Politechnika Śląska. PROBLEM OF CRACKS IN PRESTRESSED ..... rozciągających na powstanie rys w strefie zakotwienia. Następnie, zaprezentowano.
Vadzim PARKHATS* Politechnika Śląska
PROBLEM OF CRACKS IN PRESTRESSED HOLLOW CORE SLABS
1. Cracking due to tensile forces in the anchorage zone Cracks in prestressed hollow core (HC) slabs should be carefully examined, because they can reduce the shear capacity, affect the load distribution, stimulate corrosion of steel reinforcement or even lead to the slab failure. Cracks in the anchorage zone can be especially harmful, since they have a great impact on bonding of the prestressing tendons. Tensile forces as bursting, splitting and spalling are the general reasons for cracking in the transmission zone [1]. 1.1. Bursting
Bursting tensile forces are created by broadening of the end of the strand due to strand slippage (Fig. 1a). The result of bursting is the extension of the transmission zone. It should be noted that cracks caused by bursting are relatively small (usually the crack length is up to 10 cm). Bursting is typical for HC slabs with insufficient concrete cover and distance of prestressed tendons. 1.2. Splitting
If traction stresses, progressively transmitted to the concrete by the tensioned strands, are more than the tensile strength of the concrete, splitting cracks are developed (Fig. 1b). Cracking takes place mainly between strands and in time may lead to separation of the lower part of a HC slab. Splitting cracks are quite small and may be prevented by satisfying the requirements concerning concrete cover and distance between strands specified in PN-EN 1992-1-1:2008 [2].
* Supervisor: Prof. dr hab. inż. Jan Kubica
Fig. 1. Tensile forces in the anchorage zone [1]: a) bursting, b) splitting, c) spalling, d) principal stresses in the end of a prestressed HC slab having depth 20 cm with 3Ø3 mm strands Rys. 1. Siły rozciągające w strefie zakotwienia [1]: a) siła rozrywająca, b) siła rozwarstwiająca, c) siła rozszczepiająca, d) naprężenia główne na końcu sprężonej płyty kanałowej o wysokości 20 cm ze splotami 3Ø3 mm
1.3. Spalling
Whereas bursting and splitting cracks are restricted in size and a slab still can be used, spalling fissures are more dangerous (Fig. 1c). When, due to transfer of prestressing forces to the concrete, the lower part of a slab starts being subjected to a combined bending and compressive stress, the upper part still remains almost unstressed. This condition may cause spalling in the thinnest part of the web, where tensile stresses are the greatest (Fig. 1d). These cracks rapidly reach large size. According to the FIP guide [3], when more than one web undergoes spalling a slab is not acceptable. The strands sheathing for a length of 50-70 cm from the ends of the prestressed elements lead to reduce the spalling tensile stresses. Spalling stress s sp shall be checked according to the PN-EN 1168+A3:2011 [4] equation (1). The calculation shall be carried out for the most reinforced web. Moreover, the reinforcement in the upper part of a slab shall be ignored in the calculation. s sp =
Po ´ bw eo
15a e2,3 + 0, 07 1,5
æl ö 1 + ç pt1 ÷ è eo ø
(1,3a e + 0,1)
£ f ct
(1)
where: bw - thickness of an individual web; Po - initial prestressing force just after release in the considered web; eo - eccentricity of the prestressing steel; lpt1 - lower design value of the length of transmission; a e = ( eo - k ) / h ³ 0 - eccentricity ratio; h - height of the web;
k = Wb / Ac - core radius taken equal to the ratio of the section modulus of the bottom
fibre Wb and the net area of the cross section Ac ; f ct - value of the tensile strength of the concrete deduced at the time that the prestress
is released on the basis of tests. According to clause J.5 of [4], actual tensile strength f ct may be measured directly by tests or derived by the correlations of the table 3.1 of [2], where characteristic compressive cylinder strength of concrete at 28 days f ck is replaced by fc and characteristic axial tensile strength of concrete f ctk is replaced by f ct through the following calculations: - for concrete classes ≤ C50/60
-
f ctm = 0,30 f c2/3
(2)
f ctm = 2,12 ln éë1 + ( f c + 8 ) / 10ùû
(3)
f ct = 0,8 f ctm
(4)
for concrete classes > C50/60
and
where: f ctm - mean value of axial tensile strength of concrete; f c - actual compressive strength.
