Abstract
The main goal of this research is to evaluate the flexural strength of segmental reinforced concrete beams with different strengthens methods. Four reinforced concrete beams with different strengthens methods were constructed and tested by applying static loading. The experimental variables considered in this study include epoxy strengthens and using one or two layers of CFRP. All beams were geometrically similar with the dimension of (300mm in flange width, 100mm in flange thickness, 300mm in total depth, and 1100mm in effective span) and the applied loads were subjected to the beams with two point loads and simply supported span. In general using epoxy strengthens lead to decrease the ultimate load capacity and using one layer of CFRP give an economical solution while using two layer lead to about twice the ultimate load capacity.
. . . 300 100 300 1100 . .
1: General
Bridges have been built since thousands of years. Nevertheless, new innovative, construction methods are still required. In recent decades, the precast segmental method of bridge construction has become increasingly popular. Increased speed of erection, improved aesthetics, and mitigation of environmental disturbance are all positive qualities contributing to the expanded use of this method. Additionally, the various techniques within segmental construction are adaptable to a variety of span lengths, and quality control is improved while deflections from creep and shrinkage are reduced [1].
Although segmental method provides many advantages, drawbacks arise as well. For instance, joints between the precast segments require special attention in design and construction. These joints introduce a structural discontinuity in the bridge, yet they must transmit the large compressive and shear stresses in the joint locations. Another concern is that no continuous mild reinforcement crosses the joint in precast concrete segmental bridges. This lack of steel crosses the joint and could lead to sudden regions under overloads or earthquake loadings [1].
Despite their importance, at present, no established methodology exists for the design of joints in precast segmental bridges. The existing body of research in this area is limited, and the behavior of the joints under different loading is not fully understood. The Post Tensioning Institute (PTI) [2], (1988) has recently published recommendations for the design and construction of segmental concrete bridges, but the recommendations emphasize the need for added research in many areas including the joint regions.
Hence it is evident that continued research is necessary to evaluate the ultimate strength of the bridge joints and to understand their deformation behavior under service and ultimate loadings.
The segmental method of concrete bridge construction was originated in post War World II- Europe in response to the shortages of supplies and manpower. In early segmental bridge construction, the gap between segments was filled with mortar or concrete. Through improvements such as the match-casting process, these mortar joints are only required at the closure of a span. The bonding agent currently used between segments is usually a thin layer of epoxy. The benefits provided by epoxy are: during the construction phase it
(a) Facilitates placement of segments by lubricating joint regions;
(b) Eliminates unevenness of the joint surfaces, in the completed bridge it
(c) Contributes to the strength of the structure by transmitting shear stresses across the joint and
(d) Prevents corrosion of the tendons by forming a waterproof seal [3],[4].
In (1972), Bishara and Mahmoud[5] tested a 33m two span continuous girder. The girder was composed of three precast segments which were joined near the inflection points by keyed scarf connections. Scarf connections are characterized by an inclined rather than a vertical section at the joint. Pre-stressed keys, epoxy-sand mortar and high tension bolts were used to secure the scarf connections. Under a loading pattern that produced moment and high shear in the scarf connections, the beam behaved as a monolithic pre-stressed continuous beam. It was found that, cracking in the scarf connection initiates in the concrete rather than in the epoxy mortar.
Rabbat and Sowlat in(1987)[6] performed ultimate bond tests on three segmental concrete girders to examine the effect of external placement of tendons. Each of the three T-shaped girders was composed of 22 segments which covered a 10-0 mm. span, and dry-keyed joint connections (without epoxy) were provided to transfer shear forces at the joints. The bonded and modified unbonded (tendons embedded in subsequent concrete pour) tendons attained similar strengths in the elastic and early inelastic ranges, but the un-bonded tendon girder was only able to achieve 75% of the moment of the other two girders.
In 2005, Turmo et al. presented a study of the behavior of segmental bridges[7], focusing on the response of dry castellated joints under shear, in service and ultimate limit states. Tests have been performed on panels with different levels of prestressing, evaluating the behavior of castellated dry joints under direct shear. The possible benefit of using steel fiber reinforced concrete (SFRC) is also evaluated, by casting and testing reinforced and SFRC panels. The results obtained in these tests, as well as those found in literature, have been compared with several design formulae for assessing the load-carrying capacity of dry interlocked joints without epoxy, identifying the formula that gives the best predictions.
2: Materials
2.1: Concrete Materials
Ordinary Portland cement (type 1) has been used in this study which is conforms to the Iraqi Standard Specification No. 5/1984[8]. Al-Akhaidher natural sand of (4.75mm) maximum size and fineness modulus of 2.84 was used. The grading of the sand conforms to the Iraqi Standard Specification No. 45/1984 [9]. Crashed gravel from Al-nibaey region was used with maximum size of (10 mm). The crushed gravel coarse aggregate were washed and stored in air. The grading of this aggregate and the limit specified by Iraqi Specification No. 45/1984[9]. The tap water has been used for mixing concrete. Mix proportions can be seen in Table(1).
Table(1): Mix proportions of concrete
water
Liter/m3 Gravel
Kg/m3 Sand
Kg/m3 cement
Kg/m3 Material
180 1200 600 400 Weight
2.2: Steel Reinforcement
In this study the used steel reinforcement in concrete, was deformed bars (10 and 12) mm. The tensile tests are conducted on several specimens, at least three specimens. Material properties obtained from the tests for steel reinforcements static yield stress and ultimate strength, are summarized in Table (2).
