MT S THAY I CA TCXDVN 239:2006 SO VI TCXD 239:2000

TS. NGUYN C THNG Vin KHCN Xy dng

Hng nm, sn lng b tng hng nm trn th gii t khong 1,5 - 2 t mt khi v trong tng lai gn btng vn l mt trong nhng vt liu c bn dng trong xy dng. Khc bit ln nht ca btng so vi nhng vt liu xy dng khc l cc tnh cht ca btng, v c bit l cng , c hnh thnh v pht trin trong qu trnh thi cng. Do vy vic xc nh cng btng ng rn trong kt cu hoc cu kin btng so snh vi yu cu ca thit k l rt quan trng nhm m bo cht lng ca cc cng trnh btng v btng ct thp.

Nm 2000 B Xy dng ban hnh TCXD 239:2000 "B tng nng. Ch dn nh gi cng b tng trn kt cu cng trnh" phc v cho mc ch trn. y l tiu chun u tin trong h thng tiu chun xy dng hng dn tng hp v chi tit v phng php t chc, la chn, th nghim v nh gi cng btng trn cng trnh.

Tiu chun TCXD 239:2000 gp phn nng cao cht lng cng tc kim tra cht lng btng, phc v cng tc kho st v nghim thu cht lng cng trnh xy dng. Tuy nhin, TCXD 239:2000 cha t c tnh h thng, cc khi nim cha thng nht v mt s cng thc cha chnh xc. Chnh v vy, tiu chun TCXD 239:2000 c sot xt vo nm 2005 v B Xy Dng ban hnh TCXDVN 239:2006 "B tng nng. Ch dn nh gi cng btng trn kt cu cng trnh".

Bi vit ny gii thiu nhng thay i trong TCXDVN 239:2006 so vi TCXD 239:2000. 1. Tnh h thng ca tiu chun

Ni dung ca TCXDVN 239:2006 bao gm 9 phn v ph lc : 1. Phm vi p dng; 2. Ti liu vin dn; 3. Thut ng, nh ngha; 4. Mc ch xc nh cng btng hin trng 5. Phm vi th nghim; 6. Cc phng php th nghim xc nh cng b tng hin trng; 7. La chn phng php th nghim; 8. Quy trnh th nghim xc nh cng b tng trn kt cu cng trnh; 9. nh gi cng b tng trn kt cu cng trnh; Cc ph lc A, B v C. So vi tiu chun TCXD 239:2000, tiu chun TCXDVN 239:2006 sau khi sot xt m bo c tnh h thng, cc phn TCXDVN 239:2006 tun th b cc ca mt tiu chun ch dn v ph hp vi trnh t thc hin cng vic, bt u t mc ch xc nh cng btng thc t trn kt cu hoc cu kin n so snh, nh gi vi yu cu v cng do thit k quy nh. Mc d ni dung ca tiu chun c 9 phn so vi 6 phn trong TCXD 239:2000, song TCXDVN 239:2006 c trnh by c ng trong 12 trang A4 so vi 16 trang trong TCXD 239:2000. Cc ni dung trng lp, cc quy nh khng lin quan n vic xc nh v nh gi cng btng trong TCXD 239:2000 c loi b. Tiu chun TCXDVN 239:2006 kt ni c nhng quy nh v cng btng trong TCXDVN 356:2005 "Kt cu btng v btng ct thp. Tiu chun thit k" thng qua khi nim cp btng v to thnh h thng ng b. 2. Nhng khc bit gia TCXDVN 239:2006 v TCXD 239:2000 2.1 Phn ti liu vin dn

Tiu chun sot xt TCXDVN 239:2006 b sung 2 tiu chun quan trng TCXDVN 356:2005 v TCVN 4453:1995 "Kt cu btng v btng ct thp ton khi. Quy phm thi cng v nghim thu". Ba tiu chun ny to thnh h thng t thit k n thi cng v nghim thu btng ca kt cu btng v btng ct thp trn c mu c v trong kt cu. 2.2 Thut ng, nh ngha Trong tiu chun sot xt nm 2006 loi b hai khi nim khng dng khi nh gi cng btng trn kt cu cng trnh v b sung 4 khi nim: mc v cp btng theo cng chu nn, cng btng yu cu v nh gi cng btng trn cu kin hoc kt cu cng trnh. Bn

