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Construction of High-Rise Concrete Masonry Buildings

  • November 2018

INTRODUCTION

Masonry structures have been used for centuries throughout the world. Concrete masonry units, however, are a relatively recent innovation. Initially, these units were made with hand operated equipment, although by the 1940’s, block production had developed to incorporate automated mixing, molding, and curing methods, resulting in consistent quality of materials. These new manufacturing processes allowed concrete masonry to be used in engineered structural systems such as multistory load-bearing structures.

In the late 1940’s, one of the first examples of engineered multistory construction was used by Professor Paul Haller in Switzerland. Today there are many examples of loadbearing masonry buildings up to 15 to 28 stories high.

The modular nature of concrete masonry units makes construction straightforward and the small unit size makes changes in plan or elevation easy. Special unit shapes are manufactured to accommodate reinforcement. Open end units, with one or both end webs removed, permit the placement of units around vertical reinforcing bars. Slots manufactured into the webs of units (termed bond beam units) are used to position horizontal reinforcement within the wall.

Concrete masonry is widely used because of the strength, durability, economy, architectural appeal, and versatility of the masonry system. A major milestone in the advancement of engineered concrete masonry was the establishment of the Specifications for Design and Construction of Load Bearing Concrete Masonry by CMHA in the late 1960’s (ref.1). This served as the building code for engineered concrete masonry structures and was adopted by the Southern Building Code Congress and other model codes. It has evolved into our present-day Building Code Requirements for Masonry Structures (ref. 2) and Specification for Masonry Structures (ref. 3).

One of the earliest wall bearing concrete masonry structures using this new technology was a nine story senior citizens building in Cleveland, Tennessee which was built in 1969 utilizing partially reinforced concrete masonry walls.

In our world of economics, the bottom line is still a deciding factor in determining a building’s construction type. The real economy of concrete masonry lies in utilizing the strength of the masonry units (making them load-bearing) and minimizing the cutting of the modular building unit by utilizing multiples of 8 in. for building dimensions and openings. Regarding finish, the most economical one of course is normally plain, painted block. However, if the owner’s budget permits enhancements, a wide variety of architectural units are available (i.e. colored, splitface, scored, fluted, burnished, and slump block). Prefaced units with a glazed finish, vibrant colors and graffiti resistance are also available. Architectural units not only provide pleasing aesthetics but also greatly reduce maintenance and upkeep costs. Additionally, stucco or a variety of proprietary finishing systems also can be applied.

BUILDING TYPES

Most concrete masonry multistory buildings fall into two main types; loadbearing shear wall-type buildings and infilled walls. The Uniform Building Code (ref. 4) has also recently approved a design method for moment-resisting masonry wall frames.

Loadbearing/Shear Wall Buildings

Loadbearing concrete masonry shear wall buildings make the most effective use of concrete masonry by relying on both the economy and the structural capacity—compressive strength and shear resistance—of the concrete masonry. The most common application uses concrete masonry walls with concrete floor and roof diaphragms. The concrete diaphragms can be poured in place, although precast hollow core slabs are the most common.

Concrete masonry/precast slab buildings provide a fast, economical construction method that has allowed some builders to construct one story each week. Floors are enclosed quickly, so that mechanical, electrical, plumbing, and other contractors can begin working on one floor while masonry wall and plank construction continues on floors above them.

Concrete Masonry Infill

Infilled concrete masonry walls utilize the concrete masonry as cladding and interior partitions between concrete or steel frames, which form the structural load-resisting system. Concrete masonry walls are often used in this application because of the cost effectiveness and ease of construction. Historically, most of these walls have been constructed using standard concrete masonry units which were painted or plastered. More recently, however, architectural units are being used to eliminate the need for finishing the walls.

Construction of infilled masonry walls is usually straightforward since the main building system is in place prior to the masonry construction. The most important consideration is whether “gapped” or “ungapped” infilled walls will be provided. Gapped infilled walls are constructed with a predetermined space between the masonry and the building frame. These gaps act as isolation joints, allowing the building frame to drift and sway under lateral loads. Ungapped infilled walls, by contrast, are constructed tightly against the building frame so that the infilled walls serve as shear walls.

DESIGN CONSIDERATIONS

The typical specified compressive strength of concrete masonry, f’m, is 1500 psi (10.3 MPa). However, using high strength concrete masonry units, f’m values up to 4000 psi (27.6 MPa) are achievable. These high strength units are often specified on high-rise loadbearing projects to minimize wall thickness. For further economy, some designers specify lower f’m values in the upper stories, where the higher compressive strength is not needed, since high strength units may cost more than standard units. For example, the four, fast-track, 28-story towers of the $300 million, 4,000 room Excalibur hotel in Las Vegas, Nevada, used an f’m of 4000 psi (27.6 MPa) for the loadbearing walls on the first thirteen floors (ref. 5). The specified compressive strength decreased in successive stories, until the top floors where standard block with an f’m of 1500 psi (10.3 MPa) was used.

Contractors prefer repetitive floor plans for high-rise construction. This important design feature allows similar construction and provides structural continuity from floor to floor both of which lend to economy in construction. The same holds true for architectural details. Designs which facilitate scheduling repetitive, “assembly-line” construction procedures improve productivity and reduce construction costs. Obviously, aesthetic and functional constraints must also be considered, so that buildings are useful and attractive as well as economical.

Connections between building elements is key to the performance of the structures and should therefore be considered carefully during the design process. Connections should be simple and easy to construct and, where necessary, should accommodate building movements from expansion and/or contraction of building materials.

Differential movement deserves particular attention on high-rises where concrete masonry is clad with clay brick. Concrete materials tend to shrink, while clay tends to expand. Over the height of many stories, these opposing movements can be significant. In one case, the seventeen story Crittenden Court in Cleveland, Ohio, these movements were accommodated by designing the exterior brick as a reinforced curtain wall supported on the foundation (ref. 6). The brick was tied to the precast concrete floor planks using slotted anchors that allow vertical but not horizontal movement. This accommodates the differential movement, and also provides enough lateral stiffness to transfer wind and seismic loads from the brick to the floor diaphragms.

Because of the large size of most multistory buildings, a predefined quality control/quality assurance plan is recommended. Inspection, to ensure that key building elements have been installed properly, is essential to assure that the building was constructed as designed. Material testing may be required by the Specifications for Masonry Structures or the contract documents to verify that supplied materials meet the project specifications. As with all construction, tolerances should be carefully monitored. Steel or concrete frames constructed out of tolerance make the mason’s job difficult and slow. Proper alignment of these elements will facilitate the construction process and provide a more appealing completed structure.

CONSTRUCTION

Construction Materials

For construction to proceed smoothly and quickly, it is necessary to carefully schedule construction procedures and supply of materials. Where space allows, it is preferable to stockpile materials on site so that they are readily available when needed. For small sites, material delivery must be timed so that the materials can be moved quickly to the place they are needed.

Materials are delivered to the masons on upper stories via crane or hoist. Materials can either be stocked from the building floors, or can be placed on the work platform, if the platform is large enough and can support the weight. Coordination with crane and elevator schedules should also be considered so that they are available when materials arrive on site.

An adequate supply of concrete masonry units for the entire story should be supplied at one time. Mortar materials can be mixed using traditional techniques, although silo mix mortar systems have become increasingly popular. These systems deliver 14 to 28 yd3 (10.7 to 21.4 m3) of mortar ingredients, and produce consistent mortar from batch to batch. Additional advantages include ability to be lifted easily from floor to floor, mortar containment, and easy cleanup.

Reinforcement cut to proper length and provided in bundles for each story level also facilitates construction. Grout is typically supplied via ready-mix trucks and is pumped to the top of the wall or is lifted using cubic yard buckets. Silo mixed grout is also supplied on some jobs. Also, as with all grouted masonry, it is vitally important that the grout has a slump between 8 and 11 in. per the Specification for Masonry Structures for proper placement and final performance of the building.

Placing the Masonry

Concrete masonry can either be laid from the inside of the building with the masons working on the interior floor area or from the outside of the building with the masons working on scaffolds or work platforms. The choice depends on the size of the job, type of construction, and mason’s preferences.

Laying Units from inside the Building

For load-bearing and infilled exterior walls, concrete masonry can often be laid from the inside of the building. This normally is the most efficient and cost effective method as this allows the masons to work on the building’s floor area providing ample room for units, mortar, and other building materials. Since the mason’s work is confined to the perimeter of the floor, other trades can also work at the same time. Laying from the interior may also be an advantage in windy conditions, when work on exterior platforms may be limited.