Table (2): Reinforce steel properties
Modulus of elasticity
(N/mm2) Ultimate
strength
(N/mm2) Yield
stress
(N/mm2) Diameter of bar
(mm) Steel specimens
210000 471 433 12 longitudinal
210000 528 482 10 shear
3: Details of Tested Beams
Four reinforced steel-concrete beams are fabricated. These beams are classified into two groups depending on type strengthens and, epoxy and carbon fiber reinforcement polymer CFRP. The first group implies three beams, the first of three is a reference one and the other are, one with epoxy strengthens and with CFRP one layer strengthens. The second group implies also three beams which have different layers of CFRP. All beams have the same dimensions of 300mm total height and 300mm flange width and 100m flange thickness. The overall span is equal to 1200mm and the effective span is 1100mm. Fabrication details of the tested beams are presented in Table(3) and Table(4), also Figures (1) to (3) and in figure (4) show specimens preparing for test.
Table (3): Beams sampling and grouping details
Type of Beam Beam name Beam number Groups
Reference Beam (without segment) B1 1 1
Segment beam (Epoxy pond) BSE 2
Segment beam (CFRP one layer) BSC1 3
Reference Beam (without segment) B1 1 2
Segment beam (CFRP one layer) BSC1 3
Segment beam (CFRP two layers) BSC2 4
Table (4): Concrete strengths of beams
N/mm2
4 fct
N/mm2
3
N/mm2
2
N/mm2
1 Type of test
beam name beam number
0.849 3.06 2.57 27.33 23.2 B1 1
0.849 3.06 2.57 27.33 23.2 BSE 2
0.852 3.22 2.91 27.93 23.8 BSC1 3
0.852 3.22 2.91 27.93 23.8 BSC2 4
Figure (1) Reference beam details
Figure (2) Segmental beam details
Figure (3) CFRP strengthens
Figure (4) specimens preparing for test
4: Ultimate load
One of major pointers for beam behaviour is ultimate load capacity; Figure (5) shows the different values of ultimate load. Clearly it can be noted that when using only epoxy strengthens lead to decrease the ultimate load, in the study decreasing of about 25% has been noted. Using CFRP one layer give an increase of ultimate load of about 29% while using two layers of CFRP lead to an increase of about 90%. It can be concluded that using two layers of CFRP give the best effect, but also it can be seen that using one layer give the nearest better value with respect to the reference beam, so from economical consideration it may good choice, from figure(6)to figure (9) show the specimens after test.
Figure (5) Ultimate load capacity for tested beams
5: Displacements
The ability of member to be deforming within limits can give clear picture of ductility. It will be herein discuses the displacement at ultimate load. From Figure (10) it can be see that an increase of about 3% in displacement if using Epoxy strengthens. An increase of about 66% if using one layer of CFRP and if using two layer of CFRP lead to an increase of 117%.
Figure (10) Ultimate Displacements for tested beams
6: First Cracks
From table (5) an increase in first crack beam capacity when used CFRP strengthens due to effect high restriction forces, this force increase beam capacity. Highest Increase in first crack beam capacity with segments has been shown when used two layers of CFRP, this is due to increase of bonded in joint due to effect strength of CFRP. Decrease in first crack beam capacity when used epoxy strengthens.
Table (5) first crack beam capacity
Ultimate load (Pu)
(kN) First crack load (Pcr)
(kN) specimen
designation
name
147.5 38 B1
110 30.5 BSE
190 46 BSC1
280 78 BSC2
7: Load deflection
From Figure (11), it can be seen that, there is a significant effect from the method of strengthens on the behavior of load-deflection curves with history of loading. The use of epoxy strengthens will lead to decrease of ultimate load capacity and decreasing of corresponding ultimate deflection. The use of CFRP one layer give a good behavior while using two layers of CFRP give a very stiff beam with approximately twice the ultimate behavior of reference beam. It can be conclude that use of one layer give an economical solution.
Figure (6) Load deflection curve for tested beams
8: Conclusions
The idea of segmental beam has been discuses in this study with foxing onto the method of strengthens, Epoxy and CFRP layers are adopted. From this study it can concluded that using only epoxy strengthens is note safe, while using one layer of CFRP is highly recommended from economical consideration and using two layer of CFRP is recommended from strength consideration. Using one layer of CFRP lead to approximately the same behaviour of reference beam while using two layers lead to approximately double ultimate load capacity.
9: References
[1] Rombach, Dipl.-Ing. A. Specker(2000) “Finite Element Analysis Of Externally Prestressed Segmental Bridges”
[2] Post-Tensioning Institute, (1988) “Design and Construction Specifications for Segmental Concrete Bridges” Final Report. Phoenix
[3] Jazlan bin Salleh “Construction of Precast Segmental Box Girder Bridge Usimg Overhead Gantry-A Case Study””, M. Sc. Thesis, University Technology Malaysia
[4] Rombach (2002) “Precast segmental box girder bridges with external prestressing- design and construction ”
[5] Bishara, A. G. and Mahmoud, M. H. (1972), “Continuous Prestressed Concrete Girders with Keyed Scarf Connections,11 ACI Journal. Vol. 69, No. 9, September, pp. 569-577.
[6] Rabbat, B. G. and Sowlat, K., (1987), “Testing of Segmental Concrete Girders with External Tendons,” PCI Journal. Vol. 32, No. 2, pp. 87-107.
[7] Turmoa, J, Ramosb G., and Apariciob A. C. (2005) ” Shear strength of dry joints of concrete panels with and without steel fibres Application to precast segmental bridges ”
[8] Iraqi Specification No. 5, Portland Cement , Baghdad, 1984.
[9] Iraqi Specification No. 45, Natural Sources for Gravel that is Used in Concrete and Construction , Baghdad, 1984.