Testing of masonry elements

Diagonal shear (diagonal compression) test

Purpose: to find the shear strength (diagonal tensile strength) and shear modulus of masonry; used for in-plane shear design (ASTM E519, RILEM TC 76).

Specimen: a square wall panel about 1.2 m $\times$ 1.2 m (or the actual wall in situ), built with the same unit, mortar, bond and workmanship as the structure, cured 28 days.

Set-up and procedure

1. The panel is turned so that one diagonal is vertical. Steel loading shoes are fixed with plaster or mortar at the two opposite corners on that diagonal.
2. A hydraulic jack and load cell apply a compressive load $P$ along the diagonal, at a uniform rate.
3. Displacement gauges (LVDTs) on both diagonals measure shortening and elongation to get shear strain.
4. The load is increased until the first diagonal crack and failure; the maximum load $P$ is recorded.

        P (jack)
         |
      +--v-----+
      |\       |
      |  \     |   diagonal crack
      |    \   |   parallel to the load
      |      \ |
      +-------^+
           reaction

Calculation

$$\tau = \frac{0.707\,P}{A_n},\qquad f_t = \frac{0.5\,P}{A_n},\qquad A_n = \frac{w+h}{2}\,t$$

where $\tau$ is the shear strength, $f_t$ the principal (diagonal) tensile strength, $w$, $h$ and $t$ the width, height and thickness of the panel. Shear modulus $G = \tau/\gamma$, with $\gamma=(\Delta_v+\Delta_h)/g$ from the diagonal deformation.

Failure: diagonal cracking through the joints and units, giving the diagonal tension capacity used in design of shear walls.

Diagonal shear test (of a masonry panel)

Aim: shear (diagonal tensile) strength and shear modulus of masonry.

• A square wall panel (about 1.2 m $\times$ 1.2 m) is rotated so a diagonal is vertical; steel shoes at the two opposite corners.
• A jack applies compression along the diagonal at a steady rate to failure; the diagonal crack and the load $P$ are noted. LVDTs on the diagonals measure the deformation.

$$\tau = \frac{0.707P}{A_n},\qquad f_t=\frac{0.5P}{A_n},\qquad A_n=\frac{w+h}{2}\,t$$

       P
       |
    +--v----+
    |\      |
    |  \    |  crack along the
    |    \  |  loaded diagonal
    +-----^-+

Push shear test (in situ)

Aim: shear strength of bed joints of existing wall.

• A brick is isolated by removing a head joint at one end and the brick at the other end is removed to place a small jack and load cell.
• The jack pushes the brick horizontally along the bed joints; the load $F$ at first slip is recorded.

$$\tau = \frac{F}{2A_b},\qquad c=\tau-\mu\,\sigma_v$$

where $A_b$ is the bed area of the brick (two joints), $\sigma_v$ the vertical stress at the joint and $\mu$ the friction coefficient (about 0.4 to 0.65).

 +----+----+----+
 |    |[jack]->[brick]   F pushes brick along bed joint
 +----+----+----+

The diagonal shear test is a laboratory test on a panel (destructive); the push-shear test is done on the actual wall with little damage.

Compressive strength test of brick masonry (wall/prism) in the laboratory

Purpose: to find the compressive strength $f_m$ of masonry (made of the same brick, mortar and workmanship), from which the basic compressive stress is derived (IS 1905 Appendix B).

Specimen: a wall panel or prism built in stretcher or English bond, at least 400 mm high, and height-to-thickness ratio ($h/t$) between 2 and 5. Tested after curing for 28 days.

Procedure

1. Construct the specimen with the actual mortar thickness, bond and moisture content of units to be used in the structure.
2. Cap the top and bottom with mortar or plywood sheets, 4 mm thick (to give uniform bearing).
3. Place in the compression testing machine with a spherically seated upper platen, centre carefully.
4. Apply load gradually at about 350 to 700 kN/m of the bed area per minute until failure; record the failure load $P$ and the failure pattern (vertical splitting cracks).
5. Optionally fix gauges to measure strain and find the stress-strain curve and modulus of elasticity.

   | load P
   v
 +-------+  <- steel platen (spherical seat)
 |  cap  |
 |=======|
 | brick |  h = 400 mm min
 |=======|  mortar joints
 | brick |
 +-------+
 | cap   |
 =========  <- machine bed

Calculation

$$f_m = \frac{P}{A}\times k_h$$

where $A$ is the loaded bed area and $k_h$ is the correction factor for the slenderness of the prism (IS 1905 Table 12, brickwork): $h/t = 2.0, 2.5, 3.0, 3.5, 4.0, 5.0$ gives $0.73, 0.80, 0.86, 0.91, 0.95, 1.00$. Basic compressive stress of masonry $f_b = 0.25\,f_m$ (IS 1905 B-2.1).

