Chips Off the Old Block -- The Type of Mortar

JNX-clusives

Chips Off the Old Block

and other ruminations about masonry, by

Larry D. Jenks

Type of Mortar

Perhaps it's obvious. But then again, perhaps it isn't...

Why do any of us care about what mortar types are, and which one(s) we should be choosing for our projects?

For me, it all boils down to what it takes to make our masonry buildings as water-tight as we can. For our structural engineers (and therefore, also to us), it probably boils down to choosing a mortar that gives them (and us) the right compressive strength. And, obviously, if the compressive strength is too low and our building falls down, then it isn't going to be very water-tight. But it is a bit more complicated than that. My goal is to make it simple.

In my last blog entry, I said that choosing the right mortar might be counter-intuitive in some ways. I said that because we have generally been led to believe that the best mortar is the strongest mortar. And to be honest, we probably haven't questioned that position very much.

One of the cases I've worked on recently involved a leaking concrete masonry wall. One of the things that caused it to leak was the fact that there was a plethora of instances of head-joint cracking. The walls were single-wythe walls, so once water got inside the wall, the methods for getting it back out again were limited. In this particular case, the water made its way to the interior, where it saturated the batt insulation and the gypsum board that formed the interior walls, caused the metal studs to rust, and produced mold growing on the paper of the gypsum board.


The lawsuit was brought to help the plaintiff's (a school district in some far away land) determine who was at fault for this leaking, and who was going to pay to have it remedied. That was really what my assignment was, although the attorney who retained me may have believed my job was to explain why it wasn't his client's fault.

One of the things I looked at was the specification for the mortar. The spec called for a Type S mortar, a mortar with a very strong compressive strength. Though I had no way of knowing, I have seen this scenario unfold many times, though often times with different results... the project architect solicits a list of spec sections from the structural engineer for which the engineer will be responsible. The engineer writes or edits those spec sections, and returns them to the architect, who binds them with the other spec originals for reproduction. The architect is usually just fine with whatever the engineer wants. 

And that's how we get Type S mortar. 

It may be very strong (from a compressive strength perspective), but that strength makes it very rigid, very inflexible. That inflexibility is what causes cracks to form between the mortar and the block. Those cracks are what allow moisture into the building... you get the idea. That illustrates why it is so important for both the architect and the engineer to understand the objectives for the masonry walls, the characteristics of mortars, and together choose a mortar that accomplishes those objectives. 

Mortar Types

It could be helpful to understand a little more about mortar types. There are five.

The five typical mortar mix designations are M,S,N,O and K. They are so labeled because each is an alternate letter in the term MASON WORK, listed in descending order of compressive strength. These designations were assigned in 1954 and replaced the mortar designations that were being used at that time (A-1, A-2, B and C).

M 2,500 psi

A

S 1,800 psi

O 

N 750 psi

W

O 350 psi

R

K 75 psi

Detailed information about mortar types and properties can be found in the Brick Industry Association (BIA) Technical Note 8.

Mortar Composition

Mortar is made up of water, aggregates, and cementitious materials. 

• Water (potable)
• Aggregates (well-graded sand containing particles of varying sizes
• Cementitious Materials
• portland cement
• masonry cement 
• mortar cement
• lime

Physical Properties and Attributes of Mortar

Mortars have two distinct, important sets of properties: those in the plastic state and those in the hardened  state. The plastic properties help to determine the mortar’s compatibility with brick or block, and its construction suitability. 

Properties of plastic mortar include... 

• Workability (rollover)
• Water Content
• Water Retention
• Initial Flow

Properties of hardened mortars help determine the performance of the finished brickwork. 

Properties of hardened mortar include... 

• Bond Strength
• Durability
• Extensibility
• Compressive Strength

Properties of plastic mortar are commonly more important to the mason, while the properties of hardened mortar are often more important to the designer and owner.

Clayford Grimm, a prominent masonry authority, made the following statement about the importance of masonry bond strength: 

“Bond strength between mortar and masonry units is the most important physical property of masonry. Low bond strength causes every problem that can happen to masonry – cracks, leaks, stains, weathering, and structural failure.”

This table shows how increasing the proportion of lime to cement in a mortar mixture can enhance its performance in a variety of ways. But what is lime?

Lime (hydrated lime or slaked lime) is limestone that is mixed with water (called slaking) and then heated to 580°C where it dehydrates to produce calcium hydroxide. Lime is the main contributing ingredient to enhance the extent of bond, workability, water retention and elasticity. These are the elements that give mortar its flexibility, and its ability to resist cracking. If small hairline cracks do develop, water and carbon dioxide from the environment that penetrate the joint will react with lime (calcium hydroxide) from the mortar and form calcium carbonate. This newly developed calcium carbonate will seal the cracks, limiting further water penetration. This self-healing process is called autogenous healing.

