1 St Andaziaran

7/27/2019 1 St Andaziaran

1/8

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tractors,

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RESSIVE

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T 180 DAYS

(days cure

ater curi

RIN

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AS % OF

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IN

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ducers) i

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ONTINUOU

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ss from

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AMPLE


2/8

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the

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ECT OFngth of c

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pt moist

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s paper ployed in

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theACI

ent,Sula

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words: C

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h it is ke

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oncrete c

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designat

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STD-, Swere plot

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ral requir

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ring, hot

ation of

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ich the su

of time a

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y of twose study

itions. Th

mpressiv

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ven days,

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d as ST

in our p

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ater curi

N PROPer of fact

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oncrete cor evalua

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which is

with a bu

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concrete

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oncreting

COEFFICIENT

C

3 7 14

g on per

ERTIESrs, one o

ustrates t

e been:

to dry o

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uring mettion of cu

nce of cu

at da

days. Th

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robabilit

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er region

, concret

OF PERMEABIL

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28

eability

F CONwhich is

is, comp

t.

hods coming effici

ring meth

s. The

methods

d as SW

, which i

ith stand

e specim

oject) dur

he ratiosms. The

of these

. was

of similar

durabilit

ITY (m/s

(W/C = 0.51)

9

f cement

RETE Tthe lengt

ring the

monlyency und

ods was

ethods w

were wa

and wat

designat

rd cured

ns were

ing the sa

of SWCean valu

ratios be

and

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.

0

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trength

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r

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esirable r

ce hydra

e. In the

SWC curi

These hi

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crete sho

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on

ure of the

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ng. Reco

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uld be cu

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concrete

of ceme

n moistur

sults in

f concret

mmendat

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as the c

ed for at

roughout

r t: . Enterrelative humi. . Move doion.

is affecte

nt and in

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rying shr

technol

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icate a de

ncrete ha

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the curin

ith air temperity. . Move rin to wind velo

by the s

tial temp

of the ce

nkage an

gy, hot w

proving

iciency i

s set eno

secutive

process

ture,ght to concretcity. . Move l

rroundin

erature o

ent past

develop

eather is

the curin

the curi

gh to avo

days afte

see techn

left; read appr

air, abs

f material

at this st

ent of c

efined as

methods

g metho

id any su

placing.

ical bulle

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rption of

s. An

ge tends

acks in t

any

in

s,

face

nsure

in on

solar

to

e


4/8

combination of high air temperature, low relative humidity, and wind velocity [ ].

The effects of hot weather are most critical during periods of rising temperature or

falling relative humidity, or both. Undesirable hot weather effects on concrete in the

plastic state may include: (a) increased water demand, (b) increased rate of slump

loss, (c) increased rate of setting and (d) increased tendency for plastic cracking [ ].

Thus, a continuous curing, particularly during the first few hours, is acutely needed.ACI [ ] and ACI [ ] recommend that concrete be maintained in a

moist condition for at least the first days after placement. Alternate cycles of

wetting and drying promote the development of pattern cracking and should be

avoided. ACI [ ] specifies that the procedure for curing concrete shall be

improved when the strength ratio of field cured specimens to the companion

laboratory cured specimens is less than . unless the field-cured strength exceeds

the specified strength by more than . MPa.

Spears [ ] indicated that proper curing maintains relative humidity above

percent and, thereby, advances hydration to the maximum attainable limit. Proper

curing decreases concrete permeability, surface dusting, thermal-shock effects,

scaling tendency and cracking. It increases strength development, abrasion

resistance, durability, pozzolanic activity and weatherability. Haque [ ] investigated

the strength development of concrete under the conditions of fog, temperate dry,

warm-wet and warm-dry weather conditions. He found that the lack of any moist

curing adversely affects the compressive strength of plain concrete at all ages.

Martin [ ] demonstrated that rising placing temperatures do not, as a rule, lead

to lower strengths. With favorable combinations of cementitious materials and

admixtures, the strength performance of concrete can remain unaffected by higher

placing temperatures, or it can even improve over that at lower temperatures.Malvin

and Odd [ ] conducted a large-scale field investigation of high-strength light-weight

concrete and concluded that maximum curing temperatures of up to C ( F) did

not adversely affect the mechanical properties of the concrete. On the contrary, theyobserved a slight increase in compressive strength.

Khan [ ] quantified the effect of interrupted curing. He found that the losses

in strength of concrete due to an interruption in moist curing can be regained

significantly by recuring the concrete.

Carrier [ ] indicated that a short period of drying early in the curing life of

concrete specimens prevents water molecules from reaching unhydrated cement

particles and prevents concrete from gaining full strength. He also indicated thatmuch of the concrete deterioration that takes place each year should be blamed on

inadequate curing. Early and rapid drying can lead to failure such as shrinkage

cracks, crazing, wear, dusting, scaling, and spalling. Once a surface has cracked,

dusted, scaled or spalled, the entire member is more susceptible to other types of

deterioration.


5/8

Research significance

Twice a day sprinkling of water with or without burlap cover for seven days are the

curing practices used . Theaverage annual maximum temperature of o C and the

average annual minimum humidity of . % in summer in the our region and the

prevalent methods of curing which are below the required standard practice call for

studying their effects on the strength of concrete.

Objective and scope

This paper presents results of an experimental program designed to investigate the

influence of the prevalent curing practice on the strength of concrete cast during ten

month period in an arid area.

Experimental program

The experimental program was designed to evaluate the influence of the prevalentcuring practices on the compressive strength of the concrete.

