Những chỉ tiêu kỹ thuật đánh giá độ bền của bê tông trong công trình xây dựng

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<b>DIEN DAN KHOA HOC CONG NGHE </b>

<b>NHffNG CHi TIEU KY THUAT OANH GIA DO B I N CUA </b>

<b>BE TONG TRONG CONG TRINH XAY DlTNG </b>

Betong la vat lieu khong hoan thien?

Chung ta thudng dat hang betdng theo cac yeu
cdii v l cudng do. Cudng do thi cd the xdc djnh bdng
con sd, chiing ta cd the do dugc.

N I U chiing ta yeu edu betdng ed do bin 100 nam
lam t h i nao de biet vd khang djnh duge?

\

<b>P G S . T S . N G U Y E N VAN C H A N H </b>
Trudng Dai Hpc Bach Khoa TP.HCM

Cdc chl tieu ky thuat neu trong bai ndy nhdm muc
dfch giup ky sube tdng vd ky suxdy dung edng trinh:

- Cai tien eae djnh mUe eh! tieu ky thuat de dat
duge be tdng vdi chat lugng tdt hon.

- Lua chgn nhung dilu kien gidi han thich hap
ngodi cdng trudng de ddnh gid ehdt lugng be tdng khi
be tdng Idm viee trong mdi trudng khae nghiet.

- Nhdn bilt vd hieu cdc thf nghiem va ede chl tieu
ky thuat ciia thf nghiem dugc sir dung de ddnh gia

chdt lugng ciia be tdng.

1. Cudng do chju nen '*'*
Cudng do chju nen Id thf nghiem thdng dung nhat

sirdung cho be tdng. Mae dii eudng do thudng khong
phdi Id duy nhdt mdt chi tieu tdt de ddnh gia do bin
ciia be tdng, nhung trong khi thiet ke ket cau ciia
cdng trinh, be tdng d i u phai dat duge mdt mirc do t6i
thieu v l cudng do. Khi cdc bd phdn kit cdu cong
trinh khdng de cap den do ben thi cUdng dp la dieu
kien tien quylt de ddnh gid chdt lugng ciia ket cau
<b>dd. ' </b>

<i>ii'--Hinh 1: Ciu Phu My va ciu Cin Thd </i>

<i>Hinh 2: Thidt kd ve eddng do l<hdng quan tam ddn do bdn </i>

Thdng thudng, be tdng thudng thf nghiem eudng
do nen d 28 ngdy, tuy nhien, nhtmg ngudi thilt ke
chuyen nghiep cung cd the linh ddng thay ddi tudi thir
cudng do nen cua be tdng sao cho thi'ch hgp vdi cac
yeu cdu cua du an. Yeu cdu v l be tdng phdt trien
nhanh cudng do d giai doan ddu cd the thfeh hgp vdi
nhufng cdng trinh xdy dung ddi hdi tiln do nhanh, tuy
nhien dilu nay cd the ldm cho do bin ciia be tdng
giam. Nlu cdng trinh thiet ke chju tai trgng ddng ve
sau thi chl tieu ddnh gia cudng do nen cua be tdng

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<b>NHffNG CHJ TIEU KY THUAT OANH GIAflOBlN.., </b>

cdn thilt phdi d l cap din eudng do d 56 vd 90 ngdy
2. Do b i n

Khi d l cap din dp bin ciia be tdng, ngudi ky su
xay dung khdng nen chl dua chu ylu vdo ede chl tieu
ky thuat v l cudng do nen tdi thieu, ti le N/X tdi da,
ham lugng xi mang tdi thieu vd ham lugng bot khf. Cd
nhirng yeu edu tdt hon de danh gid do bin eiia be
tdng. Do eo ngdt vd do thdm thdp Id hai thdng sd ky
thuat dae trung eho do ben eiia be tdng khi ket edu
lam viec trong nhtmg dilu kien khac nghiet.

Ndi tdm lai, thf nghiem cudng do nen eiia be tdng
khdng phai la thf nghiem dai dien eho dp ben ciia be
tdng. Hien nhien la, be tdng ed eudng dp cdng eao thi
kha nang khdng nut do xdm thuc gay ra edng tdt, va
cd ti le N/X cang thdp. Tuy nhien, edn phdi chdc chdn
rdng, thdnh phdn cdp phdi eiia hdn hgp be tdng phdi
dugc t i l Uu hda de cd the chdng lai duge su xam thue
ciia mdi trudng Idm viec ed gdy anh hudng din dp
bin eiia be tdng. Cu the Id:

- Phai sir dung xi mdng thfeh hgp d l i vdi dilu kien
cdu kien betdng trong mdi trudng sir dung (xi mdng
PC, xl mdng it tda nhiet, xi mdng PCB, xi mdng
ching lai an mdn sunphat...).

- Cdt lieu khdng bj phdn ung kilm.

- Cau kien betdng khdng nut do eo ngdt vd khdng
nirt do nhiet.

