Proceedings of US-Japan Workshop on Life Cycle Assessment of Sustainable Infrastructure Materials

Sapporo, Japan, October 21-22, 2009

FRP FOR SUSTAINABLE PRECAST CONCRETE STRUCTURES

Sami Rizkalla Department of Civil, Construction, and Environmental Engineering, North Carolina State University, Raleigh,

NC 27695, USA

ABSTRACT

The use of FRP for sustainable precast concrete structures has become a common practice. The precast industry has recognized the advantage of this life-cycle approach and has begun to implement FRP materials in innovative applications. This paper summarizes some of the emerging developments and established applications. The selected applications presented in this paper are double-tee beams, insulated sandwich precast load bearing panels, architectural cladding and precast concrete filled FRP tubes. Several possible future opportunities are also presented.

1 INTRODUCTION This paper presents some of the emerging developments and established applications related to the use of FRP materials for precast concrete Structures. The paper focuses on the innovative use of FRP materials in several selected precast technologies including double-tee beams, insulated precast wall panels, architectural cladding and precast concrete filled FRP tubes (CFFT). The unique advantages of the FRP reinforced systems are highlighted and the primary action mechanisms of the FRP materials are presented. The paper concludes by identifying several areas of possible future opportunities in which FRP materials can be used for further advancement of the precast industry and for civil engineering infrastructure in general. 2 DOUBLE-TEES

Precast double-tees are commonly used in construction of parking decks and roof structures in which the top flange of the members are subjected to severe environmental exposure including rain, snow and de-icing salts. Since precast members are

optimized to provide the most structurally and cost efficient use of materials, the flanges of these members are relatively thin, in the range of 2 to 4 inches. These thin top flanges are typically lightly reinforced with steel welded wire fabric to control shrinkage and thermal cracks and to transmit the load from the flange to the stems of the double-tees in one-way slab bending action. Under harsh exposure conditions, and over time, the flanges are subjected to penetration of moisture and chlorides. This can lead to corrosion of the reinforcement, cracking and spalling of the cover concrete and possible discoloration of the member. The non-corrosive properties and high strength-to-weight ratio of carbon FRP (CFRP) grids make these materials ideally suited to replace conventional steel welded wire fabric as reinforcement for the flanges of double-tees as shown in Figure 1(a). In addition to enhancing the durability of precast concrete members, lightweight CFRP grids are commercially available in rolls which can be installed using an automated process, as shown in Figure 1(b), thereby accelerating the construction process.

(a)

Figure 1: (a) Schematic of CFRP reinforced double-tees (b) Apparatus for automated installation of CFRP grid in flanges of precast double-tees (courtesy of Metromont Corporation)

CFRP grid

(b)

Due to the brittle nature of CFRP materials,

there is some concern regarding the possibility of a brittle failure of the flange reinforced with CFRP materials. To address these concerns, two full-scale 12DT(30) double-tees, with CFRP grid reinforced flanges were subjected to a uniformly distributed load and tested to failure. The uniformly distributed load was applied using atmospheric pressure by constructing a pressure chamber around the beams and applying suction within the chamber using high-powered

vacuums as shown schematically in Figure 2. The two tested double-tees were identical except that the first, DT1, had a top flange thickness of 2 inches while the second, DT2, had a flange thickness of 3.5 inches. Both beams were tested according to the load test requirements of chapter 20 of the ACI 318-08 building code [1] which requires application of a sustained factored load for 24 hours. The beams were subsequently tested monotonically to failure. The test setup is shown in Figure 3(a).

Figure 2: Schematic of pressure chamber to apply uniformly distributed load to double-tee beams

Figure 3: (a) Setup for testing precast double-tees under uniformly distributed loading (b) Cracking of DT2 under concentrated load (crack enhanced)

(a) (b)

Both of the tested double-tees were capable to

resist the maximum factored load for 24 hours while exhibiting minimal residual deflections upon recovery. The subsequent monotonic tests to failure indicated that beams DT1 and DT2 were able to sustain a maximum load equal to 1.3 and 1.9 times the full factored load respectively. DT1 failed along the entire span of the double-tee due to the formation of a large longitudinal crack in the overhanging flange near one of the stems, crushing of the concrete and rupture of the CFRP grid which was evident by total separation of the overhanging flange from the rest of the beam. The observed linear failure along the entire length was due to the nature of the uniformly distributed load that was used to test the member. The member exhibited significant deflection and cracking prior to failure.

