Faster Footings with Fabric Forms by Rick Fearn, President, Fastfoot Industries Ltd.*

Using fabric to form concrete is a proven tech-

nology: concrete-filled fabrics are used to pre-

vent erosion on riverbeds and embankments

and to add cover around existing wharf pilings.

A new application for forming concrete with

fabric makes it possible to build strip or pad

footings similar to those obtained with conven-

tional forms (see Fig. 1). The results equal or

exceed those of conventional forming.

Advantages of Fabric Forming

Fabric is superior to conventional forms (lum-

ber, plastic, metal) for many reasons:

• lighter, less expensive, and installed faster

than lumber or plywood

• adapts to the ground on which it sits,

including handling grade changes easily

(see Figs. 2 and 3)

• can double as a dampproofing

membrane

• saves trees

• kicker boards for wall alignment are part

of the footing forms

Fabric-formed footings are suitable for both

residential and commercial construction. Since

CT002 — July 2000

Vol. 21 No. 2

ContentsFaster Footings with Fabric Forms

Pavement Durability: A Case Study

What’s New in Petrography

Concrete-Related Associations

Designing Durable Concrete

New Literature

Fabric forms offer aneconomical method ofplacing concrete footingsquickly and accurately

CT002

Fig. 1. Fabric forms allow rapid and economical place-ment of footings. (69275)

While fabric-formed footings do differ in shape

from traditional footings, they meet critical depth

and width requirements of building codes in the

U.S.A. and Canada.

Concrete for the Footing

There are no special requirements for the con-

crete used with this system. All mixtures are com-

patible with the fabric. ACI 332, ResidentialConcrete, recommends the following specifica-

tions (based on a maximum coarse aggregate size

of 19 mm [3/4 in.]):

• minimum strength of 17 MPa (2500 psi) for

foundations not exposed to weather

• maximum slump of 150 ± 25 mm (6 ± 1 in.)

• at least 340 kg/m3 (574 lb/cu yd) cementi-

tious material

• if used, air content of 6 ± 1.5% (air is not

required for non-exposed members)

• where sulfates are a concern, maximum

water-cementitious materials ratio = 0.50 for

moderate exposure and w/cm = 0.45 for

severe exposures

Two Systems Available: Strip Footings and Pads

The backbone of this system is a fully adjustable

steel support bracket called a yoke. To form the

strip footings, these brackets hold 2 x 4s (or alu-

minum ledges) in position above the ground. The 2

x 4 brackets slide along the cross bar and are care-

fully positioned, as they will become the kicker

boards for the wall forms (see Fig. 4). Fabric is then

stapled into position on the 2 x 4s and filled with

concrete to form the footing. Although you need a

level surface to place wood forms, the fabric easily

handles bumps and uneven surfaces—saving time.

The strip fabric system has significant advantages

over conventional lumber:

• fast installation – up to 30 m (100 lineal ft) of

footing forms per hour

• accurate – 6-mm (1/4-in.) precision in align-

ing wall forms

• lightweight – on a square area basis, fabric is

300 times lighter than wood

Fig. 2. A wooden bulkheadis used to step the footingat grade changes. (69276)

Fig. 3. Multiple steps infootings are quick andeasy with fabric forms.(69277)

commercial footings are typically larger, the labor and material savings on these

jobs become more significant than on residential projects.

Footing Design

From an engineering standpoint, the critical measurements of a footing are the

contact width at the ground and the footing thickness at the faces of the wall. The

fabric is marked with width adjustment lines. These lines ensure that the desired

footing contact width is

achieved when the fabric is

properly filled. A single width

of fabric allows a footing

height up to 460 mm (18 in.).

Tall footings—up to 1320 mm

(52 in.) high—have also been

built with this system. The

bulbous shape of fabric-formed

footings satisfies shear and

bending moment stresses

across the footing width (see

Fig. 4). The bulbous shape

also provides a built-in safety

C O N C R E T E T E C H N O L O G Y T O D A Y

2 Concrete Technology Today / July 2000

factor: settlement of the footing into the ground increases the footing

contact area.

