ROSTURI PLATFORME INDUSTRIALE

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M

ost owners would like to eliminate or con-t rol cracking in any industrial floor slab they build, reduce the number of joints in the s l a b, and get a superior long-lasting conc rete floor. These desirable results can be achieve dthrough three separate but highly compatible factors:

expansive cement

deformed welded wire fabric a new prog ram of certified quality control

Using these three components to produce a better

floor begins in the planning/design phase of a pro j e c tand continues with close attention to detail to final acceptance by the ow n e r. A recently completed industrial floor illustrates how the three factors work together.

The project was a 384 000-square - f o o t( 1 )* ware house addition to an existing plant in Willows, California .Loadings in the range of 600 to 800 pounds per quare f o o t ,( 2 )mainly by forklift trucks, we reexpected. To handle these rolling loads high perf o rmance standards and narrow tolerances were set for the 6-inch-thick ( 3 )floor in t e rms of smoothness (18 inch in 10 feet), (4) eedom from cracks, a minimum of joints, and a long-term, virt u a l l y m a i n t e n a n c e - f ree life.

Preplanning to help achieve those standards began

with the ow n e rs stipulation that the warehouse stru ct u re it self be erected before floor slab construction. This would avoid possible damage to the newly cast concre t e f rom the heavy erection equipment moving over it and p rovide optimum curing conditions away from dire c t sunlight and hot winds.

Expansive cement concrete specified

Since the owner wanted a virtually cra c k - f ree floor, the specification of shrinkage-compensating concrete was almost obv i o u s. Such concrete is made with expansive cement. How the expansive cement functions in shri n k age-compensating concrete to offset shrinkage and m i n i m i ze cracking has been increasingly documented in recent ye a r s . He res how Type K expansive cement (an ACI designation) work s.

As concrete made with expansive cement begins to set, it bonds to the steel

re i n f o r cement embedded in it.

At the same time, the expansive reaction causes a con-t rolled vo l u m e t ric expansion of the concre t e. The concrete now is bonded to the steel, so this expansion will put the steel in tension and the concrete in compression. Thus, the concrete is pres t ressed, but at a much

l o

wer magnitude than that of conventional pr

e s t re s s i n g .

The expansive reaction is complete in the first few

days of concrete curing. Later, when the concrete is ex-

posed to drying conditions, it will shrink. But unlike

c o n v

entional portland cement concr

e t e

, this shr

i n k a g e

m e r

ely r

e l i e v

es the slight pr

e c o m p r

ession. It does not

build up tensile stresses exceeding the tensile strength of

c o n c r

etethe reason for cracking in conventional port-

land cement concr

e t e

. The pr

e s t r

essing caused by ex-

pansion of the concrete and r

e s t r

aint by the steel has

compensated in advance for drying shr

i n k a g e

. B

a s e d

on tests and observation, the net v

o l u m e t r

ic change of

s h r

inkage-compensating concrete is almost z

e r

o.

Reduction or virtual elimination of cr

a c k - p ro d u c i n g

tensile stresses in shrinkage-compensating concrete has

another important adv

a n t a g e

. That is the placement of

l a r

ger slab areas without the need for any shrinkage con-

t r

ol joints and with fewer construction joints. In some

c a s e s

, 70 to 85 percent of the total joints needed in a

c o n v

entional portland cement concrete floor may be

eliminated in a shrinkage-compensating concrete floor

of comparable area. Designers normally break up port-

land cement concrete floor areas into 15- to 25-foot

( 5 )

Shrinkage-compensating

concrete for industrial

floors on ground

How three-way combination of

expansive cement, welded wire fabric

and certified quality control was used

for warehouse floor expected to be

crack-free

B

Y

W

ILLIAM

P. L

ILJESTROM

, P

RESIDENT

5 C S

ERVICE

I

NC

.

AND

C

HEMICALLY

P

RESTRESSED

C

ONCRETE

C

ORPORATION

H

ACIENDA

H

EIGHTS

, C

ALIFORNIA

AND

R

OBERT

C. R

ICHARDSON

, T

ECHNICAL

D

IRECTOR

D

AVIS

W

ALKER

C

ORPORATION

L

OS

A

NGELES

, C

ALIFORNIA

* Numbers in parentheses refer to metric equivalents listed with this article.

panels by construction and/or control joints. U

s i n g

s h r

inkage-compensating concr

e t e

, how

e ve r

, under

c o

v

e r

a r

eas as large as 20,000 square feet

( 6 )

h a v

e been placed

without joints. For slabs exposed to weather it is nor-

mally recommended that placements be limited to 7000

to 12,000 square feet,

( 7 )

depending on what a nor

m a l

c r

ew can adequately place and finish in a day.