DT2, which had a 3.5” thick top flange did not fail and the test was halted when the applied load was equal to 1.9 times the maximum factored load of the double-tee due to an inability to increase the suction inside the test chamber. The test results indicated that the flange bending behavior of the double-tees was quite sensitive to the flange thickness. Therefore, the possible brittle flange bending failure mode can be avoided by properly designing the thickness of the top flange. For the two tested double-tees, increasing the flange thickness from 2” to 3.5” increased the flexural moment of inertia of the overhanging flange by more than five times which resulted in a corresponding increase of the cracking load and reduction of the flange deflections. The double tee was subsequently tested under the effect of concentrated load as shown in Figure 3(b) and the results

satisfied the recommendations of the PCI Design Guideline [2].

3 PRECAST CONCRETE SANDWICH WALL PANELS

These panels consist of two concrete wythes separated by a rigid foam core. These highly efficient precast members can serve multiple purposes within the same structural member. The wall panels can act as a part of the primary load carrying system of the structure to transfer gravity loads and lateral loads, due to wind or seismic events, to the foundation of the structure. Further, the presence of the foam core increases the overall insulation properties of the panel thereby improving the overall thermal characteristics of the structure. Finally, the exterior face of the precast panels can be constructed with an architectural finish to contribute to the overall aesthetics of the structure. The panels can be designed to act in fully composite action, partial-composite action or non-composite to achieve the intended purpose of the panel. The level of composite action depends on the type of shear connection provided between the two wythes. For conventional precast concrete fully composite structural wall panels, the shear connection is typically provided between the inner and outer wythes by casting concrete solid zones within the core of the panels or by using through-thickness steel ties. One series of tests was conducted on panels in which the inner and outer wythes were connected with a steel truss assembly [3]. Test results indicated that the panels behaved as nearly fully composite up to failure. Other

research has indicated that solid concrete zones with an area of 1 percent or steel pin connectors with an area of 0.1 percent of the panel area can reduce the

insulation properties (R-value) of a wall panel by up to 40 percent [4]. The locations of the shear

Figure 4: Thermal ‘leaking’ of precast concrete wall panel system due to thermal bridges at locations of concrete solid zones (courtesy of Composite Technologies Corporation)

Typical precast concrete sandwich wall panel system

‘hotspots’ at locations of concrete solid zones

connectors thus form hot spots on the exterior of the structure, as shown in Figure 4, in the thermal image of these walls. For these panels heat is transmitted out of the structure through the shear connectors. This results in a thermally inefficient structure with increased heating costs and energy consumption. Recently some of the precast producers in the United States adopted the use of FRP ‘shear truss’ type shear connectors for fully composite wall panels. These panels are structurally and thermally efficient and behave as fully composite wall panels utilizing the inner and outer wythes of precast concrete sandwich panels. One method to achieve the ‘shear truss’ mechanism consists of cutting square CFRP grid at a 45o angle as shown in Figure 5(a). The grid is then embedded through the foam core into both wythes of the sandwich panel as shown in the figure. Alternatively, GFRP bars can be pultruded in a truss configuration as shown in Figure 5(b) and embedded in the wall panels in a similar manner. In both cases the FRP grid or bars are oriented to directly resist the maximum shear stresses through a truss mechanism. The cross section of a typical panel is shown in Figure 5(c). The presence of internal pilasters helps to transmit vertical loads from the roof system to the foundation while providing additional rigidity to the overall panel. In one study a total of six full-scale wall panels were fabricated using different types of foam cores and different configurations of CFRP grid shear connectors [5]. The panels were tested under the combined effect of simulated vertical gravity loads and lateral cyclic loads to simulate wind loading over the lifespan of a typical structure. The measured deflections and

strains indicated that the CFRP shear truss provided fully composite action at service load levels. Further the testing indicated that the degree of composite action was highly dependent on the type of foam used in the core material. A companion analytical study indicated that varying the type of foam core material can approximately double the shear transfer between the two wythes [6]. Typical testing of a full-scale wall panel is shown in Figure 5(d). The type of foam core can also affect the insulating and vapour barrier characteristics of the panel and therefore should be designed in collaboration with the panel producers. The analytical study further indicated that the degree of composite action of the panels decreased with increasing load level. At the service load level the panels acted in nearly fully-composite action while near the ultimate load level the degree of composite action was slightly reduced.

