CHNG 2 NHNG TNH CHT CH YU CA VT LIU

2.1. Cc thng s trng thi v c trng cu trc ca vt liu 2.1.1. Khi lng ring

Khi lng ring l khi lng ca mt n v th tch vt liu trng thi hon ton c. Khi lng ring c tnh bng cng thc

aV

G = (g/cm3, kg/dm3, kg/l, kg/m3 hay tn/m3)

trong : G-khi lng mu vt liu trng thi hon ton kh, g. Va-th tch vt liu trng thi hon ton c, cm3. 2.1.2. Khi lng th tch

Khi lng th tch l khi lng ca mt n v th tch vt liu trng thi t nhin (k c th tch l rng). Khi lng th tch c tnh bng cng thc

0V

G= (g/cm3, kg/dm3, kg/m3 hay tn/m3)

trong : G- khi lng mu vt liu trng thi hon ton kh, g. V0-th tch ca mu vt liu trng thi t nhin, cm3. Thng thng mt loi vt liu, khi lng th tch c th bin ng trong phm vi rng hn nhiu so vi khi lng ring v n ph thuc vo cu trc chnh ca vt liu. i vi mt vt liu, khi lng th tch lun c tr s nh hn khi lng ring. Ch vi vt liu c xem l tuyt i c th hai tr s ny mi bng nhau. Khi lng th tch ca vt liu c ngha rt quan trng trong k thut. Thng qua khi lng th tch ca vt c th nh gi s b mt s tnh cht khc ca n nh: xp, ht nc, tnh truyn nhit, cng 2.1.3. xp xp r l t s gia th tch xp trong vt liu vi th tch t nhin ca n, xc nh bng cng thc

0

r

VV

r =

trong Vr-th tch xp c trong vt liu,

V0-th tch t nhin ca vt liu. Tuy nhin xp cn hay c tnh ra % theo cng thc

x100VV

r0

r= (%)

Bit rng Vr=V0-Va, trong Va-th tch vt liu trng thi hon ton c, do

=

=

=

1

VV

1V

VVr

0

a

0

a0 hoc l x1001r

= (%)

trong : -khi lng th tch ca vt liu, g/cm3,

-khi lng ring ca vt liu, g/cm3. xp l mt ch tiu k thut rt quan trng ca vt liu v n nh hng

n nhiu tnh cht khc ca chnh vt liu nh: cng , ht nc, tnh chng thm, tnh truyn nhit, kh nng chng n mn Ngoi ra, c trng cu trc ca cc l xp trong vt liu cng ng vai tr rt quan trng.

2.1.4. mn mn l ch tiu k thut nh gi kch thc ht ca vt liu dng ht

ri rc. i vi vt liu ri rc, bn cnh vic xc nh mn cn cn phi quan

tm n hm lng ca cc nhm c ht, hnh dng ht v tnh cht b mt ca ht (gc thm t, tnh nhm rp, kh nng hp ph v lin kt vi cc vt liu khc). 2.2. Tnh cht vt l c lin quan n nc 2.2.1. m m W l t l phn trm ca nc c thc trong vt liu ti thi im th nghim. 2.2.2. ht nc ht nc ca vt liu l kh nng ht v gi nc ca n iu kin bnh thng. ht nc theo khi lng Hp l t s phn trm gia khi lng nc m vt liu ht c vi khi lng ca vt liu trng thi kh. Hp c tnh ton theo cng thc sau

.100G

GG.100

GGH unp

== (%)

trong : Gn-khi lng nc m mu vt liu ht c, g, Gu-khi lng mu vt liu t sau khi ht nc, g,