3.06 thdm

Cd nhieu quan diem cho rang, de nang cao do
ben cua be tdng can phai giam do thdm trong be
tdng. Theo tieu ehuan, ti le N/X rdt quan trpng ddi vdl
dp thdm eua be tdng. Tuy nhien, trong thuc t l , nlu
chi xem xet mdi yeu td ti le N/X thi cd the gdy dnh
hudng den nhung yeu td khdc cua be tdng. Ham
lugng hd xl mdng (nude + xl mdng) cao thi nhiet
hydrat hda se eao, d l nut, modun dan hdi thdp, tU
biln cao. Ngoai ra, do bin hda hge ciia be tdng phu
thudc Idn vdo thiet ke thdnh phdn hdn hgp be tdng.

Ngdy nay, cimg vdi viec sirdung rdng rai phu gia
vd CO hoat tfnh vd phu gia hda hgc tien tiln, chiing
ta cd the tdi uu hda cap phdi hdn hgp be tdng theo
hdm lugng cd do thdm thdp bang nhilu cdch khdc
nhau thay yi chi kiem sodt ti je N/X. Cd the ddnh gid
mUc dp thdm ciia be tdng bdng cdc thf nghiem tieu
<i>chuan hien cd. ASTM C1202 thf nghiem danh gia </i>

<i>kha nang tham thiu ion do cua be tdng thdng qua </i>
<i>nhan biet dien thi, thudng dugc ggi Id thf nghiem </i>

thir thdm nhanh ion clo, day Id mdt trong nhtmg
phuong phdp dang duoc sir dung nhilu trong cdc
tieu chuan ky thuat nham ddnh gid dp b i n cua be

tdng. Theo ASTM C1202, be tdng cd gid trj
coulombs Idn hon 4000 duge xem Id cd do thdm
cap, coulombs 2000 d i n 4000 dugc xem Id cd do
thdm trung binh, coulombs 1000 d i n 2000 dugc
xem Id dp thdm thdp, coulombs nhd hon 1000 dugc
xem la dp thdm rdt thdp. Ddi vdi hdu hit cdc k i t cdu,
be tdng cd gid tri coulombs 2000 d i n 2500 thi dugc
xem Id cd do thdm du thdp. Be tdng cd gid trj
coulombs thdp thi se cd kha ndng chdng lai sutham
thdu ciia ion clo tdt.

4. An mon cdt thep

An mdn edt thep Id mpt qua trinh dien hoa xay ra
khi edmat cua do am vd oxy. Qua trinh nay chuyen
hda cdt thep thdnh ri sdt, ri sat ed the tfeh gdp 6 ldn
so vdl thep ban ddu. Trong Idp be tdng bao ve cd do
kilm eao hinh thdnh Idp thu ddng xung quanh cdt
thep, Idp thu dpng nay ddng vai trd bao ve edt thep
khdng bj an mdn. Ldp mang thu dong nay bj pha hiiy
khi be tdng bao ve tai be mat edt thep bj cacbonat
hda. Ngodi ra, hdm lugng ion clo tai be mat ciia cdt
thep eung gdp phan Idm phd hiiy Idp mang thu dong
nay. De bao ve edt thep khdng bi an mdn, can phai
tien hdnh nhieu bude. Dieu ddu tien, Idp be tdng bao
ve phai dil day va ed chat lugng de cd kha nang tri
hoan su phdt sinh dn mdn edt thep. De tri hoan su
phdt sinh an mdn edt thep, cd the sir dung cac bien
phdp nhu tang dp ddc sft de gidm dp tham cua be
tdng, sir dung epoxy quet len b l mat edt thep, hoac

sirdung phu gia Uc che dn mdn trong qud trinh nhao
trpn hdn hgp be tdng. DT nhien Id, sir dung loai cdt
thep khdng an mdn Id phuong an tdt nhdt v l mat ky
thuat, tuy nhien bien phap nay lai rdt ddt tien.

<b>ã 4ô!- ^ </b>

<i>Hinh 3 va 4: Thim clorua gay ra an mon cdt thep </i>

De bao ve cdt thep khdng bj dn mdn do xdm thuc
eiia nudc bien hodc do sutham thdu ion clo, ACI 318
d l nghj sir dung be tdng cd ti le N/X=0,4, eudng dp
chju nen tdi thieu Id 350kG/em^ Trong tieu ehuan nay
cung dua ra gidi han cho hdm lugng ion clo tu do ed
trong be tdng, ham lugng nay tfnh theo phan trdm
khdi lugng cua xi mdng, hdm lugng nay gidi han d
mirc 1% ddi vdi be tdng lam viec trong dilu kien khd
rdo, vd 0,06% ddi vdi be tdng irng sudt trudc.

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ciru, tuy nhien theo cdch hieu thdng thudng thi nhimg
v l t nirt nhd (dudi 0,2mm) vudng gdc vdi edt thep thi
cd the khdng dnh hudng dang ke din su an mdn cdt
thep.

Qua trinh carbonat hda ciia be tdng cd ehdt lugng
tdt thudng Id mdt qud trinh xay ra cham vd khdng
thuc su ddng lo ngai neu Idp bao ve be tdng du dam
bao yeu cau.

