---
title: "Flat-jack and push-shear tests"
subject: "Concrete Technology and Masonry Structure"
chapter: 10
key: "semester_5/concrete_technology_and_masonry_structure:flat-jack-and-push-shear"
source: https://www.bctengineer.com/q/semester_5/concrete_technology_and_masonry_structure/flat-jack-and-push-shear
site: BCT Engineering
---

# Flat-jack and push-shear tests

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

- Asked in: 2078 Bhadra, 3+3 marks; 2075 Chaitra, 3+3 marks (asked 2 times)
- Video: [Flat Jack Test | How to perform Flat Jack Test on Stone and Concrete Masonry | Jack Flat Test |](https://www.youtube.com/watch?v=RLiDBKoyZFM) — Construction Factory
- 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-and-push-shear
- Whole chapter: https://www.bctengineer.com/md/semester_5/concrete_technology_and_masonry_structure/chapter-10

## Question

Explain with neat sketches the flat-jack test and push-shear test of masonry.

## Short answer

The flat-jack test uses a thin hydraulic jack inserted in a bed-joint slot to measure in-situ stress (single jack, cancellation pressure p, stress = Km*Ka*p) and deformability (double jack between two slots). The push-shear test pushes a freed brick along its bed joints to measure shear strength: τ = F/(2Ab), cohesion = τ - μσv. Both are minor-destructive in-situ methods.

## Answer

#### Flat-jack test

A **flat jack** is a thin rectangular steel bag (about 5 mm thick) which expands under hydraulic pressure. It measures the in-situ stress and deformability of masonry.

1. **Single-jack test (stress):** two pairs of measuring pins are fixed above and below a horizontal bed-joint line and the distance between them is measured with a mechanical gauge. A slot is cut in the bed joint with a saw; the stress is released and the pins move closer. The jack is placed in the slot and pressurised until the distance returns to its original value.
2. **Double-jack test (modulus and strength):** two parallel slots are cut about 4 to 5 courses apart and jacks fitted in both; the masonry between them is loaded like a compression specimen to get a stress-strain curve.

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

where $p$ is the cancellation pressure, $K_m$ the jack calibration constant (0.7 to 0.9) and $K_a$ the ratio of jack area to slot area.

```text
 pin  +----------+  pin
  o   |==========|   o    measure d before / after cutting
 --- [  flat jack  ] ---  <- slot in bed joint
  o   |==========|   o
      +----------+
      pump -> pressure p
```

#### Push-shear test

An in-situ test for the shear strength of bed joints: one brick is freed by removing a head joint and a jack pushes it along its bed joints until it slides. Shear strength $\tau = F/(2A_b)$ and, with the vertical stress $\sigma_v$ (from the flat-jack test), cohesion $c=\tau-\mu\sigma_v$.

```text
 +----+----+----+----+
 |    |[jack]->[brick]|   F pushes brick horizontally
 +----+----+----+----+
```

## Check yourself

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

1. What is a flat jack?
   A. A thin steel bag that expands under hydraulic pressure
   B. A hammer
   C. A radar antenna
   D. A sonic receiver
   Answer: A. It is a thin (about 5 mm) steel bag pressurised hydraulically. (Key point: Flat jack)

2. In the single-jack test, how is the stress found?
   A. Pressurise the jack until pins return to original distance; sigma = K_m K_a p
   B. Measure crack width
   C. Push until sliding
   D. Time pulse travel
   Answer: A. After cutting the slot the pins move closer; the pressure restoring them is the cancellation pressure p. (Key point: Cancellation pressure)

3. What does the double-jack test give?
   A. Stress only
   B. Modulus of elasticity and strength via stress-strain curve
   C. Joint cohesion
   D. Moisture
   Answer: B. Loading masonry between two slots acts like a compression specimen. (Key point: Double jack)

4. How is the flat-jack test used with push shear?
   A. It provides vertical stress sigma_v for c = tau - mu sigma_v
   B. It replaces the jack
   C. It measures F
   D. It is unrelated
   Answer: A. The vertical stress on the joint, from the flat jack, is needed to separate cohesion from friction. (Key point: Link to push shear)

## Related in Concrete Technology and Masonry Structure

- [Flat-jack test for brick masonry wall](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Aflat-jack-test) — Concrete Technology and Masonry Structure, chapter 10, asked 2×
- [Push shear test of masonry](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Apush-shear-test) — Concrete Technology and Masonry Structure, chapter 10, asked 2×
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- [Diagonal shear and push shear tests](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Adiagonal-shear-push-shear) — Concrete Technology and Masonry Structure, chapter 10, asked 6×
- [Compressive and diagonal shear tests of masonry](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Acompressive-diagonal-shear-tests) — Concrete Technology and Masonry Structure, chapter 10, asked 2×
- [Diagonal shear test of masonry wall](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Adiagonal-shear-test) — Concrete Technology and Masonry Structure, chapter 10, asked 6×

## Related in other subjects

- [Diagonal shear test and flat-jack test calculations](https://www.bctengineer.com/question.md?key=semester_5%2Fdesign_of_timber_and_masonry_structures%3Adiagonal-shear-flat-jack-numerical) — Design of Timber and Masonry Structures, chapter 8
- [Shear tests; direct shear calculation](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Ashear-tests-direct-shear-strength-calc) — Soil Mechanics, chapter 10
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