Bricks (separately): compressive strength of bricks is found by testing a brick (flat, frog up, with mortar bedding) in the compression machine; the strength is the average of five bricks, e.g. 3.5 to 10 N/mm$^2$ for common burnt clay brick classes (IS 3495).

Non-destructive testing (NDT) is used to assess the condition, strength and defects of existing masonry without damaging the structure. Common techniques:

1. Visual inspection and crack mapping with crack gauges to monitor crack width.
2. Rebound (Schmidt) hammer test: surface hardness of the masonry unit or mortar joint, correlated with strength; quick and cheap, but surface-limited.
3. Ultrasonic pulse velocity (UPV) and sonic tests: the speed of a pulse through the wall relates to quality, density and voids; detects cracks, voids and delamination.
4. Elastic wave (sonic) tomography: many sonic readings in a grid across the wall are processed to form an image of the internal condition (voids, repairs, damage).
5. Flat-jack test: a thin steel bag is placed in a slot cut in a mortar joint to measure the in-situ stress (single jack) and deformability and strength (two jacks). This is a minor-destructive test.
6. Push shear test: a brick is pushed horizontally to measure the shear strength of bed joints (minor-destructive).
7. Ground penetrating radar (GPR): detects voids, moisture, steel bars and layers.
8. Infrared thermography: detects moisture, voids and different materials from surface temperature differences.
9. Borescope / endoscope: a small camera in a drilled hole to see internal voids and cavities.
10. Core sampling and Windsor probe/pull-out: minor-destructive tests for mortar strength.

 NDT of masonry
 |-- Surface: visual, rebound hammer, thermography
 |-- Wave: UPV, sonic, tomography, radar
 `-- In-situ minor-destructive: flat jack, push shear, core

Compressive strength test of a brick masonry prism (IS 1905 Appendix B)

Aim: to determine the compressive strength of masonry $f_m$ built with the actual brick, mortar and workmanship, so that the basic compressive stress can be fixed.

Specimen: a prism of bricks laid in a stack bond or the actual bond, at least 400 mm high, with height-to-thickness ratio ($h/t$) of 2 to 5 (preferably 4 to 5), joints and mortar as in the work. Moisture content of the bricks and mortar consistency are the same as will be used on site. Test after 28 days.

Procedure

1. Build prisms on a flat base; cure under damp cloth and polythene for 28 days.
2. Cap the bed faces with sheets of plywood (4 mm) slightly larger than the prism.
3. Centre the prism in the compression testing machine with a spherically seated platen so that load is applied evenly over the whole top and bottom surfaces.
4. Apply load at a rate of 350 to 700 kN/m per minute (uniform) to failure; record failure load $P$ and the mode of failure (vertical splitting).
5. Test at least 5 prisms and average.

    P
    v
  +-----+ <- ply/cap
  |=====|
  |brick|   h
  |=====|   (h/t = 2 to 5)
  |brick|
  +-----+
  ply cap

Calculation

$$f_m = \frac{P}{A}\times k,\qquad f_b = 0.25\,f_m$$

where $A$ = bed area, $k$ = correction factor for $h/t$ (Table 12): 0.73 for $h/t=2.0$, 0.80 for 2.5, 0.86 for 3.0, 0.91 for 3.5, 0.95 for 4.0 and 1.00 for 5.0. The basic compressive stress is 0.25 times the prism strength.

Compressive strength test

• A wall panel or prism (at least 400 mm high, $h/t$ between 2 and 5) is loaded vertically in a compression machine till failure.
• $f_m = \dfrac{P}{A}\times k$, with $k$ from IS 1905 Table 12; basic compressive stress $f_b=0.25f_m$.
• Failure mode: vertical splitting cracks in units. Gives compressive strength and modulus of elasticity.

   P                    P (along diagonal)
   v                     \
 +-----+               +--\--+
 |     |               |    \|
 |prism|               |wall /|
 +-----+               +----/+
 compressive test      diagonal shear test

Diagonal shear test

• A square wall panel (about 1.2 m $\times$ 1.2 m) is loaded in compression along one diagonal through steel shoes at opposite corners.
• Shear strength $\tau = \dfrac{0.707P}{A_n}$, with $A_n=\dfrac{w+h}{2}t$; diagonal tensile strength $f_t=0.5P/A_n$.
• Failure mode: diagonal tension cracking along the loaded diagonal.

Basic difference

The push shear test is an in-situ, minor-destructive test to find the shear strength of the bed joints of existing masonry, i.e. the cohesion and friction of the unit-mortar interface.

