-
TIU CHUN QUC GIA TCVN 9357:2012
B TNG NNG - PHNG PHP TH KHNG PH HY - NH GI CHT LNG B TNG BNG VN
TC XUNG SIU M
Normal concrete - Nondestructive methods - Assessment of
concrete quality using ultrasonic pulse velocity
Li ni u
TCVN 9357:2012 c chuyn i t TCXD 225:1998 thnh Tiu chun Quc gia
theo quy nh ti Khon 1 iu 69 ca Lut Tiu chun v Quy chun k thut v im
b khon 2 iu 7 Ngh nh s 127/2007/N-CP ngy 01/08/2007 ca Chnh ph quy
nh chi tit thi hnh mt s iu ca Lut Tiu chun v Quy chun k thut.
TCVN 9357:2012 do Vin Khoa hc Cng ngh Xy dng - B Xy dng bin son,
B Xy dng ngh, Tng cc Tiu chun o lng Cht lng thm nh, B Khoa hc v Cng
ngh cng b.
B TNG NNG - PHNG PHP TH KHNG PH HY - NH GI CHT LNG B TNG BNG VN
TC XUNG SIU M
Normal concrete - Nondestructive methods - Assessment of
concrete quality using ultrasonic pulse velocity
1 Phm vi p dng
Tiu chun ny hng dn phng php xc nh vn tc xung siu m nh gi cc tnh
cht ca b tng, b tng ct thp v b tng ct thp ng sut trc.
Tiu chun ny c p dng trong cc trng hp sau:
- Xc nh ng nht ca b tng trong mt cu kin hoc gia nhiu cu kin (iu
8);
- Xc nh s hin din v d on s pht trin ca vt nt, xc nh cc l rng v
cc khuyt tt khc (iu 9);
- Xc nh s thay i c tnh ca b tng theo thi gian (iu 10);
- Kim tra cht lng b tng da trn mi quan h gia vn tc xung siu m v
cng (iu 11);
- Xc nh mun n hi tnh v h s Poisson ng ca b tng (iu 12).
m bo tin cy ca phng php, cn thit lp trc mi quan h gia vn tc xung
siu m vi c tnh ca loi b tng cn nh gi da trn cc mu c sn hoc trong qu
trnh thi cng (iu 11, iu 12).
Tiu chun ny p dng cho b tng c cng khng ln hn 60 MPa.
Tiu chun ny c th p dng cho b tng c cng ln hn 60 MPa. Khi cn cn
nhc mt s yu t c nh hng ti mi quan h gia vn tc xung v cng nh loi v
hm lng xi mng, cc ph gia, loi v c ct liu, cc iu kin dng h, tui ca b
tng v thn trng khi x l kt qu.
2 Ti liu vin dn
Cc ti liu vin dn sau l cn thit cho vic p dng tiu chun ny. i vi
cc ti liu vin dn ghi nm cng b th p dng phin bn c nu. i vi ti liu
vin dn khng ghi nm cng b th p dng phin bn mi nht, bao gm c cc sa i,
b sung (nu c).
TCVN 3105:1993, Hn hp b tng nng v b tng nng - Ly mu, ch to v bo
dng mu th.
TCVN 3115:1993, B tng nng - Phng php xc nh khi lng th tch.
TCVN 3118:1993, B tng nng - Phng php xc nh cng nn.
-
TCVN 9334:2012, B tng nng - Phng php xc nh cng nn bng sng bt
ny.
BS 1881, Testing concrete. Part 209 Recommendations for the
measurement of dynamic mudulus of elasticity.
3 Thut ng v nh ngha
3.1 Thi gian truyn (Transit time)
Thi gian cn thit cho mt xung siu m truyn t u d pht ti u d thu
xuyn qua lp b tng nm gia hai u d.
3.2 Gc dng m thi gian (Onset)
Mt trc ca xung do b m thi gian ca thit b pht hin ra.
4 Nguyn l
Xung ca dao ng dc c to ra nh mt b phn bin i in m - sau y gi tt l
u d - c gi tip xc vi mt mt ca phn b tng chu kim tra. Sau khi i qua
chiu di L bit ca b tng, xung dao ng c chuyn thnh tn hiu in nh u d
th hai. Thi gian truyn T ca xung o c nh cc mch in m thi gian. Vn tc
xung V (km/s hoc m/s) c tnh bng cng thc:
V = TL (1)
trong :
L l chiu di ng truyn, c gi l y o, tnh bng kilmt (km) hoc mt
(m);
T l thi gian cn thit xung dao ng truyn qua ht chiu di L, tnh bng
giy (s).
Xung dao ng ca siu m c s dng nhiu hn so vi m v hai l do sau:
a) Cho xung c nh nhn.
b) Pht ra nng lng cc i theo phng truyn ca xung.
Khi xung c truyn vo b tng, n b phn x nhiu ln ti cc mt tip gip
gia cc loi vt liu khc nhau nm trong b tng v to ra mt h thng tng hp
cc sng ng sut, trong bao gm c sng dc v sng ngang lan truyn trong b
tng.
5 Thit b, dng c
5.1 Tng qut
Cc b phn ch yu ca thit b bao gm b phn to xung in, mt i u d, b
phn khuch i v b phn thit b in m thi gian gia thi im lc xung bt u
pht ra t u d pht v thi im xung bt u n u do thu - lc mt trc ca xung
u tin chm ti u thu. C hai loi thit b in m thi gian v hin th kt qu
m, mt loi dng mn hnh hin sng v hin th xung nhn c trn mt thang o thi
gian thch hp, loi kia dng b m thi gian v hin th bng s c trc
tip.
5.2 Cc c tnh
Cc thit b cn c nhng c tnh sau:
a) C kh nng o thi gian truyn qua cc di trong phm vi t 100 mm n 3
000 mm (xem 5.7) vi chnh xc l 1%, c xc nh theo trnh t miu t
5.6.
b) Xung kch thch c dc khng ln hn 1/4 chu k dao ng ca u pht (xem
5.3.2). iu ny nhm to c xung c mt trc r nt.
c) Khong ngt gia cc xung phi ln m bo rng vi cc mu b tng kim tra
c kch thc nh th mt trc ca tn hiu xung nhn c khng b nh hng do s di
li ca cc xung c to ra trong chu k pht trc .
-
d) Qu phm vi gii hn v nhit , m ca mi trng xung quanh v in p ca
ngun in m ngi ch to my yu cu, thit b vn gi c cc c tnh ca mnh.
5.3 u d
5.3.1 Loi u d
Cc u d c tn s nh quy nh 5.3.2 u c th dng c. Loi u d in p v t gio
u dng c, song, vi cc di tn s thp th loi u d t gio thch hp hn.
5.3.2 Tn s ca u d
Thng thng tn s ca u d nm trong phm vi t 20 KHz n 150 KHz, khi
qung ng truyn rt di c th dng loi u d tn s thp n 10 KHz v ngc li c
th dng loi u d c tn s n 1 MHz cho va v h xi mng. Cc u d c tn s t 50
KHz n 60 KHz l thng dng nht.
5.4 Xc nh thi im n ca mt trc ca xung
5.4.1 Tng qut
Mc ch ca vic xc nh ny l o thi gian cn thit cho mt trc xung truyn
xuyn qua b tng. Cc thit b o phi c kh nng xc nh c thi im n phn xung
n sm nht. V mt k thut, c th phn bit c tn hiu xung vi nhiu khi t l
tn hiu xung trn nhiu nh hn 1, song chnh xc yu cu 1 % ca thi gian
truyn li s ch t c khi t s ny ln hn 1.