The above table is based on testing and other research performed by:

• H.H. Holmes Testing Laboratories/Dr. Russell Brown (1962)
• Clayford Grimm, a prominent masonry authority
• Rune Hedin, National Lime Association (1962)
• National Bureau of Standards (1920s and 30s)

Moisture Penetration in Masonry Walls

When water passes through brick masonry walls, it does so through separations that form between the brick  and the mortar at the time of laying or through cracks that form after the mortar has cured.

Some of these separations can be ameliorated by choosing the right mortar for the job (see Choosing the Right Mortar for the Job by Michael Schuller, PE). But some may be caused by the way the head joints are made by the masons.

Workmanship

The key to providing strong, durable masonry depends more upon workmanship than any other single factor. Don’t underestimate the mason’s skill: whereas mortar strength contributes 2 to 5 percent of wall strength, studies have shown workmanship to be responsible for up to 40 percent of wall strength. One of the keys to good workmanship is mortar workability - a term which embodies water retention, flow, and segregation resistance. Mortar acquires most of its workability properties from lime. The plasticity provided by lime results in a mortar which flows into the texture of the unit to provide good bond contact and also makes it easier to apply head joints. Workable mortar has a longer board life, is easier to spread, and less likely to form bond line delaminations upon curing. Easy to use Type N and O mortars will usually result in better built walls. 

My friend, Diane Travis, Technical Director for the Rocky Mountain Masonry Institute, would emphasize the need to attain a filled head joint. So, I scoured YouTube for videos that show real-life masons demonstrating all the proper techniques. I found plenty of videos of masons demonstrating their techniques. And many of them are very skilled, very fast, very competent. But I couldn't find a single one that showed the proper way to butter the end of a brick. What I found were several masons (no names from this corner) that butter only about half the brick end. This video is fairly typical...

If you find (or have) a video showing the proper technique, please let me know.

Extent of Bond

In general, the term bond refers to two different properties: (1) the extent of bond, or the degree of contact between mortar and masonry units, and (2) the tensile bond strength, or the force required to separate the units. 

The dominant property affecting the amount of water entering brickwork is the extent of bond between the brick and the mortar. 

Good extent of bond means complete and intimate contact, which is important to watertightness and to tensile bond strength. Good  extent of bond is obtained with a workable, water-retentive mortar, competent workmanship, full joints, and masonry units that have a medium initial rate of absorption with proper pore size and distribution. A stiff mortar will produce poor bond, whereas a wetter mortar will produce better, stronger bond, even though the compressive strength may decrease.

Extent of bond is a measure of the area of contact at the interface between brick and mortar surfaces.

Neither extent of bond nor tensile bond strength is a property of the mortar alone. Both of these important aspects of masonry construction are also dependent on workmanship in laying the units, ambient conditions, and the surface characteristics of the masonry units, such as pore structure, texture, and absorption.

I don't want to disparage our mason friends, but I need to point out the importance of completely filling the head joints in order to minimize water penetration. 

If you do field observation work, there's a good chance you will see masons at work (assuming your project includes masonry). Based on what I see on YouTube, it would appear that masons spend years, even entire careers, never being involved in lawsuits where the walls leaked because of only partially-filled head joints. But let's not let your project be the first. Make your expectations clear at the bidding stage. And when you visit the job site, take a peak at what is actually happening. You could be in for a surprise.

Moisture penetration can be a problem with any type of wall construction. The ability of masonry walls to resist water penetration is a key indication of quality for both architects and contractors. We need to engage the structural engineers in this discussion so that the decision doesn't begin and end with compressive strength. No specifier, contractor or owner of a masonry building wants the walls to leak.  

Conclusions

So, what can we conclude from this?

Hydrated lime enhances the ability of masonry walls to resist water penetration by making the mortar more compatible with the masonry unit. Further, it improves bonding because it increases water retention, extent of bond, flexibility, long-term weather resistance thru autogenous healing, and reduces shrinkage cracking.

One inescapable conclusion from the studies I've read is that an overwhelming majority opinion among independent authorities consistently substantiated the need for both lime and portland cement in a well balanced, all-purpose mortar. The lime referred to is either hydrated lime or lime putty made from quicklime and may be either dolomitic or high calcium types. This should never be confused with pulverized limestone (calcium carbonate) that is sometimes erroneously called "lime", and which is inert in mortar and has none of the properties inherent with burned lime products. 

The Portland Cement Association (PCA) makes the following recommendation:

"Use a Type N mortar for all masonry work unless there is a compelling reason to choose another mortar."

The PCA recommends the use of Type N mortar for above-grade bearing walls, but also lists Type S mortar as an alternative. ASTM C270 lists Type N mortar as the recommended choice.

That seems like a good place to stop.