By David Biggs
This month’s questions come from a masonry contractor and two architects. What questions do you have? Send them to email@example.com, attention Technical Talk.
Q. A mason writes that they have a project with 10-inch partially-reinforced CMU stairwell walls that are heavily reinforced adjacent to openings. They are concerned there is too much reinforcement to properly grout the walls where there are bar laps and bond beams. They ask if the amount of steel exceeds the code requirements.
The drawings show 2- #7 bars vertically in each of the first two block cells next to an opening and at corners. A detail note also includes 2-#7 bars at 4’-0” oc horizontally but is not shown in the detail. The detail is shown below.
The mason also notes that the specified f’m is 1,500 psi and that the horizontal bars were to be hooked around the vertical end bars.
- Thanks for the question. This opens up several interesting issues.
Let’s look at the code (TMS 402-16, Building Code Requirements for Masonry Structures) that provides guidance to engineers. However, requirements depend upon whether the wall is designed using Allowable Stress Design or Strength Design.
For Allowable Stress Design (ASD): The maximum area of steel is based upon Chapter 6, Reinforcement, Metal Accessories, and Anchors Bolts.
- Maximum reinforcement size = No.11
- Maximum reinforcement diameter = one-half the clear cell dimension
- Maximum area of vertical reinforcement = 6 percent of grout space.
- Maximum nominal bar diameter = one-eighth nominal member dimension.
For Strength Design (SD): The limitations imposed by Chapter 6 also apply. However, Chapter 9, Strength Design of Masonry, modifies two of the Chapter 6 provisions giving:
a) Maximum reinforcement size = No.9 (Section 220.127.116.11)
b) Maximum reinforcement diameter = one-half the clear cell dimension
c) Maximum area of vertical reinforcement = 4 percent of cell or course grout space (Section 18.104.22.168). The commentary notes the area is allowed to increase to 8 percent at lap splices.
d) Maximum nominal bar diameter = one-eighth nominal member dimension.
Looking at both methods: a), b) and d) are clearly satisfied for the detail provided. Now let’s check c).
Using minimum ASTM C90 dimensions for 10-inch CMU, the area of a vertical cell ~6.8 in. wide x ~6.0 in. long = 40.8 sq.in.
Area of a bond beam ~6 in. wide x ~5.8 in. high = 34.8 sq.in.
For c): Vertically, area of bars (2-#7) = 1.16 sq.in. or 2.8% of cell area.
Horizontally, area of bars (2-#7) = 1.16 sq.in. or 3.3% of grout area.
Since these values don’t exceed 4%, the reinforcement is acceptable by code for both ASD and SD. If larger unit sizes are provided, these need to be rechecked.
While the code provisions are satisfactory for area and size of reinforcement, we must still check the placement requirements. These are also are given in Chapter 6.
22.214.171.124 Clear distance between parallel bars: not less than nominal diameter of bars, nor less than 1 in.
126.96.36.199 Grout clearance to masonry: ¼ in for fine grout, ½ in. for coarse grout.
The next two sketches show two possible layouts for the 10” CMU with the vertical bars in the end block with the 2-#7 vertical and horizontal bars but not the hooked ends using “fine grout” for minimum clearances. (Note: The design detail did not show the horizontal bars or give the “d” distances for bar placement. These should be provided!)
Using the second layout spreads the horizontal bars the furthest apart. Adding in U-shaped hooked ends to this layout (see next figure, only the hooked end is shown to define the width of the hook) indicates the hooks will not fit in the cell without rotating the bars.
The net result is that by rotating the hooked extension, the cell is further congested by the hooked ends of the bars and further restricts grouting in the end cells.
Another option is to use J hooks at the end of each horizontal #7. The J portion would again have to be rolled up to fit if the bars are drawn to scale.
Every option for bar placement shows significant congestion at the end cells.
Finally, let’s look at the code again for possible problems consolidating the grout. Table 6 from TMS 602, Specifications for Masonry Structures (courtesy of The Masonry Society) is used to determine grouting limitations.
Footnote 3 is the important portion of this table. The horizontal bars are subtracted from the clear width of the grout space to determine the maximum grout pour height.
Using the bond beam information, the 2 #7 bars reduce the available grout width from ~6 in. wide to ~6 in.– 2(0.875 in.) = ~4.25 in. Based upon Table 6, the maximum pour height is 24 feet using either fine or coarse. If you consider the impact of the J bar ends, the width further reduces to 2.5 in. This limits the pour height to 12.67 feet at the ends.
What began as what seems to be a very straightforward question (Does the amount of steel exceed the code requirements?), ends up being a complex review of code standards. There were multiple issues to consider including:
- Maximum allowable area of vertical reinforcement in a cell. This was further subdivided based upon the design methodology used.
- Prescriptive spacing requirements based upon bar size and grout type.
- Constructability to physically place the horizontal reinforcement in the wall to fit with the vertical steel.
- Available space for grouting the wall and the possible grout height.
After all is said and done, code issues are satisfied but the horizontal reinforcement is a challenge to install and would be best modified for constructability. This would have been a good detail to consult with a mason contractor before placing it on the drawings!