It is important to note that it would be incorrect to use in Eq. (1) the value of the characteristic tensile strength of concrete from the table 3.1 of [2]. The point is that the values presented in the table are related to the common concrete tested on specimens with standard dimensions, whereas HC slabs are usually produced from the concrete with V0 consistence class to the Vebe test [5] subjected to the heat treatment in the first hours after casting. Moreover, the HC webs should be considered as thin-walled
elements, since only a few coarse aggregate grains can be contained at the width of their cross-section. These factors cause a considerable difference between the concrete strength provided by the table 3.1 of [2] and the actual strength of the concrete of HC slabs [6]. Molnar [7] examined the reasons of the spalling cracks in HC slabs. In the research, nevertheless the calculated spalling stress was less than the tensile strength of the concrete of the HC slabs, the spalling cracks in the webs appeared. The author found that Eq. (1) leaves out of account the changes in the bond conditions of the strands because of the production technology and the early strength development of the concrete, therefore the result of Eq. (1) may be improper. Besides, it was established that the tensing of the sawing blade is the principal production cause of the cracks. Two suggestions to eliminate the effect of the concrete sawing blade were proposed. Firstly, the cracks do not appear if slabs have higher concrete strength at the moment of the sawing. Nevertheless this solution is unpractical, because reduces the productivity. The other suggestion is the loading of a slab during sawing against camber.
2. Casting and shipping cracks The PCI prepared a catalogue of cracks [8] that can happen in prestressed HC slabs during their casting and shipping. Later the fib published the report [9] partially based on [8]. The causes of cracking and preventative measures according to the documents mentioned above are presented in Table 1. Description of the cracking effects on serviceability and repair methods in accordance with [8], [9] is presented below. Web cracks above the strands (Fig. 2a) are horizontal cracks of width up to 6.3 mm. These cracks can reduce the shear capacity of a slab. Since the use of shear reinforcement in prestressed HC slabs are restricted, it is especially important for the webs to be undamaged. The shear failure is considered as a brittle failure mode. To repair hairline cracks epoxy may be used. In the case of large cracks, filling solid the cores is recommended. Longitudinal cracks at the voids (Fig. 2b) can take place in either the top or bottom flange of a slab. These cracks influence the load distribution in untopped slabs where there are concentrated loads, openings, and transverse cantilevers. Grouting the voids
solid can be used to repair the defect. In the case of topped systems, repair may not be necessary.
Fig. 2. Cracks in prestressed HC slabs [8]: a) web cracks above the strands, b) longitudinal cracks at the voids, c) longitudinal cracks at the web, d) corner cracks, e) web cracks at or near the strands, f) transverse cracks, g) miscellaneous cracks Rys. 2. Rysy w sprężonych płytach kanałowych [8]: a) rysy w żeberku powyżej splotów, b) rysy podłużne powyżej i poniżej kanałów, c) rysy podłużne w żeberku, d) rysy narożne, e) rysy w żeberku koło splotów, f) rysy poprzeczne, g) rysy przypadkowe
Another type of cracks that influences the load distribution in untopped slabs with concentrated loads is longitudinal cracking at the web (Fig. 2c). However, hairline longitudinal cracks in the top surface are usually not dangerous. Slabs with large longitudinal cracks at the web may be used in topped systems or may be cut along its length. Corner cracking (Fig. 2d) has the same effect as an opening at the end of a slab. If the webs are undamaged, the impact on the shear capacity is generally small. To repair these cracks epoxide resin or filling solid the voids is suggested. If cracking happens at or near the strands (Fig. 2e), the shear capacity decreases because of the web defect. These cracks, like those at corners, may be assessed in the same way as an opening at the end of a slab. Epoxide resin or filling solid the voids can increase the shear capacity, but a complete repair is impractical. Transverse cracks (Fig. 2f) mainly take place in the top of a slab, but sometimes can apply to the whole member. The cracks across a slab reduce the moment of inertia and generate changes in the camber and deflection. In the case of cantilevers, the shear and moment capacities decrease greatly. For small cracks at areas of positive moment in the top flange, repair is not necessary. Epoxy resin or grouting solid the cores is recommended for the shear capacity improvement. Miscellaneous cracks in the top surface arising randomly (Fig. 2g) in most cases have an insignificant impact on the slab capacity. However, the effect of the cracks can be great in a severe exposure situation.