Based on the experimental and analytical study, a design guide was established to design the required amount of shear connection required for a given wall panel using CFRP grid as shown in Figure 5(e) [7]. The required shear force at the interface between the two wythes, and therefore the overall amount of CFRP required is based on the required moment capacity of the wall panel and the level of composite action desired. The design of a wall panel with GFRP truss connectors is based on the assumption that the panel exhibits 80 percent composite action under service loading conditions and 50 percent composite action under ultimate loading conditions [7].

Figure 5: FRP shear connectors for fully composite precast concrete wall panels (a) CFRP grid ‘shear truss’ connectors (b) GFRP shear truss connectors (courtesy Hughes Brothers Inc.) (c) Typical wall panel cross-section (courtesy Altus Group) (d) Testing of a full-scale composite sandwich wall panel (e) Proposed guideline for design of CFRP shear connection for precast concrete wall panels [7].

(a)

(d)

(e)

(b)

(c)

4 ARCHITECTURAL CLADDING In some applications it is desirable that the

precast sandwich panels act in non-composite action. In this case one wythe of the panel, typically the inner wythe, is load bearing while the outer wythe is non-load bearing and provides the architectural finish for the structure. This type of behaviour is desirable, for example, in regions where a high thermal gradient exists between the exterior and interior of the structure. In this case fully composite panels can exhibit thermal bowing caused by differential expansion of the two wythes due to the thermal gradient across the panel. This thermal bowing can result in several undesirable effects including increased P-delta effects, lack of fit between adjacent panesl due to differential thermal bowing and intentional cracking of wythes. By eliminated the shear connection between the two wythes, each wythe in the panel is free to deform independently thereby foregoing the problems associated with thermal bowing. It is still necessary, however, to provide a limited degree of connectivity between the wythes to prevent wrinkling of the wythes and through thickness separation of the panel from the core. Conventionally wythes are connected to each other with discrete metallic pins or connectors. However, as discussed, these connectors provide thermal breaks through the core of the panel resulting in thermal inefficiency. Alternatively, glass fiber reinforced vinyl ester resin pins have been developed which facilitate the manufacture of thermally efficient, non-composite precast sandwich panels. A typical GFRP pin configuration is shown in Figure 6.

FRP materials have also been used as secondary

reinforcement for non-structural, architectural cladding. For architectural products uniformity of color, crack mitigation, and thermal efficiency are primary considerations and structural demands are minimal. Precast architectural cladding panels typically consist of a thin exterior concrete diaphragm face, on the order of 1 to 2 inches thick. This layer is supported by a steel reinforced Vierendeel concrete frame with intermediate vertical and/or horizontal members, as shown schematically in Figure 7(a). This frame is attached to the primary load carrying frame of the structure. The two components of the frame are separated by an insulating foam core layer and connected by a 45o CFRP shear grid similar to that used for composite wall panels. CFRP grids are also used as the secondary reinforcement for the exterior architectural diaphragm to prevent thermal and shrinkage induced cracking. The use of non-corrosive CFRP materials prevents staining and cracking of the architectural finish which can occur due to corrosion and expansion of conventional steel reinforcements. The CFRP reinforced panels can weigh as little as 30 percent of the weight of similar conventional steel reinforced precast architectural cladding panels. The primary weight savings are due to the reduction of the required concrete cover thickness to protect the secondary reinforcement from corrosion for conventional steel-reinforced cladding.

Figure 6: Typical GFRP pin for construction of non-composite sandwich wall panels

CFRP grid reinforcement of outer diaphragm

Triangular rib of supporting Vierendeel truss-like frame CFRP shear-grid connection

between outer diaphragm and supporting frame

Foam insulation layer

Exterior architectural diaphragm

Plan view (not to scale)

Figure 7: (a) Schematic representation of CFRP reinforced architectural cladding panels (courtesy High Concrete Group) (b) Different configurations of the Vierendeel truss-like supporting frame to accommodate different panel configurations (courtesy Altus Group). 5 PRECAST CONCRETE PILES (CFFT)

