Not all brick are created equally. Brick will vary in density, weight, shape, size, color and most importantly.... Strengths. Some brick are squishier, stretchier, more twistable and/or cuttable than others. Yep!, brick have a certain amount of elasticity. The same goes for cements, mortars, and all the other types of masonry units. (IE. terra cotta, block, stone etc.)
Using the wrong type of mortar for any given project will result in a rapid premature deterioration of the materials or worse, failure of the the structure built.
The mortar used for pointing, tuckpointing, or laying any particular brick, block or stone should be guided by the inherent strengths of the particular brick, block or stone. The strengths are based on how much directional force the brick/block/stone can take before permanently deforming or failing. The amount of stress at which the material fails is the yield point of each strength.
There are many strength values of any material. The most important strength to factor in your decision for the mortar to use will be compressive strength. Other strengths may be considered, but, in my experience, often are not.
Worth mentioning, retain it or not, 4 commonly considered strength values.
- Compressive - How much can I squish it, before it breaks?
- Tensile - How much can I pull it before it breaks?
- Shear - How easy/hard is it to cut or tear?
- Torsional - How much can I twist it before it breaks?
The compressive strength of mortar used should be less than the compressive strength of the surrounding masonry units, but still strong enough to support the structure. (That's in red to signify the importance, And IT IS the underlying theme of this entire article)
How to choose the right mortar for your masonry application:
FIRST, we must consider the compressive strength of the masonry units being used. SECONDLY, we choose the strongest mortar that will least likely cause damage the surrounding brick-block-stone etc.
Choosing the proper mortar in new construction is pretty simple. Many times, the manufacturer will specify the type of mortar for intended use. Easy peasy, huh? When it's not that easy, it's still pretty easy. The strengths of the brick, block, stone, terra-cotta etc. should be readily available from the supplier and/or manufacturer. Choose the type of mortar based on that information. We'll talk about the mortar types below, just hold your horses and keep reading.
Choosing the proper mortar for pointing/tuckpointing or any other masonry restoration work can be more difficult, especially for older brick homes where bricks may have been made on site or close by because either
- There never was specified strengths OR
- The specifications are long gone, lost in time.
The most precise way to know what type of mortar mix to use for tuck pointing or laying is to have a lab analysis of the original mortar. Yeah, you can pay dudes in lab coats to chemically deconstruct the mortar and they can tell you the ratio of Portland cement, lime and sand (Those 3 ingredients will determine mortar type) that was used in mortar used for original construction, even if that mortar is over a 100 years old.
While I understand the process has value, I personally feel it's over-rated. Wondering where you can get a lab analysis of your mortar? LMGTFY (Let me google that for you) Here's 2:
The lab analysis would really only be beneficial for preserving a structure to its historical specifications, providing the original mortar used was the appropriate kind. How would you know if the wrong mortar was used? Evidence of improper mortar used will be premature deterioration of the mortar itself or failure of the surrounding masonry units. (Brick, block and stone will spall, crack or deteriorate if the surrounding mortar's compressive strength is too high.)
Below is a table of the compressive and tensile strengths of some common masonry units for consideration in choosing the correct mortar type masonry restoration work.
|Material||Compression Strength||Tension Strength|
|Brickwork, common quality||1000||7000||50||350|
|Brickwork, best quality||2000||14000||300||2100|
|Portland Cement, less than one month old||2000||14000||400||2800|
|Portland Cement, more than one year old||3000||21000||500||3500|
|Portland Concrete, 28 days old||5000||35000||200||1400|
|Portland Concrete, more than one year old||6200||43000||400||2800|
I'm going to keep this short and sweet, because I already wrote a pretty sweet blog on this very topic a while back. Check it out: MORTAR TYPES: M, S, O, N, K: TUCKPOINTING AND LAYING MASONRY
3 Basic ingredients
- Portland cement
In different specific ratios, will produce the
5 different types of mortar.
- Type M
- Type S
- Type N
- Type O
- Type K
The types were named based on alternating letters of the phrase MaSoN wOrK. Clever, ain't it? But you'd already know that if you read the other blog I linked to. Seriously, check it out.
Back on point.... below is a table specifying the compressive strength of each mortar type in pounds per square inch (psi).
COMPRESSIVE STRENGTH (PSI)