On the W

i l l o

ws job, the o

w n e r

s engineers selected 50-

by 150-foot

( 8 )

slabs as the nearly optimum panel size for

their needs and column configuration. These typical

7 5 0 0 - s q u a re - f o o t

( 9 )

slabs of shr

i n k a g e - c o m p e n s a t i n g

c o n c r

ete each saved a total of 400 feet

( 1 0 )

of extra joints

by eliminating the need for two 50-foot-long

( 1 1 )

c o n s t ru c

-

tion joints across the width of the slab and making un-

n e c e s s a r

y 300 feet

( 1 2 )

of sawed control joints which would

h a v

e divided the area into 25-foot

( 1 3 )

s q u a r

e panels (F

i g

-

u r

e 1). Use of shrinkage-compensating concrete for the

258,750 square feet

( 1 4 )

of slabs was to eliminate 13,800 lin-

ear feet

( 1 5 )

of jointsa saving of over 2 miles of potential

t r

ouble and maintenance.

The highly visible benefits of crack control and joint

reduction r

e q u i r

e more than just a decision to use

s h r


e t e

. Inattention to neces-

s a r

y design-details and construction pr

o c e d u r

es can

mar those r

e s u l t s

. Among those details and pr

o c e d u re s :

C

o n s t r

uction joints in industrial floors should be dow-

eled or keyed to allow thermal contraction and expan-

sion. D

e f o r

med bars across these joints would cause

c r

acking of nearby slab ar

e a s

.

Rectangular column boxouts that form r

e e n t r

ant cor-

ners may create stresses that initiate cracking. Cir

c u l a r

b o

xouts are r

e c o m m e n d e d .

Slabs should be cast leaving at least two adjacent sides

u n re s t r

ained and free to permit the desired initial ex-

pansion. Plan the casting schedule carefullyand re-

lease the edge forms or stakes as soon as the concr

e t e

has been finished.

Expansion against restraint essential

A related and equally important factor is the steel re-

i n f o r

cement in the slabs. The concept of providing re-

s t r

aint against expansion is essential to any use of

s h r


e t e

. ACI 223 (see R

e f e r

-

ence 1) states that r

e i n f o r

cement, generally based on the

ACI 318 Building C

o d e

, is pr

e f e r r

ed to other types of re-

s t r

aint such as adjacent str

u c t u r

al elements or subgr

a d e

f r

iction, which are largely indeterminate and which may

p r

ovide either too much or too little r

e s t r

aint .

The engineers on the W

i l l o

ws job first specified N

u m

-

ber 4

( 1 7 )

Grade 40

( 1 8 )

r

e b a r s

, 18 inches

( 1 9 )

on centers in each

d i r

ection. This provided the r

e s t r

aint r

e q u i r

ed to con-

t r

ol the expansion reaction and, at a percentage of 0.185

for the 6-inch-thick

( 3 )

s l a b s

, more than met the minimum

r

e q u i r

ements of ACI 223. The 18-inch spacing was se-

lected in order to assure, through improved accessibility

to wor

k e r s

, more accurate positioning of supports to

hold the steel in the upper third of the slab as called for

by ACI 223. This would also assure more accur

a t e

, com-

plete embedment of the r

e i n f o r

cement, while still allow-

ing adequate cov

e r

.

Deformed welded wire fabric for restraint

But when a construction contract was awarded, the

c o n t r

actor proposed substituting deformed welded wire

f a b r

ic for the r

e b a r

. The fabric proposed was designated

as 16x16-D14.4xD14.4, that is, a fabric of defor

m e d ,

0 . 4 2 8 - i n c h - d i a m e t e r

( 2 0 )

c o l d - d r

awn wir

e s

, each with a

c r

oss-sectional area of 0.144 square inch and wir

e s

spaced at 16 inches

( 2 2 )

in both dir

e c t i o n s

. H

e r

e are the ar-

guments advanced for this change:

D

e f o r

med fabric has a higher, guaranteed yield

s t r

ength of 70,000 psi.

( 2 3 )

It allows use of the minimum steel percentage of 0.15

p e r

mitted by ACI 223.

