ANALYSIS OF AND PROPOSAL FOR DEALING WITH EARLY CRACKS IN 13 M PRECAST REINFORCED CONCRETE HOLLOW-SLAB MEMBERS

Transcription

1 NLYSIS OF ND PROPOSL FOR DELING WITH ERLY CRCKS IN M PRECST REINFORCED CONCRETE HOLLOW-SLB MEMBERS Juan Huang, Rongui Wang and Liyan Liu Postdoctoral Researc Fellow, College o Traic and Communications, Sout Cina University o Tecnology, Guangzou, Guangdong 564, Cina Tel: Fa: jscut5@6.com Proessor, College o Traic and Communications, Sout Cina University o Tecnology, Guangzou, Guangdong 564, Cina Postgraduate Researc Fellow, Dept. o Civil Engineering, Sout Cina University o Tecnology, Guangzou, Guangdong 564, Cina BSTRCT Because o te inluence o te dept-span ratio, early structural cracks caused y sel-weigt loading are common in simply supported eams o m precast reinorced concrete ollow slas. However, suc cracks seldom occur in prestressed concrete memers. ltoug tere are ew researc pulications availale on te metod involving te use o moulds incorporating an internal air ag in precast ollow-sla memers, tis kind o sla is widely used in ridge projects or its convenience in construction and its cost advantage. To determine te main reasons or te early cracks in a atc o m precast reinorced concrete ollow-sla memers eing used in our ridges under construction, along wose midspans transverse cracks ave appeared, a wide-ranging investigation covering te metod o construction to te distriution o te cracks was conducted on site. Teoretical calculations, comined wit detailed numerical analysis, revealed tat te main reason or te induction o early cracks in memers is sel-weigt loading. It is sown tat te teoretical values correlate well wit te results o te numerical analysis. In order to evaluate te perormance o te ridge deck system in wic te cracks appear and to judge te eect o tese early cracks on structural eaviour, numerical analysis was used and it was concluded tat te ridge deck is ale to ulil te serviceaility requirements and to provide adequate earing capacity. Eective and economical anti-cracking measures and a tecnical proposal are recommended wit regard to te design, construction and amendment o te design speciications. Keywords: Precast reinorced concrete; ollow-sla memers; early cracks; numerical analysis Proceedings o te 5 t Soutern rican Transport Conerence STC 6 July 6 ISBN Numer: Pretoria, Sout rica Produced y: Document Transormation Tecnologies cc Conerence organised y: Conerence Planners

2 . INTRODUCTION Four reinorced concrete ridges tat span rivers were recently constructed in Inner Mongolia. Eac span o te ridge superstructure consists o pieces o simply supported precast ollow slas wit a m long reinorced concrete pavement, cm tick, and a ituminous concrete layer, 7 cm tick. Te ridge sustructure consists o columnar piers and a ried autment. Early cracks in sla memers during precasting ave led to construction eing interrupted or a time. Few researc pulications are availale on te metod involving te use o moulds incorporating an internal air ag in normal precast ollow-sla memers, altoug a considerale volume o researc as een conducted on te cracking o prestressed concrete memers Jin, Tay Coon, 987; Yang, Lin, 995; Li et al. 4. Moreover, normal precast ollow slas are widely used in ridge projects or teir convenience in construction and cost advantage, as descried in World Construction 986. Consequently, tere is a need to conduct researc to provide guidelines or te design and construction o normal precast ridge sla memers. Tis paper comines teoretical calculations wit detailed numerical analysis rom wic it is concluded tat sel-weigt loading is te main reason or te induction o early cracks in precast memers. In order to evaluate te perormance o te ridge deck system in wic te cracks appear and to judge te eect o te early cracks on structural eaviour, numerical analysis was used and it was concluded tat te ridge deck is ale to provide adequate earing capacity. However, main load-carrying memers are required not to crack under te action o sel-weigt loading, even toug te structural system composed y te memers may ulil normal serviceaility requirements. Tis work as solved te prolems in engineering practice, leading to a proposal or eective and economical anti-cracking measures. Te tecnical proposal covers design, construction and te amendment o te design speciications.. CRCKING INVESTIGTION Te ridge slas, as te main load-carrying memers o te superstructure, are normally m precast reinorced concrete ollow slas designed according to te standard drawings. Te sectional dimensions o te middle and side sla memers are sown in Fig.. Te Figure. Sectional dimensions o m middle and side slas cm. ollow sla is.96 m in lengt, wit a teoretical span o.6 m. Te concrete grade o te sla memers is C. Te tensile zones o every middle and side sla are reinorced wit main ars o 4Φ near te midspan, wit a cm concrete layer over tem to protect