5.4.2 My hin th dao ng bng mn hin sng
Trong trng hp o thi gian bng mn hnh hin sng th xung cn c khuch i
cho n khi thy xut hin xung rng ca trn trc thi gian. Giao im ca mt
trc xung vi trc thi gian c coi l thi im n ca mt trc xung. Chn giao
im ny lm mc m thi gian.
5.4.3 Thit b hin th s
Khi dng thit b hin th s, xung nhn c phi c khuch i v nh hnh theo
dc tng ng ca xung kch thch b m bt u lm vic.
B m phi m t mt trc xung v m trong khong thi gian ph hp vi chnh
xc nu 5.2. Tuy nhin, chnh xc tuyt i ca thit b lun lun b hn ch bi t
l tn hiu xung trn nhiu. Khi dng thit b hin th s c th xy ra trng hp
thit b hin th thi gian do sng th hai ch khng phi do sng u tin ca
xung khi ng b m gy ra. Cn xem xt dng tng qut ca kt qu o loi tr sai
s do nguyn nhn ny.
5.5 Chnh 0 cho thit b m thi gian
Cn chnh 0 cho thit b o v s o b nh hng bi dc ca xung truyn qua vt
liu u d v truyn trong cp ca u d. Cn iu chnh dc xung mt cch thch hp
cho thit b o khng nh hng n kt qu o.
Vic iu chnh dc xung c thc hin bng cch t hai u d ln hai u i din
ca thanh chun bit trc thi gian truyn trong n. Trong mi ln iu chnh,
phi t u d ln hai thanh chun mt cch nh nhau. Dng mt lp m truyn m mng
v n cht u d ln u mt ca thanh chun.
Mi khi thay i loi u d, hoc thay i di cp, cn chnh 0 chun xc cho
my bng cch hon v hai u d. Ty tng loi cp v s n nh ca ngun in m chnh
0 thng xuyn hn cho my.
5.6 Kim tra chnh xc ca php o thi gian truyn
chnh xc ca php o thi gian truyn xung ph thuc vo thit b in dng o
thi gian v nhy ca n trong vic pht hin mt u xung.
Cn kim tra cc c tnh ca thit b bng cch o trn hai thanh chun bit
trc thi gian truyn xung trong chng vi chnh xc 0,2 s. Cn nh k kim
tra my 5 nm mt ln, hoc cn kim tra khi my b h hng, hay b c s ch to n
a v sa cha. Vic kim tra my phi c ni ch to
-
n hoc phng kim chun cng nhn, cn c vo thanh chun c thi gian truyn
theo ng quy nh ca tiu chun quc gia.
Cc php o trn thanh chun c thc hin nh m t 5.4. Kt qu kim tra khng
c sai khc qu 0,5 % so vi tr s bit ca thanh chun.
5.7 chnh xc ca php o di ng truyn
Php o phi c sai s nh hn 1 %. nhng ch khng o trc tip c chiu di ng
truyn th dng kch thc danh ngha v dung sai ca n theo thit k v phi
ghi iu ny vo bo co. Vi nhng ng truyn c chiu di nh hn 300 mm, khng
cho php ly kch thc theo thit k v nh vy sai s s ln.
6 Xc nh vn tc xung
6.1 Cch b tr u d
thc hin c vic o vn tc xung c ba cch t u d nh sau:
a) Hai u d t trn hai mt i din (truyn trc tip);
b) Hai u d t trn hai b mt vung gc (truyn bn trc tip);
c) Hai u d t trn cng mt b mt (truyn gin tip hoc truyn b mt).
Ba cch b tr u d ny c th hin trong cc Hnh 1a, Hnh 1b v Hnh
1c.
CH THCH:
T: u d pht; R: u d thu.
Hnh 1 - Phng php truyn v nhn xung 6.2 Xc nh vn tc xung theo phng
php truyn trc tip
Nn dng phng php truyn trc tip v n c u im l nng lng truyn qua gia
hai u d t ti mc ln nht v do chnh xc ca php o vn tc xung s ch b nh
hng ch yu bi chnh xc ca php o di. Cn ph lp m cng mng cng tt trnh
hiu ng u mt do vn tc xung khc nhau trong vt liu m v trong b tng gy
nn.
6.3 Xc nh vn tc xung theo phng php truyn bn trc tip.
Phng php truyn bn trc tip c nhy nm gia nhy ca hai cch truyn kia,
mc d trong phng php o ny, chnh xc ca php o chiu di ng truyn c km hn
nhng vic ly khong cch gia tm hai mt u d lm chiu di ng truyn vn t
chnh xc cn thit.
6.4 Xc nh vn tc xung theo phng php truyn gin tip
-
Phng php truyn gin tip c dng khi b tng ch l mt b mt, khi cn xc
nh chiu su vt nt hoc khi cn xem xt mi quan h gia cht lng b mt vi
cht lng chung ca b tng (iu 9).
Phng php o ny c nhy thp nht trong ba cch o, vi cng mt chiu di ng
truyn cho trc, theo phng php o ny, bin ca tn hiu ti u thu ch bng 2
% hay 3 % bin ca tn hiu khi o theo phng php truyn trc tip. Trn cng
mt cu kin b tng, khi o gin tip th vn tc xung thng thp hn so vi khi
o trc tip t 5 % n 20 % ty thuc ch yu vo cht lng b tng kim tra.
Khi o gin tip, vic xc nh chiu di ng truyn c phn km chnh xc nn cn
thc hin mt lot cc php o vi cc y o khc nhau hn ch nhc im ny: u pht
phi c t c nh vo mt b tng ti im x v u thu c t cc im xn xa dn im x dc
theo mt tuyn chn trc trn b mt b tng. Thi gian truyn xung tng ng vi
cc v tr xn c v thnh nhng im trn biu , qua thy mi tng quan gia chng
vi y o tng ng. Xem v d trn Hnh 2 ng b.
o dc (tg) ca ng thng nht i qua cc im v v ly n l vn tc xung trung
bnh ca b tng trn tuyn chn. Trn tuyn ny, ti nhng im c s t bin v vn
tc th c vt nt b mt hoc c lp b tng b mt km cht lng (xem 9.4) v vn tc
xung o c trn y o ny ch l c bit.
a - Kt qu i vi b tng c lp km cht lng dy 50 mm (xem 9.4)
b - Kt qu i vi b tng ng nht
Hnh 2 - Xc nh vn tc xung bng phng php truyn gin tip (b mt)
6.5 p u d ln mt b tng
-
m bo cc xung siu m t u pht xuyn qua b tng ri pht hin c u thu,
phi c s ni m tt gia b tng v b mt cc u d. tip m tt, b mt b tng cn c
to phng bng cch dng cht m truyn m v ng thi phi p mnh u d ln mt b
tng. Cc cht m thng dng l du m ng, m v c, x phng nh, h cao lanh, h
glycerin Cn phi c s liu nhiu ln cho n khi thu c gi tr thi gian
truyn nh nht
Cn t u d ln b mt b tng pha tip gip vi vn khun hoc thnh khun c.