The proposed 16-inch

( 2 4 )

w i r

e spacing is almost as con-

venient for workers to step between as the spacing of

18 inches

( 1 9 )

and offers comparable advantages of easy

handling and positioning.

The reduced amount of steel needed and the r

e d u c e d

Figure 1. Comparison of joint requirements in concrete floor

slabs made with expansive cement and conventional

portland cement.

m a n p o

wer and time needed for placement of the sheets

of fabric should produce significant savings.

It was also pointed out that the wire defor

m a t i o n s

would assure the adequate bond r

e q u i r

ed to develop re-

s t r

aint against the expansive reaction. F

u rt h e rm o r

e the

rigidity of the 0.428-inch-diameter

( 2 0 )

c o l d - d r

awn wir

e s

would assure that the sheets lie flat on the supports with

full cov

e r

.

The owners accepted the proposed substitution and

found that it sped up the schedule.

Certified quality control retained

The engineers on this job took another step to help

meet their own per

f o r

mance standar

d s

. This was to con-

t r

act for a pr

o g r

am of certified quality control cov

e ri n g

e ve r

y aspect of site-cast shrinkage-compensating con-

c re t e f r

om supply of cement and aggr

e g a t e s

, batching

and testing, to project acceptance.

This r

e l a t i v

ely new concept eliminates the inability

an owner sometimes has of fixing responsibility when

c o n c r

ete does not meet design r

e q u i re m e n t s

. By this sys-

tem it is no longer necessary to go from architect to en-

gineer to contractor to testing labor

a t o r

y to concr

e t e

p r

oducer to concrete subcontractor when something

goes wrong. The sort of certified service retained on this

p r

oject can put the responsibility on a single entity.

Traditional design and construction channels r

e m a i n

unchanged under this concept, except that the cer

t i f i e d

quality control group carries out an evaluation and

t r

aining pr

o g r

am and selects all participants in the con-

c r

ete supply operation. The quality control group as-

sumes full responsibility to the owner for each step as

well as for the end product. On the W

i l l o

ws project, the

quality control group certified the producer of r

e a d y

mixed concr

e t e

, who set up a batch plant on site. The

g r

oup also retained a testing labor

a t o r

y and trained two

of its employees in testing expansive cement mixes,

t r

ained batchplant and jobsite inspectors, and r

e p o rt e d

daily to the o

w n e r

.

C

o n s t r

uction on the addition, a $3

1

2

-million pr

o j e c t ,

s t a r

ted in September 1977. Pr

e p a r

ation of the site was a

first, all-important step. As much as 2 feet

( 2 5

) of unstable

soil was dug out and up to 7 feet

( 2 6 )

of fill was impor

t e d ,

compacted to 95 percent to form a subgrade for the floor

slab to come. This was cov

e r

ed with 12 inches

( 2 7 )

of lime-

s t a b i l i z

ed material, and topped with 6 inches

( 2 8 )

of Class

2

( 2 9 )

c r

ushed stone subbase. Next came construction of

f o o t i n g s

, followed by erection of the metal-clad str

u c t u r

e

i t s e l f .

In A

p r

il of 1978, placement of shr

i n k a g e - c o m p e n s a t

-

ing concrete began for the first of thirty 7500-squar

e -

f o o t

( 9 )

and three 11,250-squar

e - f o o t

(30)

jointless slabs.

Placement continued at a rate of about 2 slabs each

w o r

king day until the middle of M

a y

, when concreting of

these slabs and over 125,000 square feet

( 3 1 )

of conv

e n

-

tional slabs was completed.

Fabric placement procedures

After final pr

e p a r

ation of the subgrade and subbase,

the first step in the slab construction sequence was

placement of the welded wire fabric sheets. For the typ-

ical 50- by 150-foot

( 8 )

slab a total of for

t y

-two 8-foot-

w i d e

( 3 2 )

sheets was r

e q u i r

ed, half of the sheets 21 feet

( 3 3 )

long and the others 30 feet

( 3 4 )

long. These w

e r

e placed in

21 rows down the length of the slabs with the 21- and 30-

foot lengths alternating so that the laps between the

pairs of sheets in each row w

e r

e stagger

e d .

