Tooling Mortar Joints Before we get too far into the topic of tooling mortar joints, let's pause for a moment to remind ourselves why we even care about mortar joints. Of course, mortar joints will affect the appearance of our masonry walls, so we obviously need to pay attention to them for that reason.
But more importantly, the mortar joints are the first line of defense against water penetration. To reduce water penetration through the wall, there is no substitute for the proper filling of the mortar joints.
Improperly filled joints...
reduce masonry strength
result in leaky walls
are susceptible to freezing and thawing damage
contribute to disintegration and cracking
reduce sound insulation
contribute to corrosion of embedded steel elements
may contribute to disintegration and cracking due to water penetration and subsequent freezing and thawing.
Figure A: Tooling a concave mortar joint
If integral water repellent is being used, as it will be for most single-wythe walls, the water repellent will reduce the absorptivity of the block, so tooled and compressed joints become even more critical. Integral water repellents make masonry materials hydrophobic, thereby significantly decreasing their water absorption and wicking characteristics. While these admixtures can limit the amount of water that can pass through units and mortar, they have little impact on moisture entering through relatively large cracks and voids in the wall. Therefore, even with the incorporation of integral water repellents, proper detailing of control joints and quality workmanship to preclude beeholes and unfilled or inadequate mortar joints is still essential.
A Specification Section on Procedures for Laying Unit Masonry As I discovered last time when I searched YouTube for videos showing the proper techniques for laying masonry, masons may have a different understanding of how various elements of the process should be carried out. The ways that they have been doing things for a generation or two may not be exactly what you are looking for in order to attain good moisture penetration resistance. A section in the masonry specifications describing the procedure for "Laying Walls and Partitions" can help. Such a specification could read:
A. Laying Procedure:
Hollow Units: In starter courses and elsewhere as noted, lay units fully bedded in mortar under both shells and webs. Other units shall be laid with fully bedded face shells, but mortar shall extend through the unit on web edges where anchors or ties occur. All head-joints, except in control joints requiring a control joint key, shall be filled solidly with mortar for a distance in from the face of the unit not less than the thickness of the longitudinal face shell. Place mortar on webs where required to contain grout in partially grouted walls.
Solid Units: Spread the mortar bed full width and relatively smooth. Do not furrow. Do not fill head joints by slushing, instead butter the end of each unit with mortar and shove it into place to completely fill the head joints.
During inclement weather, tops of unfinished walls must be covered at the end of the work day. The cover should extend two feet down both sides of the masonry and be securely help in place. After completion of the walls, immediately install the wall cap to prevent excessive amounts of water from directly entering the masonry.
NCMA TEK 19-07Characteristics of Concrete Masonry Units with Integral Water RepellentNational Concrete Masonry Association 2008http://www.ncma.org/Pages/default.aspx
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 termMASONWORK, 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).
M2,500 psi
A
S1,800 psi
O
N750 psi
W
O350 psi
R
K75 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 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 toproduce 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 Jobby 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.
"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.
Single-Wythe Masonry -- Not Just for Warehouses Anymore
As a kid growing up in rural, upstate New York in the ‘50s, I loved baseball. In fact, that love of baseball led me to one of my earliest encounters with masonry.
My dad let me paint a miniature baseball diamond on our blacktop driveway apron. Everything was meticulously measured and laid out to scale. I learned the the distance from the tip of home plate to the tip of second base was 127’-3⅜”. How many of you knew that?
For some reason, my dad had a collection of bullnose blonde bricks that had been glazed with a glossy, dark green glaze that looked like moss. They were perfect for the outfield wall around our mini-ball field. We laid them end to end, in a nice, sweeping curve that allowed the outfield to range from 350’ down both lines, to 405’ in straight-away center. Our little field was ready for a game.
As a teacher at the local high school (which was also the local elementary school, and the local junior high, all in one), my dad had access to ditto masters. In fact, he taught the class where students learned how to use ditto machines. So I made hand-drawn score sheets with a ballpoint pen and a ruler, made many ditto’d copies, and we kept score for every game.
I would play against my brother, Jim. We would use small pebbles for the ball, and a broken, shortened pencil for the bat. You would’t think you could get much bat control with irregular pebbles and a six-sided wood-clinched pencil. But it was amazing that when Mickey Mantle or Roger Maris came to bat, they were always home-run threats. It wasn’t safe for the ’49 Plymouth to be parked in the driveway when one of them came to the plate.
That was my introduction to masonry. Those were the coolest bricks I have ever seen. I still love them. But… time to move on.
Single-Wythe Masonry Walls
(where the text looks like this, hover your mouse over that text for additional information)
Right about now, you’re probably thinking… “Wow. That sounds really boring. But at least it will be short.” Well, I’m going to try my best to keep this from being boring. But you should know in advance, it definitely won’t be short.
How’s that for enticement?
The reason this series is not going to be short is that your basic, very simple, straight-forward single-wythe masonry wall is really sort of complicated. It has the following pieces and parts…
The type of brick or block
The type of mortar
How the joints are tooled
The weep system
The flashing system
Design considerations and detailing
Water repellents
Crack control
Workmanship
Joint sealants
Cleaning
I’ll be discussing each of these components over the next few weeks, and I'll use my most interesting voice. I will end up with the part about why single-wythe masonry walls are not just for warehouses anymore. I am also going to try to keep each installment short. So please bear with me…
The Type of Brick or Block
Let’s start with item 1 from the list above -- the type of brick or block. In fact, let’s start with Concrete Masonry Units (CMU in the vernacular).