3 te main ar. Te compressive zones at te midspan section are reinorced as ollows: middle sla 5Ф and side sla 7Ф. Te ridge deck system is made up o pieces o simply supported ollow slas, inged side y side. Te ridge superstructure is a composite-type pavement structure as tere are two additional layers over te ridge deck system: one is C reinorced concrete pavement cm tick, and te oter is a itumen concrete coating 7 cm tick. Te concrete grade o te joints is C, and te joint dimensions are sown in Fig.. Te precamer or te m precast ollow slas is.5 cm. Te ollow slas were initially precast on summer days at te casting yard. Te precast memers stayed on te casting ed o te internal air ag mould or aout days, ater wic tey were removed rom te ed and placed on sleepers temporary supports positioned along te lines o te teoretical supports o te m sla. Te teoretical span is.6 m. No cracks were ound in te m memers at tis stage. However, transverse cracks,.. mm in widt, occurred in te vicinity o te tensile zone near te midspan ater te sla memers ad lain on te sleepers or one or two days. Te cracks etended rom te ottom o te sla to a certain eigt and ten ecame stale. Field oservations sow tat cracking is more serious in side slas tan middle slas, wic indicates tat side slas crack more easily tan middle slas. Figure. Joint dimensions cm. Investigations were conducted on site to demonstrate tat te orming o cracks as little, i anyting, to do wit te metod o construction and te properties o te materials, wic all comply wit te appropriate norms and requirements. However, urter investigations and oservations sowed tat te distriution o cracks was concentrated on te tensile ace near te midspan and tere were no cracks near te supports. t irst sigt, it was assumed tat te cracks ad appeared due to insuicient design strengt 8% o te concrete wile te ollow slas were stored on te sleepers. To test tis assumption, ull-scale samples consisting o 8 pieces o middle sla and 8 pieces o side sla were precast in situ. Te samples were not removed rom te casting ed until % concrete strengt ad een reaced. Unortunately, similar tensile cracks appeared in te side sla samples, altoug te cracks in te middle sla samples were sligt. Evidently, it was te tensile stress caused y te weigt o te sla eam itsel eceeding te tensile strengt o te concrete tat led to te appearance o tis kind o crack.

4 Te our ridges sla memers were designed according to te standard drawing, JTG D6-4 Code or Higway Reinorced Concrete and Prestressed Concrete Bridges and Culverts, 4, in wic tere is no requirement or cecking te cracking resistance o normal reinorced concrete memers under sel-weigt loading. In engineering practice, te usual simply supported eam is indeed seldom likely to crack on sel-weigt loading ecause o te generally avourale dept-span ratio. However, te m reinorced concrete memer, wit a dept-span ratio o up to /, is a quite dierent case. s te span lengt increases, te ending moment near te midspan causing y sel-weigt loading ecomes te controlling actor o m memers. Early vertical cracks would ave occurred in zones o large moment wen te tensile stress causing y te sel-weigt loading eceeded te tensile strengt o te concrete.. CLCULTIONS FOR CHECKING THE CRCKING RESISTNCE Given te section dimensions, te reinorcement o te memer and te stress-strain relationsips etween reinorcing ar and concrete, calculation teory on omogeneous elastic materials can e applied to ceck te deormation o reinorced concrete memers. On condition tat te position o te centroid, te magnitude o te area and te moment o inertia to te centroid ais remain uncanged, te cross-section o te middle or side sla can e regarded as equivalent to an L-sape section, as sown in Fig.. Figure. Equivalent section o te sla. Te area o reinorcement in te tensile zone, 5, can e transormed into an equivalent concrete area, n 5. Compression reinorcement is ignored in te calculations ecause it as little inluence on te cecking o te cracking resistance. Fig. 4 sows te transormation section or te cracking ceck calculations. Wit te equivalent transormation o two materials etween concrete and reinorcement, Fig. 4a and Fig. 4 sow te same mecanical eaviour in cracking resistance calculations. n = Es / E Were n E 5 E = ratio o modulus o elasticity = elastic modulus o reinorcement = initial elastic modulus o concrete.

