Resources – Never Stop Learning

This summer, ACI has released the latest technical resource produced by this committee and now available for your business, ACI 332.1R-181 ‘Guide to Residential Cast-in-Place Concrete Construction’. This column discusses this latest technical document.

The American Concrete Institute has released the latest technical resource, ACI 332.1R-181 ‘Guide to Residential Cast-in-Place Concrete Construction’.
The American Concrete Institute has released the latest technical resource, ACI 332.1R-181 ‘Guide to Residential Cast-in-Place Concrete Construction’.
American Concrete Institute

Committee 332 has been a strong advocate and development force for improving the technical guidance and setting many appropriate standards for the residential industry. The first version of the Guide was published in 1984, was re-approved in 1999, updated in 2006 and re-approved in 2012. This 2018 version represents a significant update pairing with many improvements in the committee’s other major document for residential code requirements. Residential Code Requirements for Structural Concrete (ACI 3322) and Commentary was first released in 2004 with subsequent versions in 2008, 2010 and 2014 with the next version due to publish in 2019.  Together, these documents work hand in hand to identify and shape the characteristics for design and construction of quality cast-in-place residential concrete considered for foundations, slabs, above-grade housing and more.

The evolution of these documents is important to identify along with the value of the latest resource itself.  This is particularly true since the code requirements document was issued as the Guide document transitioned to a broader interpretation and practical implementation tool. In addition, both of these documents are part of the technical resources on which the ACI/CFA Residential Foundation Technician Certification was founded. Therefore, understanding some of the more significant areas of influence and update will help you prepare for certification as well as knowing what support exists for key technical issues you may face.

Backfilling

Among the topics I’ve covered in this column, backfilling and excavating come up time and time again, whether it is related to safety, the designed performance of a wall or the achieved quality. Backfilling is one operation that frequently places new residential foundations at risk for problems including cracking, water penetration and structural failure. Section 5.8 of the 332.1R-18 guide document increases the emphasis on recognizing the intended or assumed design for a residential foundation wall and how they must be treated for contractors and builders. The document states:

“…the prescriptive designs [of ACI 332] for residential foundation walls assume a fully supported condition along the top and bottom. To be considered fully supported at the bottom, the floor slab should be a) in place and poured directly against the concrete wall, b) connected to the footing by keyway, or c) connected by dowels.”

Mentioned here are important acceptable detailing methods for achieving the support conditions.  Section 4.3.6 provides a discussion of the code allowance for dowels to be wet set or pushed into the fresh concrete unless it is specifically prohibited by a local code or the project documents. Likewise, according to the code, these dowels can be preset in their location by driving them into the grade prior to concrete placement.  Also found in section 4.3.6 is a discussion of the minimum spacing, the depth of extension into the footing and the minimum height of extension into the connected wall as required by the code.

Section 5.8 continues the discussion for wall support stating the top condition “assumes that the floor framing and sub-floor sheathing are in place.” This is not new to our discussion (see Rationalizing the Standard Hook in a Residential Footing, December 2016) but one might ask the question, why does this matter?  Foundation walls are designed like a beam and described as simply-supported members, hence the positive connection at the top and bottom of the member. The pressure against the wall on only one side is the primary force it must resist as rarely is the weight of the house significant enough to consider in the design. As this section then states:

“Walls that are backfilled without bracing or the floor framing in place are vulnerable to cracking, horizontal displacement, or bowing.”

This is a big factor in the construction of foundations to deliver a quality wall, built to the intended design and specifications that has the capacity to perform outliving the above-grade structure of most residences.  Therefore, backfilling is a key area of knowledge for certification.

Pourlines or Cracks

Some information found in the Guide is intended to support the contractor in discussions with customers when facing concerns of performance. Among the most often visited topics is wall aesthetics and a common feature labeled a “pourline”. These are created when a delay occurs between concrete truck placements and the contractor is not able to sufficiently vibrate through the earlier placement. The owner or customer will often challenge this as a crack and a structural weakness in the wall. Section 5.5.10.3 states:

“Pourlines are not cracks and do not result in propagation behavior nor do they present structural weakness in the wall, provided the surface of the concrete did not reach initial set before the next placement.”