As anticipated by the decision to conv

e r

t from r

e b a r s

to fabric, significant manpower savings w

e r

e r

e a l i z

ed in

placing the sheets. A single ir

o n w o rk e r

, assisted by a

man operating a front-end loader equipped with a 14-

f o o t - l o n g

( 3 5 )

s p r

eader beam, was easily able to place up to

two bays of steel a day. The loader brought stacks of sev-

en sheets to a point at or near their final position on the

s u b b a s e

. From there, the ir

o n w o r

ker was able to slide

the 177-pound

( 3 6 )

sheets singly into final position. It took

just 26 man-days to place 313,750 square feet

( 3 7 )

of fab-

ric, or over 1500 square feet

( 3 8 )

an hourmuch faster than

placing rebars individually.

Close attention to splicing of the sheets was neces-

s a r

y to assure that the steel was continuous and thus ca-

pable of providing consistent r

e s t r

aint to the expansive

reaction, and also to get the greatest benefit from the

splicing system designed into the sheets. This consisted

of a pair of D7.2 wires 8 inches

( 3 9 )

on centers along each

edge or side of the 8-foot-wide

( 3 2 )

s h e e t s

. By placing these

pairs of wires over each other, an 8

5

8

- i n c h

( 4 0 )

lap was pro-

vided without an attendant buildup of steel. The combi-

nation of the edge wires added up to the same r

e q u i s i t e

steel area as given by the main D14.4 wir

e s w i t h o u t

ov

e r

design, and with savings in steel.

Reducing the wire size in the splice zone also pro

v i d e d

for uniform distribution of steel across the slab in the

longitudinal direction of the fabric sheets, thus eliminat-

ing steel buildup that often induces stiffness in the con-

c re t e

. It is also desirable to avoid buildup at the splices so

that the fabric sheets lie flatter and are assured of ade-

Figure 2. Lapped splices used at sides and ends of welded

wire fabric.

quate cov

e r

. The sheets w

e r

e overlapped at the ends a

minimum of 13 inches.

( 4 1 )

Both side and end lapped

splices conformed to cr

i t e r

ia established by the W

i r

e Re-

i n f o r

cement Institute (see F

i g u r

e 2 and R

e f e r

ence 7).

The sheets r

e q u i r

ed only a minimum amount of

tieing, just enough to guard against shifting during con-

c r

ete placement. The final step was positioning the fab-

ric accurately at the proper level in the slab. ACI 223

points out that because most of the drying occurs in the

top portion of slabs on gr

a d e

, the r

e i n f o r

cement should

be placed there, pr

e f e r

ably one-third the distance fr

o m

the top.

This was accomplished by supporting the sheets on 3

1

2

- i n c h

( 4 2 )

cubes of concrete adobe brick located about 4

f e e t

( 4 3 )

on centers. The cubes w

e r

e distributed about the

subbase prior to concreting, but w

e r

e not actually

placed under the wires as supports until concreting was

in pr

o g re s s

. This permitted the truck mixers to be dri-

ven onto the subbase without fear of bending or distort-

ing the sheets between the suppor

t s

. While the tr

u c k

m o

ved down the lane between columns depositing the

mix by chute onto the subbase, one man moved along

b e t w

een the truck and the end of the chute and placed

the adobe cubes under the wir

e s

. The sheet was thus ac-

c u r

ately positioned just before being embedded in con-

c re t e

.

Concrete placement procedures

This same worker accomplished another impor

t a n t

o p e r

ation. To be effective, the expansive cement mix re-

q u i r

es proper wetting of the subbase. It is suggested that

this be done with a soaking the night before, followed by

s p r

inkling just ahead of the concrete placing oper

a t i o n .

The adobe man at W

i l l o

ws had the responsibility for

s p r

inkling just ahead of concrete placement .

Batching and mixing of the 4000-psi-design-

s t re n g t h

( 4 4 )

c o n c r

ete was closely observed and checked

by the certified quality control people. D

e l i ve r

ies w

e r

e

b r

ought under the warehouse roof from the nearby

batch plant by a fleet of five 8-cubic-yard

( 4 5 )

t r

uck mixers.

The drum of each truck turned at least 70 r

e v

olutions be-

f o r

e dischar

g e

. A 13-man crew placed the mix at a r

a t e

of about 62

1

2

s q u a r

e feet

( 4 6 )

a minute. They completely

finished two slabs, or 15,000 square feet,

( 4 7 )

in 11 hours. A

5 2 - f o o t - w i d e

( 4 8 )

a i r- p re s s u re - o p e r

ated vibr

a t o r

y scr

e e d

with 26 vibrating heads struck off the 50-foot-wide

( 4 9 )

slabs and produced a flat, level floor meeting the tight

s u r

face tolerance specification and assuring complete

embedment of the deformed wir

e s

.

