Craig Schriner

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  1. ASTM C1372, Standard Specification for Dry-Cast Segmental Retaining Wall Units, is the governing ASTM standard that all SRW manufacturers must ensure their segmental retaining wall products meet if their units are to be used in any application (non-structural or structural). ASTM C1372 is described in further detail in TEK Note 2-4C (see attached). ASTM standards contain the absolute minimum requirements a product must attain to achieve the necessary properties for quality performance. The properties defined in these standards include compressive strength, absorption, density classification, permissible variations in dimensions, and finish/appearance criteria. The standards do not define the color or texture of the unit as these are defined by the purchaser. For application with potential for freeze thaw damage and deicing salts, ASTM C1372 recommends testing in water following ASTM C1262. THE SRW Bets Practices Guide gets into the details of exposure and project location for the commercial and transportation applications. TEK 02-04C.pdf NCMA SRW Best Practices 2016-Final-3-31-16 (8.5x11) (1).pdf
  2. This white substance is known as efflorescence. Efflorescence is caused by a combination of the following three circumstances: 1) A soluble compound exists in the masonry unit; 2) Moisture is present within the masonry capable of dissolving these soluble salts; and 3) As the masonry dries, the dissolved solids are carried to the surface where the moisture evaporates leaving the efflorescence on the surface of the unit. If any one of these circumstances is prevented, then efflorescence will not occur. To learn more about efflorescence, how to account for it in design and construction, and how to remove it from a masonry wall, refer to TEK Note 8-3A (see attached). TEK 08-03A (1).PDF
  3. There are two general types of water repellents for CMUs: 1) Surface Treatments; and 2) Integral Water Repellents. Surface treatment repellents are applied to the weather-exposed side of the wall after the wall is constructed. In addition to water repellency, surface treatment repellents can also improve the stain resistance of the wall. Integral water repellents are admixtures added to the CMU and mortar materials before the wall is constructed. The water repellent admixture is incorporated into the concrete mix at the block plant. This way, each block has water repellent throughout the concrete in the unit. For mortar, the water repellent is added to the mortar mix. It is critical when using integral water repellents that the repellent is incorporated into both the block and the mortar to ensure proper performance of the wall. For more information on the two types of the repellents, refer to TEK Note 19-1A (see attached). TEK 19-01 (2).pdf
  4. In accordance with TMS 602, the minimum amount of masonry cover over reinforcing bars for corrosion protection depends on the environment in which the masonry assembly is exposed to, as follows: Masonry exposed to weather or earth: - Bars > No. 5 (M #16) = 2 in. (51 mm) - Bars ≤ No. 5 (M #16) = 1 ½ in. (38 mm) Masonry not exposed to weather or earth: - Any size bar = 1 ½ in. (38 mm) The cover is measured from the nearest exterior masonry surface to the outermost surface of the reinforcement, thus including the thicknesses of the masonry face shell, mortar, and grout, as depicted below. In addition to the corrosion protection requirements, a clear distance must be maintained between the outermost surface of the reinforcement and the interior of the masonry unit or formed surfaced in order to ensure the grout is able to flow properly through the assembly when placed. The clear distance requirements are as follows: Fine Grout - Clear Distance ≥ ¼ in. (6.4 mm) Course Grout - Clear Distance ≥ ½ in. (12.7 mm) For more concrete masonry details, similar to the ones provided above, visit the “Details” page on NCMA’s Solutions Center ( ).
  5. Aside from altering the unit, the fire rating for an assembly can also be increased by one of two methods: 1) Utilizing a multi-wythe wall instead of a single wythe wall 2) Adding finishes to the CMU wall 3) Filling all of the empty cells of the assembly with grout, sand, or similar approved aggregate. Note that when finishes are used to achieve the required fire rating, the masonry alone must provide at least one half of the total required rating and the contribution of the finish on the non-fire-exposed side cannot be more than one-half of the contribution of the masonry alone. This is to assure structural integrity of the assembly during a fire. The finish material must also be continuous over the entire wall. For more information on these two methods and how to calculate the fire rating using these methods, refer to TEK Note 7-1C (see attached). TEK 07-01C3.pdf
  6. In accordance with ASTM C90, all non-split sides (typically the height and length of a unit) must comply with the ± 1/8 in. (3.2 mm) dimensional tolerance, however, the split face side of the unit is not held to this tolerance. Because the split surface is intended to show variability and irregularity, ASTM C90 waives the tolerance requirements for any split dimension
  7. A CMU is broken down into three parts, as depicted below: 1) Face shell – Material that creates the front and back of the unit 2) Webs – Material that connects the face shells together 3) Cells – Open areas between the webs that allow for the placement of reinforcement, grout or insulation
  8. Per ASTM C140 Annex A1, the normalized web area is calculated as follows: When calculating Awt, one must be cognizant of the unit’s web thicknesses. If the thickness of a whole web(s) or portion of the web(s) is less than 0.75 in. (19 mm), then that web/portion of web must be disregarded as it typically does not contribute to the unit’s structural stability. The web area is then calculated from the remaining portion containing a web thickness of at least 0.75 in. (19 mm). This is done for each web present in the unit. Once all of the web areas have been calculated for each web, sum up the web areas to determine the total minimum web area. For units with rectangular webs greater or equal to 0.75 in. (19 mm) in thickness, the web area is calculated by multiplying the width of the web (tw) by the height of the web (th). For units with non-rectangular webs, disregard portions that contain a web thickness less than 0.75 in. (19 mm) and multiplying the remaining portions width of the web (tw) by the height of the web (th). For simplicity when dealing with non-rectangular webs, it is recommended that the remaining area is broken up into simple elements (i.e. triangles, squares, and rectangles) to determine the web area. The figure below shows an example of how only a portion of a web area can be used for the calculation due to the 0.75 in. (19 mm) web thickness requirement and how the web area was calculated using elements to calculate the minimum web area (two (2) orange triangles and one (1) rectangle).
