Tran Van Mien_Tro Bay

Tp ch Khoa hc Cng ngh Xy dng s 3+4/2013

NGHIN CU C TRNG NHIT CA BTNG S DNG HM LNG TRO BAY LN

TS. Trn Vn Min Khoa K Thut Xy Dng, Trng i Hc Bch Khoa i Hc Quc Gia TP.HCM ThS. Nguyn L Thi Phng nghip v 6 Trung tm K thut Tiu chun o lng Cht lng 3 (QUATEST 3)

Tm tt: Hin tng nt do ng sut nhit trong b tng khi ln xy ra kh ph bin, v vy trong cng tc thit k hn hp b tng cng nh thi cng cho b tng khi ln th cc bin php nhm gim thiu ti a nhit do qu trnh hydrat ha ca xi mng trong b tng lun c quan tm. Bi bo ny trnh by kt qu nghin cu c trng nhit thng qua vic nh gi nhit ln nht Tmax trong b tng v chnh lch nhit gia b tng vi mi trng xung quanh. B tng nghin cu s dng nhng hm lng tro bay khc nhau thay th xi mng. Hm lng tro bay s dng thay i t 20% n 50% nhm thay th xi mng PC50 trong cp phi b tng c cng nn t 60 MPa tui 28 ngy trong iu kin bo dng tiu chun. Kt qu nghin cu cho thy, hm lng tro bay s dng thay th xi mng PC cng tng th kh nng gim nhit trong tm ca khi b tng v gim chnh lch nhit gia tm khi b tng v nhit mi trng xung quanh cng nhiu, ng thi thi gian pht trin nhit trong b tng c ko di ra, iu ny c tc dng hn ch ng sut nhit v t lm gim nt trong b tng khi ln.

Abstract: The phenomenon of cracks due to thermal stress often occurs in mass concrete. Therefore, the hydration heat of cement is concerned seriously in concrete mix design. This paper presents results of investigation on thermal characteristic of the concrete in term of maximum temperature, Tmax, and the temperature difference between concrete and environment. The concrete under investigation had different fly ash content replacing cement. The fly ash content replacing cement PC50, which was used for concrete mix design with grade of 60MPa at 28 days, varied from 20% to 50%. The results showed that the maximum temperature of concrete and the temperature difference between concrete and the environment decreased, also the time of temperature increase was prolonged, as the fly ash content replacing PC50 cement increased. This implied that thermal stress and cracks of the mass concrete is limited as the high volume of fly ash is used in the mass concrete.

T kha: tro bay, xi mng PC, btng khi ln, nhit

1. Gii thiu

Ngnh cng nghip xy dng Vit Nam ngy nay ang tng bc to nn mt hnh nh mt t nc Vit Nam hin i vi nhng cao c tm c v c s h tng c ci thin tt hn.Vi nhng cng trnh c quy m ln nh vy trong thit k u c bin php thi cng b tng khi ln nh: cc khi mng ln cho cc cng trnh, cc p thy in v thy li, cc d n trong cu v ng. Tuy nhin, hin tng nt do ng sut nhit trong b tng khi ln thng xy ra ph bin v vy m trong cng tc thit k hn hp b tng cng nh thi cng cho b tng khi ln cc bin php nhm gim thiu ti a nhit do qu trnh hydrat ha ca xi mng trong b tng lun c quan tm.

Theo TCXDVN 305:2004 kt cu b tng hoc b tng ct thp c coi l khi ln khi c kch thc gy ng sut ko, pht sinh do hiu ng nhit thu ha ca xi mng, vt qu gii hn ko ca b tng lm nt b tng v do cn phi c bin php phng nga vt nt. Trong iu kin nng m Vit Nam, kt cu c cnh nh nht (a) v chiu cao (h) ln hn 2m c th c xem l khi ln.

Thng thng, nhit trong b tng khi ln thng c gii hn trnh hin tng nt v m bo s bn vng cho b tng. chnh lch nhit T gia cc im hoc cc vng trong khi b tng nn khng vt qu 200C v mun chnh lch nhit mi trng gia cc im trong khi b tng 500C/m. Ngc li, khi hai yu cu trn khng c m bo do khng c bin php hiu qu kim sot nhit trong b tng khi ln th hin tng nt do ng sut nhit trong b tng c th xut hin. B tng khi ln b nt do hiu ng nhit thu ha xi mng khi c 2 yu t [6]: chnh lch nhit T gia phn nng nht ca b tng v b mt, Mun chnh lch nhit mi trng gia hai im trong khi b tng cch nhau 1m.


