Research

CFRP-ECC hybrid for strengthening concrete structures

Carbon fiber reinforced polymer (CFRP) composite has attracted extensive research attention for strengthening concrete structures. International design guidelines have been published for strengthening of concrete structures using externally bonded CFRP systems. This is because CFRP has shown desirable advantages including lightweight, low thermal conductivity, and high resistance to corrosion and chemical attack.

Most of the studies used polymer based materials (like epoxy) as the adhesive for bonding the CFRP to the surface of the concrete structures. Polymer adhesives can be very effective for strengthening when there are no fire related issues. Otherwise, polymer adhesives undergo significant degradation in their mechanical properties (like modulus and strength) when subjected to elevated temperatures in a fire. This is because when the glass transition temperature (Tg) of a polymer is exceeded, the polymer will change from the glassy state to the rubbery state with a significant reduction in mechanical properties. This largely hindered the application of CFRP strengthening for concrete structures which may have fire concerns in their service lives.

This research proposed a CFRP-ECC hybrid as the external bonding layer for strengthening of concrete structures when a structural fire was a concern. Firstly the compression and tension tests were conducted on ECC exposed to elevated temperatures up to 500 degree Celsius. The effects of temperature on the compressive and tensile properties of ECC were investigated. Direct pulled-out tests were conducted to study the interface bonding behavior between CFRP and ECC under elevated temperatures up to 500 degree Celsius. Various CFRP bond lengths were used in the pull-out tests in order to find out the effective bond length of the CFRP-ECC hybrid. Finally, four-point bending tests were conducted on steel reinforced concrete beams strengthened with the CFRP-ECC hybrid.

After 28 days curing time, dogbone-shaped specimens were heated in a muffle furnace. When the target temperature was reached, it was held for one hour to make sure the temperature of the specimens became uniform and stable. Then the specimens were taken out of the furnace and cooled down to the room temperature, after which the tension tests were conducted.

 

 

 

Fig 1. Comparison between normal concrete and ECC (left) and the tensile stress-strain curves of ECC (right)

 

 

 

Fig 2. Temperature effect on the tensile stress-strain curves (left) and tensile strength and tensile strain capacity (right)

 

Direct pull-out tests were conducted in order to investigate the interface behavior between CFRP and ECC of the hybrid system. In order to study the effect of the embedded length of CFRP on the interface behavior between CFRP and ECC, six embedded lengths were selected as 25 mm, 50 mm, 75 mm, 100 mm, 135 mm and 170 mm. The pull-out tests were conducted on a MTS 810 material testing system with a loading capacity of 100 kN.

 

 

 

Fig 3. (a) Peak pull-out stress versus CFRP embedded length of ECC specimens and (b) temperature effect on the peak pull-out stress of the ECC specimens

 

Three steel reinforced concrete beams were prepared. The first beam was used as reference. The second beam was strengthened with CFRP-ECC hybrid system. The third beam was the same as the second beam except that ECC was replaced with mortar (ECC without PVA fiber).

 

 

 

Fig 4. Failure modes of concrete beams: (a) reference beam; (b) beam with CFRP-mortar hybrid strengthening and (c) beam with CFRP-ECC hybrid strengthening

 

It was found that CFRP-ECC hybrid has improved performance under elevated temperatures up to 500 degree Celsius. From the theoretical analysis and the flexural testing of the CFRP-ECC hybrid alone, CFRP-ECC hybrid exhibited good potential for strengthening the concrete structures. The results indicate that direct casting of the fresh ECC to the concrete surface is not sufficient to ensure the bonding of the two, even though the surface of the concrete has been prepared with surface treatments beforehand. Therefore, it is highly recommended that shear keys be designed and installed on the concrete surface before attaching the CFRP-ECC hybrid layer. This would effectively secure the bond of ECC and concrete for not only room temperature applications, but also for conditions concerning elevated temperatures.

 

 

Chao Wu, professor, school of transportation science and engineering, Beihang University, E-mail: wuchao@buaa.edu.cn

 

References

[1]Wu, C., Li, V. C. (2017). CFRP-ECC hybrid for strengthening of the concrete structures, Composite Structures, 178, 372-382.

[2]Wu, C., Li, V. C. (2017). Thermal-mechanical behaviors of CFRP-ECC hybrid under elevated temperatures, Composites Part B, 110, 255-266.