5 a Figure 4. Transormation section or cracking ceck calculations. Te total area o te transormed section,, is: s s n n = = Te eigt o te tensile region,, is: n s = Te moment o inertia to te neutral ais o te transormed section, l, is: n I s = 4 Te ending stiness, W, can e written as: I W = 5 For reinorced concrete ollow slas, let te unit weigt 6 / 5 mm N = ρ, te tensile strengt o concrete C MPa R =, and tus te critical cracking moment cr M is: m cr R W M γ = 6 Were m γ is te plastic coeicient;.45 is recommended.

6 Te ending moment at te midspan on sel-weigt loading, M, is: M ql 8 = ρl 8 Te cracking resistance coeicient, = 7 K, can e written as: M cr K = 8 M Te calculation results o te cracking ceck or te m precast reinorced concrete ollow slas on sel-weigt loading in Inner Mongolia are presented in Tale. Te results sow tat te critical cracking moment 9.85 kn.m o te middle sla is larger tan its midspan ending moment 8.4 kn.m, wic is caused y sel-weigt loading. Te results also sow tat te critical cracking moment 5.8 kn.m o te side sla is smaller tan its midspan ending moment.5 kn.m. It is apparent tat te middle and side slas do not conorm to te standard or cracking resistance on sel-weigt loading. Wen te precast memers are eing moved and stored, altoug 8% o te concrete strengt or te m sla does satisy te design requirement, tey do not ave te aility to resist cracking on sel-weigt loading. Consequently, transverse cracks near te midspan are oreseeale. Tale indicates tat te cracking resistance coeicient o te middle sla is muc larger tan tat o side sla, wic coincides wit te ield oservation tat cracking is more serious in side slas tan in middle slas. Tale. Calculation results o cracking ceck or m precast ollow sla memers. s te critical cracking moment is proportional to te tensile strengt o concrete, 8% o te concrete strengt o precast memers is muc easier to crack tan % strengt o te concrete. Te middle sla memers, wic are moved ater reacing % o te concrete strengt, would not crack y and large, altoug tere is no adequate margin o resistance cracking. Side sla memers can oviously not resist cracking eiter in tis case, wic is in agreement wit te results rom te in situ investigation o te samples. I te concrete grade increases to C4, ten E = GPa, R =.4 MPa. Let us repeat te cracking ceck calculations listed in Tale. Te middle and side slas, wit adequate margins to resist cracking, are all unlikely to crack on sel-weigt loading. It is indicated tat te use o a iger concrete grade would e an eicient metod o preventing suc cracking.

7 4. NUMERICL NLYSIS 4. Finite element modelling Finite element modelling was used to eamine te eaviour o te m slas sujected to sel-weigt loading. Based on te mecanically symmetrical properties o te structure, a al-span was considered in te analysis. ltoug tere are a variety o inite elements availale in NSYS, only solid 45 and pipe were used to represent te concrete and reinorcement ecause o te simple, yet numerically stale, perormance. Te inite element mes o te concrete model and te reinorcement cage model are sown in Fig. 5 and Fig. 6 respectively. Figure 5. Te concrete inite element Figure 6. Te reinorcement cage inite model or al-span o sla. element model or al-span o sla. 4. Results o analysis ccording to te analysis results, te cross-section at midspan is te controlling section were te most adverse stress occurred. Fig. 7 sows te stress distriution in te vertical direction at midspan. It can e seen rom Fig. 7 tat te maimum tensile stress.4 MPa caused y sel-weigt loading eceeds te tensile strengt o te concrete. MPa. Te numerical analysis results are in ecellent agreement wit te cracking ceck calculations. Figure 7. Stress distriution along te vertical direction at midspan. Figure 8. Model o te ridge deck system.