Gii php ra hn ch nt trong b tng khi ln l: hn ch nhit ln nht trong khi b tng thng qua s dng xi mng t ta nhit gim nhit hydrat ha ca xi mng; lm mt ct liu v s dng nc lnh trn b tng, hoc hn ch s chnh lch nhit trong khi b tng thng qua phng php dng h bo n cho khi b tng [1, 2, 3, 4].

Bi bo ny trnh by kt qu nghin cu c trng nhit trong b tng khi s dng nhng hm lng tro bay khc nhau thay th xi mng. Hm lng tro bay s dng thay i t 20% n 50% nhm thay th xi mng PC50 trong cp phi b tng c cng nn t 60 MPa tui 28 ngy trong iu kin bo dng tiu chun.

2. Nguyn vt liu s dng cho nghin cu

Nguyn vt liu s dng trong nghin cu gm: ct liu 1x2 c kch thc c ht ln nht Dmax = 20mm, xi mng PC50, ct sng ht ln v sch c Ml = 2,51 v tro bay loi F. Ct liu ln v nh s dng trong nghin cu c thnh phn c ht ph hp vi tiu chun. Kt qu phn tch thnh phn ha hc ca xi mng PC50 v tro bay th hin ln lt Bng 1 v Bng 2.

Cp phi b tng mc M60 (600 daN/cm2) s dng trong nghin cu c thit k tham kho theo phng php Bolomey- Skramtaev, v c trnh by Bng 3.

Bng 1. Thnh phn khong v ha ca xi mng PC50

STT Thnh phn ha v khong n v Kt qu

1 SiO2 % 21,66 2 Al2O3 % 5,58 3 Fe2O3 % 2,79 4 CaO % 63,92 5 MgO % 2,10 6 SO3 % 2,60 7 TiO2 % 0,27 8 MnO % 0,13 9 Na2O % 0,12 10 K2O % 0,58 11 P2O5 % 0,25 12 C3S % 58,30 13 C2S % 22,60 14 C3A % 6,40 15 C4AF % 10,10

Bng 2. Thnh phn ha ca tro bay

STT Thnh phn ha n v Kt qu

1 SiO2 % 59,62

2 Al2O3 % 15,76

3 Fe2O3 % 10,00

4 CaO % 0,70

5 MgO % 3,33

6 SO3 % 0,10

7 Na2O % 0,09

8 K2O % 2,40

9 TiO2 % 1,17

10 MnO % 0,06


Hnh 1. Hnh dng ht tro bay (FA) [4, 5]

Bng 3. Cp phi btng s dng trong nghin cu

STT

K hiu

mu

FA/ (FA+XM)

%

XM (kg)

FA (kg)

Ct (kg)

(kg)

Nc

(L)

PGSD

(L)

1 1-M60 0 471 0 711 1166 140 2,83

2 2-M60 20 377 94 715 1174 139 2,83

3 3-M60 30 330 141 714 1163 133 2,83

4 4-M60 40 283 188 717 1172 132 2,83

5 5-M60 50 235,5 235,5 716 1168 127 2,83

FA: tro bay ; XM: xi mng PC50; PGSD: ph gia siu do.

Qu trnh hydrat ha trong b tng thc cht chnh l phn ng hydrat ha ca xi mng trong qu trnh ng rn. Phn ng hydrat ha ca xi mng ta nhiu nhit, khi nhit ta ra cng ln th s gy ra ng sut nhit lm bin dng kt cu, nt n trong b tng t nh hng n cht lng cng nh tui th ca cng trnh.

S ta nhit ca x imng c nh hng n tnh cht ca b tng rn chc. Khi xi mng ta nhit ln, nhit hn hp s cao, do qu trnh hydrat ha ca cc khong xi mng xy ra cng nhanh hn, v vy qu trnh ninh kt v rn chc ca h tng ln nhanh. Tuy nhin khi nhit ta ra cng ln, ng sut nhit ca h cng ln, iu ny nh hng xu n cu trc ca h. V vy cn ch n nhit trong qu trnh hydrat ha ca xi mng s dng hp l trong cc cng trnh xy dng.

Mc tiu ca th nghim l o nhit trong b tng t xc nh c nhit ln nht ta ra trong sut qu trnh ng rn ca b tng. y l phng php o nhit c lp trong khi b tng. Theo phng php ny th nhit c ghi trc tip trong mu b tng thng qua b ghi d liu v cc u cm bin nhit sensor.