The shape has another benefit: it encourages drainage away from the joint with

the base of the wall (see Fig. 4). Unlike conventionally formed footings, which are

flat across the top (in cross-section), the fabric-formed footings are sloped away

from the sides of the wall they support , thereby preventing the pooling of water,

and minimizing the potential of leakage inside the foundation.

Concrete Technology Today / July 2000 3

• less labor – no ground leveling or trenching,

no stakes, minimal stripping, no secondary

wall layout

• no concrete leakage, no concrete contami-

nation with the ground

• environmentally friendly – system minimizes

lumber consumption and yokes are reusable

• cheaper – fabric is about 5% of the cost of

wood

Installation Method

The system—ideally suited to forming footings

where accurate wall location is important—is

compatible with all common types of wall forms,

such as insulated concrete forms (ICFs), steel,

aluminum, and plywood. A typical strip footing

installation requires a few easy steps:

Step 1 – Lay out the 2 x 4s

The 2 x 4s are laid out on the ground in position

of the wall forms beginning with the outside

face. Corner brackets join the corners and union

brackets (or 460-mm [18-in.] pieces of 1 x 4)

join the butt ends. Lay the inside 2 x 4s with a

spacer using the outside ones as a guide, again

joining corners and butt joints.

Step 2 – Install the support braces (yokes)

With the legs in the contracted position, lay the

yokes around the perimeter of the foundation at

2- to 3-m (6- to 10-ft) spacings. (Deep footings

increase the load on the fabric and require clos-

er yoke spacing to prevent excess deflection of

the 2 x 4s.) Attach 2 x 4s to the yoke with a 50-

mm (2-in.) nail. Use a laser or builders level to

lift the yoke to the proper grade. Then slide the 2

x 4 hangers along the cross bar until they are

located in the correct position for the wall forms.

Step 3 – Install the fabric

Fabric comes in bundles 37 m (120 ft) long, and

in standard and wide widths, 1575 and 1880

mm (62 and 74 in.). The accordion C-fold

enables one-person installation.

Place a bag bundle on the ground near a corner.

Slit the bag, pull the fabric under the yokes to

the next corner, and cut the fabric at 45 degrees

(see Fig. 5). Continue around the perimeter of

the foundation, using a 150-mm (6-in.) overlap at all joints to prevent concrete

leakage. Nail the centerline of the fabric to the ground to ensure the footing is

centered under the wall. Staple the fabric to the 2 x 4s with a tacker-stapler.

Step 4 – Install steel reinforcement

After the fabric is stapled to the 2 x 4s, suspend individual steel bars from the yokes

using tie wire. For more complex reinforcement, attach only one side of the fabric,

slide the steel into place, then staple the second edge of the fabric to the boards.

Step 5 – Place concrete

Concrete placement into the fabric form is usually done with a pump or from a

chute. Placing concrete in the direction of the fabric overlaps prevents leakage. To

consolidate, give the fabric form a light kick for more fluid mixes or use a spud

vibrator, especially if the footing contains reinforcement. Trowel the concrete to the

bottom of the 2 x 4s to create the base of the walls.

Step 6 – Remove the yokes

If the 2 x 4s are used as kicker boards for wall alignment, nail them to the green

concrete prior to removing the yokes. The wall forms are then placed between

the 2 x 4s.

Pad Footings

Sometimes, pad footings are needed instead of strip footings. Self-supporting fabric

bags are used to form concrete pad footings. These bags are available in sizes from

squares 460 to 1220 mm (18 to 48 in.) as well as custom sizes. Zippered bags

enable placement of steel reinforcement and are available in sizes greater than 760

mm (30 in.).

The bags are labeled to indicate the contact width and height of the formed con-

crete. Proper bag filling will form a concrete footing pad with the correct length,

width, and height. If the bag is underfilled, the height will be less than indicated,

and the contact width will be larger. Conversely, if the bag is overfilled, the contact

width will be less than indicated, and the height will be greater.

Fig. 4. Cross section of footing shows the unique bulbous shape. The rounded toppromotes drainage away from the wall.

C O N C R E T E T E C H N O L O G Y T O D A Y

4 Concrete Technology Today / July 2000

The National Institute of Standards and

Technology (NIST) recently studied premature

deterioration of several concrete pavements in

Iowa. Several previous studies had provided

conflicting opinions as to the cause of the deteri-

oration. The NIST study’s aim was to provide

guidance to prevent similar problems in future

construction.