Block for the next story are normally stacked on the concrete floor as soon as it has hardened enough to prevent damaging the surface, usually a couple of hours after the steel troweling is completed. An example of this is a hotel structure where the wall between each room is a bearing wall and the floor system is a concrete, one-way, continuous slab. To ensure structural adequacy and maximum economy, two practices must be observed: 1) no shoring can be removed until the next story of walls has been laid up, and 2) sand must be spread over the new slab to facilitate cleanup of any dropped mortar.

For masonry veneers laid from the interior, the building design and construction must accommodate the construction technique. For example, if the walls are masonry veneer with concrete masonry backup, both masonry wythes can easily be laid at the same time. If, on the other hand, the interior wythe is steel studs with sheathing, the veneer would have to be placed from the exterior. Similarly, large columns and deep beams may interfere with masonry veneer placement from the interior.

One drawback to laying units from the inside of the building is that more time is typically required to place the units to assure they align on the exterior since the masons are facing the interior, unexposed, side of the wall. However, this decrease in productivity is often offset by large reductions in scaffolding costs, which can be substantial. Although some scaffolding is needed to lay the top portion of each wall, only one level of scaffold is required.

Laying Units from Work Platforms

Scaffolds and other temporary work platforms allow the masons to work facing the exposed side of the masonry, making it easier to ensure the exposed side is laid plumb and level. Most mason contractors own a supply of scaffolding, but often must rent supplemental scaffolds for high-rise construction. Time should be allotted for placing, dismantling, and moving scaffolds on the job.

Two alternatives to traditional scaffolding for high-rise construction are powered mast-climbing platforms and suspended scaffolds. Both eliminate the labor required to construct multiple levels of conventional scaffolding.

Powered mast-climbing work platforms are erected on the ground and use electric or hydraulic power to move the platform up and down the supporting mast or masts (ref. 7). The masts are attached to the building using adjustable ties or anchors.

One advantage of these systems is that the platform can be easily moved in small increments. This means the platform can be adjusted as the wall is laid to keep it at the mason’s optimum working height. This reduces the amount of lifting of individual units and improves productivity. Powered mast-climbing platforms have maximum heights ranging from 300 to almost 700 ft (91 to 213 m), depending on the type chosen. Other variables include maximum safe wind exposure, attachment requirements, speed, and optional equipment such as overhead protection.

Suspended scaffolds (ref. 8) are work platforms that are suspended from either the roof of the building or from an intermediate floor and therefore would mainly be limited to use on infill projects where the structural frame precedes the wall. Like the mast-climbing platforms, the suspended scaffolds are adjustable in small increments to keep the platform at the optimum working height for the masons. Most suspended scaffolds are raised and lowered by hand, rather than by a powered system. The attachment requirements for suspended scaffolds are fairly complex, and are typically designed for each project and installed by the scaffold supplier.

Suspended scaffolds have the advantage of keeping the lower floors of the building accessible once the work has progressed above this point. They may also be preferable on sloping sites where erection of frame scaffolding would be difficult.

Suspended scaffolds typically become cost effective at building heights of seven to ten stories. Below this height, traditional or power mast scaffolding is probably more cost effective.

CONCLUSION

Many economical concrete masonry structures have been built around the country ranging from buildings to over twenty stories in height to fifteen foot high retaining walls. Rapid growth in areas like that of Orlando, Florida, spurred by the arrival of Disney World produced a market for quality, economical building systems. Concrete masonry construction and the early CMHA Specification for Design and Construction of Load-Bearing Concrete Masonry were ready with the technology to allow engineers and architects to design economical and aesthetically pleasing structures. High-rise buildings have seen an unprecedented growth with modern, innovative construction methods, proper engineering design and use of concrete masonry materials.

REFERENCES

  1. Specification for Design and Construction of Load-Bearing
    Concrete Masonry, Concrete Masonry & Hardscapes
    Association, 1970.
  2. Building Code Requirements for Masonry Structures, ACI
    530-95/ASCE 5-95/TMS 402-95. Reported by the Masonry
    Standards Joint Committee, 1995.
  3. Specification for Masonry Structures, ACI 530.1-95/ASCE
    6-95/ TMS 602-95. Reported by the Masonry Standards
    Joint Committee, 1995.
  4. Uniform Building Code. Whittier, CA: International
    Conference of Building Officials (ICBO), 1997.
  5. Keating, Elizabeth. “A Floor a Week per Tower.” Masonry
    Construction, November 1989.
  6. Keating, Elizabeth. “Powered Mast-Climbing Work
    Platforms.” Masonry Construction, May 1997.
  7. Wallace, Mark A. “Loadbearing Masonry Rises High in
    Cleveland.” Masonry Construction, May 1997.
  8. Hooker, Kenneth A. “Suspended Scaffolds Cut High-Rise
    Masonry Costs.” Masonry Construction, March 1991.

Key Installation Checkpoints for Manufactured Stone Veneer

  • August 2018

INTRODUCTION

Manufactured stone veneer has the appearance of natural stone, but is manufactured from concrete. The veneer is increasing in popularity and being used aesthetically for commercial and residential applications, giving buildings a rich, upscale look.

Typical installations are shown in Figures 1 and 2 for concrete masonry and wood frame applications, respectively. There are a number of key installation/inspection points that must be followed to provide a properly performing system. This TEK addresses a number of those key points.

KEY INSTALLATION/ INSPECTION POINTS

1. Substrate Preparation

Wood- and Metal-Framed Applications

According to The Engineered Wood Association’s (APA) Installation of Stucco Exterior Over Wood Structural Panel Wall Sheathing (ref. 1), when wood structural panel sheathing (plywood or oriented strand board – OSB) is used as the substrate material, it must have a ⅛ in. (3 mm) gap between the sheets on all sides to accommodate wood sheathing expansion when it gets wet. Without the gap, wood swelling will cause the adhered masonry veneer finish to crack, compromising the water resistance of the finish. This will allow water entry and degradation of the substrate, the veneer and the framing. Additionally, the wood framing should be relatively dry at the time of installation, as drying of the wet substrate causes shrinkage which could lead to cracking of the finish. Also note that the International Building Code (ref. 11) limits the deflection of wall systems to L/240 for brittle finishes.

Concrete and Concrete Masonry Walls

Manufactured stone veneer can be directly applied to surfaces of concrete and concrete masonry if they are free of dirt, waterproofing, paint, form oil, or any other substance that could inhibit the mortar bond. They must have a rough texture to ensure a mortar bond. ICRI guideline number 03732 (ref. 5) discusses Concrete Surface Profile (CSP), a standardized method to measure concrete surface roughness. An ICRI CSP equal to or greater than 2 is usually acceptable. Typically, cleaning may be done with power washing and/or mechanical methods (i.e. shot or bead blasting). If a bondable surface cannot be achieved, attach lath and a scratch coat before installing manufactured stone veneer. Do not bond manufactured stone masonry veneer to clay masonry surfaces.

Figure 1—Application of Adhered Concrete Masonry Veneer Over Concrete Masonry With Lath
2. Water Management

ASTM C1780, Standard Practice for Installation Methods for Adhered Manufactured Stone Masonry Veneer, (ref. 3) requires two separate layers of water-resistive barrier (WRB) to be installed over wood sheathing in exterior applications. The standard also requires that the two separate layers be installed in shingle fashion. Starting from the bottom of the wall, the inner layer of WRB should be installed, along with flashings, to create a drainage plane. The upper layer of the WRB should lap the top of the lower layer by a minimum of 2 in. (51 mm). The vertical joints of the WRB must be lapped a minimum of 6 in. (152 mm). Inside and outside corners must be overlapped a minimum of 16 in. (406 mm) past the corner in both directions. The WRB should be installed in accordance with the manufacturer’s recommendations and be integrated with all flashing accessories, adjacent WRBs, doors, windows, penetrations, and cladding transitions. The outer layer of WRB is intended to keep the scratch coat from contacting the inner layer of WRB and may be of a different material than the inner WRB.

Drainage Wall Systems: Drainage wall systems also are a very effective means of keeping water from penetrating to the interior and diverting it to the exterior. These systems have a minimum drainage gap of 3/16 in. (5 mm) and a maximum drainage gap of ¾ in. (19 mm). When a system of this type is used, some building codes permit the use of a single WRB.

