7/26/2019 FAQ no. 3 for precast

1/2

FrequentlyAsked

u e s t i ~ n s

Slab on Ground Construction

nswers from the PT

Slab

on Ground Committee

QU STION

Is the placement

of

a

vapor

retarder

recommended beneath

a post-tensioned sl

ab

-on

ground fo

und

ation and , if

it

is, should it be placed on

top of or

below

the le

ve

ling sand?

AN

sw

There have b

ee

n

seve

ral

.

J; a atticles ' published

con

cerning the u

se

of vapor retarders

o

ften in

co

rrectly

referred to as vapor barriers), with a ran

ge of

con

clusions

fr

om absolutely no to ''yes, without excep

tion

,

a

nd

diff

erent

opinions

as to

it

s pl

ace

me

nt

on t

op

of or

below the l

eve

ling sa

nd

. There is no clear-cut

answer to this often-asked question.

This is not a question that has a definitive

co

nsensus

rec

om

mendation from regulatoty and/ or advisory or-

ganizations. The International Residential Code (IRC)

2000 (with exceptions) requir

es

a vapor retarder

placed between the concrete floor slab

and

the base

cou

rse

or the prepared

su bg

rade where

no

base course

exists.

(R506.2.3).

The

u

se

of

vapor

retarders

is

also

addressed in

seve

ral ACI

co

mmittee repotts (including

co

mmittees 302, 311, 332 and 360).

ACI

has modifi

ed

it

s p

os

ition, previously expressed in 302.1R-96, Section

4.

1.

5, that vapor retarders be placed under granular

fill , pointing out

prob

lems that have occurred with such

placement and reco

nun

ending instead that each pro

po

sed installation be independently evaluated based

upon project

co

nditions and the potential effects

of

sl

ab

curling a

nd

cr

ac

king

(Concrete International,

April

2001, p. 72-73). ACI Conunitt

ee

302 is revis

in

g 302.

1R

-

96, a

ddr

essing the various facto rs that may affect va

por

retarder placement,

but th

e current

draf

t of this

document (March , 2001) makes no definitive recom

mendation as to a

un

iversal place

ment

; instead, it pro

vides guidance for designers based on the specifics of

the

slab

under

co

nsi

de r

ation. In

li

ght

of

the above,

com

ments

wer

e

so

lic

ited

from PTI Slab-on-Ground

committee members, as we

ll

as consultants and asso

ciates of these

co

mmittee me

mb

ers.

Of the

respons

es

r

ece

ived, the clear majority reconunended the use

of

a

vapor

retarder

an

d its placement directly ben

ea

th

the

concrete .

Uniformly, the consistent positive comment was the

benefit that the vapor retarder provided in minimizing

July

2

1

Issue No . 3

vapor

transmission through the

co

ncrete. The place

ment

of the

vapor

retard

er

on

top of the

leveling sand

also provided a better base f

or the

supp01t

of

the ten

don

s a

nd

eliminated the possibility for

fi

eld problems

that occur

when

the leveling sa nd is

on

top. When the

sand

is on top of

the vapor

retarder, displace m

en

t of

the sa

nd

during

co

ncrete placement can result in re

du

ced slab thickness and/

or

beam properties, mixing

of

sa

nd

with

co

ncrete , and an

uneven

unders

id

e sur

face of the concrete slab, increasing the effec ts of sub

grade

fr

ic

ti

on.

Uniformly, the

co

nsiste

nt

negative

comm

ent was that

the

vapor

re t

arde

r cau

se

d the retention

of

moisture

in

the bottom

of

the slab, allowing the top exposed to

the air) to cure differentially. This cau

sed

the sl

ab

edges

to curl and, in some cases, shrinkage cracks to form.

Curling

and

cracking

co

uld

be

minimized by placing

the sa

nd

la

ye

r on t

op

of the

vapo

r retarder;

ho

wever,

this causes greater concerns f

or

the performance

of

the

foundation, as list

ed

above. As recommended

in

Sec

ti

on 4.3

of

PTI's

''Constructi

on and

Maintenance Pro

ce

dur

es

anual for

Post-Tensioned Slab-on-Ground

Construction,

2 < Edition, cutting

the

vapor re tarder in

the bottom of the ribs w ill aid in

water eg

ress from

the bottom of the concrete during curing. The major

concern f

or

the placement of

the

sa

nd

la

ye

r

on top of

the vapor barrier is

th

e displacement of the sa

nd

dur

ing

co

ncrete placement.

