CRACKING IN CONCRETE NEAR JOINTS IN STEEL- CONCRETE COMPOSITE SLAB

Transcription

1 CIVIL AND ENVIRONMENTAL ENGINEERING REPORTS ISSN CEER 2015; 16 (1): DOI: /ceer CRACKING IN CONCRETE NEAR JOINTS IN STEEL- CONCRETE COMPOSITE SLAB Marcin NIEDOŚPIAŁ 1, Michał KNAUFF, Wioleta BARCEWICZ Waraw Univerity of Technology, The Faculty of Civil Engineering Abtract In thi paper reult of the experimental tet of four full-cale compoite teel-concrete element are reported. In the teel-concrete compoite element, a teel beam wa connected with a lab cat on profiled heeting, by hear tud. The end-plate were (the thickne of 8 mm, 10 mm and 12 mm) thinner than in ordinary deign. Joint between the column and the beam have been deigned a emi-rigid, i.e. the deformation of endplate affect the ditribution of force in the adjacent part of the lab. The paper preent the theory of cracking in reinforced concrete and teel-concrete compoite member (according to the code), view of crack pattern on the urface of the lab and a comparion of the tet reult and the code calculation. It wa oberved, that ome factor influencing on crack width are not taken in Eurocode 4 (which i baed on Eurocode 2 with taking into account the phenomenon called tenion tiffening ). Keyword: cracking, teel-concrete compoite lab, emi-rigid joint 1. INTRODUCTION Cracking phenomenon in reinforced concrete tructure wa noticed and conidered by reearcher at the beginning of the development of concrete tructure. The firt paper conidering the extenion of reinforced concrete and cracking date back to the turn of 19th and 20th century (e.g.: [3, 1]). Nowaday, the literature concerning thi ubject i very rich. Among the Polih invetigation on problem imilar to thoe examined in thi paper, there are e.g. 1 Correponding author: Waraw Univerity of Technology, The Faculty of Civil Engineering, Armii Ludowej t 16, Warzawa, Poland, m.niedopial@il.pw.edu.pl, tel

2 168 Marcin NIEDOŚPIAŁ, Michał KNAUFF, Wioleta BARCEWICZ article written by K. Furtak [4, 5] and an article written by K. Flaga, M. Pańtak [6]. The baic model ued in the theory of cracking i an element ubjected to axial tenion, which can alo be regarded a a model of tenion zone in bending element and eccentrically loaded element with a tenion zone [7]. It i aumed that the firt crack i formed in the cro-ection, in which the concrete trength will be the lowet. Then, the load increae caue additional crack (at imilar ditance). In the ection through the crack, the tree in the concrete fall to zero and, on both ide of crack, o called "relaxation zone" are formed. The tree in thi part of the concrete are too mall to caue the appearance of the next crack. To both ide of the crack the load i partially tranmited to the concrete. It i aumed that after reaching the tage of tabilized cracking, load increment do not caue any new crack but only an increae in width of exiting crack. Cracking model ued to calculate the width of the crack i hown in Fig. 1. It wa derived from [7]. Fig. 1. The model of cracking in the reinforced concrete element under tenion at the time of the appearance of the firt crack (a) and at the tage of tabilized cracking (b) [7] 2. CRACKING EFFECT ACCORDING TO STANDARDS Until the eightie of the twentieth century, problem of compoite tructure had not been reflected in the Polih tandardization. The firt tandard for the deign of compoite teel - concrete tructure wa developed and etablihed between 1982 and 1991 [10, 11, 14]. In 2006, a new tandard for deign of compoite