Other checkpoints for effective water management include:

  1. flashing at all penetrations, terminations and transitions, integrated with the WRB and water directed out of the system,
  2. proper overhang of capping mateials, and
  3. soft joints at dissimilar materials to accommod and soft joints at dissimilar materials to accommodate some movement and minimize incidental water penetration.
3. Proper Selection of Metal Lath by Weight and Style for Each Span and Application

ASTM C1063, Standard Specification for Installation of Lathing and Furring to Receive Interior and Exterior Portland Cement-Based Plaster (ref. 4) must be followed for properly performing manufactured stone veneer system. All lath must be self-furred or use self-furring fasteners to allow the mortar to completely fill and encase the lath.

All lath and lath accessories must be corrosion resistant, consisting of either galvanized or stainless steel materials or nonmetallic lath with a published evaluation report from an ANSI accredited evaluation service and be rated for use behind manufactured stone veneer. More detailed recommendations can be found in the Installation Guide and Detailing Options for Compliance with ASTM 1780 for Adhered Concrete Masonry Veneer (ref. 2).

Figure 2—Application of Adhered Concrete Masonry Veneer Over Wood Frame With Structural Wood Sheathing
4. Proper Installation of Metal Lath

The installation of lath should be in accordance with ASTM C1063-14a (current version), Standard Specification for Installation of Lathing and Furring to Receive Interior and Exterior Portland Cement-Based Plaster (ref. 4). Metal lath should be applied horizontally (perpendicular to framing, if present) per manufacturer’s instructions, and should over- lap a minimum of 1 in. (25 mm) at the vertical seams and a minimum of ½ in. (13 mm) at the horizontal seams. The ends of adjoining lath places should be staggered. Lath should be wrapped around inside and outside corners a minimum of 12 in. (305 mm). Lath should be fastened every 7 in. (178 mm) vertically on each stud. The spacing of studs should not exceed 16 in. (406 mm). A similar spacing should be used on concrete or masonry wall surfaces. Do not end lath at inside/outside corner framing.

If not installed in accordance with ASTM C1063, alternate lath installation practices should be in accordance with manufacturer’s instructions. Acceptable installation practices for metal lath should be evaluated in accordance with AC191, Acceptance Criteria for Metal Plaster Bases (Lath) (ref. 12).

While recommendations vary, existing codes and standards do not stipulate the orientation of the lath “cups” (keys) once installed. More important than the orientation of the lath cups is ensuring the lath is embedded within, and bonded to, the mortar scratch coat for a successful AMSV installation. Lath is considered to be embedded within the mortar scratch coat when there is a ¼ in. (6 mm) nominal thickness of mortar between the back plane of the lath and the back plane of the scratch coat for at least one-half (50%) of the surface area of the installation.

When lapping paperbacked lath, be sure that lath is against lath and paper against paper. Paper backing inserted between lath at laps will prevent the mortar from going into the second lath and won’t lock the two sheets together. This can cause cracking in the manufactured stone veneer at the lath joint.

In the summer months, paperbacked lath must be protected from the sun and extreme heat to prevent the glue that attaches the paper to the lath from melting.

5. Proper Fastening of Lath

Proper fastener spacing and penetration is critical. Corrosion resistant fasteners (ref. 4) require a minimum ¾ in. (19.1 mm) nail penetration into wood framing members, a minimum ¾ in. (19.1 mm) staple penetration into wood framing members, or minimum a ⅜ in. (9.5 mm) penetration of metal framing members. Fasteners must have heads large enough to properly engage the lath.

Note that fasteners must be anchored into the framing members and are to be spaced no more than 7 in. (178 mm) on center per ASTM C1063 (ref. 4). Wood and gypsum sheathing do not have enough holding power to fully support the lath and manufactured stone veneer. A fastener attached only to the sheathing can work loose, particularly if the sheathing becomes wet. Proper fastening will help ensure that the veneer does not become detached during high wind events.

Fastener type and size is also very important. For installation directly to wood framing members, ASTM C1063 allows for the use of  1 ½ in. (38 mm) roofing nails to horizontal framing members. Vertical applications require the use of 1-in. (25-mm) roofing nails, 1-in. (25-mm) staples with minimum ¾-in. (19-mm) crowns, or 6d common nails driven to a penetration of at least ¾ in. (19 mm) and bent over to engage at least three strands of lath.

Where welded or woven wire lath is installed, rest the wire on the fastener for best performance rather than installing the fastener above the wire.

For fastening lath to steel supports, reference is made to ASTM C954 (ref. 6) by ASTM C1063 for screw information. Per that standard, lath can be wire-tied to the member with 18-gauge tie wire. Whether wire or screws are used, the maximum allowed spacing should be maintained, and once again all fastening must be corrosion resistant and penetrate into the structural member. ASTM C954 also states that the screw shall have a minimum head size of 7/16 in. (11 mm) with either a pan or wafer head large enough to engage at least three strands of lath.

6. Clearances

The following minimum clearances are critical to the proper performance of manufactured stone veneer:

Exterior Stud Walls or Where Manufactured Stone Veneer Continues Down a CMU Foundation Wall with WRB and Lath:

     a. 4 in. (102 mm) from grade/earth

     b. 2 in. (51 mm) above paved surfaces such as driveways, patios, etc. This minimum can be reduced to ½ in. (13 mm) if the paved surface is a walking surface supported by the same foundation that supports the wall.

Exterior Concrete or Masonry Walls with or without Lath and Weep Screeds:

      a. 2 in. (51 mm) clearance from grade or ½ in. (13 mm) from a paved surface.

7. Mortar Selection, Mixing and Application
  1. Choose the proper mortar Type for scratch coats and pointing mortars: ASTM C270 (ref. 7) Type N or S for sitemixed or ASTM C1714 (ref. 8) Type N or S for premixed mortar. Setting mortars are the same except that ANSI A118.1 or ANSI A118.4 also may be used (refs. 9, 10)).
  2. Mix the mortar properly and employ hot weather provisions when the ambient temperature is above 90°F (32°C) and cold weather provisions when the temperature is below 40°F (4°C).
  3. Apply the scratch coat with sufficient material and pressure to completely encapsulate the lath with a nominal thickness of ½ in. (13 mm), ensuring that the lath is completely encapsulated with mortar. Horizontally score the surface after the scratch coat is somewhat firm.
  4. If applying to a scratch coat on open studs, non-solid sheathing, or metal building panels, allow the scratch coat to cure 48 hours, then dampen it before applying the setting bed. The setting bed mortar can be applied directly to the scratch coat, to the back of the manufactured stone veneer units (back-buttering), or a combination of both application methods.
8. Setting Manufactured Stone Veneer Units
  1. Dampen the unit’s bonding surface and apply enough setting bed mortar to fully cover the back of the unit with ample squeeze-out between the units.
  2. Take care not to bump previously installed stones. If a unit is inadvertently moved after initial set has begun, it should be removed, mortar scraped off the unit and the scratch coat, and then reinstalled following the application process.
  3. Pay attention to weather conditions and adjust installation procedures for hot or cold weather as needed.
  4. If filled mortar joints are to be provided, add pointing mortar to fill in the joints after there is sufficient cure time of the installed units, when mild contact will not break the bond to the backup system. Tool the pointing mortar when thumbprint hard. Concave or V-groove tooling provides the best water penetration resistance. Filled mortar joints are recommended at schools and other public places as children tend to try climbing walls with unfilled joints.
  5. Clean off remaining mortar debris on the veneer surface with a dry, soft-bristled brush. To prevent mortar smearing, DO NOT use a wet brush to treat uncured mortar joints.
9. Environmental, Chemical, Cleaning and Other Abuse

Avoid exposing manufactured stone veneer to the following as they can result in discoloring or surface damage:

  1. de-icing chemicals, salt, or other harsh chemicals such as acid cleaners and pool chemicals,
  2. sprinklers and roof downspouts should be positioned to prevent frequent moistening of the units, and
  3. avoid installing in areas where the units may be kicked, scraped, or scuffed such as on stair risers.