It is clear that the design detail was not complete by not showing the horizontal bars or the hooked ends. This illustrates that details need to show all the bars to scale to appreciate the problems with constructability and dimensions for bar placement should be given.
Q. An architect writes that they use a metal coping for the top of on their masonry walls yet often get streaks from water staining. What could be causing this? We follow the detailing advice of the National Roofing Contractor Association (NRCA) (see attached detail).
- This is an excellent question. Thanks for offering it.
Water staining can come from problems under the coping and from the wall surface. We’ll start with the staining created by water under the coping.
Under the Coping
The question highlights that detailing advice for a building may come from many sources. Each source is well intended, but has its limitations. In this case, the detail from the roofing industry seems at odds with those from the masonry industry.
The following detail is taken from the Brick Industry Association (BIA), Technical Notes 36A – Brick Masonry Details, Caps and Copings, Corbels and Racking. Note the recommendation to add sealant to the bottom edge of the coping.
Excellent masonry details are also available on-line from the Masonry Institute of Michigan. The next figure again shows the sealant at the bottom of the coping.
Finally, we see a detail from the International Masonry Institute (IMI) which is available on the Sketch-up 3D Warehouse. This detail reaffirms the recommendation to use sealant with the coping (author added sealant note and arrow to detail for emphasis).
So we have three masonry sources that recommend sealant whereas the roofing detail did not. The point here is that designers should look to a) multiple sources for recommendations and b) be sure one of those sources is from the industry most affected. We would not expect the masonry industry to be the definitive source for roofing details, nor should we expect the roofing industry to be the source for masonry details. In reality, aspects of both sources would probably be best.
While the preceding discussion was all about references of details, let’s discuss why the masonry industry recommends sealant. There have been reported cases, similar to the one that is the source of this question, whereby wind-driven rain has blown up and under the coping and gotten into top of the wall. The sealant stops that from occurring.
There are further recommendations for preventing water infiltration:
- The NRCA detail uses a membrane underneath the coping to protect the top of the wall. This is a good suggestion that is not specifically shown on the masonry details. It will protect the wall should leaks occur in the joints of the coping.
- Some sources recommend the coping extend down at least 4 inches to 6 inches over the exposed masonry. No mention is often made of the sealant.
I was once told that wind can cause rain to rise up under coping about 1 inch for every 10 mph. The source was a Portland Cement Association study from 1952, Building Watertight Concrete Masonry Walls that was reported in the 1990 ASTM publication STP 1063, titled Masonry: Components to Assemblages, edited by John H. Matthys. Using this information, 90 mph winds would require a 9 inch or more extension over the veneer. Most architects would object aesthetically to detailing such a large coping overhang.
- In her book, Designing the Exterior Wall: An Architectural Guide to the Vertical Envelope, Linda Brock recommends designers overlap the veneer by at least 2 inches and use a long-lasting sealant to avoid upward driven rain that could saturate the masonry from the interior. She also emphasizes the importance of securely anchoring the coping so that same wind does not uplift the coping itself.
A reasonable recommendation is to provide a membrane under the coping in addition to 2 in. to 6 of coping overlap with sealant to prevent water infiltration under the coping.
Staining on the wall surface can also occur due to water running down the wall just below the coping. Two considerations to avoid this are a) slope the coping toward the roof and b) provide a sufficient drip edge on the coping to keep the water away from the wall.
Neither the NRCA nor masonry industry recommendations offer a dimension for the drip edge for metal coping. One suggestion comes from the Sheet Metal and Air Conditioning Contractors’ National Association (SMACNA) Architectural Sheet Metal Manual.
This detail is primarily intended to physically create the hemmed edge and gives ¼ inch to 3/8 inch projection away from the wall. Is that sufficient?
A published article from the 14th Canadian Conference on Building Science and Technology titled Quantitatively Evaluating the Effectiveness of Different Drip Edges Profiles (https://www.protradecraft.com/sites/protradecraft/files/Effectiveness-Different-Drip-Edge%20Designs.pdf) by Smegal, et al studied various configurations including drips that extend 0.8 in. versus 1.75 in. away from the wall.
The following figure shows the effect of a hemmed edge whereby runoff partially runs back to the wall compared to a straight edge.
Some conclusions of the authors were that:
a) larger drips are better for keeping runoff away from the wall.
b) a straight edge performs better than a hemmed edge.
c) 20 gauge metal requires a hemmed edge to maintain stiffness; 12 gauge metal needs no hem.
d) a 45 degree drip is preferred.
1. Coping designs would be improved by including a membrane under the coping and 2 in. to 6 of veneer overlap with sealant to prevent water infiltration under the coping.
2. Look to multiple sources for detailing recommendations. Look to the masonry industry for detailing recommendations pertaining to masonry. Evaluate the masonry details carefully when they include non-masonry features.
3. Consider the size and the type of drip when detailing to avoid runoff staining of the wall.
Q. The next question came from an architect, but we have received similar ones from structural engineers.
“My budgets on projects are very restrictive. Sometimes it seems like the test reports I receive from the pre-construction tests I specify for mortar are a poor investment of my client’s money. I understand the issues behind why masonry mortar samples taken at the jobsite do not have the same strength values as material tested in the laboratory, but then why test mortar in pre-construction or at the jobsite?