3 CRACKING IN A CONCRETE NEAR JOINTS IN STEEL-CONCRETE COMPOSITE SLAB 169 teel - concrete tructure [12] wa releaed that replaced the previou three tandard. It wa baed on the proviion of the Eurocode 4. In addition to the principle of deign of beam and column, thi tandard contained information and guidance on how to analyze the tructure, how to deign compoite lab on teel decking and compoite connection, how to tet connector and compoite lab and how to deign element due to the fatigue. Thi tandard wa replaced in 2008 by PN-EN :2008 Eurocode 4 [16]. Until 2006, the Polih tandard for compoite tructure had ued a theory for deign of cracking in reinforced concrete tructure drawn from tandard prior to the firt Polih tandard containing proviion of the Eurocode (1999) [9]. A in the current tandard, crack width wa calculated a the product of the average train of the reinforcement and a ditance between crack. Characteritic feature of the older tandard i factor called Ψ a, determining the ratio of average train of the reinforcement to the maximum train occurring in crack. For the firt time the theory baed on the Eurocode appeared in 2006 in the tandard for deign of compoite tructure. According to [13] the width of the crack wa teted in compliance with the tandard [10]. Concept of the effective tenion area and tenion tiffening taken from Eurocode were etablihed. Tenion tiffening phenomenon involve the fact that the average train of the reinforcement i maller than the train calculated from the tre achieved in the crack. In order to reduce the crack width to it width limit and avoid uncontrolled crack between rarely paced bar, it mut be aured that at the time of the firt crack appearance, reinforcement remain in it elatic tate and a ditance and a diameter of the bar are limited. According to the current tandard [16] the width of the crack can be evaluated uing [15] with regard to the phenomenon of "tenion tiffening". Tenile tree in the reinforcement can be determined from the following formula:, 0 (2.1) f ctm (2.2) t in which f ctm i the average tenile trength of concrete, α t i the ratio of the product of the cro-ectional area and the moment of inertia of the effective compoite ection excluding the tenion concrete and profiled heet (if any) to the product of the ectional area and the moment of inertia of the compoite

4 170 Marcin NIEDOŚPIAŁ, Michał KNAUFF, Wioleta BARCEWICZ ection, and ρ i the degree of reinforcement in the element. Formula (2.3) and 2.4) are ued to calculate the pacing and width of crack: m cm k t f w ct, eff k r, max m cm (2.3) p, eff E 1 e p, eff, but not le than 0,6 E (2.4) in which rmax i the maximum crack pacing, ε m i the average train of the reinforcement, ε cm i the average train of the concrete between the crack, k t i a coefficient depending on the duration of the load, ρ p,eff i the ratio of area of tenion reinforcement to the effective tenion area of concrete around the reinforcement, α e i the ratio of the modulu of elaticity of reinforcement to the modulu of elaticity of concrete, a E i the modulu of elaticity of reinforcement. If in the tenion zone pacing of the reinforcement, with a bond to the concrete, i not higher than 5(c+/2)), the maximum final crack pacing can be calculated a: r,max k3c k1k2 k4 (2.5) p, eff In (2.5) k 1, k 2, k 3, k 4 are coefficient given in [15], c i the concrete cover. 3. PROGRAM AND COURSE OF INVESTIGATIONS Experimental tet were carried out in the laboratory of the Intitute of Building Engineering at the Faculty of Civil Engineering at Waraw Univerity of Technology. The ummary of data analyzed in thi paper i given in Table 1. Moreover, tet of the following material - concrete, tructural teel and reinforcing teel were made. Table 1. The ummary of data Name of the pecimen Thickne of the end-plate [mm] Reinforcement Number of bolt EZ5 6ø12 4 M20 10 EZ6 6ø12 6 M20 10 EZ7 6ø12 6 M20 8 EZ8 6ø12 4 M20 12

5 CRACKING IN A CONCRETE NEAR JOINTS IN STEEL-CONCRETE COMPOSITE SLAB 171 Specimen conited of a compoite lab on profiled heet (the trapezoidal teel heet with open rib and pecific emboment for compoite lab - Cofraplu 60, fold pacing 207 mm) attached to teel beam by hear tud (Ø19x100 mm, pacing 207 mm). The teel beam were attached to a teel column through the fluh end-plate (element EZ5, EZ8) or the extended end-plate (element EZ6, EZ7). Longitudinal reinforcement of the lab conited of 6 rebar (teel BSt500S) with a diameter of 12 mm (1.37% degree of reinforcement) ard for three on both ide of the beam. The longitudinal rebar are connected together by tranvere bar made of mild teel (teel S235JR) of a diameter of 6 mm, placed at a pacing of mm (two bar in each fold). The lab wa made uing concrete cla C25/30. The view of the element and the arment of meauring equipment i hown in Fig. 2. The view of the lab urface, the reading level of crack width and the location of the train gauge on the reinforcement i hown on Fig. 3. Detailed information can be found in [8] and [2] (PhD thei made under the uperviion of prof. M. Giżejowki). Fig. 2. Scheme of the teted element

