Bulletins

Technical bulletins on pertinent masonry topics. 

NOVEMBER 2012: This Bulletin serves to remind whoever is placing the order for concrete masonry units to first review the contract documents (drawings and specifications) to see what the designer (typically, the structural engineer) has specified for f’m and mortar type.

For years, designers have been specifying an f’m equal to 1500 psi with Type S mortar using the unit strength method.  According to the Code this would require a block compressive strength of 1900 psi.

Recently on projects, some designers are now specifying a higher f’m, i.e., 2000 psi, 2500 psi and 3000 psi with Type S mortar.  According to the Code this would require a block compressive strength of 2800 psi, 3750 psi and 4800 psi, respectively.

Some of the benefits for specifying the increased strength are less grout required for partially grouted walls, less reinforcement required for walls, and reinforcement lap lengths reduced.  In fact, a recent article in the Masonry Edge/the Story Pole, Vol. 7, No. 1,

Embodied Energy of Concrete Masonry , David Biggs, PE, SE, Dist M ASCE, HTMS, pages 12-15, states; “In each case, upgrading f’m from 1500 psi to 2000 psi reduces the amount of grout and reinforcement by 50% for partially grouted walls…”

ORIGINAL BULLETIN

The decision to utilize this information is not within the purview of the MIM, and persons making use of this information do so at their own risk.  MIM makes no representation or warranties, expressed or implied, with respect to the accuracy or suitability of this information.  MIM and its members disclaim liability for damages of any kind, including any special, indirect, incidental, or consequential damages, which may result from the use of this information.  This information is not to be interpreted as indicating compliance with, or waiver of, any provision of any applicable building code, ordinance, standard or law.

Updated: February 2015
This Bulletin serves to caution against using exposed metal drip edges at all locations within pedestrian reach. Reachable locations, typically, include base of wall, first floor window sills, garden walls and site walls. Exposed metal and metal drip edges, including laps and corners, could result in unforeseen injuries due to sharp edges.  Even those with hemmed edges can be dangerous at laps and outside corners when they are within reach. Durable, fully adhered flexible membrane flashing, is recommended at these locations. Extreme caution should be taken with exposed metal flashings.

The use of exposed drip edges becomes more important with wall height. By definition, drip edges serve to deflect downward cascading moisture away from the masonry substrate and other building components below. Since there is little to no substrate below the base flashing course and little below first floor window sills or garden walls, no metal drip edge is necessary. At these locations a fully adhered flexible flashing membrane will serve well to divert drainage cavity moisture to the exterior, and furthermore, will prevent exterior moisture from entering under the membrane.

The Masonry Institute of Michigan further recommends that all flashing be capable of withstanding harsh all-weather conditions including wind, ultraviolet degradation and extreme temperature cycles. This applies to all necessary locations, including but not limited to base of wall, sills, above window, door and mechanical openings, relief angles, high walls at low roofs and at copings.

All exposed metal flashing requires hemmed edges with laps sealed with non-skinning butyl sealant and a continuous compatible caulk sealant between the underside of the sloped hemmed drip edge and the substrate below. A continuous bead of sealant under the drip edge further prevents exterior moisture from entering beneath the metal drip edge at locations that include but are not limited to above window, door and mechanical openings, relief angles and top of wall copings.

Originally posted: February 2006

ORIGINAL BULLETIN 

The decision to utilize this information is not within the purview of the MIM, and persons making use of this information do so at their own risk.  MIM makes no representation or warranties, expressed or implied, with respect to the accuracy or suitability of this information.  MIM and its members disclaim liability for damages of any kind, including any special, indirect, incidental, or consequential damages, which may result from the use of this information.  This information is not to be interpreted as indicating compliance with, or waiver of, any provision of any applicable building code, ordinance, standard or law.

Date: January 2015

Please use this Bulletin as a guide when specifying concrete masonry units (CMUs).

Typically Concrete Product Producers manufacture their CMU’s out of locally available aggregates.  These aggregates when mixed with cement and water produce the specific block physical properties inherent to the locally available aggregates.

When it comes to block making, the State of Michigan is abundant with normal weight aggregates of sand, gravel, and limestone (ASTM C-33) throughout the state, which when utilized in the concrete block manufacturing process will result in concrete masonry units (CMU’s) that ASTM classifies as normal weight (density of 125 lbs/cf and above).

For many years, expanded slag, a lightweight aggregate (ASTM C-331) has been available in the southeast Michigan area, which offered CMU manufacturers the option of blending the expanded slag  with normal-weight aggregates to produce a medium-weight CMU (density of 105 lbs/cf to less than 125 lbs/cf).  Compared to normal-weight units, these medium-weight units offered the mason contractor a lighter weight unit, and offered the designer potentially increased fire ratings and reduced coefficients of thermal expansion, all for a reasonably priced premium compared to normal-weight units.

Over the years, some Michigan manufacturers chose to inventory only normal-weight units, some have inventoried both normal-weight and medium-weight units, and some have chosen to inventory only medium-weight units.  Typically, at least in southeast Michigan, a medium-weight unit came to be known as the “standard” unit that is inventoried in most block yards.

