viscosity and filling ability), the J-ring test (site-friendly passing ability), and the slump flow test — plus the self-compacting concrete introduction for the background.

Frequently Asked Questions

What is a good L-box blocking ratio?
H₂/H₁ ≥ 0.80 is the EFNARC requirement — with 2 rebars for PA1 (moderate reinforcement) or 3 rebars for PA2 (congested reinforcement). Closer to 1.0 is better.

What is the difference between the two-bar and three-bar L-box test?
The number of 12 mm bars at the gate. Two bars leave ~59 mm gaps (less severe); three bars leave ~41 mm gaps (more severe, for congested reinforcement). The ratio limit is 0.80 in both cases.

What do T20 and T40 mean in the L-box test?
The times for the concrete’s leading edge to travel 200 mm and 400 mm down the horizontal section. They are an extra measure of filling ability — shorter times mean a more fluid mix.

Can the L-box test be done on site?
It can, but the apparatus is heavy and awkward compared with the J-ring, which needs only a ring and a slump cone. Most site QC uses the J-ring; the L-box stays in the lab for mix development.

Is there an ASTM standard for the L-box?
No. The L-box is covered by EN 12350-10. ASTM’s passing-ability test is the J-ring (ASTM C1621) — a different apparatus and procedure.

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Written by Mohsin Raza Adil — BSc Civil Engineering Technologist (NTC-registered), Pakistan.