8 5. EVLUTION OF BRIDGE DECK SYSTEM In order to evaluate te perormance o te ridge deck on wic te cracks appear and to judge te eect o tese early cracks on te structural eaviour, numerical analysis was used to ceck te ultimate carrying capacity o te ridge sla system. 5. Transverse distriution actor inite element model or te ridge sla system, as sown in Fig. 8, was constructed to eamine te eect on te ridge deck sujected to service loading. Te ridge deck system is made up o pieces o simply supported ollow slas, inged side y side. Te transverse distriution actors o veicle weel loads or every sla are listed in Tale. It can e seen rom Tale tat te maimum transverse distriution actors o veicle weel loads or te side and middle slas are, respectively,.84 and.48. Tese actors are signiicant in te ollowing ceck o te ultimate carrying capacity o te side and middle slas. Figure 9. Load arrangement. 5. Ceck o ultimate carrying capacity ccording to drawing JTG D6-4 Code, 4, te ridge sla system is sujected to te ollowing loading, as sown in Fig. 9: uniorm load q k =.5KN / m and concentrated P load k = KN. Tales and sow te cecking results on ending resistance and sear capacity or te sla eam. It is ovious rom Tales and tat te sla eam can satisy te equation γ S R wit an adequate margin o saety. Since te slas considered in te ceck o ultimate carrying capacity are te most adverse condition or te ridge deck system, we can conclude tat te ridge sla system is ale to ulil te serviceaility requirements and to provide adequate earing capacity. Tale. Cecking results o ending resistance.

9 Tale. Cecking results o sear capacity. 6. CONCLUSIONS ND RECOMMENDTIONS In ridge engineering, wit regard to mainly load-carrying memers, it is required tat memers do not crack under te action o sel-weigt loading, even toug te structural system composed y te memers is ale to provide normal serviceaility. However, tere is no corresponding requirement in te present design code. In order to meet te requirement, it is necessary to amend te crack cecking calculations or normal precast reinorced concrete memers in design speciications. Wen reinorced concrete memers are aout to crack, te tensile stress o te reinorcement is around MPa. So tere are two main ways o improving te memers' cracking resistance: one is to enlarge te section size o te memers and te oter is to enance te concrete strengt grade. For m precast reinorced concrete ollow slas, i te section size is not enlarged, it is suggested tat te concrete strengt grade e increased to C4. Wit te cement grade and te eectiveness improved, it is possile to improve te concrete strengt witout increasing te cost. For rapid construction, precast memers are usually removed rom te casting ed wit 8% concrete strengt. In tis case, an eicient approac to preventing memers rom cracking would e to sorten te distance etween te temporary supports sleepers or te m precast slas. Tis sould e applied witin proper limits, oterwise new cracks close to te supports would appear in te top surace o te sla eams. Wit igway ridge overloading increasing, or ridge engineers tere is no etter coice ut to sustitute m precast sla memers wit prestressed concrete ollow slas to control te cracks caused y sel-weigt loading. However, m precast sla memers ave een widely used in current ridge projects ecause o teir convenience in construction and teir lower cost. It would seem tat te proposed metods presented in tis paper may result in a new approac to controlling cracks in m precast reinorced concrete ollow-sla memers. 7. REFERENCES [] Code or Higway Reinorced Concrete and Prestressed Concrete Bridges and Culverts, 4. JTG D6-4. [] Jin, Tay Coon, 987. Jurnal Institusi Jurutera Malaysia, 4, pp [] Li, T, Yan, Y and Yan, W, 4. Progress in saety science and tecnology. Proc. 4 International Symposium on Saety Science and Tecnology, Part, Vol. 4, pp [4] World Construction, , pp. 8, 4-4. [5] Yang, Lin, 995. Structural Engineering Review, 74, pp