Khi phi o trn b mt c to hnh bng cch khc (nh trt tay) cn o trn mt
tuyn di hn so vi tuyn o bnh thng. Vi b mt khng c to hnh bng khun th
dng chiu di ng truyn ti thiu l 150 mm khi truyn trc tip v ti thiu l
400 mm khi truyn gin tip. Khi b mt b tng qu x x v g gh th phi lm
cho phng v mi nhn vng s p u d. C th dng mt s loi cht to phng nh nha
epoxy ng rn nhanh hoc va trt, song phi m bo s bm dnh tt gia chng vi
b mt b tng xung c truyn hon ton vo b tng kim tra. Lp to phng cng
mng cng tt. Nu lp ny kh dy th phi k n vn tc xung trong n khi tnh
ton vn tc xung trong b trong. trnh nhng nh hng rc ri n vn tip m tt
gia u d v b mt khng phng, cho php dng mt lp m mng v dng loi u d c
bit c th pht v nhn xung qua mi nhn c ng knh 6 mm. Khi dng loi u d c
bit, bt buc phi chnh 0. Khi p u d khng cn thn, s c s bin ng lin tc,
khi p u d tt th s c s nhanh chng n nh.
7 Cc yu t c nh hng n vic o vn tc xung 7.1 Tng qut
m bo php o vn tc xung c th lp li v ch ph thuc v ph thuc ch yu vo
tnh cht ca b tng kim tra, cn phi xem xt cc yu t nh hng ti vn tc v
mi quan h ca cc yu t vi c tnh vt l ca b tng.
7.2 m
m c hai tc ng n vn tc xung: tc ng ha hc v tc ng vt l. Trong vic
thit lp ng chun nhm d on cng ca b tng, hai tc ng ny c vai tr quan
trng. Vn tc xung o trn mu lp phng c dng h chun v vn tc xung o trn
mt b phn kt cu c ch to cng bng loi b tng y c th khc nhau nhiu. S
khc nhau ny phn ln l do iu kin dng h khc nhau tc ng ln s thy ha xi
mng gy nn, cn phn nh l do lng nc t do trong l rng gy nn. Cn xem xt
k cc tc ng ny khi nh gi cng ca b tng.
7.3 Nhit ca b tng
S thay i nhit ca b tng trong khong t 10 oC n 30 oC khng gy ra
nhng bin i ln v cc c trng cng v tnh n hi. Vic hiu chnh kt qu o vn
tc xung ch cn thc hin i vi khong nhit nm ngoi phm vi ny, nh cho Bng
1.
Bng 1 - nh hng ca nhit ti s truyn xung
Nhit , oC
Hiu chnh vn tc xung o c, %
B tng kh B tng bo ha nc
60
40
20
0
-4
+ 5
+ 2
0
- 0,5
- 1,5
+ 4
+ 1,7
0
- 1
- 7,5
7.4 Chiu di ng truyn
ng truyn phi di vn tc xung o c khng b nh hng nhiu bi tnh khng ng
nht t nhin ca b tng. Ngoi nhng iu quy nh trong 7.5, ng truyn phi di
ti thiu l 100 mm i vi b tng dng ct liu th l 20 mm v di ti thiu l
150 mm i vi b tng dng ct liu th t 20 mm n 40 mm. Khi thay i chiu di
ng truyn vn tc xung khng b nh hng nhiu, mc d n
-
c xu hng gim i cht khi chiu di ng truyn tng. Vn tc xung gim i l
do kh xc nh chnh xc mt trc ca xung v n ph thuc vo phng php c th dng
xc nh mt trc xung. S gim vn tc xung ny thng nh v nm trong phm v
chnh xc cho php khi o thi gian truyn nh nu trong 5.2.
7.5 Hnh dng v kch thc mu
Khi kch thc nh nht ca mu kim tra nh hn mt gi tr cc tiu nht nh th
vn tc xung s b gim nhng nu t s gia chiu di bc sng ca xung v cnh nh
nht ca mu nh hn 1 th mc gim s t i. Bng 2 cho mi quan h gia vn tc
xung trong b tng, tn s ca u d v kch thc ti thiu cho php ca cnh
mu.
Nu kch thc ti thiu ny nh hn chiu di bc sng hoc nu dng cch o gin
tip th dng lan truyn sng s thay i v vn tc xung thu c s khc i. Khi
so snh cc cu kin b tng c kch thc khc nhau nhiu th iu ny c ngha c
bit quan trng.
Bng 2 - nh hng ca kch thc mu ti s truyn xung
Tn s ca u d,
KHz
Kch thc ti thiu cho php ca cnh mu, mm
Khi vn tc xung trong b tng, km/s
Vb = 3,5 Vb = 4,0 Vb = 4,5
24
54
82
150
146
65
43
23
167
74
49
27
188
83
55
30
7.6 nh hng ca ct thp 7.6.1 Tng qut
Vn tc xung siu m vng b tng ln cn ct thp s cao hn vn tc xung vng
b tng n thun. Mc nh hng ca thp n vn tc xung ph thuc vo khong cch t
v tr o n ct thp, vo ng knh ct thp, vo s lng thp v phng t ca chng so
vi ng truyn, n cng b chi phi bi c tn s xung v c iu kin b mt ct thp.
Vic iu chnh cc gi tr o do k n ct thp s lm gim chnh xc khi tnh vn tc
xung trong b tng, do nn nhng v tr khng c thp nm trn hoc nm gn ng
truyn gia hai u d. Nn s dng thit b o lp bo v bng phng php t pht hin
v tr ct thp.
7.6.2 Trng hp trc thp song song vi phng truyn xung
Nu khng chn c v tr o sao cho ct thp khng nm song song vi ng
truyn xung th trong qu trnh tnh ton phi k n s c mt ca ct thp bng
cch hiu chnh gi tr vn tc xung.
Khi , vn tc xung trong b tng c tnh theo cng thc:
Vb = 2
t2
t
)LVT(a4
Va2
(2)
Vi iu kin: Vt > Vb
trong :
Vb l vn tc xung siu m trong b tng, tnh bng kilmt trn giy
(km/s);
Vt l vn tc xung trong thp, tnh bng kilmt trn giy (km/s);
a l khong cch t mp thanh thp n ng ni hai im gn nht ca hai u d,
tnh bng milimt, (mm), xem Hnh 3;
T l thi gian truyn, tnh bng microgiy (s);
-
L l chiu di ca ng truyn trc tip gia hai u d, tnh bng milimt
(mm).
Thp s khng c nh hng khi:
bt
bt
VVVV
21
La
V lc ny khng s dng c cng thc (2) na. Vng m ct thp c nh hng n vn
tc xung ph thuc vo gi tr tng i ca vn tc xung trong thp v trong b
tng, trong trng hp thp c ng knh ln nm trong b tng cht lng km th gii
hn trn ca t s a/L c th c gi tr khong 0,25. Trong trng hp b tng cht
lng cao, gii hn a/L khng th ln hn 0,15 nhng s nh i nhiu khi thp c
ng knh nh hn hoc bng 12 mm. Trong thc t khng pht hin c vng nh hng
ca thp c ng knh 6 mm v c th b qua.
Kh khn chnh khi s dng cng thc (2) l vic xc nh gi tr Vt v n b nh
hng bi c ng knh thp v c vn tc xung trong vng b tng bao quanh thp. C
th o vn tc Vt bng cch truyn xung dc theo trc thanh thp nm trong b
tng v b qua lp b tng bo v hai u thanh thp.
Cng thc (2) c th bin i thnh dng sau:
Vb = k x Vm (3)
Trong :
Vm l vn tc xung o c (L/T), tnh bng kilmt trn giy (km/s);
k l h s hiu chnh tnh theo cng thc:
k = + 2 x 21xLa
vi t
bVV
Hnh 3 trnh by cc gi tr ca ng vi nhng gi tr Vb v ng knh thp thng
gp khi dng loi u d c tn s xung 54 KHz. ng vi mt gi tr Vb gi thit,
kt hp tr s tng ng tra trn Hnh 3 vi biu trn Hnh 4 d on gi tr k trong
cng thc (3). c c gi tr Vb gn ng nht cn lp li nhiu ln qu trnh gi
thit Vb, tra v xc nh k.
Cc cng thc ny ch c gi tr khi khong cch a ln hn 2 ln lp bo v u mt
thanh thp. Khi khong cch a nh hn th xung s truyn qua ton b thanh
thp. Trng hp cc thanh thp nm ngay trn ng truyn xung gia hai u d th
h s iu chnh tnh nh sau:
k = 1 - )1(xLL t (4)
trong :
Lt l chiu di thanh thp, tnh bng milimet (mm).
Khi b tng v ct thp bm dnh tt v vng b tng kim tra khng c vt rn nt
th Vb c sai s trong phm v 3 %.
7.6.3 Trng hp trc thanh thp vung gc vi phng truyn xung
Lc ny, c th dng l thuyt tnh ton nh hng ln nht ca thp vi gi thit
rng xung truyn ngang qua ton b cc ng knh d ca tng thanh thp trn ng
truyn ca n, xem minh ha trn Hnh 5.
Trong thc t, khi dng cc u d 54 KHz, c th b qua nh hng ca cc
thanh thp c ng knh nh hn 20 mm. Trng hp cc thanh thp c ng knh t 20
mm n 50 mm, bm dnh tt vi b tng, nh hng ca chng c k n bng cch coi
chng l thanh thp c chiu di bng chiu di tng th L 't (xem Hnh 5). Lc
ny dng h s k 7.6.2 (cng thc(4)) trong tr s ly t Hnh 5 k n vic gim
vn tc trong thp. Trng hp cc thanh thp ngang lin kt khng tt vi b tng
th
-
nh hng ca n s gim v khi thp khng nm trc tip trn ng truyn xung
gia hai u d th nh hng ca n s kh xc nh mt cch chnh xc.
Hnh 3 - nh hng ca ct thp n vn tc xung khi thp nm song song vi ng
truyn
xung
Hnh 4 - nh hng ca ct thp n vn tc xung. H s hiu chnh khi thp nm
song song vi
ng truyn xung (vi a>2b) (xem Hnh 3a)
-
Hnh 5 - nh hng ca ct thp n vn tc xung khi thp nm vung gc vi ng
truyn
xung
8 Xc nh ng nht ca b tng
Vic o vn tc xung cho php nghin cu v ng nht ca b tng. t mc ch ny
phi la chn mt h thng im o phn b u trn mt b mt b tng nht nh ca kt
cu.
S lng im kim tra ph thuc vo kch thc ca kt cu, vo chnh xc yu cu v
tnh bin ng ca b tng. Khi kt cu ln, lm bng b tng kh ng nht, nn dng
mng li im o c khong cch l 1 m, nhng khi cu kin nh v b tng c bin ng
ln th cn gim kch thc li im o. Khi dng chiu di ng truyn T khng i
trong sut qu trnh o th c th s dng ngay gi tr thi gian truyn nh gi
ng nht ca b tng m khng cn phi chuyn i qua vn tc. Trong thc t, cch
lm ny ch thch hp vi cch truyn trc tip.
ng nht ca b tng c biu din di dng mt i lng thng knh lch chun hay
h s bin ng ca vn tc xung o c trn li o. Tuy nhin, ch c th dng nhng
thng s ny so snh s bin ng trong cc cu kin b tng c kch thc hon ton
ging nhau.
Khi nh gi tm quan trng ca bin ng, cn k n nhng yu t c th tc ng n
b phn kt cu kim tra. V d nh s phn b ng sut trong kt cu iu kin ti
trng ti hn hoc l iu kin b mt kt cu.
9 Xc nh cc khuyt tt
9.1 Tng qut
Vic s dng vn tc xung siu m d tm v vch r quy m khuyt tt bn trong
b tng phi do cc chuyn gia c kinh nghim thc hin.
Nu ch n thun cn c vo nhng kt qu o vn tc xung siu m m a ra cc kt
lun chung l rt nguy him. Cc khuyt tt nm gia hai u d, c kch thc ln
hn b rng ca u d v ln hn bc sng ca xung siu m s lm cho thi gian
truyn xung trong b tng b ko di do xung b nhiu x nhng vng khuyt tt.
Hiu ng ny c s dng xc nh v tr cc vt rn nt, cc l rng hoc khuyt tt khc
c kch thc ln hn khong 100 mm su khong hn 100 mm. Vic xc nh v tr
khuyt tt c cn c trn cc ng ng mc ca cc xung siu m. Ti nhng ch b nt
nhng vn gn kt vi nhau do c lc nn (nh cc chu lc) th xung vn truyn
qua c. Nu
-
vt nt b lp y bng cht lng c tnh truyn nng lng xung nh nc bin th
khng pht hin vt nt bng thit b hin s c m phi o s suy gim nng lng d
tm vt nt.
9.2 D tm cc l rng hoc cc hc kh ln
Trn phn b tng chu kim tra, t mt li o c kch thc li ph hp vi kch
thc ca l rng. Bng cch o thi gian truyn xung gia cc u d trn mng li
im o, s nghin cu c quy m cc hc kh ln khi n nm trn ng truyn xung.
Kch thc ca cc hc ny c d on trn c s tha nhn rng xung c truyn theo ng
ngn nht gia hai u d v i xung quanh cc hc kh . Vic d on ny ch c gi
tr khi b tng xung quanh hc kh l ng nht, chc c v c th o c vn tc xung
loi b tng .
9.3 D on chiu su vt nt b mt
o chiu su vt nt b mt (nhn thy c) c hai cch t u d. Hnh 6a th hin
cch t u d thch hp vi mc ch ny. Ly hai gi tr x l 150 mm v 300 mm, o
thi gian truyn tng ng vi chng. Vi nhng tr s x nh trn th chiu su ca
vt nt cha y khng kh c tnh bng cng thc:
21
22
22
21
tttt4150C
(5)
trong :
C l chiu su vt nt, tnh bng milimt (mm);
t1 l thi gian truyn khi x = 150 mm, tnh bng microgiy (s);
t2 l thi gian truyn khi x = 300 mm, tnh bng microgiy (s).
Cng thc (5) c c t gi thit rng vt nt nm vung gc vi b mt b tng v b
tng vng ln cn vt nt l c cht lng ng nht.
C th kim tra xem vt nt c vung gc vi b mt b tng hay khng bng cch
t hai u d gn vt nt nh Hnh 6b v dch chuyn dn mt u d ra xa vt nt. Khi
u d dch chuyn m thy thi gian truyn gim i th chng t vt nt kt thc vo
khong v tr ca u d.
C mt cch b tr u d khc l u pht c t cch tm vt nt 2,5Y v c thi gian
truyn xung ba ln ng vi cc khong cch Y, 2Y, 3Y, k t u pht theo phng
ca vt nt. Thi gian truyn c v ln tng ng vi khong cch nh trn Hnh 6c,
trong Y = 150 mm. Nu phn ko di ca ng thng i qua 2 im (Y,t1) v (2Y,
t2) i qua gc 0 th khng c vt nt ngm, cn chiu su ca vt nt nhn thy s c
tnh theo cng thc:
C=
25T.T
T2T32Y
32
23
22 (6)
Trong :
T2 l thi gian truyn ng vi khong cch 2Y, tnh bng microgiy
(s);
T3 l thi gian truyn ng vi khong cch 3Y, tnh bng microgiy
(s).
Hnh 6c cho thy khi u thu ri xa dn vt nt th thi gian truyn c xu
hng dn t ti tr s thi gian truyn ng vi b tng khng c vt nt.
-
Hnh 6 - Cch b tr u d xc nh chiu su ca vt nt
9.4 D on chiu dy lp b tng km cht lng
C th d on chiu dy lp b tng b mt b km cht lng (do b chy hay do
tip xc vi sunfat, hoc do qu trnh sn xut) qua thi gian truyn xung
siu m dc theo b mt b tng. Dng trnh t nu iu 10 o v kt qu c v trn biu
nh Hnh 2. Vi y o ngn, xung s truyn qua lp b mt v dc tg ca ng thc
nghim cho ta vn tc xung trong lp b mt ny. Vi y o c ln nht nh, xung
u tin sau khi i qua mt phn cch gia hai lp s truyn trong lp b tng c
cht lng cao di n u thu v dc ca ng thc nghim lc ny cho ta vn tc xung
ca lp b tng di. D on chiu dy lp b mt b km cht lng theo khong cch x0
m ti thy dc thay i (Hnh 2) v theo vn tc xung o c trong hai lp b tng
khc nhau bng cng thc sau:
t=hb
hb0
VVVVx
2x
(7)
trong :
t l chiu dy lp b tng km cht lng, tnh bng milimt (mm);
Vh l vn tc xung trong lp b tng b h hng, tnh bng kilmt trn giy
(km/s);
Vb l vn tc xung ca lp b tng tt di tnh bng kilmt trn giy
(km/s);
x0 l khong cch t u pht ti im c dc thay i, tnh bng milimt
(mm).
-
Phng php ny c p dng cho nhng b mt b tng rng ln, trn lp b tng km
cht lng c chiu dy tng i ng u v c Vh nh hn hn Vb. Nhng vng h hng hoc
r t ong cc b th kh xc nh hn, nhng nu dng c hai phng php truyn trc
tip v truyn b mt th vn c th tm c chiu dy gn ng ca lp .
10 Xc nh s thay i tnh cht ca b tng
C th xc nh s thay i tnh cht ca b tng bng cch o vn tc xung nhiu
ln ti nhng thi im khc nhau vi cng mt loi u d v trn cng mt v tr.
S thay i vn tc xung thng biu th s thay i cng v c thun li l c th
o c trong sut thi gian nghin cu trn cng mt mu th nghim.
Vic o vn tc xung c bit thch hp theo di qu trnh ng rn ca b tng,
nht l trong thi gian 36 h u tin. Lc ny, vn tc xung thay i nhanh l
do s thay i ha l trong cu trc va xi mng v nu mun theo di cht ch qu
trnh thay i ny nn o cch 1 h hay 2 h mt ln. Sau 36 h u tin ny, khong
cch gia hai ln o c th ko di n 1 ngy hoc hn na.
11 Quan h gia vn tc xung v cng
11.1 Tng qut
Cht lng b tng thng biu hin bng cng v do vy vic o vn tc xung siu
m c dng d on cng .
Quan h gia vn tc xung siu m v cng b chi phi bi mt s yu t bao gm
tui, iu kin dng h, iu kin m, t l cp phi, loi ct liu v loi xi mng.
Nu yu cu d on cng ca loi b tng no th cn xy dng quan h gia cng v vn
tc xung (sau y gi l ng chun V-R) cho ring loi b tng . ng chun ny c
thit lp bng thc nghim trn c s th nghim mt lng mu ln bao trm phm vi
cng cn c v tin cy v mt thng k. Vic thit lp ng chun gia vn tc xung
vi cng c tin hnh hoc bng mt trong nhng phng php th nghim c m t
trong TCVN 3118:1993 hoc bng cch tin hnh th nghim trn mt kt cu hay
cu kin hon chnh. tin cy ca ng chun ph thuc vo lng mu i din cho cc
kt cu kim tra. thun tin, ngi ta xy dng ng chun ny bng cch th nghim
cc mu c. Thc nghim cho thy ng chun da trn cc mu c cho d on cng thp
hn so vi cng thu c t cc mu ct hoc khoan kt cu.
11.2 ng chun da trn cc mu c
Cng ca b tng c th bin thin theo s thay i ca:
a) T l nc xi mng;
b) Tui lc th nghim.
Yu cu ch s dng mt phng php thay i cng nht nh xy dng ng chun c th
v phng php phi ph hp vi yu cu ra. ng chun V-R s km tin cy khi cng b
tng tng dn ln. Khi yu cu gim st s pht trin cng th dng quan h gia vn
tc xung v s thay i tui b tng, cn khi yu cu kim tra cht lng b tng th
dng ng chun gia vn tc xung v s thay i t l nc xi mng l thch hp.
Vic to mu c tin hnh theo TCVN 3105:1993. Mi m trn nn to t nht 3
mu. Vn tc xung nn o trn mu th nghim hai mt gip thnh khun. Vi mu dm,
t chnh xc cao hn, phi o vn tc xung sut dc chiu di ca dm. Vi mi mu
th nghim nn o t nht 3 ln vi v tr u d t nh, gia v y mu (xem Bng 2 v
kch thc mu). Thi gian truyn xung o c trn tng mu ch c bin thin trong
khong 5 % gi tr trung bnh ca 3 ln o, nu khng th loi b, coi nh mu d
thng. Sau nn th nghim xc nh cng chu nn ca mu theo phng php c m t
trong TCVN 3118:1993.
Tr s trung bnh ca vn tc xung v tr s trung bnh ca cng thu c trn
tng t mu gm 3 mu ging nhau s l nhng s liu xy dng ng chun. ng chun
ny ch c gi tr vi nhng mu c ch to, bo dng, th nghim bng cng mt phng
php. Nu dng cch dng h kh thay cho cch dng h nc th phi c ng chun khc
thay th.
-
11.3 ng chun da trn cc li khoan
Khi xy dng ng chun bng cch th nghim cc li khoan th khng th thay
i cng b tng mt cch ch ng c. Cho nn phi o vn tc xung ring cho tng
vng c cht lng khc nhau v dng gii hn cng ca li khoan vng tng ng xy
dng ng chun. Khi xy dng ng chun lu l phi dng vn tc xung o c trn kt
cu ti vng li khoan, khng c dng vn tc xung o c trn li khoan sau khi
ct bng u v ngm nc v vn tc xung ny thng cao hn vn tc xung o kt cu.
Khoan v th nghim li khoan theo TCVN 3118:1993 v ng chun c xy dng nh
m t 11.2. Vi loi b tng bt k khng c dng h chun th ng chun V-R c dng
ging nhau. Bi vy c th dng ng chun khc ngoi suy ra phn ng chun b
thiu khi xy dng t cc li khoan.
11.4 ng chun i vi cng cc cu kin c sn
Khi i hi cc b phn c sn phi t cng yu cu no th xy dng ng chun gia
vn tc xung vi ring loi cng . Khi thit lp ng chun ny cn tin hnh o vn
tc xung nhng vng d on l b tng s b h hng khi chu ti th nghim. S dng
trnh t m t 11.2 xy dng biu quan h .
11.5 Kt hp vn tc xung vi cc php o khc
Vic d on cng c th t c chnh xc cao hn khi kt hp kt qu o vn tc
xung vi kt qu o tr s bt ny ca sng nh m t trong TCVN 9334:2012. chnh
xc s t cao hn na nu kt hp vn tc xung siu m vi kt qu o khi lng th
tch. kt cu, nn o khi lng th tch ngay trn tuyn o vn tc xung. o khi
lng th tch dng phng php c m t trong TCVN 3115:1993, vi iu kin loi
tr nh hng ca ct thp. Sau c th xy dng ng chun ring cho tng phm vi
khi lng th tch yu cu.
12 Xc nh mun n hi v h s Poisson ng
Mi quan h gia hng s n hi v vn tc xung truyn trong mi trng n hi
ng hng, kch thc v hn c cho bi cng thc sau:
E = )v1()v21()v1(V
22
(8)
Trong :
E l m un n hi ng, tnh bng megapascan (MPa);
v l h s Poisson ng;
l khi lng th tch, tnh bng kilgam trn mt khi (kg/m3);
V l vn tc xung, tnh bng kilmt trn giy (km/s).
trong phng th nghim, t s E/ c tnh t cng thc:
622 10Ln4E
(9)
Trong :
n l tn s cng hng, tnh bng hc (Hz), xc nh theo BS 1881 Part
209;
L l chiu di mu th, tnh bng mt (m).
Kt hp cng thc (8) v (9) ta c:
2
6222
V10Ln4
)v1()v21()v1(
(10)
Tr s v c th xc nh t Bng 3.
-
Thng khng th tin hnh th nghim cng hng trn cc b phn ca kt cu xc
nh cc c tnh ca chng. C th s dng kinh nghim d on gi tr ca mun n hi
tnh v ng t vn tc xung o c mt im bt k trn kt cu. Vi cc loi b tng
thng thng ch to t ct liu t nhin c th ly mun n hi tnh v ng nh Bng 4.
Khi d on mun n hi theo bng ny th t c sai s nh hn 10%.
Bng 3 - Tr s v
V10Ln 3 v
0,257
0,342
0,395
0,431
0,456
0,474
0,487
0,494
0,499
0,45
0,40
0,35
0,30
0,25
0,20
0,15
0,10
0,05
Bng 4 - Mi quan h theo kinh nghim gia mun n hi
tnh v ng vi vn tc xung
Vn tc xung
(km/s)
Mun n hi
(MPa)
ng Tnh
3,6
3,8
4,0
4,2
4,4
4,6
4,8
5,0
24000
26000
29000
32000
36000
42000
49000
58000
13000
15000
18000
22000
27000
34000
43000
52000
13 Bo co th nghim
Bo co cn khng nh rng vn tc xung siu m c xc nh theo ch dn ca tiu
chun ny v nn bao gm cc thng tin sau:
a) Ngy gi v a im lm th nghim;
b) M t kt cu hay mu chu kim tra;
c) Thnh phn ca b tng, bao gm;
1. Loi xi mng;
2. Hm lng xi mng;
-
3. T l nc/xi mng;
4. Loi ct liu v kch thc;
5. Loi ph gia (nu c);
d) iu kin dng h v tui ca b tng ti thi im th nghim;
e) c im k thut ca mi trng bao quanh b tng;
f) S v tr t u d v ng truyn ca xung. Trn s ny nn th hin chi tit v
ct thp hoc ng dn ln cn vng th nghim;
g) iu kin b mt ti im th nghim ( nhn, trt th cng, rp, c vt nt hay
mnh vn do h hng v ha hon);
h) D on m bn trong ca b tng ti thi im th nghim v iu kin dng h lu
di ( y hiu l b mt t, lm kh b mt (ngay khi tho khun) hay kh trong
khng kh (khi tho khun trong iu kin kh));
i) Loi v cu to ca thit b o, chnh xc, tn s ca xung v bt k mt c
tnh c bit no;
j) Chiu di ng truyn, phng php o v chnh xc d on ca php o;
k) Tr s vn tc xung o c (k c nhng hiu chnh cn thit v nhit , nh
hng ca thp);
l) Kt qu d on v cng , khuyt tt, ng nhtty theo yu cu kim tra.
Ph lc A
(Tham kho)
Phng php xc nh h s chuyn i vn tc xung xc nh theo phng php gin
tip sang vn tc xung xc nh bng phng php trc tip
A1 Vic xc nh h s chuyn i (K) t vn tc xung siu m o theo phng php
gin tip thnh vn tc xung o theo phng php trc tip c thc hin khi chun
b th nghim kt cu v thc hin t nht mi nm mt ln.
A2 Ch to t nht 6 mu lng tr c kch thc 100 mm x 100 mm x 200 mm
theo yu cu ca TCVN 3105:1993 t cc m trn khc nhau. Cc m trn ny c cp
phi, cng ngh ch to v ch bo dng nh kt cu c kim tra theo phng php gin
tip.
A.3 o vn tc xung siu m cho tng mu, vi cng mt chun o, theo s nh
cho Hnh A1. Chun o nh nht l 120 mm. B mt b tng trn mu v kt cu, ti
ni o vn tc xung theo phng php gin tip, phi c v tr tng i nh nhau so
vi vn khun v so vi phng b tng.
o thi gian truyn xung theo phng php b mt t nht 3 ln cho mt vng
o.
A.4 H s chuyn i (K) c tnh theo cng thc:
K =
n
1iiKn
1 (A.1)
trong
Ki l h s chuyn i c xc nh theo kt qu th nghim ca mu th i, tnh
theo cng thc:
Ki = mi
iVV (A.2)
Vi l vn tc xung trung bnh ca mu th i, o bng phng php trc
tip;
Vmi l vn tc xung trung bnh ca mu th i, o bng phng php gin
tip;
n l s lng mu th nghim xc nh h s K.
-
A.5 lch bnh phng trung bnh (Sk) ca h s chuyn i K c tnh theo cng
thc:
Sk = W x dn (A.3)
trong :
W = Kmax - Kmin ;
Kmax l gi tr ln nht ca Ki (1 i n);
Kmin l gi tr nh nht ca Ki (1 i n);
dn l h s, ly theo Bng A1 ty theo s lng mu n.
Bng A1 - Gi tr ca h s dn ty theo lng mu th nghim
n 6 7 8 9 10
dn 2,51 3,0 3,47 3,92 4,35
Tr s Sk c s dng tnh sai s ca ng chun, c ni n B.3 - Ph lc B.
CH THCH:
a - S th nghim mu lng tr theo phng php truyn trc tip;
b - S th nghim mu lng tr theo phng php gin tip (b mt);
1 - Phng c mu.
Hnh A.1 S th nghim mu xc nh h s chuyn i K
A.6 Cho php tin hnh o vn tc xung siu m theo A.3 i vi cc vng kt
cu m iu kin k thut cho php o theo c phng php trc tip v gin tip.
S lng vng o trn kt cu t nht l 6 vng.
A.7 Vn tc xung dng xy dng ng chun R-V c k n h s K c tnh theo cng
thc:
-
V = K x mTL x 103 (A.4)
Trong :
Tm l thi gian truyn xung khi o theo phng php gin tip, tnh bng
micro giy (s);
L l chun o, tnh bng milimt (mm);
V tnh bng mt trn giy (m/s).
Chun o ging nhau khi xc nh h s K v khi o trn kt cu v khng c ln
hn 400 mm.
Ph lc B
(Tham kho)
Phng php xy dng ng chun V-R
xy dng ng chun V-R cn c t 30 n 60 mu lp phng cnh 15 cm hoc cnh
10 cm, dng h mu cho n tui th nghim, o vn tc xung siu m Vi. Th nghim
nn mu xc nh cng Rmi ca tng t mu (loi b cc kt qu th nghim d thng),
tin hnh xy dng ng chun theo trnh t sau:
B.1 Xc nh quan h gia V v R:
Quan h gia V v R c 2 dng l tuyn tnh v phi tuyn
- Khi Rmimax - Rmimin (60 - 0,1 x mR )% th dng phng trnh dng
tuyn tnh:
R = a0 + a1 x V (B.1)
- Trng hp cn li th dng phng trnh dng phi tuyn tuyn (hm s m):
R = b0 x eb1V (B.2)
trong :
V l vn tc xung siu m trong mu;
R l cng b tng xc nh theo phng trnh;
mR l cng nn trung bnh ca tt c cc t mu;
Rmimax l cng nn ln nht ca tt c cc t mu;
Rmimin l cng nn nh nht ca tt c cc t mu;
Cc h s a0, a1, b0, b1 c tnh nh sau:
a0 = mR - a1 x V (B.3)
a1 =
n
1i
2i
imin
1im
)VV(
)VV()RR( (B.4)
b1 =
n
1i
2i
n
1imimi
)VV(
)RlnRln()VV( (B.5)
b0 = e VbRmln 1 (B.6)
-
nRR
n
1imi
m
(B.7)
n
VV
n
1ii
(B.8)
n
RlnRln
n
1imi
m
(B.9)
Trong :
Rmi l cng ca mu th i;
Vi l vn tc ca t mu th i;
n l s t mu th nghim xy dng ng chun.
B.2 Vic hiu chnh ng chun c thc hin bng cch loi b mu khng tha mn
iu kin:
2SRR mii
(B.10)
Trong :
S l lch bnh phng trung bnh, xc nh theo cng thc:
S =
n (Rmi R i )2 i1
n 2 (B.11)
trong :
Ri l cng ca t mu th i xc nh theo ng chun xy dng;
Ri = a0 + a1 x Vi i vi phng trnh (B.1) hoc Ri = b0 x e i vi phng
trnh (B.2).
Sau ng chun c xy dng li theo kt qu cc t mu cn li.
Vic hiu chnh ng chun c thc hin cho n khi kt qu tng t mu tha mn
iu kin (B.10).
B.3 Sai s ca cng b tng c xc nh bng ng chun va xy dng, c tnh theo
cng thc:
Sc = 2k22 SqS (B.12)
Trong :
S 2k l lch bnh phng trung bnh ca h s chuyn i K t vn tc o theo
phng php b mt sang vn tc o theo phng php xuyn m. Nu khng c h s
chuyn i th Sk = 0;
q l h s c tnh bng Rm - a0 khi s dng phng trnh (B.1) v bng 0
mm b
RlnR khi dng phng
trnh (B.2).
Nu Sc/Rm x 100% > 12 % th khng c php dng ng chun ny.
B.4 Vn cho php dng phng trnh (B.1) xy dng khi Rmimax - Rmimin
> (60 - 0,1 x mR ) nu Sc/Rmx100%12%
-
B.5 Kim tra ng chun cn c tin hnh nh k t nht hai thng mt ln bng
cch sau:
B.5.1 Ch to t nht su t mu. Xc nh vn tc Vi v cng Rmi ca tng t mu
theo 11.2 ca tiu chun ny. ng vi Vi ca tng t mu, xc nh Ri tng ng bng
ng chun ang s dng.
Tnh vn tc trung bnh V ca tt c cc t mu kim tra ng chun.
Chia cc t mu thnh hai nhm:
- Nhm th nht gm cc t mu c vn tc xung nh hn hoc bng V ;
- Nhm th hai gm cc t mu cn li.
B.5.2 ng chun s c tip tc s dng nu ng thi tha mn cc iu kin
sau:
- Chnh lch Rmi - Ri ca 5 trn 6 t mu phi khc du.
- Phi tha mn bt ng thc: Sn < 1,5 x Sc
trong :
Sn =
n (Rmi R i )2 i1
n 2 (B.14)
n l s t mu c th nghim kim tra ng chun.
- Chnh lch Rmi - Ri ca nhm mu 1 v nhm mu 2 khng c cng du.
B.6 Khi dng loi thit b v tn s u d khc cng phi kim tra ng chun
theo B.5 ni trn.
B.7 V d v vic xy dng ng chun. B.7.1 Thit lp ng chun vn tc-cng
kim tra cng b tng lc tho khun. Yu cu kim tra cng b tng trn kt cu
bng phng php siu m b tng mc 25.
xy dng ng chun "R-V", to 20 t mu lp phng cnh 10 cm trong 5 ngy.
Mu c dng h nhit v th nghim tui 4 h n 8 h sau khi dng h. Sau khi loi
b cc s liu d thng, kt qu th nghim cc mu cho Bng B.1.
Cng mR (MPa) v vn tc xung siu m trung bnh V (m/s) l:
79,2720
3,33...0,266,20Rm
4,423920
4436...43714029V
Cng ln nht v nh nht ca mu l Rmmin = 20,6 MPa; Rmmax = 36,9 MPa
(t mu s 1 v t mu s 17).
V Rmmax - Rmmin = 16,3 MPa < 2 x 27,79 x (60 - 27,79)/100 =
17,9 MPa nn dng ng chun dng tuyn tnh Ri = a0 + a1 x V.
Cc h s a0, a1 c tnh ton theo cng thc (B.3),(B.4):
a1 = 0301,0)44364,4239(...)43714,4239()40294,4239(
......)43714,4239(x)0,2679,27()40294,4239(x)6,2079,27(222
a0 = 27,79 - 0,0301 x 4239,4 = -99,92
Bi vy ng chun c m t bng phng trnh:
Ri = 0,0301 x V - 99,92
-
Cc gi tr cng Ri tnh theo phng trnh ng chun c ghi trong Bng B.1.
lch bnh phng trung bnh xc nh theo cng thc (B.11) bng:
S = 8,218
)6,333,33(...)65,310,26()35,216,20( 222
MPa
Khi so snh gi tr Rmi ca cc t mu vi Ri xc nh bng ng chun (Bng
B.1) thy rng t mu th hai khng tha mn iu kin (B.10), do loi b t mu
ny.