INSPECTION CHECKLIST

For Wood or Steel Stud Wall Systems:
  • Minimum ⅛ in. (3 mm) gap between sheathing panels.
  • Minimum of two layers of a water resistive barrier (WRB) for exterior applications.
  • Proper and sufficient lap of WRBs.
  • Fasteners for lath placed into framing members with sufficient penetration.
  • Corrosion-resistant lath, flashing, fasteners, and accessories.
  • Lath cups facing up with at least ¼ in. (6 mm) space to backing.
  • Proper and sufficient lap of lath and no WRB between lath at laps.
  • Scratch coat of proper materials, proper thickness and completely encapsulating lath.
  • Scratch coat scored horizontally after it is somewhat firm.
For Concrete Masonry or Concrete Wall Systems:
  • Use lath if surfaces are not clean and free from release agents, paints and other bond breakers for bonding directly. See checklist items for lath above.
  • If using lath and a WRB is needed, see checklist items for WRB above.
  • If a scratch coat is needed if not using lath, see checklist items for scratch coat above.
For Concrete Masonry, Concrete and Stud Wall Systems:
  • Dampen scratch coat and units before applying setting mortar with full coverage and squeeze-out between units.
  • For placed units inadvertently bumped, remove stones and mortar and reinstall.
  • For filled joints, apply pointing mortar after setting mortar has sufficiently cured. Tool joints when pointing mortar is thumbprint hard.
  • Properly clean pointing mortar debris from veneer surface.

REFERENCES

  1. Installation of Stucco Exterior Over Wood Structural Panel Wall Sheathing. The Engineered Wood Association (APA), 2006.
  2. Installation Guide and Detailing Options for Compliance with ASTM C1780 for Adhered Manufactured Stone Veneer, 5th Concrete Masonry and Hardscapes Association, 2023.
  3. Standard Practice for Installation Methods for Adhered Manufactured Stone Masonry Veneer, ASTM C1780-14. ASTM International, 2014.
  4. Standard Specification for Installation of Lathing and Furring to Receive Interior and Exterior Portland Cement- Based Plaster, ASTM C1063-14a, ASTM International,
  5. Selecting and Specifying Concrete Surface Preparation for Coatings, Sealers, and Polymer Overlays, ICRI Technical Guideline No. 03732. International Concrete Repair Institute, 2009.
  6. Standard Specification for Steel Drill Screws for the Application of Gypsum Panel Products or Metal Plaster Bases to Steel Studs from 033 in. (0.84 mm) to 0.112 in. (2.84 mm) in Thickness, ASTM C954-11. ASTM International, 2011.
  7. Standard Specification for Mortar for Unit Masonry, ASTM C270-14. ASTM International, 2014.
  8. Standard Specification for Preblended Dry Mortar Mix for Unit Masonry, ASTM C1714/C1714M-13a. ASTM International, 2013.
  9. American National Standard Specification for Standard Dry-Set Cement Mortar, 1. American National Standards Institute, 2013.
  10. American National Standard Specification for Modified Dry-Set Cement Mortar, A118.4. American National Standards Institute, 2013.
  11. International Building Code. International Code Council,
  12. Acceptance Criteria for Metal Plaster Bases (Lath), AC191. ICC Evaluation Service, Inc., 2012.
  13.  

Inspection Guide for Segmental Retaining Walls

  • August 2018

INTRODUCTION

Segmental retaining walls (SRWs) are gravity retaining walls which can be classified as either: conventional (structures that resist external destabilizing forces due to retained soils solely through the self-weight and batter of the SRW units); or geosynthetic reinforced soil SRWs (composite systems consisting of SRW units in combination with a mass of reinforced soil stabilized by horizontal layers of geosynthetic reinforcement materials). Both types of SRWs use dry-stacked segmental units that are typically constructed in a running bond configuration. The majority of available SRW units are dry-cast machine-produced concrete.

Conventional SRWs are classified as either single depth or multiple depth. The maximum wall height that can be constructed using a single depth unit is directly proportional to its weight, width, unit-to-unit shear strength and batter for any given soil and site geometry conditions. The maximum height can be increased by implementing a conventional crib wall approach, using multiple depths of units to increase the weight and width of the wall.

Reinforced soil SRWs utilize geosynthetic reinforcement to enlarge the effective width and weight of the gravity mass. Geosynthetic reinforcement materials are high tensile strength polymeric sheet materials. Geosynthetic reinforcement products may be geogrids or geotextiles, although most SRW construction has used geogrids. The geosynthetic reinforcement extends through the interface between the SRW units and into the soil to create a composite gravity mass structure. This enlarged composite gravity wall system, comprised of the SRW units and the reinforced soil mass, can provide the required resistance to external forces associated with taller walls, surcharged structures or more difficult soil conditions.

Segmental retaining walls afford many advantages, including design flexibility, aesthetics, economics, ease of installation, structural performance and durability. To function as planned, SRWs must be properly designed and installed. Inspection is one means of verifying that the project is constructed as designed using the specified materials.

This Tech Note is intended to provide minimum levels of design and construction inspection for segmental retaining walls. The inspection parameters follow the Design Manual for Segmental Retaining Walls (ref. 1) design methodology. This information does not replace proper design practice, but rather is intended to provide a basic outline for field use by installers, designers and inspectors.

INSPECTION

Many masonry projects of substantial size require a quality assurance program, which includes the owner’s or designer’s efforts to require a specified level of quality and to determine the acceptability of the final construction. As part of a quality assurance program, inspection includes the actions taken to ensure that the established quality assurance program is met. As a counterpart to inspection, quality control includes the contractor’s or manufacturer’s efforts to ensure that a product’s properties achieve a specified requirement. Together, inspection and quality control comprise the bulk of the procedural requirements of a typical quality assurance program.

SRW UNIT PROPERTIES

SRW units comply with the requirements of ASTM C1372, Standard Specification for Dry-Cast Segmental Retaining Wall Units (ref. 2), which governs dimensional tolerances, finish and appearance, compressive strength, absorption, and, where applicable, freeze-thaw durability. These requirements are briefly summarized below. A more thorough discussion is included in SRW-TEC-001-15, Segmental Retaining Wall Units (ref. 3). The user should refer to the most recent edition of ASTM C1372 to ensure full compliance with the standard.

  • Dimensional tolerances: ±1/8 in. (3.2 mm) from the specified standard overall dimensions for width, height and length (waived for architectural surfaces).
  • Finish and appearance:
    • free of cracks or other defects that interfere with proper placement or significantly impair the strength or permanence of the construction (minor chipping excepted),
    • when used in exposed construction, the exposed face or faces are required to not show chips, cracks or other imperfections when viewed from at least 20 ft (6.1 m) under diffused lighting,
    • 5% of a shipment may contain chips 1 in. (25.4 mm) or smaller, or cracks less than 0.02 in. (0.5 mm) wide and not longer than 25% of the nominal unit height,
    • the finished exposed surface is required to conform to an approved sample of at least four units, representing the range of texture and color permitted
  • Minimum net area compressive strength: 3,000 psi (20.7 MPa) for an average of three units with a minimum of 2,500 psi (17.2 MPa) for an individual unit. When higher compressive strengths are specified, the tested average net area compressive strength of three units is required to equal or exceed the specified compressive strength, and the minimum required single unit strength is:
    • the specified compressive strength minus 500 psi (3.4 MPa) for specified compressive strengths less than 5,000 psi (34.4 MPa), or
    • 90% of the specified compressive strength when the specified compressive strength is 5,000 psi (34.4 MPa) or greater.
  • Maximum water absorption:
    • 18 lb/ft3 (288 kg/m3) for lightweight units (< 105 pcf (1,680 kg/m3))
    • 15 lb/ft3 (240 kg/m3) for medium weight units (105 to less than 125 pcf (1,680 to 2,000 kg/m3))
    • 13 lb/ft3 (208 kg/m3) for normal weight units ( > 125 pcf (2,000 kg/m3 or more))
    • Freeze-thaw durability—In areas where repeated freezing and thawing under saturated conditions occur, freeze- thaw durability is required to be demonstrated by test or by proven field performance. When testing is required, the units are required to meet the following when tested in accordance with ASTM C 1262, Standard Test Method for Evaluating the Freeze-Thaw Durability of Manufactured Concrete Masonry Units and Related Concrete Units (ref. 4):
  • weight loss of each of five test specimens at the conclusion of 100 cycles < 1% of its initial weight; or
  • weight loss of each of four of the five test specimens at the end of 150 cycles < 1.5 % of its initial weight.