Mu th nghim c kch thc hnh khi lp phng (1500x1500x1500) mm. Khun c mu c cu to nh Hnh 2. Lp cc sensor o nhit ti cc v tr dc theo trung tm ca mu th bao gm: cch y khun 25mm, tm v cch mt trn 25mm.

Nhit trong khi b tng ti im 1, 2, 3 v nhit mi trng c o bng sensor nhit v ghi nhn li theo thi gian 7 ngy lin tc vi chu k ghi s liu l 2 gi, thit b ghi nhn s liu th hin Hnh 3.


Hnh 2. V tr lp t cc sensor nhit

1: im trn; 2: im gia; 3: im di

Hnh 3. Hin th ca b iu khin nhit Autonics-T4WM

3. nh hng ca hm lng tro bay n nhit trong b tng

Trong cc cu kin b tng khi ln, nhit thy ha ca xi mng ti tm khi s tng t bin. Trong qu trnh ng rn, nhit ny c th ln n 850C i vi cc khi c chiu dy ln v s dng xi mng thng thng. S chnh lch nhit trong lng khi b tng vi nhng v tr gn mt thong mi trng v vi nhit mi trng, to ra ng sut ni trong cu kin b tng, t hnh thnh cc vt nt trong khi b tng. Cc vt nt do nhit xut hin do s chnh lch nhit ln gia nhng v tr khc nhau trong khi b tng. S chnh lch nhit ny c th pht sinh do b tng l vt liu c tnh dn nhit tng i thp. iu ny thng xy ra i vi nhng khi ln, nhit tng trong sut qu tnh thy ha ca xi mng.

Trong nghin cu ny nhm kho st nhit trong qu trnh hydrat ha ca cc mu b tng vi cc t l phn trm tro bay thay th cho lng dng xi mng ln lt l: 0, 20, 30, 40, 50 %. V tr cn o nhit l t dc theo trung tm ca mu th gm 3 v tr: v tr 1 trn cch b mt trn 25mm, v tr 2 gia v v tr s 3 di cch b mt di 25mm. Song song vi o nhit trong khi ta cng tin hnh o nhit mi trng xung quanh khi .

Kt qu kho st nh hng ca hm lng tro bay n c trng nhit trong btng khi ln th hin hnh 4, 5, 6, 7, 8 v 9.

Hnh 4. c trng nhit trong khi b tng s dng 100% xi mng PC50 theo thi gian


Hnh 5. c trng nhit trong khi b tng s dng 80% xi mng PC50 + 20%FA theo thi gian





Hnh 9. c trng nhit ti tm khi b tng s dng cc hm lng tro bay khc nhau theo thi gian

Mu i chng 1-M60 s dng 100% xi mng PC50 c nhit cao nht c th l: nhit ti v tr gia c nhit cao nht Tmax = 810C sau 15 gi. Mu 2-M60 s dng 20% tro bay thay th lng dng xi mng PC50 c nhit cao nht ti v tr gia l Tmax = 730C sau 24 gi (hnh 9). Mu 3-M60 s dng 30% tro bay thay th hm lng xi mng PC50 t nhit cao nht ti v tr gia l Tmax = 710C sau 26 gi. Mu 4-M60 s dng 40% tro bay thay th lng dng xi mng t gi tr nhit ln nht ti v tr gia l Tmax = 610C sau 29 gi.