Recently, however, expanded slag has become virtually unavailable to southeast Michigan producers, unless trucking the aggregate in from neighboring states or even Ontario.  The premium for a medium-weight block compared to normal-weight has increased approximately four times from what is was just a year ago, a premium that most manufacturers feel the market will not bear.  As a result, many manufacturers have either changed or are in the process of changing their everyday inventory over to normal-weight units.  Medium-weight units will still be available in the Michigan market, but on a much more limited basis than in the past and also at the higher cost premium.

To assist the designer and specifier, the following are suggested guidelines.  CMUs should be specified to meet ASTM C90, the Standard Specification for Loadbearing Concrete Masonry Units.  There are three density classifications for CMUs: 1) normal weight (125 pcf or more), 2) medium weight (105 pcf to less than 125 pcf) and 3) lightweight (less than 105 pcf).

  1. All three density classifications have to meet a minimum net area compressive strength of 1900 psi.  Depending on the mortar type, the compressive strength of the wall increases with higher density CMUs.
  2. Based on the calculation procedure, the sound transmission class (STC) increases with higher density walls.  Code, typically, requires STCs of not less than 50.
  3. When designing for moisture management and mitigation in single wythe CMU walls, three levels of defense should be considered: surface protection (properly constructed mortar joints, surface water repellents, surface coatings), internal protection (integral water repellents), and drainage/drying (flashing, weeps, vents).
  4. The coefficients of thermal expansion of CMUs depend to some degree on density and on the type of aggregate used.  Normal weight units have an expansion coefficient of about 5 x 10-6/F and the value for lightweight units is about 4 x 10-6/F.  The TMS 402-11 Building Code Requirements for Masonry Structures shows 4.5 x 10-6/F.
  5. As the density of the unit increases, the unit’s weight increases.
  6. In general, fire resistance ratings with lower density walls provide more fire resistance.
  7. Thermal resistance with lower density walls provide higher R-values.
  8. The heat capacity (thermal mass) increases with higher density walls.

Note, when particular features are desired such as higher compressive strength, fire resistance, thermal performance or acoustical performance, these features should be specified separately.  Your local block manufacturer should be consulted as to the availability of CMUs having the desired features.  If you have any questions regarding this Bulletin and/or block specifications, please feel free to contact the Masonry Institute of Michigan at 248-663-0415.

ORIGINAL BULLETIN

The decision to utilize this information is not within the purview of the MIM, and persons making use of this information do so at their own risk.  MIM makes no representation or warranties, expressed or implied, with respect to the accuracy or suitability of this information.  MIM and its members disclaim liability for damages of any kind, including any special, indirect, incidental, or consequential damages, which may result from the use of this information.  This information is not to be interpreted as indicating compliance with, or waiver of, any provision of any applicable building code, ordinance, standard or law.

Date: April 2015

In the last several years, considerable attention has been given to energy lost through the building envelope.   In response to increasingly demanding energy code requirements, professionals are now incorporating control layers (water, air, thermal and vapor), whenever possible, in their masonry wall designs.  Spray polyurethane foam (SPF) insulation is somewhat unique in that manufacturers of this product claim that it can provide all four control layers in one application.  For that reason, there has been interest in using SPF.

The Generic Wall Design Committee (GWDC) operates under the auspices of the Masonry Institute of Michigan (MIM).  As a complement to the sets of details that address masonry cavity walls with rigid insulation and mineral wool insulation, GWDC began developing a set of masonry cavity wall details utilizing SPF insulation.  However, this effort was suspended in 2013, when a paper that revealed some concerns with SPF came to the attention of the MIM.  The paper was titled, Dimensional Stability Considerations in Spray Polyurethane Foam Air Barriers, and was presented at an ASTM Symposium on Building Walls Subject to Water Intrusion and Accumulation: Lessons from the Past and Recommendations for the Future, after which it was published in ASTM STP 1549.

The following considerations were presented in the referenced paper:

SPF undergoes short and long term shrinkage, which must be considered in the design and detailing.

The potential for shrinkage and curling must be considered where the foam interfaces with other wall and roof system components (i.e., flashing membranes, transition membranes, closures, terminations, air barriers, water barriers and other accessory materials).

Because SPF is site-mixed, its physical properties can vary.  Factors that affect the physical properties of SPF include mixture proportions, thicknesses, ambient and substrate temperatures, moisture or humidity conditions, as well as other factors.

Additional concerns that have come to the attention of MIM include:

Required protection of other building components, pedestrians, vehicles, and landscaping in the vicinity where SPF is being applied is more extensive than with other products.

Guides for how to repair out-of-compliance SPF are not available.

Guidance for design, detailing, and application of SPF to address the considerations and concerns raised have not been developed by the spray foam industry.  Until the time when such guidance is available, the GWDC masonry cavity wall details that incorporate SPF will not be finalized nor published by MIM.

If you have any questions regarding this Bulletin, please feel free to contact the Masonry Institute of Michigan at 248-663-0415.

ORIGINAL BULLETIN

The decision to utilize this information is not within the purview of the MIM, and persons making use of this information do so at their own risk.  MIM makes no representation or warranties, expressed or implied, with respect to the accuracy or suitability of this information.  MIM and its members disclaim liability for damages of any kind, including any special, indirect, incidental, or consequential damages, which may result from the use of this information.  This information is not to be interpreted as indicating compliance with, or waiver of, any provision of any applicable building code, ordinance, standard or law.