Procedure

1. Select a brick in a plain wall region (not near openings) and remove the brick on one end (head joint) and clean the head (vertical) joint on the other end to free the test brick.
2. Place a small hydraulic jack and load cell in the gap, with a steel plate bearing on the end of the test brick.
3. Push the brick horizontally (parallel to the wall) at a slow, steady rate until it slides along the bed joints. Record the load $F$ at first movement (sliding).
4. Estimate the vertical compressive stress $\sigma_v$ on the joints from the load above (or from a flat-jack test).
5. Repeat on several bricks and average.

 wall elevation
 +----+----+----+----+
 |    |  jack |    |    <- removed brick position
 +----+--[F]->[brick]--+   brick pushed along bed joint
 |    |    |    |    |
 +----+----+----+----+

Calculation

$$\tau = \frac{F}{2A_b},\qquad \tau = c + \mu\sigma_v$$

where $F$ is the sliding load, $A_b$ the bed-joint area of the brick (length $\times$ thickness; there are two joints, above and below), $c$ the cohesion and $\mu$ the coefficient of friction (about 0.4 to 0.65, from tests). The cohesion $c=\tau-\mu\sigma_v$ is used in the shear design of the wall.

Merits: quick, cheap, nearly non-destructive (only one brick is replaced); demerits: scatter in results, affected by local mortar quality, gives strength of joint only.

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.

 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 $c=\tau-\mu\sigma_v$.

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

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

 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.

  ====[ 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).

Simple tests that can be done at site to judge the quality of bricks:

1. Visual / shape test: bricks should be uniform in size and colour (deep red or cherry), with sharp edges and plane faces, free from cracks, lumps and flaws.
2. Hardness test: a scratch with a fingernail should not leave an impression.
3. Soundness (ring) test: two bricks struck together should give a clear metallic ringing sound and not break.
4. Structure test: a broken brick should show a homogeneous, compact, uniform structure without voids or lime lumps.
5. Drop test: a brick dropped from about 1 m onto hard ground should not break.
6. Water absorption test: a dry brick soaked in water for 24 hours should not absorb more than 20 percent of its weight (15 percent for first class, 22 percent for second class).
7. Efflorescence test: a brick placed in a dish of water should show little or no white salt patches after drying.
8. Dimension test: measure 20 bricks in a line to check size tolerance.

Elastic wave (sonic) tomography

Principle: elastic (stress) waves travel faster in sound, dense masonry and slower through voids, cracks and moisture. The travel time between many source and receiver positions is measured and processed to give a velocity image (a tomogram) of the wall interior.

Procedure

1. Mark a grid of points on both faces of the wall (e.g. every 0.25 to 0.5 m).
2. A small instrumented hammer (or piezo transmitter) at one point generates a pulse; accelerometers or receivers at other points record the arrival time $t$.
3. Velocity along each path $v = d/t$ ($d$ path length). Many crossing paths are collected.
4. Software (inversion, e.g. SIRT) divides the wall into cells and solves for a velocity in each cell, giving a colour map.
5. Interpretation: low-velocity zones are voids, cracks or moist or weak masonry; high velocity means sound masonry. Used to detect internal damage, repairs and leaf separation without damaging the wall.

 Transmitter (hammer)        Receivers
  T o ----------> o R1       Wall section grid
    o ----------> o R2       paths cross the wall;
    o ----------> o R3       slow zone = void or crack

Push shear test

An in-situ test of the shear strength of bed joints. A brick is freed by removing the vertical joints on both sides (one brick removed to fit a jack), then pushed horizontally with a hydraulic jack until it slides. Shear strength $\tau=F/(2A_b)$ and cohesion $c = \tau-\mu\sigma_v$, with $\mu\approx0.4$ to $0.65$ and $\sigma_v$ the vertical compressive stress on the joint.

 +----+----+----+
 |    |[jack]->[brick]|    F slides the brick
 +----+----+----+

Destructive (and minor-destructive) tests on masonry wall

1. Compressive strength test of wall panel or prism (laboratory).
2. Diagonal shear (diagonal compression) test of a panel.
3. Flexural (bond wrench or beam) tests; tensile bond strength.
4. Flat-jack test (minor-destructive, in situ): slot cut in mortar joint.
5. Push shear test (minor-destructive, in situ).
6. Core extraction and testing, and mortar sampling.

Non-destructive tests (NDT)

1. Visual inspection and crack monitoring.
2. Rebound (Schmidt) hammer test.
3. Ultrasonic pulse velocity (UPV) and sonic tests.
4. Elastic wave (sonic) tomography.
5. Ground penetrating radar (GPR).
6. Infrared thermography.
7. Borescope / endoscopy; moisture meters.