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Understanding Fabric Formwork

The use of fabric to form concrete is a very new technology. In fact, one of the first articles published on this method appeared in the October 1998 issue of “Concrete Facts”. Since that article appeared, considerable progress has been made, and fabric forming is finding various economic applications throughout North America.

This article will examine fabric forming, the progress made over the last two years, where fabric has an economic advantage, and what future it’s future will be.

Understanding Fabric Forming

Rigid materials such as dimensional lumber, plywood, or aluminum are conventionally used to restrain the fluid forces of concrete. In order to restrain these forces, the form will have tension forces on one surface, compressive forces on the other surface, and a neutral plane between the two surfaces.

Fabric, however, can only operate in tension and as a result can be an extremely efficient method of forming concrete. Fabric weighs approximately 1/300th that of dimensional lumber. However, in the world of concrete, nothing is “free”. Pressure is pressure, and pressure in fabric results in bulging or deflection. This means that fabric can only be used where the bulging is not detrimental to the finished concrete.

Using Fabric to Form Footings

To use fabric to form footings, the two edges of the fabric are supported with 2x4s or #5 rebar using a bracing system (called Fast-Yokes), and the concrete is placed into the fabric. In this case, the bulging of the fabric is not a disadvantage, and indeed the ability of the fabric to deform to the uneven ground becomes an advantage compared with rigid forming materials. The diagram below shows the shape the footing fabric takes when under the concrete pressure.

Advantages of Fabric in Forming Footings

  • Less weight – fabric weights approximately 1/300th that of a conventional rigid form
  • Less expensive – fabric is approximately 1/25th the cost of dimensional lumber. This may make the use of fabric a viable option, especially when the cost of moving the lumber from job to job is taken into consideration
  • Stay in place form – As the fabric is left in place, the cost of stripping the forming lumber is eliminated.
  • Waterproofing membrane – the fabric prevents the wicking of water up into the footing (testing of this factor has not been completed).
  • Adaptable to uneven ground conditions – the weight of the concrete on the fabric ensures that the footing forms conform perfectly with uneven ground conditions. This makes the system ideal for use on uneven ground.
  • Improves the quality of concrete – in the summer, the fabric prevents the rapid drying of the concrete, and in the winter, the fabric prevents the contamination of the concrete with mud and excess water.
  • Stakeless system – as the concrete pressures are held by the fabric and the Fast-Yokes, there is no requirement for stakes to hold the concrete pressure. Small stakes are only required to locate the Fast-Yokes.

Where is Fabric Forming of Footings Competitive

  • Deeper footings – With deeper footings, the cost of lumber and labor to install and strip the forms increases significantly. With fabric, the weight of the fabric and the labor to install remain constant, regardless of the footing height. Footing heights of up to 30″ are achieved with fabric.
  • Uneven ground – Using lumber to conform to uneven ground is expensive, both from a lumber and labor standpoint. fabric adapts perfectly to uneven ground with minimal labor and inexpensive fabric. As well, the Fast-Yokes are easily adjusted both vertically and horizontally.
  • Rocky ground – Driving stakes in rocky ground is extremely difficult, and sometimes impossible. Fastfoot is a stakeless system, and in rocky conditions can save thousands of dollars.
  • Accuracy – The Fast-Yokes are easily adjusted and enable a high degree of footing accuracy.
  • Curved footings – #5 or # rebar is bent to the desired radius, and placed in the Fast-Yokes. The Fastfabric is attached to the rebar by wrapping around the bar, and stitching with a 3″ nail. Fabric forming is about 1/3 the cost of conventional methods.
  • Footing pads – Fastbags are an economic solution to forming footing pads, as they replace short lengths of lumber which are normally wasted on the jobsite

Where Fabric Forming of Footings is NOT Competitive

  • Shallow footings with level excavations – The contractor will not experience significant savings if his footings are shallow, and he can reuse his 2x4s or 2x6s on the jobsite. Form-A-Drain is an excellent product for this situation.
  • Trenched footings – Fabric forming is not suitable in this situation.
  • Production forming – If a contractor has already invested in conventional footing forms, he may be reluctant to invest in the Fast-Yoke bracing system.

The Future of Fabric Forming

The following items provide some indication as to the future on fabric forming in the concrete industry:

  • Carbon reinforced fabric – carbon fibers woven into the fabric will eliminate the need for steel reinforcing in the concrete. As the strength of the reinforcing is directly proportional to the square of the distance from the neutral plane, the fabric becomes an excellent location for the concrete reinforcement.
  • Fastfabric DPM – Using the fabric as a damp proof membrane (DPM) that ‘blankets’ the footing and prevents water from wicking into the concrete will become an important feature of footing forming design in the future; this will allow builders to construct drier, healthier concrete structures and interior spaces.
  • Fabwall – This wall forming system uses a waterproof fabric on the outside, and rigid insulation on the inside. A drop stitch in the fabric acts as the tie. This fabric based wall form has significant advantages:
  1. The thickness of EPS or XPS is determined by insulation requirements, not concrete pressure (i.e., the tie spacing can be reduced to accommodate concrete pressure)
  2. The form can be shipped flat, minimizing storage and shipping costs
  3. The form is pre-manufactured – no labor is needed to install ties on the job site
  4. The form panel size is not limited (in sizes up to 10′ high and 4′ wide), further reducing labor costs
  5. No stripping, as the forms are left in place
  6. No damp proofing, as the fabric is a damp proofing membrane.

 

 

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