1Vt liu Thit k cu b tng Phm Tun L2009B Tng Cng chu nn Cng chu ko M un n hi Tnh cht lu di ca b tng2Cng chu nn ca b tng fc : cng chu nn 1 trc, ca mu b tng hnh tr 15cmx30cm, tui 28 ngyfcfcc1cc'', fc' cc'fc0cEng sut,fcBin dng, cc Kin ngh dng fc = 16 MPa 70 MPa Bn mt cu: fc >= 28 MPa Thng thng: fc = 30 50 MPaAASHTONotes:1 Pa = 1 N/m21 MPa = 10.2 kg/cm23Cng chu ko ca b tngCc th nghim xc nh cng chu ko ca b tngKo trc tip(Direct Tension)Un(Modulus of Rupture)p ch(Splitting Test)Cng chu ko un, MPa (Modulus of Rupture) B tng t trng thng thng (AASHTO-07 5.4.2.6) :'.c rf f 63 0 =Khi tnh nt v vng'97 . 0c rf f =Khi tnh ct thp chu un ti thiu 5.7.3.3.2 B tng t trng thp (AASHTO 5.4.2.6) :'52 . 0c rf f ='45 . 0c rf f =Khi tnh vngKhi tnh ct thp chu un ti thiu'54 . 0c rf f =Khi tnh sc khng ct theo 5.8.3.4.34Cng chu ko, MPa (Direct Tensile Strength)'62 . 0c rf f =tng ng vi CT sau, tnh bng psi'5 . 7c rf f = C.5.4.2.7: Khi xt nt do co ngt, vng neo b p v,hay cc trng hp chu ko khc khng do un, nn s dng cng chu ko trc tipB tng chu S 2 trc, 3 trc a- mt trcfcfcfcfcfrf11f2ff2c11cc2b- hai trc c- ba trc5B tng chu S 2 trc failure stressfailure strainB tng chu S 3 trc3 11 . 4 ' o o + =cf3 11 . 4 ' o o + =cf6B tng c kim ch(Confined concrete) B tng c kim ch c cng v gii hn ng sut ln hn b tng khng kim chKim chBin dng,ng sut,Khng kim chcofcccf''f'cc cc ccf0 cocfcfcfrc- ba trcM un n hi, EcAASHTO 5.4.2.4' 5 . 10043 . 0c cf E =Ec: MPacl t trng ca b tng (kg/m3) fcl cng chu nn ca b tng (MPa)i vi b tng t trng thng thng, bng 2320 kg/m3, Ec c th ly nh sau:' 4800c cf E =7Cc c tnh ph thuc vo thi gian ca b tng* Cng nn ca b tng thay i theo thi giana,b hng s, ph thuc vo thnh phn ca b tng v phng php bo dngttui ca b tng (ngy)S thay i cng ca b tng cn c quan tm khi xt kh nng chu lc ca kt cu trong qu trnh thi cngACI 209R-92, 2.1ACI 209R-92Prediction of Creep, Shrinkage, and Temperature Effects inConcrete Structures8Cng chu nn ca b tng theo thi gian0.000.200.400.600.801.001.201.400 100 200 300 400Thi gian, ngyCng chu nn, fc'(t)/fc'(28)a=4; b=0.85Co ngt(Shrinkage)* Co ngt ca b tng l hin tng gim th tch theo thi gian sau khi b tng ng cng, do s mt nc trong b tng.* Co ngt ph thuc vo hm lng N/XM, loi xi mng, ct liu, qu trnh bo dng, m, nhit , t l th tch/din tch b mt.* Co ngt c th gy:- Nt b tng- ng sut ph* Bin php gim co ngt v nt do co ngt:- Gim hm lng N/XM- Bo dng b tng: gi m, lm mt, gi kn gi- B tr thp chng co ngt9T bin(Creep)* T bin ca b tng l hin tng bin dng tng theo theo thi gian di ng sut khng i.* T bin ph thuc vo nhiu yu t nh m, t l th tch/din tch b mt, cng ca b tng, tr s ng sut lu di trong b tng v tui ca b tng ti thi im bt u chu ng sut lau di* T bin c th gy:- Gim S trc trong thp DL- Bin dng ln trong kt cu theo thi gian* Bin php gim nh hng ca t bin:- Gim hm lng N/XM- Bo dng b tng: gi m, lm mt, gi kn gi- B tr thpH s t binci i i CRt t t t c c ) , ( ) , ( =AASHTO 20075.4.3.3.210ci i i CR) t , t ( ) t , t ( c c =( )( )0,6i0,6i0,118i f ct t 10.0t tt120H1.58 k 3,5k +|.|

\| =) , (it t Collins and Mitchel, 1991H - m tng i (%)kc- h s xt n anh hng ca t l th tch/ din tch b mt, xc nh theo th trn hinh 1.7kf- h s xt n anh hng ca cng b tngcff 4262k' +=f'c- cng chu nn 28 ngy ca b tng (MPa)11Xc nh kcCt thp thng Quan h S Bin dng S chy (Yield stress-fy) S cc hn (Ultimate stress-fu) M un n hi12Quan h S Bin dng thp thanhEs= 200 000 MPaThp d ng lc Cc loi thp DL: cp, thanh Quan h S Bin dng S chy, S cc hn Chng S13Quan h S Bin dng thp STCc c trng ca thp STVt liu Chng loiDng knh(mm)Cng chu ko fpu(MPa)Gii hn chy fpy(MPa)Tao thp 1725 MPa (Cp 250)1860 MPa (Cp 270)6.35 n 15.249.53 n 15.241725186085% ca fpu90% ca fpuvi cpc chng thpThanh bar Loi 1, thp trnLoi 2, thp c g19 n 3515 n 361035103585%ca fpu80%ca fpuTr s mun n hi ca thp d ng lc Epc kin ngh trong AASHTO [A.5.4.4.2] nh sau:- Di vi tao cp: Ep = 197 000 MPa- Di vi thp thanh: Ep= 207 000 MPa14Loi b tng(Class of Concrete)- B tng loi A c dng cho mi b phn ktcu v c bit l b tng trong nc mn- B tng loi B dng cho mng, b cc, m trnng v cc tng trng lc.- B tng loi C dng cho cc tit din c chiudy nh hn 100mm, nh lan can hoc lpy trong cc sn li thp.- B tng loi P c s dng khi cng yucu ln hn 28 MPa.c tnh ca cc loi b tng