Fly Ash
Concrete masonry units, or concrete block, is a very common building material, and is often incorrectly referred to as cinder block (this term “cinder block” is accurate only if cinders… ash — a waste product of coal combustion, often from electrical generation plants… is used as the aggregate.
Fly Ash Mounds
Concrete block is made up of portland cement, water, sand, and a coarse aggregate that is usually less than 3/8 inch. Masonry block producers thus use coal combustion byproducts in two ways: fly ash as a cement replacement, and bottom ash as a partial replacement for the sand and/or coarse aggregate.
Type I and Type II Units Moisture-controlled and Non-moisture controlled Units Prior to the year 2000, ASTM C90 (the standard specification for load-bearing concrete block) included two different type designations for concrete masonry units... Type I units were defined as moisture-controlled units, and Type II units were defined as non-moisture controlled units. The requirements for these two different unit types were identical in all respects with one exception: the moisture content of the unit at the time of delivery. Project specifications prior to 2000 commonly required Type I, moisture-controlled units, because that was one fairly effective way to help control cracking caused by shrinkage. Historically, ASTM C90 stipulated a maximum moisture content for Type I units at the time of delivery. Conversely, no such moisture content requirements were specified for Type II units. The Type I/Type II unit designations have not appeared in ASTM standards covering concrete masonry units since 2000.
In theory, by limiting the moisture content of a concrete masonry unit to a relatively low level (based on the environmental conditions at the job site and the physical properties of the unit) would in turn reduce a unit’s potential drying shrinkage, which in turn would translate to a reduced potential for shrinkage cracks from forming in the masonry assembly. As such, designers that wanted to maximize the distance between control joints, or possibly remove the need for control joints altogether, would specify the use of Type I concrete masonry units. While sound in theory, the effective use of Type I, moisture-controlled units was difficult to implement primarily because the drying shrinkage potential is largely a function of a unit’s moisture content at the time of installation, not the time the unit was delivered to the jobsite.
The phrase “at the time of delivery” contained in ASTM C90 is central to the reason for the removal of moisture-controlled and non-moisture controlled concrete masonry units. Once the concrete masonry units have been delivered to a customer, the producer of the units has lost control over how they will be used or how they will be protected from the environment. Herein lies the disconnect between using ASTM C90 as a manufacturing specification – as it is intended – and using ASTM C90 as a construction specification – for which it is not intended. Because the “time of delivery” rarely coincides with the time of installation, units delivered within the moisture content limitations of a Type I unit may no longer meet these moisture requirements at the time of installation; having potentially been exposed to a myriad of varying environmental conditions during the time between delivery and installation.
As such, a unit that is delivered to the jobsite meeting the requirements for a Type I unit may in fact have become a Type II unit by the time it was installed, which could compromise critical design assumptions and result in increased potential for shrinkage cracks. To alleviate the confusion and potential misuse of Type I/Type II concrete masonry units, these designations and their associated requirements were removed from ASTM specifications for concrete masonry units. While they are not designated as such with ASTM standards, this action effectively classifies all concrete masonry units as non-moisture controlled.
Concrete Block Classes Categorized by Weight
Heavy Weight Block
Normal Weight Block
Medium Weight Block
Light Weight Block
CMU are categorized into three weight classes perASTMC90: normal (heavy) weight, medium weight and lightweight. Classes are defined by the weight per cubic foot (pcf) of material. Heavyweight units are 125 lb pcf or more. Medium weight units are between 105 and 125 lb pcf. Lightweight units are less than 105 lb pcf.
In my first job after graduating from high school, I was a laborer for a small general contractor. Remember I said I grew up in rural upstate New York? We were building a barn for one of the local dairy farmers. The foundation was made of 12" x 8" x 16" concrete blocks. Since I was not a skilled mason, and both of my co-workers were, I was assigned to carry the blocks from the spot where they were dropped off of the truck on pallets to spots all around the barn where the masons would be able to access them conveniently and put them in place. These blocks weighed 56 pounds apiece, and my boss told me that if I was a member of a labor union, we would have to have each block carried by two people. I'm assuming that he knew what he was talking about. After a day of carrying blocks around the barn with no gloves, my hands looked like hamburger, only bloodier. I understand why lightweight block is so popular with masons.
Figure A
When building with hollow units, it is VERY important to fully bed the face shells and webs of starting courses on the foundation. Structural piers, columns and pilasters should also be laid with full head and bed joints. Webs may need to be mortared on either side of grouted cells to prevent grout flow to adjacent cells and cavities. Face shell bedding is typically specified for all other hollow unit applications. "Face shell bedding" requires that head and bed joints be filled solidly with mortar a distance in from each face of the unit equal to the thickness of the face shell (figure A).
Figure B
When I see concrete block walls that have been singled out in lawsuits because they leak, they seem to have some things in common. One of those things is cracking where the mortar and the block come together (figure B). Sometimes this is caused by not filling the joints completely with mortar. It's easy to understand if the mason is trying to maneuver a 56-pound block into position with one hand, and butter the head joints with the other.
Filling the head joints becomes even more important with single-wythe walls. In order to make certain the head joints are completely filled, the Rocky Mountain Masonry Institute recommends "double-buttering" (buttering is the term masons use to describe the application of mortar to the edges of a concrete block). You load your trowel from the mortar hawk with a ribbon of mortar... never mind. Just watch this...
Next time, I'll be discussing mortar. Your selection of the right mortar may be counter-intuitive in some ways, so be sure to stay tuned. Also, if you find this blog helpful, please take a moment to "like" it, and to "subscribe" to it. And your comments would be much appreciated. Thanks.
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