REFERENCES

  1. Design Manual for Segmental Retaining Walls (Third Edition), TR 127B. Concrete Masonry & Hardscapes Association, 2009.
  2. Standard Specification for Dry-Cast Segmental Retaining Wall Units, ASTM C1372. ASTM International, Inc., 2017.
  3. Segmental Retaining Wall Units, SRW-TEC-001-15, Concrete Masonry & Hardscapes Association, 2008.
  4. Standard Test Method for Evaluating the Freeze-Thaw Durability of Dry Cast Segmental Retaining Wall Units and Related Concrete Units, ASTM C1262. ASTM International, Inc., 2017.
  5. International Building Code. International Code Council, 2012.
  6. Segmental Retaining Wall Installation Guide, SRW- MAN-003-10, Concrete Masonry & Hardscapes Association, 2010.

Design Checklist

Construction Checklist

Concrete Masonry Inspection

  • August 2018

INTRODUCTION

Concrete masonry is a popular building material in part because of its strength, versatility, durability, economy and resistance to fire, impact, noise and termites. To function as designed, however, concrete masonry buildings must be constructed properly.

Concrete masonry is used in projects ranging from small single story buildings to multistory loadbearing projects and is used in every building type and occupancy, including institutional, residential, commercial and manufacturing facilities. Because of the varying nature of these facilities, masonry construction continues to evolve, becoming more detailed and multifaceted. Reinforced masonry requires masons to not only lay masonry units, but to also properly place reinforcing steel and grout. As the intricacy and variety of masonry systems continues to expand, so does the need for educated and knowledgable inspectors to verify that masonry is being constructed as designed. Likewise, ensuring that the physical properties of the masonry materials comply with project specifications requires detailed knowledge of testing procedures.

Many masonry projects of substantial size requires the implementation of a quality assurance program. A quality assurance program includes the owner’s or designer’s efforts to require a specified level of quality and to determine the acceptability of the final construction. As part of a quality assurance program, inspection includes the actions taken to ensure that the established quality assurance program is met. As a counterpart to inspection, quality control includes the contractor’s or manufacturer’s efforts to ensure that the final properties of a product achieve a specified goal under a quality assurance program. Together, inspection and quality control comprise the bulk of the procedural requirements of a typical quality assurance program.

INSPECTION

Inspection is one part of a quality assurance program, which are the administrative and procedural requirements set up by the architect or engineer to assure the owner that the project is constructed in accordance with the contract documents. Inspection is one means of verifying that the project is constructed as designed using the specified materials.

Inspection assures that masonry materials and construction practices comply with the requirements of the contract documents. Inspectors, the inspection program, and inspection records should be addressed in the quality assurance program. Local municipalities may have minimum inspection requirements that augment or complement minimum code requirements to ensure the safety of the public. Additionally, the amount of inspection required depends on the owner’s needs. The architect or engineer will typically specify the degree of inspection necessary to meet the owner’s quality assurance program, local ordinances and code requirements. (See Required Levels of Inspection below.)

Concrete Masonry Inspectors

A variety of individuals may review the progress of masonry construction. The mason, general contractor, and often the architect, engineer and owner will periodically observe the progress to verify that the masonry construction is proceeding as planned. Municipal or jurisdictional building inspectors may also be required to verify that the constructed project meets local building code requirements. In addition to these individuals, special masonry inspectors are sometimes required by the local building code or by the owner through the architect or engineer.

Each of these “inspectors” tends to look at the masonry construction differently. For example, architects, owners, and masons and general contractors may focus on aesthetic aspects of the masonry, such as color of units, color and size of mortar joints, tolerances, etc. Municipal building inspectors and engineers may concentrate more on verifying structural-related items, such as proper connections, reinforcing steel size and location and connector spacing. Individuals designated as masonry inspectors also closely inspect structural-related items but may also inspect aesthetic, weatherproofing and serviceability aspects of the masonry project as outlined in the contract documents.

The following helps address the level of inspection that may be required by masonry inspectors. It can also serve as a guide for engineers, architects, contractors and building officials engaged in masonry construction or inspection.

Required Levels of Inspection

Local municipalities may have minimum inspection requirements to ensure public safety. Additionally, the amount of inspection required depends on the owner’s needs. The architect or engineer will typically specify the degree of inspection necessary to meet the owner’s quality assurance program and local code requirements.

How long an inspector should be on a job site and what should be inspected has, however, been a source of confusion in many areas of the country. To clarify how much inspection should be required on masonry projects, Specification for Masonry Structures (ref. 1) includes detailed inspection guidelines that provide an excellent basis for the degree of inspection that should be provided on masonry projects.

The 2003 International Building Code (IBC) (ref. 2) Section 1704.5 inspection requirements are virtually identical to those in Specification for Masonry Structures. The corresponding designations are:

  • IBC special inspection Level 1 requirements correspond to Specification for Masonry Structures Level B.
  • IBC special inspection Level 2 requirements correspond to Specification for Masonry Structures Level C.
  • Although there is no special inspection requirement corresponding to Specification for Masonry Structures Level A, this basic requirement is covered in IBC section 109.

In addition, in the 2002 edition of Specification for Masonry Structures the three levels of quality assurance were designated Levels 1, 2 and 3, which were replaced by Levels A, B and C, respectively, in the 2005 edition. This change in nomenclature is wholly editorial and does not affect the requirements specified for each level.

Three levels of inspection are defined within Specification for Masonry Structures:

  • Level A (IBC Basic) – These requirements are the least stringent, requiring verification that the masonry construction complies with the plans and specifications (see Table 1). This level of inspection can only be applied to empirically designed masonry, glass unit masonry and masonry veneer used in facilities defined as nonessential by the building code. When masonry is designed by engineered methods or is part of an essential facility, Level B or C inspection is required.
  • Level B (IBC Level 1) – These requirements provide a periodic-type inspection for engineered masonry used in nonessential facilities (as defined in the building code) and for empirically designed masonry, glass unit masonry and masonry veneer used in essential facilities. Key inspection items include assurance that required reinforcement, anchors, ties and connectors are in place and that appropriate grouting procedures are used (see Table 2).
  • Level C (IBC Level 2) – The most comprehensive inspection procedures are required for essential facilities (as defined in the building code) that are designed by engineered design methods (see Table 3). Items inspected under a Level C quality assurance program are similar to those of Level B, with the added requirement that inspection be continuous during all phases of masonry construction.

These inspection levels are minimum criteria and may be increased when deemed necessary by the owner or designer. In this case, the contract documents must indicate the inspection level and tests that are required to assure that the masonry work conforms with the project requirements. Due to their relative importance or potential hazard, more significant inspection and quality assurance measures are required for essential facilities.

Table 1—Level A Quality Assurance (IBC Basic Inspection)
Table 2—Level B Quality Assurance (IBC Level 1 Special Inspection)
Table 3—Level C Quality Assurance (IBC Level 2 Special Inspection)

Responsibilities and Qualifications of Masonry Inspectors

Proper construction techniques are essential for a building to function as designed. Unfortunately, buildings are sometimes poorly constructed because of oversight, miscommunication, or occasionally because of unscrupulous behavior. Accordingly, inspection of the construction process can be vital to the success of a project.

An inspector’s main duty is to observe the construction to verify that the materials and completed project are, to the best of the inspector’s knowledge, in conformance wit h the contract documents and applicable building code. The inspector is not required to determine the adequacy of either the design or application of products and cannot revoke or modify any requirement nor accept or reject any portion of the work. To function effectively, the inspector must be familiar with proper construction techniques and materials, with the requirements of the local building codes, Building Code Requirements for Masonry Structures (ref. 3) and Specification for Masonry Structures. Although not required by Specification for Masonry Structures or the International Building Code, inspectors may be qualified or certified under nationally recognized education programs offered through such organizations as the International Code Council. Completion of such a program may be required by a local jurisdiction or by a building official.

Although vague, Section 1704.1 of the 2003 International Building Code provides general guidance on the minimum qualifications for inspectors, as follows:

“The special inspector shall be a qualified person who shall demonstrate competence, to the satisfaction of the building official, for inspection of the particular type of construction or operation requiring special inspection.”

The nonspecific nature of this code provision has been a source of confusion on various construction projects due to the wide variety of interpretations of a ‘qualified person.’ Some equate qualification with a nationally recognized certification, while others have allowed a noncertified individual with sufficient experience to serve as an inspector.

As a minimum, however, a masonry inspector must be familiar with masonry construction and be able to read plans and specifications effectively in order to judge whether the construction is in conformance with the contract documents. As part of this task, an inspector should always review the contract documents thoroughly before construction begins.