Table 1 Fabrication and shipping cracks in prestressed HC slabs [8] Cause
Prevention
a) Web cracks above the strands A. Excessive prestress force relative to cross A. Reduce shear lag through webs. section of member. B. Increase release strength. B. Insufficient release strength. C. Bleed rubber void forms earlier. C. Expansion of rubber void forms due to D. Clean and oil form properly or ensure a dry increased temperature. contact surface when stack casting. D. Bottom surfaces of member sticking to form E. Cut completely through strands and as close during stripping. as possible to the bottom of the member. E. Sawcut not deep enough. F. Place lifting inserts and stirrups properly. F. Lifting inserts or end stirrups misplaced. G. Adjust water content. G. Mix too wet or too dry. H. Improve vibration. H. Insufficient vibration. b) Longitudinal cracks at the voids A. Transverse shrinkage; improper handling. A. Proper mix and curing; proper handling. C. Lack of differential compaction. C. Improve vibration. D. Placement of prestressing steel. D. Proper positioning and design of E. Flange too thin due to movement or prestressing steel. misalignment of void forms. E. Correct and maintain void form position. F. Overinflation of rubber void forms. F. Maintain proper inflation. c) Longitudinal cracks at the web A. Subsidence over void. A. Prevent subsidence over void. B. Shrinkage due to improper curing and mix. B. Improve curing procedures and mix. d) Corner cracks A. Saw blade pinches when member cambers. A. Place weight on member to restrict camber. B. Sawcut not deep enough. B. Cut completely through strands and as close C. Saw blade “wobble” due to excessive use. as possible to the bottom of the member. D. Uneven dunnage. C. Proper saw maintenance. E. Support blocks not providing even D. Provide level supports. distribution of load during storage. E. Make sure support blocks do not transfer F. Uneven handling due to pickup devices not loads through flanges. being level. F. Use spreader beams to minimize uneven G. Excessive tension stress during stripping. handling. G. Anneal strand and employ proper cutting sequence. e) Web cracks at or near the strands A. Excessive bursting stresses. A. Reduce bursting stresses. B. Lack of concrete consolidation around B. Improve consolidation around strands. strands. C. Secure masking to prevent movement. C. Strand masking improperly placed. D. Revise production procedures to avoid cold D. Layers not bonded. joints. E. Sawcut not deep enough or not complete E. Saw completely through or across member. across sides. f) Transverse cracks A. Longitudinal shrinkage. A. Proper mix and curing. B. Contraction due to delayed detensioning B. Detention as soon as strength is verified, after uncovering heat cured product. before product cools. C. Excessive top fiber tension. C. Reduce top fiber tension. D. Insufficient cover on transverse bar. D. Increase reinforcing bar cover. g) Miscellaneous cracks A. Surface shrinkage; improper troweling. A. Proper mix and curing; reduce troweling. B. Improper mixes or operating of equipment.
3. Cracking in prestressed hollow core slabs of slim floor constructions Slim floor construction is a system in which shallow steel beams bear prestressed HC slabs (Fig. 3). This system provides with an opportunity to minimize the dead load and thickness of the floor. Nevertheless, slim floor constructions also have some disadvantages [10]. Deflections of the steel beams may generate a great reduction in shear strength of the HC slabs. The edge slabs of the floor undergo significant shear deformations, while the slabs at the midspan principally experience transverse bending. Concentration of the support reaction in the outermost webs of the slabs at the midspan may lead to longitudinal cracks at the strands that affect the transfer length. At the same time because of shear deformations, longitudinal web cracks may occur in the edge slabs.
Fig. 3. Stresses of HC slabs supported on flexible steel beam [10] Rys. 3. Naprężenia w płytach kanałowych opartych na podatnej belce stalowej [10]
Concrete topping or filling solid the voids of the edge slabs can reduce the shear stresses [11]. However, it should be noted that the continuous reinforcement in the topping might cause cracks in the support zone of the slabs (Fig. 4). Cracking patterns B and C in Figure 4 decrease the slab capacity, so they should be avoided, while cracking pattern A is not harmful.
