---
title: "Flat-jack test for brick masonry wall"
subject: "Concrete Technology and Masonry Structure"
chapter: 10
key: "semester_5/concrete_technology_and_masonry_structure:flat-jack-test"
source: https://www.bctengineer.com/q/semester_5/concrete_technology_and_masonry_structure/flat-jack-test
site: BCT Engineering
---

# Flat-jack test for brick masonry wall

Concrete Technology and Masonry Structure · Chapter 10: Testing of masonry elements

- Asked in: 2079 Baisakh, 6 marks; 2071 Chaitra, 6 marks (asked 2 times)
- Video: [flat jack test single double](https://www.youtube.com/watch?v=NtIC4rRH7Q4) — DIPAK  MANDAL
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/concrete_technology_and_masonry_structure/flat-jack-test
- Whole chapter: https://www.bctengineer.com/md/semester_5/concrete_technology_and_masonry_structure/chapter-10

## Question

Describe the flat-jack test for brick masonry wall with neat sketch of the set-up.

## Short answer

A flat jack is a thin oil-filled steel bag inserted in a bed-joint slot. Reference pins on the wall surface are monitored while cutting the slot (stress release) and pressurizing until original distance is restored; the cancellation pressure p gives in-situ stress σm = Km Ka p (Km ≈ 0.8-0.9, Ka ≈ 0.9-1.0). Double jacks separated by 4-5 courses yield stress-strain curve and modulus.

## Answer

The **flat-jack test** is a minor-destructive in-situ test to find the compressive stress, modulus of elasticity and strength of existing masonry. A flat jack is a thin steel bag filled with oil that acts as a hydraulic pressure cell.

#### Test set-up

```text
 wall surface
  o     reference pins (demec points)
 ----- ----- ----- -----
  o  [  flat jack in slot ]  o      <- horizontal bed joint slot
 ----- ----- ----- -----
        |
      pump + pressure gauge
```

#### Procedure (single-jack test: in-situ vertical stress)

1. Fix 4 to 6 pairs of measuring points (pins) across the mortar joint, above and below the line of the slot; measure initial distance $d_0$ with a mechanical gauge.
2. Cut a slot in the mortar bed joint with a masonry saw or by drilling; stress is released and the distance reduces to $d_1<d_0$.
3. Insert the flat jack into the slot and increase pressure slowly with a hydraulic pump until the pins return to the original distance $d_0$. Note the cancellation pressure $p$.
4. Compute the in-situ stress in the wall.

$$
\sigma_m = K_m\,K_a\,p
$$

with $K_m$ = calibration factor of the jack (about 0.8 to 0.9) and $K_a$ = ratio of area of jack to area of slot (about 0.9 to 1.0).

#### Double flat-jack test (modulus and strength)

1. Cut two parallel slots 400 to 500 mm apart (4 to 5 courses) in the same vertical line and install a jack in each.
2. Increase pressure in steps; measure the vertical deformation of the masonry between the jacks, giving the stress-strain curve and modulus of elasticity $E=\Delta\sigma/\Delta\varepsilon$.
3. Continue to the first cracking to get the compressive strength.

```text
  ====[ jack 2 ]====   slot
   |                 |
   | masonry panel   |   stress and strain measured
   |                 |
  ====[ jack 1 ]====   slot
```

**Uses:** determine the actual stress state in old buildings (e.g. heritage structures), compressive strength and elastic modulus for seismic assessment, with little damage (the slots are refilled with mortar).

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What can a single flat-jack test find?
   A. In-situ vertical stress in masonry
   B. Shear modulus
   C. Water absorption
   D. Brick colour
   Answer: A. The single jack gives in-situ stress; double jack gives modulus and strength. (Key point: Purpose)

2. What happens when the slot is cut?
   A. Stress is released and pin distance reduces from d0 to d1
   B. Pin distance increases
   C. Nothing changes
   D. The wall cracks
   Answer: A. Cutting releases stress so the masonry above and below moves together. (Key point: Slot effect)

3. How is the stress computed?
   A. Pressure p raises until distance returns to d0; sigma = K_m K_a p
   B. sigma = p / 2
   C. sigma = 0.707 p
   D. sigma = d0 - d1
   Answer: A. p is the cancellation pressure; K_m is jack calibration and K_a area ratio. (Key point: Cancellation)

4. How are slots arranged in the double-jack test?
   A. Two parallel slots 400 to 500 mm apart (4 to 5 courses)
   B. Two slots side by side
   C. One vertical slot
   D. Slots 2 m apart
   Answer: A. The masonry between them is loaded to get stress-strain curve and E. (Key point: Double jack)

## Related in Concrete Technology and Masonry Structure

- [Flat-jack and push-shear tests](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Aflat-jack-and-push-shear) — Concrete Technology and Masonry Structure, chapter 10, asked 2×
- [Compressive strength test of brick wall](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Acompressive-test-masonry-wall) — Concrete Technology and Masonry Structure, chapter 10, asked 4×
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- [Brick and mortar for 230 mm wall (100 kN/m)](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Awall-eccentric-230mm-100kn-5m) — Concrete Technology and Masonry Structure, chapter 8, asked 7×
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