The Three curingmethods which were employed are described and designated as in

Table .

Concrete samples were collected from randomly selected construction sites inSulaimanyah . The sampling was done during ten

Table . Curing methods used and their designation

Designation Curing method

SWC Twice a day sprinkling without cover for seven days

SBC Twice a day sprinkling with burlap cover for seven days.

STD Twenty eight day immersion in water, considered standard curing.

A total of concrete samples were collected at construction sites in Sulaimanyah

during the sampling period and cast into standard cubes of x x mm. Each

sample consisted of three pairs of cubes and each cube in a pair was collected from a

separate truck(norsoy ready mix ). The cubes were left at the site for about hours

and then transferredto the laboratory. A pair in a sample was cured by SWC, thesecond by SBC and thethird by STD method. The cubes were tested for strength at age

of days.

Results, analysis and discussion

The ratios R and R of the compressive strength of the SWC and SBC cured cubes,


6/8

res

Th

wit

and

bur.

Ta

Ri

R

R

Inve

pro

wit

.

ectively,

results a

the best

. , res

ap cover, respec

le . Bas

Min

.

.

rse of stand

It is int

ess can c

probabi

, respect

to the S

e present

fit by lin

ectively.

in dry-hoively, an

ic statisti

ercentile

.

.

ard normal

Fig. Infl

eresting t

ause thes

ity of p

vely.

D cured

d in Tab

ar regres

This clea

weather.their mi

s of the s

mean

.

.

istribution

-

-

-

ence of cu

note tha

ratios to

rcent of

ubes wer

e and p

ion in Fi

rly indica

The maxima are

rength ra

ax CO

.

.

. .

Ratio of fiel

ing method

the vari

be highe

eing not

e subjecte

lotted on

. . The

tes the be

mum val. and

ios R an

% P

.

.

R

R

.

d to standa

on concret

bility in c

than unit

exceeded

d to analy

normal

ean val

eficial e

es of the. .

d R in th

(R< )

.

.

. .

d cured str

e strength i

ompressi

y. The fi

of the t

sis by or

robabilit

es of R

fect of cu

two ratio

Sulaima

. .

ngth , R

the centra

e strengt

e percen

o ratios a

er statisti

paper al

nd R are

ring with

are .

nyah Ar

l province

and cur

iles (the

re . a

cs.

ong

.

and

a.

ng

alues

d


7/8

ACI [ ] specifies that procedures for protecting and curing concrete shall be

improved when strength of field cured cylinders at test age designated for

determination of fc is less than percent of that of companion laboratory cured

cylinders. In the presence of strength variability, there is a possibility of having this

ratio less than . which is very small with good curing practices, however, this

probability will increase when poor practices are employed.

Results indicate that the mean values of R are less than those for R while the

coefficients of variation of R are higher than that of R . As a result, the

probability of being R less than . is %, which is very high. This indicates that

the SWC curing method does not meet the ACI- requirement in arid areas. The

probability of having R less than . in sulaimanyah area is about %, which is

also relatively high.

The authors suggest that the efficiency of curing methods should be based on the percentile of the distribution function of the ratio R which is affected by both its mean

and COV. The results indicate that the percentiles are . and . , respectively.

The curing methods should be improved to bring these values to . .

The ACI- recommends that concrete be maintained in a moist condition for at

least the first days after placement. In arid areas, it is impossible to meet this

recommendation using SWC where the available water for concrete curing is very

limited. The SBC curing method is more efficient; however, the frequency of water

sprinkling per day should be increased so that the percentile of the distribution of is

at least equal to . . The efficiency of water sprinkling three times a day with a

burlap cover is under investigation.

. Conclusion

The effect of curing practice was evaluated by statistical analysis of strength ratio, R,

of field cured to standard cured cubes. The field curing methods used were sprinkling

without cover and with burlap cover. The results in the sulaimanyah area showed thatthe mean value of R & R are . and . , respectively. The higher value of

R indicates the effectiveness of burlap cover in improving the curing process. The

probability of having R less than . , specified by ACI- , is percent,

indicating that curing by water sprinkling without cover twice a day for days will not

satisfy the ACI- requirement. The SBC curing method is more efficient; however,

the frequency of water sprinkling per day should be increased so that the percentile of

the distribution of R is at least equal to . . The efficiency of water sprinkling

three times a day with a burlap cover is under investigation.


8/8

References

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Concrete Institute, Detroit.

. ACI Committee . ( )

Concrete . ACI- , American Concrete Institute, Detroit.. ACI Committee . ( )

ACI- , American Concrete Institute, Detroit.

. Spears, R. E. ( )

Concrete International. pp. - .

. Haque, M.N. ( )

International. Vol. , No. , pp. - .

. Mittelaeher, M. ( )

Field Concrete.Concrete International. pp. - .

. Sandvik, M. and Gjorv, O. E. ( )High Curing Temperatures in Light

Weight High-Strength Concrete . Concrete International. pp. - .

. Khan, M., S. and Ayers, M., E., ( ) Interrupted Concrete Curing. Concrete

International. pp. - .

. Carrier, R. E. ( )Concrete Curing Tests concrete international pp -

. ACI Committee . ( ) Hot Weather Concreting . ACI- , American

Building Code Requirements for Reinforced

Recommended practice for curing concrete

The Percent Solution to Inadequate Curing Problems

Some Concretes Need Days Initial Curing. Concrete

Compressive Strength and the Rising Temperature of

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