Inspectors must keep complete and thorough records of observations regarding the construction process. An effective way to accomplish this is by keeping a daily log when the inspector visits the project. Items such as the date, weather, temperature, work in progress (location and what was accomplished), meetings (attendees and topics of discussion), as well as overall observations and test results should be recorded in a neat, orderly manner since these notes may be needed later.

At the completion of the project or at predetermined stages of construction, inspectors must submit a signed report stating whether the construction requiring inspection was, to the best of the inspector’s knowledge, in conformance with the contract documents and applicable workmanship standards. Specific services and duties required by an inspection agency are outlined in Article 1.6 B of Specification for Masonry Structures.

TESTING AND QUALITY CONTROL

Material testing may be necessary either before, during or after the construction of a building. For example, preconstruction testing may be requested to verify compliance of materials with the contract documents and is typically the responsibility of the contractor or producer of the product. Testing during construction, as part of the owner’s quality assurance program, may also be required to ensure that materials supplied throughout the construction process comply with the contract documents. These tests are the owner’s responsibility. Additionally, testing may be necessary to determine the in-place condition of the building materials after the building is complete or during the building’s life.

Standards for sampling and testing concrete masonry materials and assemblages are developed by the technical committees of ASTM International in accordance with consensus procedures. These standards reflect the expertise of researchers, concrete masonry manufacturers, designers, contractors and others with an interest in quality standards for masonry.

Specific testing procedures for concrete masonry units and related materials are covered in detail in references 4 through 8.

REFERENCES

  1. Specification for Masonry Structures, ACI 530.1-05/ASCE 6-05/TMS 602-05. Reported by the Masonry Standards Joint Committee, 2005.
  2. 2003 International Building Code. International Code Council, 2003.
  3. Building Code Requirements for Masonry Structures, ACI 530-05/ASCE 5-05/TMS 402-05. Reported by the Masonry Standards Joint Committee, 2005.
  4. Evaluating the Compressive Strength of CM based on 2012IBC/2011 MSJC, TEK 18-01B, Concrete Masonry & Hardscapes Association, 2011.
  5. Sampling and Testing Concrete Masonry Units, TEK 1802C, Concrete Masonry & Hardscapes Association, 2014.
  6. Masonry Mortar Testing, TEK 18-05B, Concrete Masonry & Hardscapes Association, 2014.
  7. Compressive Strength Testing Variables for CM Units, TEK 18-07, Concrete Masonry & Hardscapes Association, 2004.
  8. Grout Quality Assurance, TEK 18-08B, Concrete Masonry & Hardscapes Association, 2005.

Roles and Responsibilities on Segmental Retaining Wall Projects

  • August 2018

INTRODUCTION

On all construction projects, including those involving segmental retaining walls (SRWs), it is the owner’s responsibility to achieve coordination between construction and design professionals that ensures all required design, engineering analysis, and inspection is provided. In many cases, a design professional such as a site civil engineer or an architect acts as the owner’s representative. In either case, the owner or owner’s representative should ensure that the engineering design professionals’ scope of work, roles and responsibilities are clearly defined so that there is no ambiguity regarding responsibility for investigation, analysis and design, and that all required testing is performed.

The roles outlined in this TEK are typical industry roles for various engineering disciplines. SRW design and construction should generally follow these traditional roles. However, these roles may vary from project to project, depending on the contractual obligations of each consultant. For example, for simpler projects, such as residential landscapes, one design professional may take on the responsibility of several roles, if acceptable to local building code requirements.

For tall or complex walls and for commercial projects, each of these roles is likely to be provided by separate firms, each with expertise in a particular discipline. The discussion in this Tech Note is generally oriented towards projects where several design professionals are contracted.

Reinforced SRWs, because of their nature as composite soil structures, may have unique design and inspection considerations for the site civil engineer, the geotechnical engineer, and the independent testing agency. These considerations are discussed in further detail in the following sections.

Detailed guidance on SRW design, construction and inspection can be found in references 1 through 3.

Figure 1—Reinforced Segmental Retaining Wall System Components

OVERVIEW OF ROLES

The owner/developer, or a designated representative, is ultimately responsible for ensuring that all applicable requirements of governing authorities for the permitting, design, construction and safety on the project are addressed. The owner or owners’ representative should ensure that the types of retaining walls specified are appropriate for the site conditions and ensure the wall alignment fits within the site’s space limitations. It is the owner’s or owner’s representative’s responsibility to contract an engineer to provide site civil engineering including site layout, drainage and grading. The owner must also ensure that a geotechnical engineer and testing agency are contracted to provide all necessary and required soils exploration, analysis and earthwork inspection for the entire project, including in the vicinity of the SRWs, just as they do in the vicinity of building structures. The owner or owner’s representative must also ensure that a qualified wall design engineer provides an SRW structural design.

The most straightforward means for the owner or owner’s representative to ensure all engineering roles are well-defined is for the SRW design engineer’s assigned roles to be the same as those traditionally given to a structural engineer designing a cast-in-place concrete retaining wall, and for the other design professionals, such as site civil and geotechnical engineers, to also provide the same roles and services as they would for a cast-in-place retaining wall.

Table 1 contains an itemized list of the suggested roles for each professional discipline for larger walls and commercial projects involving SRWs. A more thorough explanation of the site civil engineer’s, geotechnical engineer’s and SRW engineer’s roles, and construction observation and testing roles is provided in the following sections. The actual responsibilities for each discipline should be contractually based.

Table 1—Suggested Roles for a Segmental Retaining Wall Project

SITE CIVIL ENGINEER SUGGESTED ROLES OVERVIEW

It is suggested that the site civil engineer be contracted for all traditional site civil duties, including the design of surface drainage, storm drainage collection structures, utility layout, erosion control and scour protection. The site civil engineer is also typically responsible for site layout and grading plans, including slopes and retaining wall locations. The site civil engineer should, in consultation with the geotechnical engineer, ensure that all planned grades, including those at the top and bottom of SRWs, do not exceed the stable slope angles and do not cause surface drainage or erosion problems.

The site civil engineer should also plan the wall alignment so that the SRW structure does not encroach on any easements. In addition, the site civil engineer should be responsible for any other issues related to the wall location, such as proximity to property lines, utilities, watersheds, wetlands, or any other easements. In some cases, the site civil engineer may also act as the SRW Design Engineer and take on suggested roles for the SRW Engineer discussed below.

The site civil engineer should evaluate and design for any hydrologic issues and structures such as: culverts, open channels, detention/retention ponds, scour and erosion control details, as well as defining high water levels, flow volumes, flood areas and scour depths. The site civil engineer should provide any pertinent hydrologic data that may affect the SRW to the SRW engineer.

Often, when not designing the SRW in-house, the site civil engineer specifies the engineering design of SRWs to be part of the SRW construction contract (a design/build bid). While a common practice, this type of bid can place the SRW engineer in a different position than other project engineers. Unlike other engineers working directly for the owner, the SRW engineer in this design/build case is often working directly for a contractor, who is often a subcontractor to other contractors. This can cause design coordination issues because the SRW engineer may not be included in project discussions with other engineers, such as pre-construction meetings. Therefore, it is suggested that the site civil first determine if it is appropriate to have the SRW engineering specified as part of the wall construction contract. For some more complicated projects, it may be preferable to have the SRW design engineer perform the design prior to bidding the construction rather than as part of a design/build bid. If the site civil engineer chooses to specify the SRW design as part of the construction bid, it is recommended that the site civil engineer ensure that the SRW design engineer is involved in any required design and construction observation services before and during construction, similar to the way geotechnical engineers are often contracted for their services during construction.

GEOTECHNICAL ENGINEER SUGGESTED ROLES OVERVIEW

The geotechnical engineer should typically be contracted to provide the same engineering roles in the vicinity of the SRW as they do for all other structures on site. The geotechnical engineer’s typical roles are the investigation, analysis and testing of the site soil materials and groundwater conditions. Just as geotechnical engineers traditionally provide bearing capacity, settlement estimates and slope stability analysis for building structures, it is suggested they do the same for SRWs. The geotechnical engineer’s role should include providing soil properties such as soil shear strength parameters, ground water elevation, seismic conditions, and bearing capacities to the SRW engineer.

Responsibility for slope stability evaluation around an SRW can be a source of confusion, because the SRW engineer can often address slope stability issues near a geosynthetic-reinforced SRW by modifying the geosynthetic reinforcement layout. Thus, the SRW engineer is sometimes requested to evaluate and design for slope stability by the civil engineer’s specifications. However, involving the SRW engineer in addressing slope stability should not remove ultimate global/slope stability responsibility from the geotechnical engineer.

