Building beyond the basement
By ROSS MONSOURCommercial Construction Institutional Residential
The use of insulating concrete forms has evolved to address many of today’s construction challenges and ambitions.
Around for more than 40 years, Insulating Concrete Forms (ICFs) represent a simple solution to achieve many government- and industry-led climate or environmental goals. Among their benefits are air tightness, labour savings, energy efficiency, resiliency, low carbon intensity, and more.
Most ICF systems can be described as two sections of expanded polystyrene insulation connected by a series of plastic ties. These may arrive on a jobsite in block form, or they may be assembled on site. Blocks have interlocking joints on their edges to connect them together to form the structure. Steel rebar is placed horizontally on the webs and then concrete is cast in the form to create a structural wall. The plastic webs are embedded in the expanded polystyrene insulation and create flanges that the contractor can use to attach exterior and interior finishes. Many ICF companies have specially designed forms for brick ledges and corners.
The initial introduction of ICF focussed mainly on residential basements. This was due, in part, to engineering restrictions imposed by building codes. Gradually, the market moved to above-grade residential and commercial construction. As concrete pumping has allowed the ICF universe to expand, today’s ICF industry also serves high-rise buildings, schools, commercial buildings and retirement homes.
In the Kitchener-Waterloo region of Ontario, more than 80 eight-storey buildings have been constructed over the past 20 years. The main driver for many of these builds was time. For example, the use of ICF allowed student housing to be constructed and occupied before the next school year. This advantage allowed the owners considerate improvement in obtaining fully occupied residences in a short amount of time.
Another driver is winter construction. Once the ICF shell is constructed, winter heating is often not necessary as the insulated thermal mass often provides sufficient heat to continue the indoor work to complete the building.
The latest edition of the National Building Code (NBC) 2020 has recognized the national standard for ICFs: “ULC S717.1 Standard for Flat Wall Insulating Concrete Form (ICF) Units – Material Properties.” Other relatively recent changes to the NBC allow for interior and exterior attachment to the plastic webs. Tables have been created to outline the type of fasteners and under which wind and seismic conditions they can be used.
Another significant change is that under Part 9, ICF builds will be allowed up to two storeys without additional engineering. Also revised over the past little while is the allowance for the use of 10M rebar rather than 15M, which is lighter and more manageable to use on site.
To support these changes, the Insulating Concrete Form Manufacturers Association (ICFMA) has created a free document, available on its website, that explains the prescriptive designs for residential buildings, and is stamped for all the provinces. All commercial and industrial ICFs must still be designed to Part 4 of the NBC. These requirements are dictated by the “CAN/CSA A23.3 Design of Concrete Structures.”
Tall and thin
Reinforced concrete is one of the strongest structural systems for walls, its used in wall that spans vertically between lateral supports allows for a thinner design. Depending on the project design requirements, some multi-storey buildings up to five storeys have been constructed with a 6” (150 mm) concrete core ICF. Other projects, such as box big stores with walls up to 32-foot (9.75 m) in height have been constructed using an 8” (200 mm) ICF. These ICF walls have load bearing capacities that can support any type of floor or roof system.
For large, multi-storey buildings, ICF walls may be designed as shear walls, elevator shafts and interior load-bearing walls. There is a simplicity with the design and installation of an ICF wall system – the insulating form, the reinforcement, and the concrete working together as one in the structural building envelope. Designs can exceed 20 storeys and beyond.
Concrete contractors should treat ICF construction as typical concrete wall construction for both commercial and residential applications. Vibration, in order to achieve consolidation, is a requirement.
Curing is achieved by the nature of the insulating properties of the ICF form. In cold weather the only concern is the exposed top of the pour, which can be protected by thermal blankets or speciality products recommended by the manufacturer. Since the concrete is never exposed to the weather, it retains a higher moisture content. This slow cure results in a high early strength and longevity. By not being exposed to the weather and having the concrete insulated, the requirement for expansion or control joints is eliminated on most walls.
The responsibility for the concrete mix design should remain with the ready-mix producer. Most producers have a concrete mix design for ICF walls, but they may have to be adjusted due to weather or site conditions. This should always be done in conjunction with your producer.
Concrete specifications for ICFs have two main requirements that are important to the placement and flow of concrete within the forms. The recommended concrete slump for commercial work should be a minimum of 5” to 6” (102 mm to 152 mm), subject to climate and engineering design. This higher slump allows the concrete to flow easily within the forms and prevents damage to the cross-ties during placement. Slump verification by testing upon delivery is recommended. In some instances, higher concentrations of reinforcement may dictate higher slump than this minimum.
Furthermore, a smaller aggregate size is recommended to alleviate congestion in the walls between the reinforcement bars and the inside face of the ICF. A smaller aggregate also improves the flow rate within the forms. The recommended aggregate size for a 6” (150 mm) ICF core size is either 3/8” to 1/2” (9.5 to 12.5 mm). For larger ICF forms with 8” (200 mm) and higher core sizes, 1/2” to 3/4” (12.5 to 19 mm) aggregate may be used.
Concrete placement is typically done from a pump truck or line pump with the placement rate controlled and regulated by multiple lift heights, minimizing the hydrostatic pressure within the forms. All ICF wall applications are required to be consolidated by an internal vibrator. Typically, concrete is placed and consolidated in the ICF walls from floor level to floor level as a continuous pour in four-foot (1,220 mm) lifts.
In today’s world, low-carbon concrete and embodied energy have become a major part of design criteria, and ICFs can augment the industry’s efforts toward producing high-performing buildings.
Concrete mix designs have improved the reduction in carbon production by making use of supplemental cement materials such as fly ash and slag – waste materials that provide additional qualities to the long-term performance of the concrete. Chemical admixtures such as water reducers and strengtheners may reduce the need for additional cement, which also lowers the carbon footprint of a project. And cement producers are making use of PLC cements, which lower the carbon footprint of the end product.
ICFs take these performance characteristics and add an airtight building envelope, with higher insulation values and thermal mass performance to create a solid base for the construction of Net Zero buildings going forward. And the sector is not standing still. Exciting developments exist, such as the introduction of fibreglass reinforced rebar and polypropylene fibres as replacement for rebar.
Ross Monsour is the director of ICFMA. For additional information on ICF technology, visit www.icf-ma.org.
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