Pavement Construction

The pavements were non-reinforced, two-lane

roads between 200 mm and 300 mm (8 in. to

12 in.) thick, with dowel-reinforced, skewed

joints. Though the pavements studied were all

constructed in the mid-1980s, they did not share

common material characteristics.

Analysis and Interpretation of Deterioration

Three methods were used to determine the cause

of the deterioration:

• a thorough review of construction records

and field evaluation of the pavements

• a comprehensive petrographic examination

of cores for both distressed and non-dis-

tressed sections of pavements (see Fig. 1)

• computer simulation of various proposed

degradation mechanisms at a microstructur-

al level

The air-void systems in the deteriorated pave-

ments were "marginal to substandard," according

to guidelines in ACI 212.3R, ChemicalAdmixtures for Concrete. The conclusion: the

pavements were victims of freeze-thaw damage

to mortar and aggregates.

Installation of Pad Footings

Place the bag on the ground in the desired location. A screed stake is usually driven

into the center of the bag, both to locate the pad and to provide a screed for the

proposed concrete height. If using a zippered bag, unzip it, place the rebar cage on

chairs inside the bag, and zip up.

To place concrete in the bag, hold up the edge of the bag, and pump or chute the

concrete inside the bag to the desired height. Kick the bag to level the concrete, or

lightly vibrate if reinforcement is present.

Good for both business and the environment, fabric forms for concrete footings

offer a sensible option for concrete contractors.

*Fastfoot Industries Ltd., Unit 212, 6333 148th St., Surrey BC, V3S 3C3, Canada,

phone: 888.303.3278, fax: 604.501.6090, Internet: www.fastfoot.com

Fig. 5. The fabric isquickly installed bylaying it at one cornerand opening the bag,pulling the fabricunder the 2 x 4s.(69278)

Pavement Durability: A Case Study Study reveals causes fordeterioration of Iowaconcrete pavements

Concrete Technology Today /July 2000 5

In addition, the mix design—notably the high

sand content—may have contributed to the

problems. Thorough mixing may be difficult

when concrete contains large amounts of sand.

This makes it harder to develop a good air-void

system, which is generated by the shearing of Serial No. 2224a

Fig. 1. A poor air-void system is responsiblefor deterioration of this pavement. Note thelarge bubble sizes, the clustering of air voidsin certain areas, and the lack of any bubblesin some locations. (68887)

What’s New in Petrography

the paste during mixing. The high sand content may also have led to difficulty in

placing, consolidating, and finishing the pavements, increasing the potential for dry-

ing shrinkage. If cracks form as a result, it becomes easier for moisture to enter the

concrete interior. Accelerated deterioration is usually associated with moisture.

While the research uncovered the probable cause of damage, it also indicated that

alkali-silica reaction (ASR), while present, did not appear to play a major role in the

degradation of these pavements. No evidence of delayed ettringite formation (DEF)

was found.

Conclusions

As a result of this study, two points can be made. First, basic practices are important

for concrete durability. This includes establishing an adequate air-void system through

proper proportioning, thorough mixing, and good construction techniques. Second,

when concrete durability issues arise, petrography is a helpful analytical tool.

Reference

Stutzman, Paul E., Deterioration of Iowa H ighway Concrete Pavement: A PetrographicStudy, IR6399, NIST, www.fire.nist.gov/bfrlpubs, 2000, 84 pages.

Already an established science, the petrograph-

ic (microscopic) study of hardened concrete

and related materials is emerging as an impor-

tant field in concrete construction. By studying

prematurely deteriorated concrete, petrogra-

phers have helped identify what makes con-

crete durable. Recent activities in petrography

include a book and the formation of a society.

A 1998 reference, Concrete Petrography: Ahandbook of investigative techniques, provides

help with every aspect of this science, includ-

ing sample selection, equipment, and test

methods. It is published by Arnold and is avail-

able from PCA as LT226.

Another valuable document, PetrographicMethods of Examining Hardened Concrete: APetrographic Manual, is available at

www.tfhrc.gov/pavement/pccp/petro.htm.