These members consist of a filament wound GFRP tube which acts as a stay-in-place form for a concrete core. In addition to acting as a stay-in-place form, the GFRP tube acts as the primary longitudinal reinforcement of the piles and provides confinement for the concrete core. The presence of the GFRP tube eliminates the need for internal reinforcement and prevents ingress of moisture and sulphates into the concrete thereby enhancing the environmental durability of the piles. The enhanced durability of the CFFT piles makes them

well suited for application as bridge piles and also in marine applications including as supports for light marine structures, fender piles and pile clusters. Due to their enhanced durability and their similar performance to conventional piles, ten CFFT piles were used to construct one bent of the Route 40 bridge in Virginia is shown in Figure 8(a) [8]. The piles exhibited similar axial load response and similar driving characteristics to conventional prestressed precast concrete piles. The CFFT piles were connected to the pile bent cap using a similar

(a)

CFRP shear-grid

Vertical back rib configuration

Horizontal back rib configuration

Back ribs designed to accommodate punched openings

(b)

detail to that used for the conventional piles. After two years in service the piles did not exhibit any indication of deterioration.

To increase the flexural cracking load of the piles and reduce lateral deflections, prestressed CFFT (PCFFT) piles have also been developed. The piles can be internally prestressed using similar materials and methods to those used for conventional prestressing as shown in Figure 8(b). An experimental investigation was conducted to evaluate the behaviour of the PCFFT piles [9]. Two types of precast concrete piles were tested. The first tested pile was a circular PCFFT pile while the second was a conventional precast concrete pile that was designed with similar dimensions and configuration of prestressing as the PCFFT. The conventional pile was reinforced with a transverse

steel spiral to compare the confinement effect of the GFRP tube and a conventional steel spiral. The normalized moment-deflection responses of the two piles are shown in Figure 8(c). Comparison of the behaviour indicates that both piles exhibited similar initial stiffnesses. However the PCFFT pile exhibited a higher post-cracking stiffness, ultimate capacity and energy absorption prior to failure as compared to the conventional pile. This is primarily attributed to two mechanisms: (1) the GFRP tube provided additional reinforcement to the pile in the longitudinal direction and (2) the presence of the GFRP tube on the outer surface of the pile provided confinement to a greater volume of concrete as compared to the internal steel spiral typically used in conventional piles.

CFFT pile bent(a)

(b)

(c)

Figure 8: Prestressed CFFT piles (a) Route 40 CFFT pile bridge bent (b) Fabrication of typical PCFFT piles (c) Comparison of flexural behavior of PCFFT and conventional prestressed concrete circular piles (courtesy Dr. A. Fam, Queens University)

Hollow CFFT, with a circular void within the central core region of the pole, represent an efficient use of concrete materials and can be used to significantly reduce the weight of CFFT members. These lighter weight members can be effectively used in applications in which the poles are subjected primarily to flexural loading conditions such as for lighting poles, highway sign support structures and as support structures for wind turbines. The hollow CFFT are commonly fabricated using the spin-casting fabrication method in which the concrete is placed into the circular GFRP tubes which are subsequently spun at a high rate of revolution to consolidate the concrete and to form the central void. In one study a series of spun-cast CFFT poles were fabricated with different sizes of interior voids and subsequently tested to failure [10]. The results indicated that, for the specific type of CFFT considered, the diameter of the inner void could be increased to 60 percent of the pole outer diameter without reducing the flexural strength of the pole as compared to a similar CFFT member with a solid cross--section. The study further indicated that the ultimate strength of the poles can be increased and the strain demand on the external GFRP tube can be reduced by reinforcing the concrete portion of the member with additional internal mild steel reinforcement. 6 FUTURE OPPORTUNITIES

Recent developments of construction materials have highlighted the advantages of using FRP materials and revealed a number of opportunities in which FRP materials can be used for sustainable precast concrete Structures.

FRP materials can be used as a reinforcement for several precast sections, including precast deck panels, hollow core slabs and hollow core concrete plank bridges. The use of FRP reinforcement for these profiles could potentially reduce the required cover concrete thereby resulting in lighter, more efficient and sustainable members. FRP grids can also be used as combined longitudinal and transverse reinforcement for precast concrete piles thereby eliminating the possibility of corrosion and potentially extending their serviceable life.