6 172 Marcin NIEDOŚPIAŁ, Michał KNAUFF, Wioleta BARCEWICZ Fig. 3. The view of the teted lab - the ditribution of benchmark on the lab urface, the train gauge on the reinforcement and crack width reading level In order to permit reading the train in the reinforcement directly in a crack in the axi of ymmetry, an artificial crack - thin oiled heet forcing a dicontinuity of lab wa introduced (Fig. 4). Fig. 4. The view of the artificial crack The detailed decription of the teted element and method of meaurement i preented in [8]. 4. RESULTS The greatet width achieved the crack extending over the entire width of the element, in the direct vicinity of the column and the crack extending from the corner of the column. Each crack, from the moment of appearance, croed the entire thickne of the lab. It wa due to the fact that the entire lab wa in tenion zone. Fig. 5 how the view of crack on the urface of the lab directly before failture.

7 CRACKING IN A CONCRETE NEAR JOINTS IN STEEL-CONCRETE COMPOSITE SLAB 173 EZ5 10mm, 4 M20 EZ6 10mm, 6 M20 EZ7 8mm, 6 M20 EZ8 12mm, 4 M20 Fig. 5. The view of crack on the urface of the lab directly before failture

8 Crack pacing rm [mm] Crack pacing rm [mm] 174 Marcin NIEDOŚPIAŁ, Michał KNAUFF, Wioleta BARCEWICZ 5. ANALYSIS OF THE RESULTS In order to analyze cracking, the lab wa divided into three zone hown in Fig. 6. Fig. 6. The diviion of the lab into zone for analyi cracking Fig. 7 and Fig. 8 how the average crack pacing depending on the load. The graph preent value calculated according to [15] (marked a rm,tandard ) and value characterizing the hape of the heet - the pacing between fold (equal pacing of hear tud) and the pacing of the thinner part of the lab, equal to the half of the ditance between the fold. EZ5 400 EZ8 400 rm,tandard rm,tandard 200 pacing between fold 200 pacing between fold pacing of the thinner part of the lab 0 90% 100% zone I zone II zone III pacing of the thinner part of the lab 0 90% 100% zone I zone II zone III Fig. 7. The average crack pacing - pecimen with the fluh end-plate connection

9 Crack pacing rm [mm] Crack pacing rm [mm] CRACKING IN A CONCRETE NEAR JOINTS IN STEEL-CONCRETE COMPOSITE SLAB 175 EZ7 400 EZ6 400 rm,tandard rm,tandard 200 pacing between fold 200 pacing between fold pacing of the thinner part of the lab 0 90% 100% zone I zone II zone III pacing of the thinner part of the lab 0 90% 100% zone I zone II zone III Fig. 8. The average crack pacing - pecimen with the extended end-plate connection It wa noticed (Fig. 5) that crack were formed principally at the interface between the lab and the rib and their pacing wa imilar to the pacing of the thinner part of the lab. The ummary of crack width i hown in Fig. 9 and Fig. 10. The figure how the crack width calculated in accordance with current tandard. Calculation were carried out in two option: uing the crack pacing in accordance with the formula given in the tandard (Eurocode (rmtandard)) and uing the crack pacing obtained from the tet (Eurocode (rmtet)). In the graph a trend line wa plotted, howing the growth of the crack width in the of the elatic reinforcement behaviour a well a the meaurement of fit to the trend line (R 2 ). Conidering the limit tate of cracking, the long-term load i analyzed, which typically from 40% to 80% of the deign reitance of the element (calculated by uing of the deign value of the trength of material), which correpond to 24% to 62% of the failure load obtained from the invetigation. Thi load wa marked on the chart a. During the tet, it wa oberved that the firt crack appeared earlier than it would be expected, calculating the cracking moment with the ue of the concrete tenile trength obtained from the tet. The compoite lab connected with a teel beam and heet ha no freedom of hrinkage deformation. Therefore, during the drying of concrete mixture, tenile tree and crack appear in the lab. It eem that due to thi phenomenon, in the calculation of tree in the reinforcement of the compoite tructure, a maller value of the tenile trength of concrete hould be taken into conideration (f ctm i preent in the formula for the tree in the teel taking into account the phenomenon of tenion tiffening ). In thee calculation, after the comparion with the tre calculated uing the balance of the joint and Δσ addition, a good agreement with the experimental reult wa obtained, after taking the concrete tenile trength at the level of 50% of the plitting tenile trength.