This section serves as advocacy for the contractor as well as a statement of practical application and caution for best practice…that is, be sure to not let the concrete reach initial set between load placements.  The pourlines are most often very easily distinguished when the wall forms are removed as they appear nearly horizontal or at very slight angles, shaped to the natural leveling action of the concrete flow (see Visible Pour Lines – Are They A Problem?, May 2014). 

In addition to the discussion of pourlines, 332.1R-18 also provides the reader with some recommendations on repairs for visible defects, indicating what are perhaps “normal” and what are features to be corrected.  In section 5.5.10 crack widths less than 0.125 in. (3mm) are considered “normal” or acceptable as well as surface air pockets less than 0.75 in. (19mm). Beyond these conditions, the contractor should take care to patch or remedy these visible defects for the best customer relation and quality performance.

Freezing weather

A condition faced by most every contractor building foundations in regions north of 40˚ latitude and many more with experience above 35˚ latitude is freezing weather conditions. This affects water in the concrete mix and slows the rate of curing and is also another of the most significant concerns for customers. Experienced contractors know the behavior of concrete and the variation in curing performance between a residential foundation wall and a residential slab-on-ground.  ACI 3063 is a very broad guidance document that presents a multitude of methods for protecting all concrete applications for proper curing and strength gain.  However, it is important for the residential contractor to identify with the specific information effective for the two primary applications they face, walls and slabs. Therefore, to identify the most practical and effective methods to implement, 332.1R-18 states:

“Protective measures include the addition of blankets or protective sheets, auxiliary heat, tents, admixtures, increased cement content, use of high-early strength cement (Types III or HE), and higher concrete delivery temperatures.”

The “why” for this issue is that it is imperative for the concrete to reach 500psi before freezing one time.  If achieved, the durability of the concrete will not suffer the freezing and the contractor is then cautioned to maintain loading caution until they are certain the in-place concrete has reached sufficient strength to meet the requirements. For more information see Understanding Concrete Maturity for Curing Cold Weather, December 2014 or Tips for Pouring Concrete Foundations in Cold Weather, December 2013.  It is also important to know that 332.1R-18 provides the guidance for form removal related to this same 500psi concrete strength. It is at this early-age that can be considered self-supporting. The guide provides:

“Supporting forms, braces, and shoring should not be removed from wall forms until adequate strength is attained.  Depending on climate conditions, curing procedures and the mixture proportions, the concrete should develop a minimum compressive strength of 500psi, which is typically sufficient for the wall to sustain its own weight in as little as a few hours after initial set.  It is recommended, however, that the forms be left in place for a minimum of 12 hours.”

This is not the time restriction for when the wall can support the real loads, that of the backfill and the vertical load of the residential structure, but it is when the wall will not be structurally impacted by form removal or at risk without the forms in place.

Consolidation

The topics of pourlines and surface air pockets are part of a larger construction issue for quality concrete referred to as consolidation. This is the process of densifying the concrete by putting energy into the flowable concrete mix to ensure it completely fills all spaces of the formed condition, sufficiently encompasses the reinforcement and embeds and moves entrapped air vertically through the mix to the top surface so that formed surfaces are as smooth, detailed or consistent as intended. The guide offers that the use of mechanical vibrators is the most common method for consolidation. However, it also recognizes that it is also common for contractors to “rod” the concrete with a two-by-four piece of lumber or a steel reinforcing rod.

Tolerances

Conflict over the final tolerance of a structural element is not uncommon to residential concrete. Wall forms are erected according to the designed specification with great precision using laser stations. They are then filled with concrete and the forms removed 24 hours or so later revealing the hardened (or hardening) concrete wall. Movement in the forms, shrinkage from curing and other factors including the accuracy of the forming, shoring and bracing process can lead to conflicting dimensions and geometric relationships of walls and corners to the intended design.  However, framing contractors may be working on a concrete foundation with relatively exacting tolerances and end up with framed features that do not match the designed specifications. These conflicts are managed through guidance on acceptable tolerances. ACI 1174 is the general tolerance specification document from ACI referenced in codes and project drawings. As a guide, 332.1R-18 interprets and provides further application of these tolerances specific to the common features of residential concrete projects and thoroughly describes how most out-of-tolerance conditions are not indications of structural deficiency or failure and should be handled amicably for moving forward as an entire team. It is therefore important to know these tolerances exist in Table 5.5.1.1 and that they represent indications for when contractors should consider ways to work with the customer and/or builder to remedy conditions that may present definite challenges to the continued construction process. For example, table 5.5.1.1 provides for a foundation wall supporting non-masonry framing, there is an acceptable deviation of +/- 2 in. (50mm) for the edge of the wall from its designed location (see Out of Plumb May Not Mean Damaged and Unsafe, August 2015). Tolerances are also provided in section 4.5 for footings, which can be another source of anxiety in project conversations (see Acceptable Tolerances for Residential Footings, August 2017).