Fig. 4. Cracking in HC slabs due to unintentional clamping moment in the support [11] Rys. 4. Zarysowanie w płytach kanałowych wywołane momentem niezamierzonego zamocowania na podporze [11]
4. Conclusions Cracks in prestressed HC slabs may lead to serious problems as a decrease in the shear capacity, corrosion propagation, breaking the bond between the concrete and the strands, and even the failure of a HC slab. In the paper, the causes and consequences of cracking in prestressed HC slabs due to improper design, casting, and shipping are presented and discussed. It is found that the main factors of cracking in the transmission zone are tensile forces as bursting, splitting and spalling [1]. Splitting and bursting cracks are limited in dimensions, so a slab remains acceptable. Spalling cracks are more detrimental, because they rapidly reach large size. Spalling stresses shall be checked according to Eq. (1). HC slabs have thin-walled webs and are generally produced from the concrete with V0 consistence class to the Vebe test [5] subjected to the heat treatment in the first hours after casting, so there may be a great difference between the concrete strength provided by the table 3.1 of [2] and the actual tensile strength of the concrete of a HC slab [6]. Moreover, it is identified that the changes in the bond conditions of the strands as a result of the production technology and the early strength development of the concrete are not properly allowed for in Eq. (1), so the result of the equation may be inaccurate [7]. The fabrication and shipping cracks are divided into groups and recommendations concerning their acceptability are provided. It is established that longitudinal web cracks in the edge slabs of slim floor constructions appear because of shear deformations that decrease the shear capacity.
Additionally, the slabs at the midspan of the steel beam may crack due to concentration of the support reaction in the outermost webs [10]. REFERENCES
1. ASSAP: The Hollow Core Floor Design and Applications, Manual. Verona, 2002. 2. PN-EN 1992-1-1:2008. Eurokod 2 - Projektowanie konstrukcji z betonu Część 1-1: Reguły ogólne i reguły dla budynków. PKN, Warszawa, 2008. 3. FIP: Guide to good practice. Quality Assurance of Hollow Core Slab Floors. SETO, 1992. 4. PN-EN 1168+A3:2011. Prefabrykaty z betonu - Płyty kanałowe. PKN, Warszawa, 2011. 5. PN-EN 206+A1:2016-12. Beton - Wymagania, właściwości, produkcja i zgodność. PKN, Warszawa, 2016. 6. Flaga K., Derkowski W., Surma M.: Concrete strength and elasticity of precast thin-walled elements. Cement Wapno Beton, Vol. 21/83 (5), 2016, pp. 310-317. 7. Molnar G.: Analyses of the end-block of a high performance hollow core concrete slab with high initial prestressing stressed strands. Proceedings of the 9th fib International PhD Symposium in Civil Engineering, Karlsruhe, 2012, pp. 79-84. 8. PCI Committee on Quality Control Performance Criteria: Fabrication and Shipment Cracks in Prestressed Hollow-Core Slabs and Double Tees. PCI Journal, Vol. 28, 1983, pp. 18-39. 9. fib: Bulletin No. 41. Treatment of imperfections in precast structural elements: State-of-the-art Report. Lausanne, 2007. 10. Hegger J., Roggendorf T., Kerkeni N.: Shear capacity of prestressed hollow core slabs in slim floor constructions. Eng. Struct., Vol. 31, 2009, pp. 551-559. 11. Derkowski W., Surma M.: Shear capacity of prestressed hollow core slabs on flexible supports. Technical Transactions iss. 8. Civil Engineering iss. 2-B, 2013 (110), pp. 3-12. This paper was created as a part of the implementation of the BKM 504/RB6/2017 project at the Department of Structural Engineering of the Silesian University of Technology.
PROBLEM OF CRACKS IN PRESTRESSED HOLLOW CORE SLABS Summary The aim of this paper is to summarize the essential information from literature on cracking in prestressed hollow core slabs. In the beginning of the paper, the influence of tensile forces in the transmission zone on the cracking development is analysed. Next, the causes, prevention and repair of the casting and shipping cracks are presented. At the end of the paper, sources of cracking in prestressed hollow core slabs of slim floor constructions are considered.
PROBLEM RYS W SPRĘŻONYCH PŁYTACH KANAŁOWYCH Streszczenie Celem referatu jest przedstawienie najważniejszych informacji na temat rys w sprężonych płytach kanałowych. Na początku referatu przeanalizowano wpływ sił rozciągających na powstanie rys w strefie zakotwienia. Następnie, zaprezentowano i przedyskutowano przyczyny oraz skutki zarysowań będących wynikiem błędów projektowych i wykonawczych. Szczegółowo opisano wpływ pęknięć na nośność płyt, metody ich naprawy oraz sposoby zabezpieczenia płyt przed powstaniem takich zarysowań. Końcowa część referatu zawiera dyskusję na temat zarysowań sprężonych płyt kanałowych w konstrukcjach Slim Floor.
