Single Wythe Wall: Structural Reinforced Concrete Block

Single Wythe Wall: Structural Reinforced Concrete Block2022-02-28T20:18:19-07:00

Sponsored by:

  • Warehouses, garages, storage units, sound fences and industrial buildings
  • Interior partition walls
  • Single wythe walls are the most cost-effective of all masonry wall systems
  • A single layer provides structure, weather-resistance, exterior wall finish and (in some cases) also the interior wall finish
  • Minimal long-term maintenance
  • Inherently fireproof with fire ratings up to 4 hours
  • Excellent choice for interior partition walls where security, noise abatement, fire separation and durability are important in schools, offices, and hotels.
  • This wall system does not have a drainage cavity. It can be difficult to make this wall completely weather-tight unless you coat the exterior surface of the wall.
  • Fully grouting the wall minimizes water penetration that typically comes through the head joints between blocks.
  • Do not use batt insulation behind a single-wythe wall. If water leaks through the wall, it will saturate the batt insulation, inviting mold and reducing the R-value of the insulation. Rigid insulation boards can tolerate occasional moisture exposure.
  • Exercise caution when using this wall system in climates with above average precipitation.
  • In Colorado’s climate, single-wythe concrete block walls have difficulty meeting the energy requirements of the International Energy Conservation Code unless you add insulation to the assembly.  You can either build the wall with specialty units that include insulation within the wall, or you can add furring and rigid insulation on the interior face of the wall. If you use Hi-R block, the wall will need to be at least 10” or 12” wide to allow enough room for the insulation inserts and the space for grout and rebar. Compliance with the IECC is not required for interior partitions that are not exposed to weather.
  • Masonry Units: Lightweight plain gray concrete block (CMU), 8” x 8” x 16” (nominal)
  • Vertical Reinforcement: #6 rebar vertical reinforcement @ 32” on center
  • Horizontal Reinforcement: 9-gauge joint reinforcement @ 16” on center (every other course)
  • Insulation: Polyurethane foam in un-grouted cores of the backup wall OR 2-inch rigid extruded polystyrene on the inside face of the CMU wall (Reference: Colorado Masonry Systems Design Guide, Chapter 7, Table 7-1 and Figure 7-3.)
  • Flashing: Proprietary flashing systems like Block Net from MortarNet or Cavity Vent from Masonry Technologies, Inc. at the base of the wall.
  • Mortar: Type N, Portland cement/lime, plain gray
  • Joints: Concave tooled
  • In high seismic zones, CMU walls are required to have additional horizontal bond beams to resist earthquake loads. Check with your structural engineer to verify requirements in your area. These bond beams can take the place of horizonal reinforcement in the structural back-up. Horizontal reinforcement is still required in the veneer to resist shrinkage.
  • Type S mortar is required in areas of high seismic risk. Consult with your structural engineer to see what requirements apply in your area.
  • Grouted 32” on center: 3 hours
  • Fully grouted: 4 hours

Reference: NCMA TEK Manual 07-01D

  • CMU grouted 32” on center: 43 pounds per square foot
  • CMU fully grouted: 78 pounds per square foot
  • Grouted 32” on center:  STC = 51.65 dB
  • Fully grouted: STC = 59.33 dB

Reference: NCMA TEK Note 13-01C

  • R-value with 2” rigid insulation on inside face of the wall = 13.72
  • Calculations
    • Outside air film                                        0.17
    • 8” lightweight CMU, grouted 32” on center                                                                            1.45
    • 2” rigid insulation board                        0.97
    • Air gap                                                    10.00
    • ½” gypsum board                                    0.45
    • Interior air film                                         0.68
    • TOTAL                                                      13.72

Thermal mass benefit:  “Thermal Advantages of Masonry Walls” by David Woodham, PE
Reference: NCMA, Thermal Catalog of Concrete Masonry Assemblies