10 176 Marcin NIEDOŚPIAŁ, Michał KNAUFF, Wioleta BARCEWICZ EZ5 - zone I R² = 8 tet (max) tet (average) Eurocode (rm-tandard) Eurocode (rm-tet) EZ8 - zone I EZ5 - zone II fluh end-plate 10mm, 4 M20, EZ8 - zone II R² = 0,83 tet (max) tet (average) Eurocode (rm-tandard) Eurocode (rm-tet) R² = 0,93 tet (max) tet (average) Eurocode (rm-tandard) Eurocode (rm-tet) R² = 0,97 Load-capacity ratio[%] tet (max) tet (average) Eurocode (rm-tandard) Eurocode (rm-tet) fluh end-plate 12 mm, 4 M20, Fig. 9. The crack pacing - pecimen with the fluh end-plate connection EZ7 - zone I R² = 0,95 tet (max) tet (average) Eurocode (rm-tandard) Eurocode (rm-tet) EZ6 - zone I EZ7 - zone II extended end-plate 8 mm, 6 M20 EZ6 - zone II R² = 0,94 tet (max) tet (average) Eurocode (rm-tandard) Eurocode (rm-tet) R² = 0,93 tet (max) tet (average) Eurocode (rm-tandard) Eurocode (rm-tet) R² = 0,85 tet (max) tet (average) Eurocode (rm-tandard) Eurocode (rm-tet) extended end-plate 10mm, 6 M20 Fig. 10. The crack pacing - pecimen with the extended end-plate connection

11 CRACKING IN A CONCRETE NEAR JOINTS IN STEEL-CONCRETE COMPOSITE SLAB SUMMARY AND CONCLUSIONS In the pecimen with the fluh end-plate and 4 M20 bolt, influence of the endplate thickne on the crack width became ignificant only in the cae of the maximum crack width in zone II (away from the joint). For element with the extended end-plate, lightly wider crack (either maximum or average) were oberved while uing thinner end-plate. Analyzing the influence of the number of bolt (at a contant thickne of the end-plate) on crack width it can be oberved that in the cae of the fluh end-plate and 4 M20 bolt maximum crack, located in zone I, were lightly wider than for the element with the extended end-plate. However, in the cae of maximum crack width in zone II and the average crack width in zone I and II, there wa no uch influence. The crack width depend on the pacing of crack in the final tage of tabilized cracking. The tandard [16] doe not provide additional guidance related to the compoite lab on teel decking, refering a deigner to [15], which refer to the reinforced concrete lab. In fact, in addition to the factor lited in the tandard, the final crack pacing in a compoite element i alo affected by the hape of the teel heeting (ditance between fold), the ratio of the thickne of the concrete lab above the fold to the thickne of the concrete in the fold and a pacing of hear connector auming compoite action between the lab and the beam. In the analyed element, fold pacing effect on the crack pacing wa oberved. In all invetigated element the ame teel decking wa ued, which did not allow to wider recognition of thi phenomenon. Better compatibility between the calculated crack width and crack width determined experimentally i obtained by ubtituting in the calculation of the crack width, the tandard value of crack pacing, different from that obtained in the tudy. In the calculation of reinforced concrete tructure, it i aumed that the ratio of the maximum crack width to it average value i 1.7. In the tudied compoite element, the ratio d from 1.5 to 2.3 (average 1.8) in zone I and from 1.6 to 2.8 (average 2.1) in zone II. The eemingly greater difference between the maximum and the average crack width in zone II than in zone I i due to a greater difference between the bending moment at the beginning of the zone and the bending moment at the end of zone II (42%) than in the cae of zone I (22%). REFERENCES 1. Bach R.: Mitteilungen über Forchungarbeiten, Zeitchr. D. Vereindeutcher Ingenieura, Berlin 1907.