G-khi lng mu vt liu trng thi hon ton kh, g. 2.2.3. Tnh thm nc Tnh thm nc l tnh cht cho nc thm qua chiu dy ca n khi gia hai b mt vt liu c chnh lch p sut thu tnh. 2.2.4. co dn m Mt s vt liu rng c ngun gc hu c hoc v c nh g, b tng khi m thay i th th tch v kch thc ca chngcng thay i: co khi m gim, n khi m tng ln. 2.3. Tnh cht vt l c lin quan n nhit 2.3.1.Tnh truyn nhit Tnh truyn nhit ca vt liu l tnh cht cho nhit truyn qua chiu dy ca khi vt liu, t pha mt gii hn c nhit cao sang pha mt gii hn c nhit thp. Tnh truyn nhit c ngha rt quan trng i vi nhng vt liu dng trong cc b phn cng trnh xy dng dn dng (nh tng bao che, mi, trn) v c bit l i vi nhng vt liu cch nhit chuyn dng gi nhit cho cc bung v thit b nhit. 2.3.2.Tnh chng chy v tnh chu la Tnh chng chy l kh nng ca vt liu chu c tc dng ca ngn la trong mt thi gian nht nh. Vt liu khng chy l vt liu di tc dng ca ngn la hay nhit cao cng khng b bt la, khng chy m v cng khng b cacbon ho. N c th c bin dng nh khng ng k nh gch ngi, b tng, aming hay nhng vt liu c bin dng ln nh thp. Vt liu kh chy di tc dng ca ngn la hay nhit cao c th b bt la, chy m hay b cacbon ho mt cch kh khn. Vt liu d chy di tc dng ca ngn la hay nhit cao s bt la v tip tc chy sau khi b ngun chy. Tnh chu la l tnh cht ca vt liu chu c tc dng lu di ca nhit cao m khng b chy v bin hnh. Vt liu chu la c kh nng tc dng lu di ca nhit cao hn 15800C nh gch chu la samt, inas Cc vt liu ny dng lt bn trong cc l cng nghip. Vt liu kh chy chu c nhit t 13500C n 15800C gm nhng loi gch c bit xy l v xy ng khi.

Vt liu d chy chu c nhit thp hn 13500C nh gch t st nung thng thng. 2.4. Tnh cht c hc 2.4.1. Tnh bin dng ca vt liu Tnh bin dng l tnh cht thay i hnh dng v kch thc ca vt liu khi chu tc dng ca ngoi lc. Thc cht ca bin dng l ngoi lc tc dng ln vt lm thay i hay ph v v tr cn bng ca cht im trong vt liu v lm cho cc cht im ny c chuyn v tng i. Bin dng ca vt liu c xc nh bng bin ng tuyt i v bin dng tng i.

Bin dng tuyt i l (mm) tnh theo cng thc l = l1 l0, trong l0 v l1 l chiu di ban u v chiu di sau khi bin dng ca mu th.

Bin dng tng i tnh bng cng thc

x100ll

0

= (%)

Bin dng n hi: sau khi b ngoi lc th bin dng mt i hon ton. Tnh cht hi phc hnh dng v kch thc ban u ca vt liu sau khi d b ngoi lc gi l tnh n hi. iu kin bin dng n hi l: xut hin khi ngoi lc tc dng cha vt qu lc tng tc gia cc cht im ca vt liu. Cng c ngoi lc s chuyn ho thnh ni nng nng lng n hi. Nu b ngoi lc i, tc l nng lng n hi s sinh cng khi phc v tr cn bng ban u cho cc cht im. Khi bin dng ca vt liu s trit tiu. Bin dng n hi thng xy ra khi ti trng nh v tc dng ngn hn. Tnh n hi ca vt liu c c trng bng mun n hi Eh.

Edh = (daN/cm

2 hay MPa)

trong -ng sut giai on n hi, daN/cm2 hay MPa.

-bin dng n hi tng i, %. Khi ngoi lc tc dng ln vt liu ln hn lc tng h gia cc cht im ca vt liu s gy nn s ph hoi cc b hay ton b i vi cu trc ca vt liu. Lc ny cng do ngoi lc gy ra khng chuyn ho thnh ni nng v gy ph hoi cu trc ni b vt liu v do bin dng khng th b trit tiu. Vt liu dn: cho ti trc khi ph hoi vn khng c hin tng bin dng do r rt (gang, thin nhin, b tng).

Bin dng do: sau khi d b ngoi lc vt liu khng tr li hnh dng v kch thc ban u. Tnh cht ny gi l tnh do.