It is therefore suggested that, regardless of the SRW engineer’s involvement, the geotechnical engineer be contracted to have the ultimate responsibility for the site’s slope stability, including: determining when and where global stability analyses are required, determining the appropriate soils and groundwater properties to be used for the analyses, and ensuring that all required failure planes are analyzed. While the geotechnical engineer may need to coordinate with the SRW engineer for evaluating potential failure planes that pass through the reinforced soil (compound failures), the geotechnical engineer has the primary responsibility for these analyses.

When the geotechnical consultant is retained to provide construction observation and soils testing for a project, the contract should include inspection and testing of SRW earthwork along with all other earthwork on site. See TEC-008-12, Inspection Guide for Segmental Retaining Walls (ref. 3) for further discussion of inspection roles.

While geotechnical engineers should be contracted for the same traditional roles regarding SRWs as for other structures, the soils engineering for SRWs may require some slightly different methods of analysis compared to evaluating soils below rigid structures on spread footings. Design guidelines for SRWs are provided in Reference 1.

SRW DESIGN ENGINEER SUGGESTED ROLES OVERVIEW

As noted previously, the SRW design engineer should serve the same roles for SRWs as a structural engineer would for the design of a cast-in-place concrete retaining wall. In some cases, the site civil engineering firm may also act as the SRW engineer, while in others, the SRW design engineer will be a separate firm. The SRW design engineer should design a stable SRW, given the specified wall geometry and site conditions provided by the site civil and geotechnical engineers. The SRW engineer’s duties typically include determining the SRW’s maximum stable unreinforced height and providing a geosynthetic reinforcement layout design when required.

The SRW design engineer is typically responsible for preparing the SRW construction drawings, and for determining the internal stability, facial stability of the SRW units, internal drainage of the SRW (both at the face of the wall and at the rear of the reinforced soil mass, if required), external stability (sliding and overturning), and internal compound stability.

The SRW designer engineer’s output generally consists of specifications of wall components, a wall elevation detail, typical cross sections, details for any required drainage materials within or just behind the wall system, and details for how to incorporate any other structures (utilities, pipe penetrations, posts, etc.), if feasible, within the reinforced zone and wall face.

The SRW design engineer should typically not assume any duties typically relegated to the geotechnical engineer elsewhere on site. While an SRW engineer may be asked to participate in addressing the slope stability immediately around the SRW or foundation improvements in the soil below an SRW, it is recommended that the geotechnical engineer be clearly contracted to have ultimate responsibility for all slope stability and bearing capacity/settlement concerns on site, including those below and around SRWs.

It is appropriate that the SRW engineer be contracted to provide services during construction, especially on larger projects, but it is recommended that these not be included in a design/build contract for the wall construction. Time lag between design and construction can make it impractical to expect the designer to be available for services during construction and, given the often unpredictable extent and timing of construction, it is inappropriate to have services during construction be in a lump-sum design/build contract. Rather, it is suggested that the SRW engineer be hired under a separate contract directly with the owner or owner’s representative to provide services during construction. These services may include preconstruction correspondences and meetings, review of materials submittals, review of earthwork testing performed by the geotechnical engineer, and review of the wall contractor’s building practices.

CONSTRUCTION OBSERVATION AND TESTING SUGGESTED ROLES OVERVIEW

The soil in the reinforced zone should be checked to ensure it meets specifications; just as concrete and steel are inspected in a cast-in-place concrete retaining wall.

The wall contractor is responsible for quality control of the wall installation: performing necessary observation and testing to verify that the work performed meets minimum standards.

It is the owner’s or owner’s representative’s responsibility to perform quality assurance: auditing and verifying that the quality control program is being performed properly.

Just as is done for building structures and cast-in-place concrete retaining walls, foundation and retained soils should be evaluated for consistency with the soil properties used in the design. Generally, the geotechnical engineer evaluates the onsite soil conditions and performs earthwork testing. It is suggested that the geotechnical engineer perform any field and laboratory testing they deem required to verify soil conditions. The geotechnical engineer should confer with the SRW engineer regarding the reinforced soil specifications and provide the SRW engineer with the fill soil test results. The geotechnical engineer should also determine the frequency of tests required to ensure that compaction of the SRW reinforced fill meets the project specifications.

OWNER SUGGESTED ROLES OVERVIEW

Segmental retaining walls are designed to provide a long life with little to no maintenance required. After the SRW installation is complete, some very basic maintenance will help maximize the SRW project’s beauty and durability.

The most basic maintenance task is a periodic visual assessment of the SRW units and overall wall. If coatings have been applied to the wall, the need for re-coating should be assessed based on the coating manufacturer’s recommendations and the exposure conditions of the wall. Table 2 lists regular inspection tasks that can be performed on SRWs and their suggested frequency.

Periodic cleaning of SRWs may be desired to maintain the wall’s aesthetics. Cleaning recommendations for SRWs are essentially the same as those for other concrete masonry walls. The reader is referred to: TEK 8-04A, Cleaning Concrete Masonry; TEK 08-02A, Removal of Stains from Concrete Masonry; and TEK 08-03A, Control and Removal of Efflorescence (refs. 5, 6, 7), for more detailed guidance.

In addition to maintenance and cleaning, the owner is also responsible for ensuring that subsequent digging or trenching, such as for landscaping, does not impact the SRW installation. During any excavation, care should be taken to leave a zone of undisturbed soil behind the segmental retaining wall. Particular care should be taken to ensure that excavation does not damage, cut or remove the geosynthetic soil reinforcement, if present. For this reason, the owner should maintain a record of the installation, including the locations of geosynthetic reinforcement.

Once established, tree roots do not typically damage an SRW. The roots will typically not damage the wall face from behind because the drainage aggregate behind the SRW face does not support root growth. In fact, the root system can act as additional soil reinforcement, helping to further stabilize the soil. When newly planted, trees and other large vegetation should be adequately supported to prevent them from toppling and potentially damaging the SRW.

Table 2—Example SRW Maintenance Schedule (ref. 4)

REFERENCES

  1. Design Manual for Segmental Retaining Walls, Third Edition, SRW-MAN-001-10, Concrete Masonry & Hardscapes Association, 2010.
  2. Segmental Retaining Wall Installation Guide, SRWMAN-003-10, Concrete Masonry & Hardscapes Association, 2010. 
  3. Inspection Guide for Segmental Retaining Walls, SRW-TEC-008-12, Concrete Masonry & Hardscapes Association, 2012. 
  4. Maintenance of Concrete Masonry Walls, TEK 08-01A, Concrete Masonry & Hardscapes Association, 2004. 
  5. Cleaning Concrete Masonry, TEK 08-04A, Concrete Masonry & Hardscapes Association, 2005.
  6. Removal of Stains from Concrete Masonry, TEK 08-02A, Concrete Masonry & Hardscapes Association, 1998.
  7. Control and Removal of Efflorescence, TEK 08-03A, Concrete Masonry & Hardscapes Association, 2003.

Evaluating Fire-Exposed Concrete Masonry Walls After a Fire

  • August 2018

INTRODUCTION

Fire safety requires that a wall not only halt the spread of fire from one area to another, but also retain its structural integrity throughout the fire and fire-fighting operations. If occupants, firefighters and building contents are to be fully protected, the structure must not collapse, add fuel to the fire nor emit toxic gases during the fire.

Concrete masonry fire walls provide maximum safety during and after severe fire exposure. Because concrete masonry is a noncombustible structural material which neither adds fuel to a fire nor emits toxic gases, it is widely used to provide compartmentation—containing a fire until it can be brought under control by fire fighters. In addition, even after severe fires, concrete masonry walls can typically be repaired by simply patching cracks and tuckpointing mortar joints, rather than requiring demolition and replacement. Experience with building fires has shown that the most damage to concrete masonry walls during a fire often occurs due to lost support rather than as a direct result of fire on the masonry.

This TEK provides general information on assessment methods and repair techniques and discusses what can be expected after concrete masonry walls have been subjected to fire.

EVALUATING FIRE-EXPOSED WALLS

Preliminary Inspection

After a fire occurs, a preliminary inspection should be conducted as soon as possible to assess: the condition of the structure, the type and severity of problems observed in the affected area(s), the feasibility of rehabilitation and the need for conducting a detailed investigation. After collecting data on the building structure and the fire event, the preliminary investigation should take place as soon as safe entry into the building can be arranged.