Material specifications

Guide documents also put in plain language many of the minimum requirements of codes such that the contractor can check off a series of details for the project. ACI 332.1R-18 discusses the minimum concrete strength of 2,500 psi found in the 332 code document and then relates this to the maintenance of strength versus slump (section 4.3.7.1 and 4.3.7.2). The guide also delivers the primary specifications for steel reinforcement.  Although it does further delineate why many of the walls found in the prescriptive code do not require structural steel reinforcement, it is present at all times for shrinkage and the guide helps rationalize its presence, the minimum type of steel (Grade 40), the common sizes (No. 4 & No. 5) and what to do when reinforcement bars overlap. It is a misconception to assume that wire-tying steel bars together where they cross or overlap is sufficient.  The guide describes the code application of “thirty bar diameters (30db) or 24 in. (610mm), whichever is greater”, in order to maintain the integrity of the intended presence.

In the 2014 edition of the 332 code, a new section on durability and exposure was included.  To residential concrete contractors, the reference to RF, RC and RS for exposure categories dealing with Freezing, Corrosion and Sulfate may seem foreign. Therefore, the guide discusses at length the determination of these exposure categories and the way mix designs should reflect the application of each particular category to the project.

Testing

The last featured item of the guide we will discuss is that of testing. During Concrete Foundations Convention 2018 in Utah this past July, Dr. Kim Basham spoke to attendees on the importance of properly taken, stored and controlled cylinders and more importantly, what happens when a bad break is returned.  Section 3.7 of 332.1R-18 delivers the summary information that all contractors should know when determining effective strengths through cylinder testing. It is this information that can make or break (figuratively) the tense discussion that comes from a statement of understrength concrete, though the reality is few residential concrete projects rely on testing cylinders.

CFA is committed to creating greater awareness and better education on the technical aspects of the concrete industry.  Rather than making assumptions on detailing or relying on the concrete producer alone to make decisions for sufficient concrete design, contractors need to recognize the resources available to them and spend enough time to understand them. This is a primary reason for the creation of the ACI/CFA Residential Foundation Technician Certification, to demonstrate and verify command of these technical resources for use in projects. This certification is also the first step toward the ability of a company to achieve CFA Certified Foundation Company status.

Want to know more?  Contact CFA Executive Director, Jim Baty at 866-232-9255 or by email at jbaty@cfawalls.org. The Concrete Foundations Association mission is to support the cast-in-place contractor as the voice and recognized authority for the residential concrete industry.

References:

1 Guide to Residential Concrete Construction (ACI 332.1R-18) published by the American Concrete Institute, 38800 Country Club Drive, Farmington Hills, MI 48331 | Phone: 248-848-3700 | www.concrete.org

2 Residential Code Requirements for Structural Concrete (ACI 332-14) and Commentary published by the American Concrete Institute, 38800 Country Club Drive, Farmington Hills, MI 48331 | Phone: 248-848-3700 | www.concrete.org

3 Guide to Cold Weather Concreting (ACI 306R-16) and Commentary published by the American Concrete Institute, 38800 Country Club Drive, Farmington Hills, MI 48331 | Phone: 248-848-3700 | www.concrete.org

4  Specification for Tolerances for Concrete Construction and Materials (ACI 117-10) and Commentary published by the American Concrete Institute, 38800 Country Club Drive, Farmington Hills, MI 48331 | Phone: 248-848-3700 | www.concrete.org


 


Latest