12 178 Marcin NIEDOŚPIAŁ, Michał KNAUFF, Wioleta BARCEWICZ 2. Barcewicz W.: Sztywność, nośność i zdolność do obrotu pewnej klay węzłów w kontrukcjach talowych ze tropami zepolonymi, rozprawa doktorka, Politechnika Warzawka, Warzawa, Conidére L.: Compte rendu de éace de l'akademie de Science, tom V, nr 127, Furtak K.: O rozwoju ry w płycie żelbetowej belek zepolonych, Inżynieria i Budownictwo, 12/ Furtak K., Jarek B.: Uwagi na temat obliczania zerokości ry w żelbetowych płytach pomotu motów zepolonych, Konferencja naukowo - techniczna "Moty zepolone", Kraków Flaga K. Pańtak M.: Ryzyko zaryowania płyt pomotów kontrukcji zepolonych betonowo-talowych, Inżynieria i Budownictwo, 8/ Knauff M.: Obliczanie kontrukcji żelbetowych według Eurokodu 2, Wydawnictwo Naukowe PWN, Niedośpiał M.: Nośność i odkztałcalność węzłów talowo - betonowych tropów zepolonych - rola żelbetu, Rozprawa doktorka, Politechnika Warzawka, Warzawa PN-B-03264:1999: Kontrukcje betonowe, żelbetowe i prężone. Obliczenia tatyczne i projektowanie. 10. PN-B-03264:2002: Kontrukcje betonowe, żelbetowe i prężone. Obliczenia tatyczne i projektowanie. 11. PN-B-03300:1982: Kontrukcje zepolone talowo-betonowe. Obliczenia tatyczne i projektowanie. Belki zepolone krępe. 12. PN-B-03300:2006: Kontrukcje zepolone talowo-betonowe. Obliczenia tatyczne i projektowanie. 13. PN-B-03300:2006: Kontrukcje zepolone talowo-betonowe. Obliczenia tatyczne i projektowanie. 14. PN-B-03302:1991: Kontrukcje zepolone talowo-betonowe. Obliczenia tatyczne i projektowanie. Słupy. 15. PN-EN :2008 Eurokod 2: Projektowanie kontrukcji z betonu, Część 1-1: Reguły ogólne i reguły dla budynków. 16. PN-EN :2008 Eurokod 4: Projektowanie zepolonych kontrukcji talowo-betonowych, Część 1-1: Reguły ogólne i reguły dla budynków.

13 CRACKING IN A CONCRETE NEAR JOINTS IN STEEL-CONCRETE COMPOSITE SLAB 179 ZARYSOWANIE PŁYTY ŻELBETOWEJ W STREFIE PRZYWĘZŁOWEJ STROPU ZESPOLONEGO S t r e z c z e n i e W artykule przedtawiono wyniki badań czterech elementów zepolonych. Kztałtownik talowy połączony był z betonowym tropem wykonanym na blaze fałdowej. W modelu zatoowano cienkie blachy czołowe (o grubości 8 mm, 10 mm i 12 mm), cieńze niż zwykle przyjmowane w praktyce projektowej. Połączenie to zaprojektowano jako podatne tzn. takie, w którym odkztałcenia blach czołowych mają itotny wpływ na rozkład ił w połączeniu. Przedtawiono normową teorię dotyczącą zaryowania elementów żelbetowych i zepolonych, obraz zaryowania tropu oraz porównano otrzymane wyniki z obliczeniami wykonanymi wg aktualnych norm. Zauważono, iż nie wzytkie czynniki obliczania zerokości ry w kontrukcjach zepolonych ą zdefiniowane w normie projektowania kontrukcji zepolonych (która w tej kwetii odwołuje ię do normy projektowania kontrukcji żelbetowych z uwzględnieniem zjawika tenion tiffening ). Słowa kluczowe: zaryowanie, trop zepolony, węzły podatne Editor received the manucript: