Bi ging: BTNG C S

Chng I. Khi nim chung v btng ct thp 1

Chng 1

KHI NIM CHUNG V BTNG CT THP (BTCT)

1.1 Tnh cht ca btng ct thp :

Btng ct thp l vt liu xy dng phc hp do hai loi vt liu l btng v thp c c trng c hc khc nhau cng phi hp chu lc vi nhau.

Btng l loi vt liu phc hp bao gm xi mng (cht kt dnh), ct, si - (ct liu) kt li vi nhau di tc dng ca nc. Cng chu ko ca btng nh hn cng chu nn rt nhiu (8 - 15 ln).

Ct thp l loi vt liu chu ko hoc chu nn u rt tt. Do nu t lng ct thp thch hp vo tit din ca kt cu th kh nng chu lc ca kt cu tng ln rt nhiu. Dm btng ct thp c th c kh nng chu lc ln hn dm btng c cng kch thc n gn 20 ln.

Btng v ct thp cng lm vic c vi nhau l do:

+ Btng khi ng rn li th dnh cht vi thp cho nn ng lc c th truyn t vt liu ny sang vt liu kia, lc dnh c c m bo y th kh nng chu lc ca thp mi c khai thc trit .

+ Gia btng v ct thp khng xy ra phn ng ha hc, ngoi ra h s gin n ca ct thp v btng sut sot bng nhau:

s = 0.000012 ; b = 0.000010-0.000015

1.2. Phn loi:

Theo phng php thi cng c th chia thnh 3 loi sau:

Btng ct thp ton khi: ghp cp pha v btng ti cng trnh, iu ny m bo tnh cht lm vic ton khi (lin tc) ca btng, lm cho cng trnh c cng v n nh cao.

Btng ct thp lp ghp: ch to tng cu kin (mng, ct, dm, sn,) ti nh my, sau em lp ghp vo cng trnh. Cch thi cng ny m bo cht lng btng trong tng cu kin, thi cng nhanh hn, t b nh hng ca thi tit, nhng cng ton khi v n nh ca c cng trnh thp.

Bi ging: BTNG C S

Chng I. Khi nim chung v btng ct thp 2

Btng ct thp bn lp ghp: c mt s cu kin c ch to ti nh my, mt s khc ti cng trnh m bo cng ton khi v n nh cho cng trnh. Thng th sn c lp ghp sau, cn mng, ct, dm c ton khi.

Nu phn loi theo trng thi ng sut khi ch to ta c:

Btng ct thp thng: khi ch to, ct thp trng thi khng c ng sut, ngoi ni ng sut do co ngt v gin n nhit ca btng. Ct thp ch chu ng sut khi cu kin chu lc ngoi (k c trng lng bn thn).

Btng ct thp ng sut trc: cng trc ct thp n ng sut cho php (sp), khi bung ct thp, n s co li, to ng sut nn trc trong tit din btng, nhm mc ch kh ng sut ko trong tit din btng khi n chu lc ngoi hn ch vt nt v vng (hnh 1.2).

Hnh 1.1 Dm btng ct thp thng vng xung

khi chu ti

Hnh 1.2 Dm btng ct thp ng sut trc

th di chu nn trc

Bi ging: BTNG C S

Chng I. Khi nim chung v btng ct thp 3

1.3. u v khuyt im ca btng ct thp :

Btng ct thp hin nay l vt liu xy dng c s dng rng ri v c cc u im sau:

R tin so vi thp khi chng cng chu ti trng nh nhau.

C kh nng chu lc ln so vi gch v g, c th chu c ti trng ng lc v lc ng t.

Bn vng, d bo dng, sa cha t tn km so vi thp v g.

Chu la tt hn so vi thp v g.

C th c thnh kt cu c hnh dng bt k theo cc yu cu v cu to, v s dng cng nh v kin trc.

Tuy nhin btng cng tn ti mt s nhc im sau:

Trng lng bn thn kh ln, do kh lm c kt cu nhp ln. Nhng nhc im ny gn y c khc phc bng cch dng btng nh, btng ct thp ng lc trc v kt cu v mng....

Di tc dng ca ti trng, btng d pht sinh khe nt lm mt thm m v gy thm cho cng trnh.

Thi cng phc tp, tn nhiu cp pha khi thi cng ton khi.

1.4. Phm vi ng dng v xu hng pht trin:

BTCT c s dng trong nhiu lnh vc, lm kt cu chu lc ca nh, cu, p, cc cng trnh cp thot nc, mng dn nc, tng chn, nh my thy in,...

BTCT ngy cng t ra chim u th trong cc lnh vc xy dng, nh vo cc tin b khoa hc k thut, khc phc c mt s nhc im chnh ca btng, btng ngy cng c kh nng chu lc tt hn, thay th c nhiu kt cu trong cc dng cng trnh khc nhau.

Bi ging: BTNG C S

Chng 2. Tnh cht c l ca vt liu 4

Chng 2 TNH CHT C L CA VT LIU

Tnh nng c l ca btng bao gm : tnh nng c hc - nghin cu v cng v tnh nng vt l - nghin cu v bin dng, co ngt, chng thm v chng n mn ca btng.

Tnh nng c l ca btng ph thuc phn ln vo cht lng xi mng, cc c trng ca ct liu (si, dm, ct liu rng,...) cp phi ca btng, t l nc, xi mng v cch thi cng. V ph thuc nhiu nhn t nn cc tnh nng khng c n nh lm, tuy vy tnh nng c l ca btng vn c th m bo tha mn cc yu cu ca thit k nu chn vt liu, tnh ton cp phi v thi cng theo ng nhng qui nh ca qui trnh ch to.

Cn c vo trng lng th tch, btng c chia ra hai loi ch yu sau:

- Btng nng : c trng lng th tch t 1800 n 2500 kgf/m3 .

- Btng nh c trng lng th tch t 800 n 1800 kgf/m3.

2.1. Tnh nng c l ca btng : 2.1.1. Cng btng

Cng l c trng c hc ch yu ca btng. Trong kt cu btng ct thp, btng ch yu chu nn, cng chu nn c th xc nh tng i chnh xc bng th nghim, v vy cng chu nn c dng lm ch tiu c bn ca btng.

2.1.1.1. Cng chu nn : Mu th khi vung 15x15x15 hoc lng tr trn ng knh 16cm (din tch

200cm2), chiu cao h=2D, c tui 28 ngy, c thnh phn v cch pha trn nh lc thi cng thc t, mu c dng h trong iu kin tiu chun:

F

NR P (MPa hoc kgf/cm2) (2.1)

Trong : NP : Lc nn ph hoi (N hoc kgf)

F : Din tch mt chu nn ca mu th (m.m2 hoc cm2).

2.1.1.2. Cng chu ko :

Thng thng ngi ta lm mu chu ko tit din vung, cnh a, hoc chu un: tit din bxh, chiu di L=6h (hnh 2.1), hoc c th nn ch mu lng tr trn (hnh 2.1.a)

Bi ging: BTNG C S

Chng 2. Tnh cht c l ca vt liu 5

Cng chu ko vi mu (a):

R(t) = LDP

2 (2.2)

Trong : P: ti trng tc dng lm ch mu L: chiu di mu D: ng knh mu Cng chu ko vi mu (b):

R(t) = FN k (2.3)

Cng chu ko vi mu (c):

R(t) = 25,3

bhM (2.4)

2.1.1.3. Quan h gia cng chu ko v cng chu nn:

Thng thng ngi ta c th tnh cng chu ko thng quan cng chu nn bng cng thc thc nghim m khng cn lm th nghim chu ko. n gin nht l quan h ng thng, theo cng thc:

R(t) = 0,6 + 0,06R (2.5) Hoc quan h ng cong:

R(t) = 130060150

RR R (2.6)

2.1.1.4. S tng cng theo thi gian:

Cng ca btng tng theo thi gian. Cng lc u tng kh nhanh, sau chm dn, n mt vi nm sau th hu nh l dng li.

L=6h

L/3

L

a

L=4a

Nk

a

D

a Nk

P

P

b

M

L/3P P

h

Hnh 2.1 Cc kiu mu th ko btng

a). Mu th ch; b). mu th ko c). mu th un

a)

b) c)

Bi ging: BTNG C S

Chng 2. Tnh cht c l ca vt liu 6

xc nh cng ca btng theo thi gian c th dng cng thc nghim sau:

tRtRR lg7,028lg

lg2828 (2.7)

Trong : t - tui ca btng tnh theo ngy.

Cng thc trn ca tc gi Lin x - Skrantaep (1935) ch cho kt qu ph hp vi thc t khi tui ca btng t 7-300 ngy, ty theo mi nc c qui nh khc nhau.

2.1.1.5. Gi tr tiu chun ca cng btng:

Gi tr tiu chun ca cng btng hay cn gi l cng tiu chun (Rbn) c tnh nh sau (thng c ly vi mu th lng tr):

Rbn = kcRch (2.8) Trong : kc - h s k n s lm vic ca btng trong thc t, c th ly

bng 0,7 0,8. Rch - cng c trng ca mu th, c tnh nh sau: Rch = Rm(1 - S) (2.9)

Vi Rm gi tr trung bnh (cng trung bnh) ca mu th = nRi

(n - s lng mu) S - h s ph thuc vo xc sut m bo, vi xc sut m bo l 95% th c th ly S = 1,64.

- h s ng cht ca btng, c th ly nh sau: = 0,135 cho btng c thnh phn v cht lng thi cng cao. = 0,150 cho btng c thnh phn v cht lng thi cng thng.

T cng thc (2.8) ta cng thy rng c th ly Rbn bng cng c trng ca mu lng tr.

2.1.1.6. Cp bn v mc ca btng: a). Mc theo cng chu nn (M):

Theo tiu chun c 5574 1991, mc btng k hiu l M l cng trung bnh ca mu th khi vung, cnh a=15cm, tnh bng kG/cm2. Btng c cc mc sau: M50, 75, 100, 150, 200, , M600.

b). Cp bn chu nn (B):

Theo tiu chun mi 356 2005 quy nh phn bit cht lng btng theo cp bn chu nn, k hiu l B l cng c trng (Rch) ca mu th khi vung, cnh

Bi ging: BTNG C S

Chng 2. Tnh cht c l ca vt liu 7

a=15cm, tnh bng MPa. Btng c cc cp bn B3,5; B5; B7,5; B10; B12,5; B15; B20; B25; B30; B35;; B60.

Tng quan gia cp bn B v mc M ca cng mt loi btng c th hin qua cng thc sau:

B = M (2.10) Vi : - l h s i n v t kG/cm2 sang MPa, c th ly = 0,1.

- l h s chuyn i t cng trung bnh sang cng c trng, theo cng thc (2.9) th = (1 - S).

2.1.1.7. Cc yu t nh hng n cng ca btng a). Yu t vt liu:

Cht lng v s lng ximng: thng thng trong 1m3 btng cn dng t 250 500kg ximng, khi dng ximng nhiu th cng btng cao hn, nhng ch to btng cng cao (B25, 30, ) ngoi vic tng lng ximng cn cn phi dng ximng mc cao (PC40, 50, ) mi em li hiu qu kinh t v s dng. Chng hn nh: ch to btng c cp bn B7,5; 10; 12,5; 15 c th s dng ximng PC30, cn khi ch to btng c cp bn B20; 25; 30 cn dng ximng PC40, nu s dng ximng PC30 th phi dng vi s lng nhiu, khng t hiu qu kinh t, ng thi lm tng tnh co ngt v t bin trong btng nh hng xu n cht lng btng.

cng, sch v t l thnh phn ct liu (cp phi): thit k cp phi hp l s em n hiu qu s dng cao v tit kim ximng.

T l nc ximng: t l ny cao s lm gim cng btng v tng tnh co ngt, t bin, nhng nu t l ny thp (va ) th kh thi cng, c bit l khi bm btng.

a). Yu t con ngi:

Ngoi vic s dng vt liu tt, sch, cn c yu t con ngi nh hng n cht lng btng, c bit l trong iu kin thi cng ton khi ti cng trnh, gm cc yu t sau:

Cht lng thi cng: thi cng k lng, m cht ng qui cch, s t c cng btng nh mong mun.

Cch thc bo dng: trong iu kin thi cng ton khi ti cng trnh, iu kin bo dng kh t c nh trong phng th nghim, nhng cn bo dng tht tt trong iu kin c th t c cht lng btng cao v gim co ngt, c bit l cho sn.

Bi ging: BTNG C S

Chng 2. Tnh cht c l ca vt liu 8

Cht lng btng qua kt qu th nghim i khi cng khng phn nh ng cht lng btng thc t, y yu t con ngi c tm nh hng ln, m c th l ngi lm th nghim, n gm cc yu t sau:

Ly mu v bo dng mu: ly mu cn tun th ng qui trnh c qui nh trong tiu chun TCVN 3105-1993. Bo dng mu c th bo dng theo iu kin tiu chun hoc trong iu kin thc t m cu kin chu nh hng ti cng trnh.

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