Tnh cc gi tr mi ca mR , V v cc h s a0, a1 theo kt qu ca 19 t mu
cn li:
88,2719
3,33...0,226,20Rm
MPa;
4,423219
4436...40804029Rm
m/s
a1 = 0325,0)44364,4232(...)40804,4232()40294,4232(
......)40804,4232(x)0,2288,27()40294,4232(x)6,2088,27(22
a0 = 27,88 - 0,0325 x 4232,4 = -109,68
S = 48,217
)49,343,33(...)92,220,22()26,216,20( 222
MPa
ng chun va c hiu chnh ny c t s 2SRR mii
vi tt c cc t mu. Bi vy khng cn
phi hiu chnh ng chun na v s dng phng trnh sau lm ng chun:
Ri = 0,0325 x V - 109,68
Bng B.1 - S liu xy dng ng chun V - R
Th t t mu
Vn tc xung Vi,
m/s
Cng , MPa SRR mii
Theo kt qu nn mu,
(Rmi)
Theo ng chun (Ri) Cha x l Sau x l Ch thch Cha x l Sau x l
1
2
3
4
5
6
7
8
9
10
11
12
4 029
4 371
4 080
4 097
4 116
4 137
4 136
4 187
4 195
4 248
4 232
4 285
20,6
26,0
22,0
26,3
21,1
23,4
26,0
26,4
29,2
25,5
28,5
25,0
21,35
31,65
22,89
23,40
23,97
24,60
24,57
26,11
26,35
27,94
27,46
29,06
21,26
-
22,92
23,47
23,09
24,77
24,74
26,40
26,66
28,38
27,86
29,58
0,27
2,02
0,32
-1,04
1,03
0,43
-0,51
-0,10
-1,02
0,87
-0,37
1,45
0,27
-
0,37
-1,14
1,21
0,55
-0,51
0
-1,03
1,16
-0,26
1,85
Loi b
-
13
14
15
16
17
18
19
20
4 267
4 037
4 316
4 352
4 398
4 393
4 475
4 436
31,6
21,7
34,3
30,5
36,9
34,5
33,0
33,3
28,52
21,59
30,00
31,08
32,46
32,31
34,78
33,60
29,00
21,52
30,59
31,76
33,26
33,09
31,76
34,49
-1,10
-0,04
-1,54
0,21
-1,59
-0,78
0,64
0,11
-1,05
-0,07
-1,50
0,51
-1,47
-0,57
1,11
0,48
ng chun trc v sau khi hiu chnh c trnh by trn Hnh B.1
CH DN:
() ng chun trc khi hiu chnh
(__) ng chun c hiu chnh
(x) T mu b loi b
Hnh B.1 - Biu ng chun trc v sau khi hiu chnh
B.7.2 nh gi sai s khi xc nh cng theo kt qu o siu m.
Cng b tng trn kt cu c kim tra theo ng chun va xy dng trn. Tin
hnh kim tra bng phng php siu m, khng dng h s chuyn i. Lc ny, sai s
cng tnh ton theo cng thc (B.12):
Sc = 02 qS
V %1290,8%100x88,27
48,2%100xRS
m
c nn c php dng ng chun ny xc nh cng
b tng.
Ph lc C
(Tham kho)
-
nh gi ng nht v d tm khuyt tt ca b tng trn cng trnh bng phng php
siu m
C.1 nh gi ng nht
nh gi ng nht ca b tng, tin hnh o vn tc xung siu m trn b tng theo
mng li o. Lu la chn mng li im o cho ph hp vi kch thc cu kin v cht
lng b tng. Sau khi c kt qu o vn tc xung, c th xem xt mc ng nht theo
cch sau:
Biu din cc kt qu o vn tc xung thnh biu . S phn tn ca biu cng hp
th ng nht ca b tng cng cao v ngc li.
Hnh C.1 cho thy biu c trng cho ng nht ca cc loi b tng khc nhau.
Da theo dng biu nhn xt s b.
Sau c th tnh ng nht ca b tng bng h s bin ng ca kt qu o vn tc
xung.
Vi tiu chun cht lng xy dng thng thng th gi tr h s khi c bin ng l
t 2 % n 3 % th c th cho l kt cu c ch to ng nht.
Hin cha c quy nh c th v gii hn gi tr bin ng khi yu cu kim tra
cng b tng trn kt cu cng trnh bng phng php siu m.
a, i vi b tng ng nht
b, i vi b tng r t ong
Hnh C.1 - Biu c trng cho ng nht ca b tng
-
c, i vi b tng gm hai loi cht lng khc nhau
Hnh C.1 - Biu c trng cho ng nht ca b tng (kt thc)
C.2 D tm l rng
d tm cc l rng c th nh c c tin hnh o vn tc xung siu m trn mt mng
li im o ph u trn b mt b tng vng nghi ng c khuyt tt. Phi lu la chn
kch thc li o cho thch hp. Sau v ng ng mc vn tc xung ca cc im trn
mng li o. ch m vn tc xung thay i nhanh trong phm vi din tch b mt b
tng nh th hin bng mc dy st ca cc ng ng mc vn tc th ti c l rng.
phc ha c c th hn v tr l rng cn phi dng php tnh gn ng. Dng phng
php thng k vch ng bin ca l rng vi lp lun nh sau:
Gi s cc kt qu o phn b thng thng v c tin cy l 95% th gii hn tin
cy thp hn c tnh theo cng thc V - 1,96 x trong V l gi tr trung bnh
ca vn tc xung, l lch chun ca vn tc xung ( loi tr cc gi tr qu thp).
Cc kt qu o V no thp hn gii hn ny th chc chn l n b tc ng bi nhng yu
t khc ( y mun ni ti l rng). ng biu din gii hn tin cy thp hn ny (trn
b mt b tng c o V) c coi l chu vi ca l rng. Phng php ny s ng tin cy
hn nu l rng c bin r rng v nm vng b tng c ng nht cao.
Phng php d tm v phc ha l rng c kim chng bng cch o V cho mt s mu
b tng c to trc cc l rng c kch thc khc nhau. Li o c kch thc l 50 m.
Kt qu o c th hin trn Hnh C.2.
Hnh v ny cho thy vn tc xung gim nhanh v pha tm l rng v cc ng ng
mc vn tc chm li vi nhau, vn tc xung bn trong l rng thp hn gi tr vn
tc xung trung bnh nhiu. V ng gii hn tin cy thp bm rt st vi bin ca l
rng.
Khi tin hnh o V, v cc ng ng mc V trn 2 mt vung gc vi nhau ta s c
c hnh nh khng gian 3 chiu ca l rng.
-
CH DN:
() Gii hn tin cy thp
(__) ng bin thc ca l rng
Hnh C.2 - ng bao vn tc xung siu m o c trn mu
MC LC
1 Phm vi p dng
2 Ti liu vin dn
3 Thut ng v nh ngha
4 Nguyn l
5 Thit b, dng c
6 Xc nh vn tc xung
7 Cc yu t nh hng n vic o vn tc xung
8 Xc nh ng nht ca b tng
9 Xc nh cc khuyt tt
10 Xc nh s thay i tnh cht ca b tng
-
11 Quan h gia vn tc xung v cng
12 Xc nh m un n hi v h s Poisson ng
13 Bo co th nghim
Ph lc A (tham kho) Phng php xc nh h s chuyn i vn tc xung xc nh
theo phng php gin tip sang vn tc xung xc nh bng phng php trc
tip
Ph lc B (Tham kho) Phng php xy dng ng chun V-R
Ph lc C (Tham kho) nh gi ng nht v d tm khuyt tt ca b tng trn cng
trnh bng phng php siu m