<b>5. Xam thuc sunphat </b>

Cae ion sunphat duge tim thdy trong nUde ngdm
va ddt. Trong hdu het eae trudng hgp, mdt sd mudi
sunphat cung phan ung vdi eae thanh phdn hydrat
hda ciia xi mang tao thanh thach eao, ket qua Id
cung gay ra sU truong nd the tfch. Trong mpt sd
trudng hgp, cac san pham hydrat hda eiia xl mang bj
pha hoai, tU dd lam mdt di cdc tfnh chat ket dfnh eiia
da xi mang. De ehdng lai sU xdm thuc do sunphat
gay ra, edn thilt phai sir dung nhtmg loai xi mdng
thfeh hgp, va gidm sutham thau eiia ion sunphat ydo
trong be tdng.

<b>NHJTNG CHi TIEU KY THUAT OANH GIA 0 0 B E N </b>

-cd the khdng hieu qua nlu mdi trudng ben ngoai ed
chira kiem.

<i>Hinh 5: Tham mudi Sunfat tao khoing ettringit gay ndt be tdng </i>

Trong mdi trudng xdm thuc sunphat trung binh, cd
the sir dung xi mdng loai II (theo each phdn loai xi
mdng ciia ASTM). Trong mdi trudng xdm thuc
sun-phat manh thi sirdung xi mdng loai V. Xi mdng loai II
ed ham lugng C3A tdi da Id 8%, trong khi dd, xi mdng
loai V cd hdm lugng C3A tdi da Id 5%. Khd ndng bin
sunphat cua xl mdng Portland dugc thf nghiem theo
tieu chuan ASTM C452. Hdu hit cdc loai phu gia yd
CO hoat tfnh nhu tro bay (loai F), xi vd mudi silic diu
cd kha ndng Idm tang kha ndng bin sunphat cho xl

mang Portland.

<b>6. Phan ifng kiim-silic </b>

Trong dilu kien cd dp am, cdc ion kilm tdn tai
trong xi mdng se phdn irng vdi nhung thdnh phdn
khodng silic hoat ddng trong elt lieu, phan irng ndy
tao ra gel kilm-sllic truong nd khi hdp thu nude, tirdd
Idm eho be tdng bj nut. De trdnh phan ung kilm-silic,
ngudi Idm tieu chuan nen yeu cdu phdi sir dung loai
edt lieu khdng cd thanh phdn silic hoat ddng. Sir
dung loai xi mdng cd ham lugng kilm thdp (ham
<i>lugng kilm tfnh tUdng duong theo Hafi nhd hon </i>
0,6% khdi lugng xi mdng) cd the ngan chdn dugc
phan iing kilm-silic. Tuy nhien, bien phdp nay cung

<i>Hinh 6 va 7: Tie dung cua kidm vdi cdt lieu gay ndt be tdng </i>

Cdt lieu dugc xem Id khdng gdy ra phan irng
kilm-silic khi dp dan nd trong 14 ngdy nhd hdn 0,1% thf
nghiem theo ASTM C1260, hodc dan nd trong 1 nam
nhd lion 0,04% theo ASTM 1293. Ngodi ra, ed the sir
dung phUdng phdp thach hpc de xdc djnh sd lugng
thdnh phdn khodng silic hoat ddng ed trong edt lieu.

Neu nhu cdt lieu dugc thf nghiem theo nhtmg thf
nghiem dtren vd cho ra kit qud Id cd khd ndng sinh
ra phdn irng kilm-silic, thi cdt lieu vdn cd the sirdung
trong be tdng khi chiing ta sirdung cdc bien phap ho
trg Idm gidm phdn ung kiem-silie nhu diing phu gia

vd eo hoat tfnh vd phu gia hda hpe gdc lithium.

<b>Kit luan </b>

-Yeu cdu tfnh chdt ky thudt ciia be tdng phai dat
dugc tijy thudc vdo tdi trgng vd mdi trudng sir dung
eua cdu kien be tdng.

- Nhiimg chi djnh ky thudt phai rd rang, mirc kit
qua phai dat dugc ciia be tdng tir nhung kiem djnh
tieu chuan - Vf du ddi vdi cdc cdt nhd cdng trinh chi
sir dung cudng do chiu nen vi do bin khdng quan
tdm, nhung tieu chuan ciia cdng trinh cd sd ha tdng
phdi bao gdm R^, R,,, gldl han do thdm vd nirt eo ngdt
vl be tdng sir dung trong mdi trudng an mdn khac
nghiet.

- Phai ehi djnh phUOng phdp thf nghiem va mirc
tieu chuan chap nhdn de nghiem thu, cae yeu cdu thi
nghiem chdt lugng be tdng trude khi thi edng va cac
thf nghiem hien trudng sau thi cdng.

- Nhd thdu vd nha sdn xudt be tdng phai thuc hien
phdn tfeh vd thilt ke cap phdi hdn hgp be tdng va
phUdng phdp thi cdng thfch hgp ddi vdl hdn hgp be
tdng de cdu kien be tdng ddng nhdt dat dugc nhOmg chl
tieu ky thudt cd ly vd dp bin quy djnh ciia be tdng.Q

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