  • There are three different pathways to meeting the requirements of the International Energy Conservation Code (IECC). You can use R-values to satisfy the prescriptive limits of Table C402.1.3 of the code. Using R-values requires continuous insulation in the cavity in all Colorado climate zones. Following Table C402.1.4 of the code uses U-values to meet the insulation requirements. This approach does not require continuous insulation in the cavity. A third approach is to use computer software like COMCheck — to prove that the building meets code. Computer software is the most complex, but it is also the most flexible option. It is often the best choice for mass walls.
  • If you build this wall with block manufactured with Integral Water Repellent (IWR), you also need to add compatible IWR chemicals to the mortar. Block walls built with Integral Water Repellent maintain their moisture resistance over their entire lifetime.
  • In addition to the IWR in the block and the mortar, RMMI advises you to finish off this wall with a spray-applied coating of RTV Silicone water repellent.
  • Use a proprietary flashing system at the base of the wall to lead moisture out of the wall without compromising the structural connection from the CMU to the foundation.  Details showing these flashing products are shown here under “System Details.”
  • Use chips of marble or Portland cement to achieve white flecks in the block. Pumice (the typical white aggregate used in block) is very porous and absorbent. Including pumice in the mix can lead to moisture issues in a Single Wythe wall.
  • All concrete products shrink over time. Do not forget to design and install control joints to minimize shrinkage cracks.
  • Untreated concrete block is too porous to be used as a coping or a sill. Call for precast concrete, cast stone, or metal units at these vulnerable locations.
  • Type N mortar works well for Single-Wythe walls, except in high seismic zones where Type S is required.
  • Using A-shaped or H-shaped blocks can make it faster and easier to build a heavily grouted wall by eliminating the need to thread block over rebar. These specialty blocks cost about 15% more than standard units but save you time and money in construction.
  • In addition to the IWR in the block and the mortar, RMMI advises you to finish off this wall with a spray-applied coating of RTV Silicone water repellent.
  • Use a proprietary flashing system at the base of the wall to lead moisture out of the wall without compromising the structural connection from the CMU to the foundation. Details showing these flashing products are shown here under “System Details”.
  • If you are doing high-lift grouting, a rebar positioning device can help you keep the rebar in place where it will give maximum structural effect. See example.

Flashing & Moisture Control

Weep System

MTI flashing/weep system for base of the wall

Block Flash with Rebar

MortarNet BlockFlash flashing/weep system for base of the wall

Block Flash pan

MortarNet BlockFlash flashing/weep system for base of the wall

Roof Parapet Detail

Roof to Wall Parapet Detail—York 304 SA Self-Adhering Stainless Steel Flashing (view from roof)

Joint Reinforcement

Ladder Horizontal Reinforcement: Adds tensile strength to the wall and helps control shrinkage cracking. The cross rods of Ladder Reinforcement are spaced at 8” on center so they align with the cross webs of the concrete block. This keeps the core holes open and makes it easier to place grout in the wall.

H-B Ladder Mesh Reinforcement
Ladder Mesh Reinforcement

Masonry Anchors & Ties

Double-Pin Stone Anchor: Allows you to tie two capstones to the top of the parapet with each anchor. Drill an over-sized hole in the ends of each piece of the masonry cap. Insert a pin into the fresh mortar and then attach the angle of the anchor to the top edge of the structural wall.

H-B Stone Anchor #433

Stone Anchor #433
Cap with Pins
Cap with Pins

Insulated Block

Hi-R Specialty Unit (image)

Hi-R Specialty Unit

For more info, see “Korfil Hi-R and Hi-R-H Construction Best Practices”

These drawings contain technical information on masonry wall systems. They provide some of the basic information required to properly design and detail these systems. This information does not cover all designs or conditions. The information presented illustrates only principles that are involved. The information contained here is based on the available data and experience of the technical staff of Atkinson-Noland & Associates and Rocky Mountain Masonry Institute. The information should be recognized as suggestions which, if followed with good judgement, should produce positive results. Final decisions on the use of information, details and materials as discussed in this guide are not within the purview of Atkinson-Noland & Associates or Rocky Mountain Masonry Institute. Final choices must rest with the project designer, owner, or both.