Mu 5- M60 s dng t l tro bay thay th 50% t gi tr nhit cao nht v tr gia khi b tng l Tmax = 590C sau 31 gi. Kt qu th hin Hnh 4 n Hnh 9 cho thy rng nhit ln nht trong khi b tng gim xung ng k t 810C khi s dng 100% xi mng PC50 xung cn 590C khi thay th 50% khi lng xi mng PC50 bng tro bay. Cc kt qu nghin cu trn cng cho thy rng, nhit ln nht trong khi b tng cng tng chm li khi hm lng xi mng PC50 c thay th bng tro bay tng ln, c th l khi s dng 100% xi mng PC50 th Tmax trong tm ca khi b tng t c sau 15 gi, trong khi Tmax trong tm ca khi b tng t c sau 31 gi khi s dng 50% tro bay thay th khi lng xi mng PC50. Ngoi ra, do nhit ln nht trong khi b tng gim xung, s chnh lch nhit gia tm khi b tng v nhit mi trng xung quanh cng gim xung khi hm lng tro bay s dng thay th xi mng PC tng ln. Chnh lch nhit ti v tr c nhit cao nht (nhit ti tm khi ) so vi nhit mi trng ca cc mu th nghim cc t l tro bay thay th cho xi mng: 0, 20, 30, 40 v 50% ln lt l: 530C, 390C, 400C, 310C v 300C. Trong xu hng hin nay, trong b tng khi ln (BTKL), ngoi vic quan tm n chnh lch nhit gia cc v tr trong khi th mt i lng khc chng ta cng cn phi quan tm l chnh lch nhit T (l s chnh lch nhit ln nht trong b tng so vi nhit ca mi trng), c c gi tr T ta c th xc nh c kh nng xut hin vt nt sau khi tnh ton c ch s nt. Thc t gi tr T gim ng k khi tng lng dng tro bay thay th, v khi gi tr T cng ln th kh nng xut hin vt nt cng cao.

C c kt qu nh trn l do gim c lng dng xi mng nn nhit ta ra trong qu trnh hydrat ha ca xi mng gim, khi s dng tro bay thay th mt phn khi lng xi mng, phn ng puzzolanic xy ra rt chm, nhit trong b tng tng t t trong mt thi gian di. iu ny c gii thch nh sau [2, 4]:

S c mt ca tro bay lm cho trnh hydrat ha ca C3S chm li trong giai on u do tr hon s hnh thnh mm Ca(OH)2 m nguyn nhn l s hp th ha hc cc ion Ca2+ ln b mt ht tro bay, lm gim nng Ca2+ trong dung dch. Ngoi ra NaOH m tro bay to ra s lm tng hm lng kim c trong va, lng kim ny c cho l nguyn nhn lm ha tan cc thnh phn silicat v aluminat trong tro bay. Cc thnh phn ny gy nh hng lm chm n s hnh thnh Ca(OH)2 v C-S-H.

i vi qu trnh hydrat ha ca C2S: s hin din ca tro bay hu nh khng c nh hng n s hydrat ha ca C2S trong khong 14 ngy u, sau tro bay mi gy nh hng t n s thy ha ca C2S.

i vi qu trnh hydrat ha ca C3A v C4AF: s c mt ca tro bay lm chm s hydrat ha ca C3A v C4AF. C kin cho rng s hydrat ha ca C3A b chm li khi c mt tro bay theo c ch tng t nh s dng thch cao lm chm qu trnh ninh kt ca xi mng.

Kt qu nghin cu khng nh xu hng rng, hm lng tro bay s dng thay th xi mng PC cng tng th tc dng gim nhit trong tm ca khi b tng v gim chnh lch nhit gia tm khi b tng v


nhit mi trng xung quanh cng nhiu, v thi gian pht trin nhit trong b tng c ko di ra, ba xu th ny c tc dng hn ch ng sut nhit trong khi b tng, v t lm gim nt trong b tng khi ln.

4. nh hng ca hm lng tro bay n cng chu nn ca b tng

Kt qu kho st nh hng ca hm lng tro bay n cng chu nn v s pht trin cng ca b tng khi ln th hin hnh 10 v 11.

Kt qu cho thy cng chu nn 7 ngy ca mu i chng s dng 100% xi mng PC50 tng ng k c th l 64,6 MPa trong khi cc mu s dng tro bay th cng chu nn 7 ngy tui pht trin rt chm. C th l t l tro bay thay th l 20% th cng chu nn l 53,8 MPa, 52,5 MPa t l tro bay thay th l 30%,v 48,2 MPa t l tro bay thay th 50%. Nguyn nhn l v giai on u ca qu trnh hydrat ha, phn ng puzzolanic ca tro bay l chm hn so vi phn ng hydrat ha ca xi mng. Mu b tng s dng 100% xi mng PC c cng chu nn pht trin sm, cng 7 ngy tng ng k, cao hn nhiu so vi cc mu s dng tro bay thay th xi mng PC. Tuy nhin, n tui 28 v 35 ngy th h s dc gia cc mu khng chnh lch ln, c xu hng xch li gn nhau hn. iu ny c ngha l, giai on sau 7 ngy, c bit l 35 ngy, cng chu nn ca b tng s dng 100% xi mng PC v 50%PC + 50%FA c s chnh lch thp, tro bay lm cho b tng pht trin cng chm giai on u 7 ngy v t t pht trin giai on sau.

cc tui 7, 28, 35 ngy khi tng t l thay th tro bay trong cht kt dnh cng, chu nn gim tng ng so vi mu i chng, nguyn nhn l do khi tro bay thay th cng nhiu th lng xi mng cng t, ng ngha vi lng khong to cng C-S-H , C-A-H t qu trnh thu ho xi mng s t lm cng b tng thp thi gian u.