  9. When it is not possible to meet the space requirements needed for the geogrid, there are several alternatives. A recommended method is the use of multidepth SRW units (as shown in the figure below). Other methods include modular walls, as shown in the figure on the left, or low fines concrete, as shown in the figure on the right. The end goal of these three methods is to create a bigger mass to handle the anticipated loads and thus reduce the length of geogrid needed if the mass was not present. For more information, please see the SRW Market Histroy article (see attached). SRWmarketHISTORY_3 Design.pdf
  10. The space required depends on the system to be built as well as the soil type of the project location. For gravity wall systems, the only space needed to be accommodated for is the width of the SRW units and gravel fill placed behind them. For reinforced systems, space for the SRW units, gravel fill, and geogrid need to be accommodated for. During excavation, the event of over excavation can be present and should be kept to a minimum to minimize work unless it is done as a result of poor soil being encountered. Whether occurred on purpose or not, all soils placed due to over-excavation must be sufficiently compacted.
  11. There are many areas where concrete masonry and hardscape products can contribute. Certain types of site hardscaping can help contribute to several Sustainable Sites credits, including reduction in heat island and stormwater management. Concrete masonry assemblies can be incorporated into the building envelope to reduce energy usage which helps to achieve Energy and Atmosphere credits. Within the Materials and Resources section, there are several areas. Recycled and reclaimed materials are readily incorporated into concrete masonry mix designs. Units are typically produced locally, well within the 500 mile radius required for LEED credit. And since CMU are modular, there is typically little construction waste. Finally, the use of concrete masonry on the interior of the building can contribute to several other credit area, as CMU do not contain volatile organic compounds (VOC’s) and can be easily used to achieve sound transmission class targets for assemblies. More details on these credits and more can be found in NCMA TEK Note 6-9C, see attached. TEK 06-09C (1).pdf
  12. In most situations, two layers of WRB complying with ASTM D226, E2556, or approved equal are required for MSV installation. However, there are some instances where this requirement can differ: - - Building codes may allow a single layer of a WRB to be used when a drainage space (rainscreen) is incorporated behind the stone assembly. Requirements for the rainscreens vary by region. Verify with the local jurisdictional requirements regarding the use an application of rainscreens. - - In the event a cladding transition occurs, the number of layers of WRB necessary behind the non-manufactured stone veneer cladding is dependent upon the material selected (i.e. zero layers, 1 layer, 2 layers, etc.). However, once the transition occurs to AMSV then two (2) layers of WRB must be present. - In the presence of concrete/masonry backup assemblies, WRBS are not required. For details and more information on WRB, please refer to the MVMA MSV Installation Guide (click here).
  13. Self-consolidating grout is an engineered grout that is specially formulated to have good flow characteristics without segregation of constituent materials. It is important to note that it is not simply watered-down grout. Instead, it utilizes carefully controlled particle-size distribution and chemical admixtures to achieve stable flow characteristics. Unlike conventional masonry grout, self-consolidating grout does not require external consolidation when pouring. Since 2008, self-consolidating grout has been included as an acceptable material within TMS 602 (Specification for Masonry Structures) so it can be used in construction – subject to a few material testing considerations. For more information, see NCMA TEK Note 9-2B, see attached. Below is self-consolidating grout undergoing a slump test in accordance with ASTM C1611. TEK 09-02B.pdf
  14. Unfortunately, no. Typical thermal resistivity (R-value) of common insulations used in construction range from about R-4.0 to R-7.0 per inch, possibly masonry unit configuration depicted below. Masonry grout has an R-value of about R-0.10 per inch, possible masonry unit configuration depicted below. Because of this, it is important to account for the grouting schedule used in a concrete masonry wall when determining R-value and U-factor. NCMA has a spreadsheet-based calculator that can assist with this (click here).
  15. The engineered method provides a more rational approach to crack control in concrete masonry (as opposed to the empirical approach detailed in TEK Note 10-2). The engineered method examines the effects of the moisture content of the unit, the effects of variations in temperature, and cement carbonation and creates three separate numerical coefficients to describe the net shrinkage movement these conditions have on a system when summed together, the three coefficients create the Crack Control Coefficient (CCC). The CCC enables a designer to control crack width to a maximum value by 1) limiting the distance between control joints when used in combination with a minimum amount of horizontal reinforcement or 2) incorporating a predetermined, higher amount of horizontal reinforcement (when needed for structural purposes) to limit crack width without the use of control joints. It must be noted that this approach requires more detailed knowledge of the concrete masonry units and site conditions to be fully effective. For more information, refer to TEK Note 10-3 (see attached). TEK 10-03.pdf