Vt liu do: vt liu trc khi ph hoi c bin dng do v rt r rt thp t cacbon, bitum Nh trn ni, iu kin ca bin dng n hi l ngoi lc tc dng ln vt liu cha vt qu lc tng tc gia cc cht im (phn t) ca vt liu. Khi lc ln v di hn th ngoi bin dng n hi cn xut hin bin dng do. Vt liu n hi l tng c bin dng n hi ca tun theo nh lut Hooke. Vt liu do l tng (nh cht lng l tng) c bin dng tng i tun theo nh lut Newton:

.t = (%)

trong -ng sut trt, daN/cm2 hay MPa, t-thi gian, s,

- nht, daN.s/cm2 hay MPa.s. Khi vt liu c c tnh n hi v tnh do (b tng atphan hay cht do), bin dng tng hp bao gm hai thnh phn: bin dng n hi h v bin dng do d.

= h + d

.t

E += hay

+=t

E1

Khi mt ngoi lc khng i tc dng lu di ln vt liu c th lm cho bin dng ca vt liu tng theo thi gian v hin tng ny c gi l t bin. Nguyn nhn gy ra t bin l do b phn phi tinh th trong vt liu c tnh cht gn ging vi cht lng v mt khc, bn thn mng li tinh th cng c nhng khuyt tt (hin tng sai lch cu trc). Hin tng bin dng ca vt liu di tc dng ca ngoa lc khng thay i theo thi gian, m ng sut trong vt liu gim dn theo thi gian, c gi l hin tng chng ng sut. Nguyn nhn ca hin tng chng ng sut l do mt b phn vt liu c bin dng n hi chuyn sang bin dng do, nng lng n hi chuyn dn thnh nhit nng v mt i. 2.4.2. Cng Cng l kh nng ln nht ca vt liu chng li s ph hoi do ti trng hoc tc ng mi trng gy ra v c xc nh bng ng sut ti hn khi mu vt liu b ph hoi.

Vt liu phi chu cc ti trng khc nhau nh: ko, nn, un, ct tng ng vi mi dng chu ti ny s c cc loi cng : cng chu ko, cng chu nn, cng chu un, cng chu ct Cng l ch tiu k thut quan trng ti thiu nh gi cht lng ca nhng vt liu dng cho cc b phn chu lc ca cng trnh. Bi vy cng c dng lm cn c ch yu nh ra mc ca cc vt liu. i vi cc vt liu c kh nng chu nn cao ( thin nhin, b tng xi mng, cng chu nn thng cao hn cng chu ko t 8 n 15 ln) thng c dng lm cc b phn chu nn ca cng trnh: cng chu nn dng nh ra mc. i vi vt liu c kh nng chu ko cao (thp xy dng): cn c vo cng chu ko nh ra mc. Cng ca vt liu thng c xc nh bng phng php th nghim ph hoi mu: t cc mu vt liu c gia cng thch hpln my gia ti tng ti cho n khi mu b ph hoi. Cng ca vt liu c tnh ton t cc kt qu xc nh trong th nghim theo cc cng thc tng ng vi dng chu lc c nghin cu trong mn sc bn vt liu. Chng hn nh cng chu nn Rn v cng chu ko Rk ca vt liu c tnh bng cng thc

F

PR maxn = (daN/cm

2 hay MPa)

hay F

PR maxk = (daN/cm

2 hay MPa)

trong : Pmax-ti trng nn hoc ko ph hoi mu, daN hay N, F-tit din chu lc ca mu, m2 hay mm2. Cng chu un Ru ca vt liu c tnh bng cng thc

WMR u = (daN/cm

2 hay MPa)

trong : M-mmen un ph hoi, daN.cm hay N.mm, W-mmen chng un ca tit din chu un, cm3 hay mm3. 2.4.3. cng cng l tnh cht ca vt liu chng li tc dng m xuyn ca vt liu khc cng hn n.

Vi vt liu khong, cng c nh gi bng thang Mohr. Thang Mohr gm c 10 khong vt mu c sp xp theo mc cng tng dn t 1n 10 (xem bng).

2.4.4. mi mn mi mn l mn khi lng trn mt n v din tch mu b mi

mn trn my th nghim. mi mn ca vt liu ph thuc vo cng, cng v cu to bn

trong ca vt liu. 2.4.5. Tnh chng va p Tnh chng va p l kh nng ca vt liu chng li s ph hu do tc

dng ca ti trng va p gy ra v c biu th bng cng cn thit p v mt n v th tch mu vt liu.