Hnh 10. nh hng ca tro bay n cng chu nn ca b tng

Hnh 11. nh hng ca tro bay n s pht trin cng ca btng 5. Kt lun

T cc kt qu nghin cu, c th rt ra cc kt lun nh sau:

a) S gia tng nhit trong b tng c th c kim sot bng cch hn ch lng dng xi mng trong hn hp b tng. Khi thay th xi mng PC bng tro bay lm gim s gia tng nhit ti a v ko di thi gian t nhit ti a trong b tng. Kt qu cho thy nhit cc i trong cc mu b tng gim tng ng khi tng t l s dng tro bay thay th v thi gian t nhit cc i cng tng theo vi cc t l tro bay c thay th. Mu i chng Mu 1-M60 s dng 100% xi mng c nhit cao nht c th l: nhit ti v tr gia c nhit cao nht Tmax = 810C sau 15 gi, mu 2-M60 s dng 20% tro bay thay th lng dng xi

C

ng

ch

u n

n, M

Pa

Hm lng tro bay thay th xi mng PC

C

ng

ch

u n

n, M

Pa

Thi gian, ngy


mng c nhit cao nht ti v tr gia l Tmax = 730C sau 24 gi, mu 4-M60 s dng 40% tro bay thay th lng dng xi mng t gi tr nhit ln nht ti v tr gia l Tmax = 610C sau 29 gi, mu 5-M60 s dng t l tro bay thay th 50% t gi tr nhit cao nht v tr gia l Tmax = 590C sau 31 gi.

b) Khi c mt tro bay trong b tng dn n s pht nhit thy ha chm v thp hn nhiu so vi mu b tng khng s dng tro bay ng thi lm gim chnh lch nhit ln nht gia tm khi b tng v nhit mi trng xung quanh. iu ny ph hp vi cng tc thi cng BTKL v s hn ch c cc vt nt do ng sut nhit v c th tnh ton b tr khi ln hn, to iu kin tng tin thi cng. Nhit ti tm ca khi b tng c t l nghch vi hm lng tro bay trong khi . Vi t l thay th l 50% xi mng PC50 bng tro bay gim c 220C ti tm khi btng.

c) S dng tro bay hm lng ln c th nh hng n s pht trin cng ca b tng. C th xem xt s dng hm lng tro bay thay th cho xi mng t 20 40% gim nhit trong khi b tng, gim s chnh lch nhit gia khi b tng v mi trng xung quanh ng thi vn t c cng thit k ph hp vi tui yu cu.

TI LIU THAM KHO

[1]. Gajda John and VanGeem Martha, Controlling temperatures in Mass Concrete, Concrete International, January 2002.

[2]. Cengiz Duran Atis, Heat Evolution of High -Volume Fly Ash Concrete, Cement and Concrete Research, 32, pp. 751 -756, 2002.

[3]. Escalante-Garcia, J. I., and J. H. Sharp, The effect of temperature on the early hydration of Portland cement and blended cements, Advances in Cement Research,12 (3), pp. 121-130, 2000.

[4]. Bamforth, P.B, In Situ Measurement of the Effect of Partial Portand Cement Replacement Using Either Fly Ash or Ground Granulated Blast Furnace Slag on the Performance of Mass Concrete, Proc. Inst. Civil Engrs. Part 2, pp. 777 800, 1980.

[5]. Takemoto, K., Uchikawa, H, Hydration of pozzolanic cement, 7th International Congress on the Chemistry of Cement, Paris, I IV-2/1-2/29, 1980.

[6]. Mehta, P.K.: Concrete: structure, properties and materials. Prentice Hall, Englewood Cliffs, 1986.

[7]. Soo Geun Kim- Iowa State University, Effect of heat generation from cement hydration on mass concrete placement, 2010.

[8]. Siddique, R, Effect of fine aggregate replacement with class F fly ash on the mechanical properties of concrete, Cement and Concrete Research, 33(4), pp. 539547, 2003.

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