Although the handling char

a c t e r

istics of shr

i n k a g e -

compensating concrete are similar to a comparable T

y p e

I or II portland cement concr

e t e

, one notable and sig-

nificant difference r

e q u i r

ed close attention by the finish-

e r s

. There is an almost total absence of bleed water after

s c r

eeding or floating. For this reason a cement mason

w o r

king with the mix the first time might be inclined to

s t a r

t finishing operations too soon. A good rule of

t h u m b

, which was followed at W

i l l ow s

, is to hold off un-

til the slab is firm enough to support the finishers.

As provided in the design/planning stage of the pro-

ject, the forms on two adjacent sides of the typical 7500-

s q u a re - f o o t

( 9 )

slab w

e r

e released as soon as possible with-

out inflicting surface damage on the concr

e t e

. This

p e r

mitted the controlled expansive reaction to go to its

limit. These sides w

e r

e kept open for at least 3 days.

C

u r

ing of the shrinkage-compensating concrete floor

f o l l o

wed accepted pr

o c e d u r

es for conventional slabs but

with an added precaution. Since any deficiencies in pro-

c e d u r

es for curing expansive slabs may reduce the

amount of initial expansion needed to offset the later

d r

ying shr

i n k a g e

, more care is r

e q u i r

ed than usual. At

the C

a l i f o r

nia project, a 27-percent chlorinated r

u b b e r

solution was spr

a y

ed over the surface of subareas of the

l a r

ge slab immediately after finishing.

References

(1) Recommended Practice for the Use of Shrinkage-Com-

pensating Concrete (ACI 223-77), 21 pages, American Con-

crete Institute, Box 19150, Detroit, Michigan 48219.

(2) Building Code Requirements for Reinforced Concrete

(ACI 318-77), American Concrete Institute, same address.

(3) Tentative Specification for Expansive Hydraulic Cement,

ASTM C 845-76T, 3 pages, American Society for Testing

and Materials, 1916 Race Street, Philadelphia, Pennsylvania

19103.

(4) Standard Test Method for Restrained Expansion of Ex-

pansive Cement Mortar, ASTM C 806-75, 4 pages, Ameri-

can Society for Testing and Materials, same address.

(5) Shrinkage-Compensating Cement Concrete, Concrete

Construction, 4 pages, February 1976, page 63.

(6) Guide for Testing, Handling and Placing ChemComp Ce-

ment Concrete, Second Edition, Chemically Prestressed

Concrete Corporation, 14656 Oxnard Street, Van Nuys, Cali-

fornia 91401

(7) Jointed Concrete Slabs and Pavements (WWF 101), Sec-

ond printing, March 1975, Wire Reinforcement Institute,

7900 Westpark Drive, McLean, Virginia 22102.

Metric equivalents

(1) 35,700-square-meter (30) 1050-square-meter

(2) 30,000 to 40,000 pascals (31) 11,600 square meters

(3) 150-millimeter-thick (32) 2.4-meter-wide

(4) 3 millimeters in 3 meters (33) 6.4 meters

(5) 4.5- to 7.5-meter

(34) 9.1 meters

(6) 2000 square meters (35) 4-meter-long

(7) 650 to 1100 square meters (36) 80-kilogram

(8) 15- by 45-meter

(37) 29,150 square meters

(9) 675-square-meter (38) 140 square meters

(10) 120 meters

(39) 200 millimeters

(11) 15-meter-long

(40) 220-millimeter

(12) 90 meters


(13) 7.5 meter

(42) 90-millimeter

(14) 24,038 square meters (43) 1.2 meters

(15) 4200 meters

(44) 28-megapascal-design-strength

(16) over 4 kilometers (45) 6-cubic-meter

(17) 12.70-millimeter-diameter (46) 5.8 square meters

(18) 276-megapascal (47) 1400 square meters


(48) 16-meter-wide

(20) 10.9-millimeter-diameter (49) 15-meter wide

(21) 93 square millimeters (50) 2.4- by 9.1 meter



(23) 480-megapascal (52) 75 cubic meters

(24) 410-millimeter

(53) 15 meters


(54) 45 meters

(26) 2 meters

(55) 30 meters




(57) 7.5 meters

(29) about 20- to 65-millimeter

P U B L I C AT I O N# C 7 9 0 11 0

Copyright 1979, The Aberdeen Gr

o u p

All rights r

e s e r v e d

ROSTURI PLATFORME INDUSTRIALE

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