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Q ;i Frequently Asked Questions

Slab On Ground

Construction

Answers from the PTI Slab On Ground Committee

QUESTION

When converting a ribbed

slab to a uniform thickness slab, does the ribbed

slab have to satisfy soil bearing stressrequirements

based upon the bearing area of the ribs alone before

the conversion can be made?

ANSWER No, the ribbed slab must

satisfy moment, shear and

diff

erential deflection

criteria before being converted (see Section 6.12),

but it need not satisfy soil bearing stress

requirements that are based upon the rib widths

only. Bearing stress requirements under the

uniform thickness slab must, of course, be satisfied

based upon the full plan area

of

the slab.

QUESTION Why does Section 7.3 of

PTI' s Design nd Construction

of

Post-Tensioned

Slabs-On-Ground

nd

edition, require the use of

encapsulated systems when the sulfate levels in the

soil are considered high?

ANSWER

The requirement for

encapsulation of tendons in elevated soil sulfate

conditions was incorrect. This oversight was

addressed during the PTI Slab-on-Ground seminar

series presented shortly after the publication of the

nd

Edition. Since sulfates are not known to cause

corrosion in steel , the presence

of

elevated levels

of

sulfates in the so il does n

ot

require the use

of

an

encapsulated (aggressive environment) post

tensioning syste

m.

Elevated levels of chlorides in

the soil,

or

environments in which the concrete is

ex

po

sed to direct

or

indirect applications

of

deicing

chemicals, seawater, brackish water, or spray from

these sources, would qualify as an aggressive

environment , requiring the use of encapsulated

systems in post-tensioned slab-on-ground

construction.

Please note that elevated levels

of

sulfates

in

the

soil may require the use of higher strength concrete

and sulfate resistant cements.

October

2 3

Issue No. 4

QUESTION

How much bare stra

nd

is

allowed on a tendon before it needs to be repaired?

What are the acceptable methods and materials that

are to be used when repairs are required?

ANSWER

The purpose

of

the sheathing

material is two-fold. First, it prevents the concrete

from bonding to the prestressing steel so that it can

be tensioned after concrete placement. Second, it

contains the P-T coating (grease) and serves as a

barrier against corrosion.

In an aggressive environment, where the designer

has specified an encapsulated system, the

requirements call for the tendon to be completely

waterti

ght

from end anchor to end anchor. T

hi

s is

strictly interpreted that Q prestressing steel is to be

exposed and that a ll rips, tears, or

gaps

in

the

tendon sheathing are to be repaired using one of the

methods described below.

In a non-aggressive environment, where the

designer has specified the use of a standard system

as is the case with

mo

st slab-on-ground

construction, the requirements are typically not as

stringent as those referenced above for aggressive

environments. T

yp

ically, for standard system

applications, a maximum of 1 inch of exposed

prestressing steel is allowed behind the stressing

end anchors and a maximum of 12 inches is

allowed at the fixed-end anchors.

n

addition, rips

and tears I inch

or

less in length may be l

eft

un

repaired; however, all gaps in the sheathing should

be repaired as described below. In some non

aggressive environment applications, the engineer

may specify more or less stringent requirements

than those noted above.

Rips and tears in the sheathing greater than 1 inch

in length should be repaired according to method 1

or 2 (described below) in order to protect the

strand and prevent excess friction during stressing.

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All gaps (total discontinuities) in the sheathing

should be repaired using only method 1 described

below.

Repair Method 1:

1.

Restore tendon P-T coating in damaged area

if

needed.

2. Place a section

of

split tubing

or

a piece of the

original tendon sheathing over the damaged

area.

It

should overlap approximately 3 inches

(75 mm) past each end of the original tendon s

sheathing.

3. Spirally wrap the entire length

of

the repair area

with tape and extend the tape past the end

of

the

repair section by 3 inches (75 rnm) in both

directions.

Repair Method

2

Taping can be used in place

of

method 1 to repair

small rips and tears in the sheathing where no

significant portion

of the original sheathing is

missing. Spirally wrap a minimum

of

two layers

of

repair tape extending a minimum

of

3 inches (75

mm) past the damaged area in both directions.

3

(7Smm)

3

(75mm)

Acceptable Material

PTI s

pecification or Unbonded ingle trand

Tendons states that the tape used should:

be self-adhesive and moisture proof

be non-reactive with sheathing, P-T

coating, and prestressing steel

have elastic properties

have a minimum width

of

2 inches

have a contrast ing color to the tendon

sheathing.

Tests run by PTI have shown that various materials,

including

PVC

pipe

wrap

tape, waterproof duct

tape, and other

PVC

and polyethylene tapes, meet

these requirements and perform as required. The

testing revealed that the most important factor in

the performance

of

any tape repair is that the

application of the tape is smooth and that seams,

joints, and tape wrinkles are kept to a minimum.

Testing showed that tapes wider than 3 inches were

very difficult to properly apply without wrinkles

and still maintain the watertight seal that is

required.

3

SPRI L WR PPED

T PE

3

(7Smm)

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2003 BY POST-TENSIONING INSTITUTE

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d i

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llaiiiNSTITUT

requently Asked

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October 2003

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