To further the efforts of a growing group of individuals using light optical microscopi-

cal methods to evaluate concrete, The Society of Concrete Petrographers was formed

in February 2000.

By providing a meeting and speaking forum, and even adopting a code of con-

duct, the group hopes to achieve many goals:

• exchange information about methods and techniques of examination

• apply these methods to evaluate concrete and related materials

• maximize information obtainable from concrete

• interpret collected data

• write efficient and effective reports

• increase awareness of an individual’s limitations

• advance petrography as a profession

• establish qualification requirements for allied associations

To find out more about the Society, contact any of the following individuals:

Bernard Erlin, The Erlin Company: 724.539.1800

Mauro Scali, Simpson, Gumpertz & Hager: 781.641.7342

Robert O’Neill, MicroChem Laboratories: 209.728.8200

Derek Cong, W. R. Grace Company: 617.498.4849

C O N C R E T E T E C H N O L O G Y T O D A Y

6 Concrete Technology Today / July 2000

Designing Durable ConcreteDurability tests to consider when working with concrete

Concrete, the most widely used building material, is both durable and economical in a broad

range of environments. Though concrete is a forgiving material, every step of manufacturing

concrete influences its properties—from design to mixing to placing and finishing—perhaps more

than with any other building product.

What Works Now? What Worked in the Past?

Local practice provides a good starting point for mixture designs. Even though many ingredients

have fairly consistent properties, materials must be constantly monitored because there is variation

over time. Ongoing testing of materials and mix designs helps reduce concerns about durability.

Typical factors of mix designs include cement content, water-cementitious material ratio, and per-

meability of hardened concrete. It is also important to assess exposure conditions for things like

sulfate levels in soil and water, and exposure to frost.

Past performance of concrete mixtures is valuable data. It helps to see how materials have per-

formed in similar environments. This could save you time and expense. Unfortunately, this data is

not always available. You should run tests if you are uncertain about a material or potential expo-

sure, but how do you decide which tests to run?

Major Durability Topics

The table on the next page guides designers, concrete contractors, and technicians through con-

crete durability topics. The tests may focus on concrete ingredients, on concrete mixtures, or on

exposure conditions (soil, water).

Virtual Design

Computer programs for designing concrete mixtures are becoming more sophisticated and

widespread. Many of these are accessible through the Internet, often focusing on the "high perfor-

mance" aspect of concrete, where the high performance refers to durability. Some sites of interest:

Partnership for High Performance Concrete Technology http://ciks.dbt.nist.gov/phcpt

CIKS for High Performance Concrete http://ciks.cbt.nist.gov/~bentz/welcome.html

Concrete Optimization Software Tool (COST) http://ciks.dbt.nist.gov/bentz/fhwa

References

1. Effects of Substances on Concrete and Guide to Protective Treatments, PCA IS001

2. Diagnosis and Control of Alkali-Silica Reactions in Concrete, PCA IS413

3. Design and Control of Concrete Mixtures, PCA EB001

4. Manual on Control of Air Content in Concrete, PCA EB116

5. Significance of Test and Properties of Concrete and Concrete-Making Materials, ASTM STP

169C (available from PCA as LT205)

Concrete-RelatedAssociationsTwo new non-profit groups will addressspecialty areas

The Silica FumeAssociation

For decades, silica fume has been

improving concrete structures, adding

strength, and reducing permeability

beyond cement-only mixtures. To

promote the use of silica fume for

infrastructure building and repair, the

Silica Fume Association (SFA) has

been formed. Contact them at:

3927 Old Lee Highway, Suite 100

Fairfax, VA 22030

Phone: 703.934.2937

Fax: 703.934.0279

Internet: www.silicafume.org

American Shotcrete Association

Likewise, shotcrete is a specialty

concrete product with definite

advantages in certain applications.

To promote increased use of shot-

crete, the American Shotcrete

Association (ASA) aims to educate

consumers and practitioners on the

proper installation and benefits of

shotcrete.