khi nim ny m bo cho s thng nht gia thit k, thi cng v nghim thu kt cu btng v btng ct thp theo h thng tiu chun xy dng Vit Nam. 2.3 Mc ch xc nh cng btng hin trng Phn ny lc b cc thng tin v khi nim khng cn thit khi xc nh cng btng hin trng v ch tp trung vo 2 mc ch c th. Cn lu rng kt qu xc nh cng btng hin trng khng thay th cho vic xc nh cng hn hp btng trn mu c. 2.4 C s la chn phng php th nghim Tu thuc vo mc ch s dng kt qu th nghim (kim tra, kim nh, hoc gim nh) c th chn phng php th nghim ph hp [1]. TCXDVN 239:2006 cho php s dng ng khoan c ng knh bng hai ln kch thc ht ln nht ca ct liu. iu ny nn p dng khi ct liu c kch thc ht ln nht t 40mm tr ln [2,3]. 2.5 Tnh ton xc nh cng btng hin trng Trong tiu chun TCXDVN 239:2006 phn tnh ton xc nh cng btng hin trng c quy nh theo phng php khoan ly mu v phng php khng ph hu. Khi s dng phng php khoan ly mu, phn ch gii trong cng thc xc nh cng btng hin trng ca mu khoan:

mkht xRDR

15.1

c chnh xc ho. Theo khi p dng cng thc ny th cng btng hin trng c qui v cng lp phng mu chun v khng c nhn thm vo cc h s v theo quy nh trong TCVN 3118:1993. Trng hp s dng phng php khng ph hu, cng btng hin trng c b sung:

)1( hththt vtRR

, trong :

- htR

l cng btng trung bnh ca cc vng kim tra;

- htv l h s bin ng cng btng ca cc vng kim tra; - t l h s ph thuc vo s lng vng kim tra; Nh vy, ng nht ca btng cng thp (gi tr htv cng cao) v s lng vng th cng t

th cng btng hin trng cng thp. 2.6 nh gi cng btng trn kt cu cng trnh

Phn ny c vit li hon ton khi sot xt TCXD 239:2000 theo cc nguyn tc: - Ph hp vi cc tiu chun cng loi ca cc nc pht trin; - Gn kt vi tiu chun thit k kt cu btng v btng ct thp TCXDVN 356:2005.

- C th p dng cng vi tiu chun thit k TCVN 5574:1991 trong giai on chuyn tip, khi s dng "cp" thay cho "mc" b tng; Gi tr ti thiu ca cng b tng hin trng c chp nhn la chn mc cao trn c s phn tch v so snh gi tr ny trong mt s tiu chun tng ng [4, 5, 6], theo :

ACI 318 -02: '85,0 cck ff

v 'if'75,0 cf ;

AS 3600: '9,0 cck ff

;

BS 6089: Rht cuf8,0 . T xc nh rng, i vi phng php khoan ly mu, btng c chp nhn t yu cu

v cung chu nn khi: Rht ycR9,0 = 0,9B = 0,7M v Ri ycR75,0 = 0,75B = 0,585M Trong trng hp s dng cc phng php khng ph hu, btng trn cu kin hoc kt cu cng trnh c chp nhn t yu cu v cng chu nn khi: Rht ycR9,0 = 0,9B = 0,7M 3. Kt lun

- TCXDVN 239:2006 sau khi c sot xt cng vi tiu chun TCXDVN 356:2005 v TCVN 4453:1995 to nn s ng b t thit k, thi cng n nh gi s ph hp i vi cc kt cu btng v btng ct thp .

- Cc cng thc v khi nim c a ra v hiu chnh trong qu trnh sot xt. - B tng trn kt cu cng trnh c chp nhn nghim thu v cng chu nn khi gi tr

cng btng hin trng khng nh hn 90% cng btng yu cu. TI LIU THAM KHO

1. ACI 301 Standard specification for structural concrete. 2. ASTM C39 Standard test method for compressive strength of cylindrical concrete specimen. 3. 28570-90 . ,

. 4. ACI 318 Building code requirements for structural concrete. 5. AS 3600. Concrete structures. 6. BS 6089:1988 Guide to assessment of concrete strength in existing structures.