What would be wrong with simply requiring submittal tests from the manufacturer demonstrating compliance to whatever ASTM C 270 mortar(s) I’m specifying? And then having the manufacturer certify that the tests they submitted are representative of the mortar that will be supplied to the project. That’s it, no tests.”
- We appreciate the question. Hopefully, we can illustrate that what you are requesting is possible.
Let’s start with the code requirements for specifying masonry mortar and the quality assurance requirements associated with each. TMS 602, Specification for Masonry Structures, Article 2.1 references ASTM C270, Standard Specification for Mortar for Unit Masonry. ASTM allows two options, Proportion Requirements and Property Requirements. Architects and engineers can select either option.
ASTM C270, Table 1 provides volumetric ratios for cement, lime and aggregates. A common example is Type N mortar which is often referred to 1-1-6 (one part cement, 1 part lime and 6 parts aggregate). The table is not so precise and actually allows a variation of the lime and aggregate portions.
For quality assurance, we turn to ASTM C1586, Standard Guide for Quality Assurance of Mortars which states “This procedure of specifying mortar requires no sampling and testing of mortar, and hence, no measurement of mortar properties in the laboratory or the field is required. All that is necessary is field confirmation of the proper proportions of the mixes used in construction.”
TMS 602, Article 1.5B.1.a.1) affirms this under Submittals:
“(Provide) Mix designs indicating type and proportions of ingredients in compliance with the proportion specification of ASTM C270.” No testing is required.
In addition, TMS 602, Article 1.5B.2.d requires mortar material certifications. No testing is required if the certifications are current.
TMS 602, Tables 3 and 4 further specify requirements for Minimum Verification Requirements and Minimum Special Inspection Requirements, respectively. For pre-construction and during construction, the quality assurance is essentially only verifying proportions.
ASTM C270, Table 2 provides the necessary properties for each type of mortar including compressive strength, water retention, air content and aggregate ratio. All of the properties must be included by the architects and engineers specifiers.
For pre-construction acceptance, ASTM C1586 requires evaluation of the mortar with a consistency (flow) of 110 +/- 5 %. This is an arbitrarily established mortar consistency that is used to approximate the water content of mortar after it is placed in a masonry assemblage with absorbent masonry units. The amount of water required in mortar produced at the construction site is normally greater than the amount used for the pre-construction evaluation.
ASTM 1586 requires that the pre-construction testing “Verify that the tested properties of the laboratory-prepared mortar meet the appropriate Property Specifications’ requirements of Specification C 270 for the specified mortar Type. Also verify that the laboratory test report includes the volumetric proportions and materials used to meet the Property Specifications.” This verification involves preparing and testing mortar samples for strength, water retention and air content. ASTM C780, Standard Test Method for Preconstruction and Construction Evaluation of Mortars for Plain and Reinforced Unit Masonry outlines the actual tests.
TMS 602, Article 1.5B.1.a.2) affirms this by specifying under Submittals:
“(Provide) Mix designs and mortar tests performed in accordance with the property specification of ASTM C270.” Testing is required as previously noted per ASTM C780.
During construction, samples are taken and tested in accordance with the project specifications created by the architect or engineer with guidance from ASTM C780.
So to answer the original question, architects and engineers can avoid the expense of testing by specifying the Proportions Requirements method of ASTM C270 for mortar. Seems easy, right? But there is more to the story.
Some specifiers allow the Proportions Requirements method but still choose to require mortar testing for compressive strength. This creates several problems:
- There is added expense that is unnecessary. Mortar testing is not required by code for the Proportions Requirements method.
- It inappropriately mixes the Proportion and Properties methods.
- Testing for compressive strength only is not sufficient to qualify for the Properties requirements method.
Again, the added testing is unnecessary for the Proportions Requirements method.
If the Proportions Requirements method is so easy and avoids testing, why use the Properties Requirements method? From my experience, some manufacturers of pre-blended mortars benefit from the Properties Requirements method. Those who do are able to provide a more efficient, economical mix than using the Proportions Requirements method and can justify it in the name of sustainability.
- Architects and engineers can choose either the Proportions Requirements method or the Properties Requirements method for specifying mortar. If neither method is specifically stated, the mason can use the Proportions Requirements method.
- The Proportions Requirements method per ASTM C270 requires no mortar testing before or during construction.
- The Properties Requirements method per ASTM C270 requires mortar testing before and during construction.
- Mixing aspects of the two methods is not necessary.
Thank you again for following this column. Remember, by bonding we get stronger! Keep the questions coming. Send them and your comments to firstname.lastname@example.org, attention Technical Talk. If you’ve missed any of the previous articles, you can find them on-line in the archives of Masonry Design (http://www.masonrydesignmagazine.com/digital-editions/) starting with the Spring 2018 edition.
David is a PE and SE with Biggs Consulting Engineering, Saratoga Springs, NY, USA (www.biggsconsulting.net). He specializes in masonry design, historic preservation, forensic evaluations, and masonry product development.