1323 Shepard Drive, Suite D

Sterling, VA 20165-4428

Phone: 703.444.9619

Fax: 703.450.0119

Internet: www.shotcrete.org

Acid attack — Cement paste or Low w/cm Depth of attack and No standard test: see Ref. 1 for effectsewers, manholes, (calcareous) Low cement content rate of disintegration of particular acid. If attack is expected,industrial facilities aggregates are Special cements or Ref. 1 run tests with concrete and acid (at

dissolved, matrix aggregates expected strength), or use recommendeddeteriorates Sacrificial cover protective measures

CoatingsAlkali-silica reactivity Alkali hydroxides Addition of pozzolans Change of length of ASTM C 295, Practice for Petrographic(ASR) — any concrete react with reactive or slags mortar bars or Examination of Aggregates for Concretein North America, aggregate to form gel, Blended cements concrete prisms ASTM C 856, Practice for Petrographicmostly exterior, but also, which grows and Low alkali loading (aggregate-cement Examination of Hardened Concretemoist interior exposures expands within the of concrete combinations) ASTM C 1260, Test Method for Potential

concrete, resulting in Mineral composition Alkali Reactivity of Aggregates cracking of aggregate (Mortar-Bar Method)

(aggregate character- ASTM C 1293, Test Method for Concrete istics/behavior) Aggregates by Determination of Refs. 2 and 5 Length Change of Concrete due to

Alkali-Silica ReactionCorrosion (deicer and Concrete spalling Low w/cm (max. 0.40) Permeability ASTM C 1202, Test Method for Electrical seawater) — marine Reinforcement Addition of pozzolans Depth of cover Indication of Concrete’s Ability to Resistenvironments, bridges, corrosion or corrosion inhibitors Refs. 1and 5 Chloride Ion Penetrationand pavements Maintain proper cover AASHTO T 259, Resistance of Concrete

over reinforcement to Chloride Ion PenetrationInspect cover over steel, should meetbuilding codes

D-cracking — any Aggregates take up Decrease aggregate Freeze-thaw testing of ASTM C 666, Test Method for Resistance pavement made with water, then crack particle size or saturated aggregates of Concrete to Rapid Freezing and susceptible aggregate after repeated choose other Monitor length change, Thawingin North America exposure to aggregates mass loss, and

freezing relative dynamic temperatures modulus

Ref. 3Freeze-thaw Concrete matrix Specify and maintain Total air content (fresh ASTM C 173, Test Method for Air Content exposure — freezing cracks or surface recommended air or hardened concrete) of Freshly Mixed Concrete by the exposures, especially scales contents (5% to 8%) Air-void system (of Volumetric Methodwhere deicers are used hardened concrete) ASTM C 231, Test Method for Air Content on structures Freeze-thaw testing of Freshly Mixed Concrete by the (pavements and to monitor length Pressure Methodbridges). Also, cold change, mass loss, ASTM C 457, Practice for Microscopical marine environments and relative dynamic Determination of Air-Void Content and

modulus Parameters of the Air-Void System inRef. 4 Hardened Concrete

ASTM C 666, Test Method for Resistance of Concrete to Rapid Freezing and ThawingASTM C 672, Test Method for Scaling Resistance of Concrete Surfaces Exposed to Deicing Chemicals

Sulfate exposure — Sulfate ions react with Make low permeability, Expansion of concrete ASTM C 1012, Test Method for Length soils in (predominantly) cement paste in an usually by keeping samples in sulfate Change of Hydraulic-Cement Mortars Western U.S. Also, expansive process low w/cm (0.40 to solutions Exposed to Sulfate Solutionseawater and some 0.50) Sulfate level of soils ASTM D 516, Test Methods for Sulfate Ion industrial environments Use sulfate-resistant or water in Water

cement Types II, V, Refs. 1 and 5MS, or HS

Concrete Technology Today / July 2000 7

Durability aspect Effect Typical Measurement Test designationsand potentially prevention criteria*

affected locations methods

What to Look for Regarding Concrete Durability

*Consult local practices, ASTM, ACI, and other standards for limits. Consult Ref. numbers in this column as a starting point.

CONCRETETECHNOLOGY

Today

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Durability of Concrete, LT237This 68-page Transportation Research Circular (No. 494) gives a general

overview of how to make durable concrete. It addresses material selec-

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