Based

on co

mments that were received, the consen

sus op

inion of

spec

ia

li

sts in the

de

sign and

co

ns

tm

c-

tion

of

pos t-tensioned slabs-on-gro

und

is that a vapor

reta rder be placed

beneath

a

ll

post-t

ens

ioned slab-on

ground

fo

und

ations used for residential applications

and that the

vapor

retarder be placed on top

of

the

l

eveling

sand. D

es

ig

ne r

s should evaluate each instal

lati

on on

a case-by-

case

basis a

nd

make their

own

de

cisions

about

vapor retarder use a

nd

placement as they

see

fit, based

upon the

information available to them

and the conditions and history that exist in their geo

graphic area.

For industrial floors

and

special-use foundations,

the

use of a

vapor

retarder may reduce slab subgrade

fric

tion;

how

ever,

th

e nega tive effec t

of

slab curling

du

e

to differential curing rates must be anticipated.

7/26/2019 FAQ no. 3 for precast

2/2

One

additional factor that may

be of

special note is

that in areas with l

ow

humidity and l

ow

annual rain

fall ,

vapor

retarders may be eliminated and in some

cases may not be all

owed

by local regulato1y

age

n

cies. In

th

ese cases, a granular fill

sho

uld

be

placed

beneath

th

e slab

to

provide a barrier against capilla1y

ac

ti

on. If

th

e va p

or

retarder is eliminated , the subgrade

should

be

pre-wetted

so

that it does not accelerate the

reduction of water from

th

e

bottom

surface of the con

crete during curing.

Co

u r

ete Cou

stm l o

u Magazlue.

Ma rch 1999. pp 2.3-31: Jul y 2000. pp ~ . 3 ~ 5 : Fehruary

1001.

p

178.

QUESTION: ~ ~ ~ t n ~ t ~

step in a fo

und

ati

on

where a continuous tendon can

be used? Figures 12 and 13

of

the Constructi

on

and

Maintenance

Ma

nual currently show this as 6 in

ches

.

A

W\. Architectural

and

existing

.

iJ I ft

grade elevations often re

quire the u

se of

steps in foundations that a re greater

than 6 inches. The use of disco

ntinuous

tendons at

these st

epped

loca tions can present several construc

tion difficultie

s:

The anchors cannot be attached

to

the floating form

since this form is

removed during

the pour once the

concrete has r

eac

hed its initial set and the vertical

face can

be

held. f the anchors are attached to the

form, they will be dislodged from

th

e

concrete

wh

en

the form is removed .

There is not

su ffi

cie

nt

suppleme

nt

al reinforcing

to

which

th

e fixed-end an

chorage can

be tied in lie u

of attaching

th

em

to the

form) in o rder

to

prevent

dislocation during the pour.

~ POST-TENSIONING

~

INSTITUTE

In residential applications, providing discontinuous

tendons would result in ve1y shmt tendons These are

difficult

to

stress and often result in l

ow

effective forces.

The committ

ee

is now aware of much successful ex

pe rience with continuous tendons through vertica l slab

steps of up

to

and, in

so

me instances, greater

than

12

inches. Based upon this successful experience , the

committee recommends a modification to Figures 12

and

13

in

the

Construction

and

Maintenance Manual

increasing the 6-inch ver

ti

cal step dimension to 12

inches.

Supplemental reinforcing may be added by the engi

neer

to

prevent spalling of the up per edge and

to

pro

vide continuity through the stepped transition.

Th

e transition ratio of step heig

ht to

transition length

shalf

be

a minimum

of

1:6 The tendon should be

placed with a straig

ht

profile through

the

stepped area

to avoid pullouts caused by

up

ward tendon forces ex

erted at the step. Minin1llm

cove

r should always be

provided for

th

e tendon through the transition.

requently

Asked Questions

JULY 2001

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