Chips Off the Old Block -- Single-Wythe Masonry

JNX-clusives

Chips Off the Old Block

and other ruminations about masonry, by

Larry D. Jenks

 






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…

1. The type of brick or block
2. The type of mortar
3. How the joints are tooled
4. The weep system
5. The flashing system
6. Design considerations and detailing
7. Water repellents
8. Crack control
9. Workmanship
10. Joint sealants
11. 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 per ASTM C90: 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.

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Thanks.