Thin FRP grid materials can also enable the construction of thin precast concrete plate and shell structures which serve both structural and architectural purposes while completely optimizing the use of the materials and enhancing building aesthetics. The potential to use FRP materials for slender concrete structural members has been highlighted in a recent study in which precast concrete lighting pylons were prestressed with 4 mm CFRP tendons [11]. The 9.2 m long poles had an outer diameter of 185 mm and a wall thickness of

only 40 mm. The cantilevered poles were capable of achieving tip deflections up to 925 mm or approximately L/100. The use of prestressed CFRP tendons in conjunction with high-strength concrete resulted in an overall flexible system despite the brittle nature of the materials themselves. Similar high-strength, slender and flexible structures can be developed by innovative application of FRP reinforcing and other new technologies including high-strength concrete, self-consolidating concrete and reactive powder concrete.

Through the ingenuity of the new generation of engineers, and based on a thorough knowledge and understanding of structural behavior, FRP materials can be used to develop a new generation of precast concrete structures and systems to address the sustainability challenges of the 21st century. 7 CONCLUSIONS

This paper summarized the recent developments and common applications of FRP materials for the construction of sustainable, durable and structurally efficient precast products. The technologies discussed in this paper represent some of the selected advancements in the precast industry. They demonstrate the distinct advantages of using FRP materials which result in improved structural, thermal and architectural characteristics of the completed precast product. Typically the use of FRP materials can result in long-term economic benefits when the life-cycle costs are considered. The advancements described in this paper represent the growing acceptance of FRP in the engineering community and symbolize what can be achieved when new technologies are combined with ingenuity. The applications described in this paper paint the picture of a bright future for composite materials, in the precast industry specifically and in civil engineering in general. These advancements lead the way towards exciting new future opportunities in the use of FRP materials and open the doors for the future success of the use of new and advanced materials, structures and systems in the years to come. 8 ACKNOWLEDGEMENTS The authors would like to acknowledge the cooperation of Altus Group, Composite Technologies Inc., High Concrete Group, Hughes Brothers Inc., and Metromont Corporation, for providing much of the necessary background information needed for the preparation of this paper. 9 REFERENCES [1] American Concrete Institute. (2008) Building

code requirements for structural concrete (ACI 318-08) and commentary. Farmington Hills, MI: American Concrete Institute. [2] Precast/Prestressed Concrete Institute. (2004). PCI design handbook: Precast and prestressed concrete (6th ed.). Chicago: Precast/Prestressed Concrete Institute [3] Benayoune, A., Samad, A. A. A., Ali, A. A., and Trikha, D. N. 2007. Response of precast reinforced composite sandwich panels to axial loading. Construction and Building Materials, 21(3), 677-685. [4] Eina A., Salmon, D.C., Fogarasi, G.J., Culp, T.D. and Tadros, M.K. (1991). State-of-the-art of precast concrete sandwich panels, PCI Journal, 36(6), 78-98. [5] Frankl B.A., Lucier, G.W., Hassan, T.K. and Rizkalla, S.H. . Behavior of insulated precast prestressed concrete wall panels reinforced with CFRP grid. ACI Structural Journal. (submitted for publication) [6] Hassan, T.K. and Rizkalla, S.H. Analysis and design guidelines of precast/prestressed composite

load-bearing sandwich wall panels. ACI Structural Journal. (submitted for publication) [7] Aslan Pacific Ltd. (2008). Nu-Tie sandwich wall connector: Aslan FRP. Hong Kong: Aslan Pacific Ltd. [8] Fam, A., Pando, M., Filz, G., and Rizkalla, S. (2003). Precast piles for Route 40 bridge in Virginia using concrete filled FRP tubes. PCI Journal, 48(3), [9] Fam, A. and Mandal, S. (2006). Prestressed concrete-filled fiber reinforced polymer circular tubes tested in flexure. PCI Journal, 51(4), 42-54. [10] Qasrawi, Y. and Fam, A., (2008). Flexural load tests on new spun-cast concrete-filled fiber-reinforced polymer tubular poles. ACI Structural Journal, 105(6), 750-759. [11] Terrasi, G.P. and Lees. J.M. (2003). CFRP prestressed concrete lighting columns. In Rizkalla,S. And Nanni, A. (Eds.), Field applications of FRP reinforcement: Case studies (ACI SP-215) (pp. 55-74). Farmington Hills, MI: American Concrete Institute.