The first step in the preliminary investigation is a visual inspection of structural members in the fire-affected areas. Indications of cracking, spalling, deflections, distortions, misalignment of elements and/or exposure of steel reinforcement should be documented. Measurements of deflections, deformations and geometry can be taken of any suspect members for comparison to unexposed members in the same structure. These observations should be recorded, documenting the type of damage and its severity for each affected member. This summary helps identify damaged members in need of more detailed investigation, as well as the extent and nature of any necessary repairs.

As an adjunct to visually assessing the structural members in fire affected areas, the building contents in these areas should be observed. The melting points of various materials (see Table 1) indicate the temperature ranges that have occurred in localized areas, providing an estimate of the maximum temperatures achieved during the fire. These estimated maximum temperatures help establish the severity of the fire relative to the Standard Test Methods for Fire Tests of Building Construction and Materials, ASTM E 119 (ref. 2) fire test, or to another recognized baseline. If the maximum temperatures during the fire are similar to those of the E 119 test, the potential damage to the concrete masonry is somewhat predictable, based on the history of E 119 testing on concrete masonry.

The ASTM E 119 fire test time/temperature protocol is shown in Figure 1.

There is a large body of data on concrete masonry walls tested according to the ASTM E 119 protocol. This test method evaluates walls subjected to the standard test fire. Performance criteria include: resistance to temperature rise on the unexposed side of the wall; resistance to the passage of hot gases or flames through the wall; structural stability during the test; and resistance of the masonry to deterioration under direct exposure to a fire hose stream immediately following the fire test. Research has shown that the fire resistance ratings of concrete masonry walls are invariably determined by the temperature rise on the cold (unexposed) side of the wall.

Field Testing Procedures

Part of the preliminary inspection is determining the need for further testing and evaluation. Nondestructive field tests, such as the use of an impact rebound hammer, are typically not used with concrete masonry, as the hollow cells interfere with obtaining meaningful results in many cases. In addition, extensive field testing is not always prudent, as removal and replacement of the fire-damaged element can sometimes be more economical than extensive testing. A solid understanding of both structural engineering and the effects of fire on building materials is invaluable to this decision-making process. When necessary, destructive test methods can be used to assess the strength of the in-situ concrete masonry (see reference 3). However, nonuniform fire damage on opposite sides of the wall and corresponding strength differences can lead to unreliable results. In most cases, strength testing is unnecessary.

ASSESSING THE CONCRETE MASONRY

In general, structural concrete masonry without excessive deformations, deflections, misalignments or large extensive cracks can typically be repaired rather than replaced. When these types of conditions are present, it indicates that the member’s load-carrying capacity may be impaired, which would require removal and replacement of the suspect members.

Fire distress such as soot and smoke deposits, pitting of aggregates, hairline cracks, shallow spalling and other surface damage generally require only cosmetic repairs. The following sections provide some more detailed guidance on assessing various concrete masonry characteristics after a fire.

Cracks

Cracks wider than about 1/16 in. (1.6 mm) should be further investigated to determine the potential structural impact. When the reinforcing steel in conventionally-reinforced masonry has not been exposed, the load-carrying capacity of the wall can typically be assumed to be relatively unaffected (see also Exposed Reinforcement, below).

Mortar Joint Damage

Mortar joints often appear to be more affected by fire exposure than the adjacent surface of the masonry units. When concrete masonry walls are subjected to a fire hose immediately after fire exposure in ASTM E 119 testing, mortar in the dehydrated state is sometimes flushed out, typically to a depth of about 1/4 in. (6.7 mm). In actual fires, mortar joints subjected to the most severe fire exposures can become softened or chalky, although this damage is typically not deeper than about 3/4 in. (19 mm). However, this loss of mortar does not affect the load-carrying ability of the concrete masonry wall (ref. 4), so can most often be adequately repaired by tuckpointing.

Exposed Reinforcement

Reinforcement exposed during or after a fire must be evaluated for quenching, buckling and/or loss of prestress. The investigator must consider that any exposed steel may have been quenched during fire fighting operations. This rapid cooling causes a loss of ductility in the steel that can reduce the load-carrying capacity of the member. A visual inspection of any exposed structural reinforcement can help asses the potential structural damage. This assessment must be tied to the element under consideration: either a conventionally-reinforced wall or prestressed wall, as follows. In a conventionally-reinforced wall, local buckling of exposed reinforcing bars usually indicates that the steel has been directly exposed to fire. When steel is exposed to temperatures of 1,100 o F (593 o C) or higher, the bars lose about half of their yield strength and buckling occurs. If the bars are exposed but not buckled or otherwise deformed, spalling may have occurred after the fire exposure. In general, flexural reinforcement that is not visibly deformed is unlikely to have suffered significant permanent damage. Similarly, if the spalling does not expose the reinforcement, i.e. the cover protection remains intact, the wall strength is unlikely to be compromised.

In prestressed concrete masonry walls, on the other hand, significant loss of prestress can occur without any visible distress to exposed tendons. Therefore, for prestressed masonry, any exposed prestressing tendons should indicate the need for a more in-depth structural evaluation. Tendon buckling is rarely observed, as the tendon typically remains in tension, even with significant loss of prestress.

EFFECT OF FIRE EXPOSURE ON WALL STRENGTH—EXPERIMENTAL RESULTS

One effect of fire exposure, as determined by testing (ref. 4), was reduced wall compressive strength due to the gradual dehydration of the cement and, depending on the aggregate type, to the expansion and changes in the physical properties of the aggregate used in the concrete masonry units. Reductions in compressive strength for 8-in. (203-mm) units exposed to 3 to 3 1/2 hours of fire varied widely, resulting in maximum reductions of 50 percent for some types of concrete masonry units. Lightweight aggregates, manufactured by expanding certain minerals in a kiln, are stable under fire exposure, so minimize loss of strength. During testing, limestone aggregate concrete masonry units also showed substantial stability and minimized loss of strength after fire exposure (ref. 4). For the wide range of masonry units tested, the wall strength after fire exposure remained directly proportional to the concrete masonry unit compressive strength before fire exposure.

A number of 8-in. (203-mm) walls underwent 2 1/2 to 3 1/2 hours of fire exposure, were cooled, then subjected to another 2 1/2 hour fire before being tested for compressive strength. These results showed that these walls were able to carry the same, or slightly higher, loads as similar walls exposed once for three to four hours, as well as serving as an effective fire barrier during the second fire.

PREPAIRING FIRE-EXPOSED CONCRETE MASONRY

For fire-exposed concrete masonry free from large cracks or deflections, repairs should be minimal. Crack repair and mortar joint tuckpointing procedures and recommendations are covered in detail in Maintenance of Concrete Masonry Walls, TEK 08-01A (ref. 5). Recommended cleaning procedures are covered in Cleaning Concrete Masonry, TEK 08-04A (ref. 6).

SUMMARY

  • In conventionally-reinforced concrete masonry, if reinforcing steel is not exposed, there is little likelihood of structural damage.
  • Lintels and beams free from excessive deflections are unlikely to be structurally impaired.
  • Softening of the top surface of mortar results in little loss of load carrying capacity and can be easily repaired by tuckpointing.
  • Walls subjected to fire one time without structural damage can be expected to perform just as well in a second fire.
  • Field tests are typically not conducted to assess fire damaged concrete masonry walls. Post-fire investigation typically consists only of visual inspection.
  • If no severe distortion, cracking or displacement of concrete masonry walls is present, complete reinstatement of the wall can usually be accomplished by patching cracks and tuckpointing mortar joints.

REFERENCES

  1. Assessing the Condition and Repair Alternatives of Fire-Exposed Concrete and Masonry Members. National Codes and Standards Council of the Concrete and Masonry Industries, August, 1994.
  2. Standard Test Methods for Fire Tests of Building Construction and Materials, ASTM E 119-05. ASTM International, 2005.
  3. Evaluating Existing Concrete Masonry Construction, TEK 18-09A. Concrete Masonry & Hardscapes Association, 2003.
  4. Menzel, Carl A. Tests of the Fire Resistance and Strength of Walls of Concrete Masonry Units. Portland Cement Association, January, 1934.
  5. Maintenance of Concrete Masonry Walls, TEK 08-01A. Concrete Masonry & Hardscapes Association, 2004.
  6. Cleaning Concrete Masonry, TEK 08-04A. Concrete Masonry & Hardscapes Association, 2005.