Journal of the Korea Academia-Industrial cooperation Society (한국산학기술학회논문지)

The Korea Academia-Industrial cooperation Society (한국산학기술학회)
권호 표지
DOI QR코드

DOI QR Code

Effect of Long-Term Load on Flexural Crack Widths in FRP-Reinforced Concrete Beams

장기하중이 FRP-보강근 콘크리트 보의 휨균열폭에 미치는 영향

  • Choi, Bong-Seob (Department of Architectural Engineering, Chungwoon University)
  • 최봉섭 (청운대학교 건축공학과)
  • Received : 2018.10.15
  • Accepted : 2018.12.07
  • Published : 2018.12.31
Download PDF
⟨ Previous Next ⟩

Abstract

Larger crack widths can be observed more in FRP-reinforced concrete members than in steel-reinforced concrete members as a result of the lower elastic modulus and bond strength of FRP reinforcement. The ACI 440.1R-15 design guide provides equations derived as the maximum bar spacing to control the crack widths indirectly. On the other hand, it is not concerned with long-term effects on the crack control design provisions. This study provides suggestions for how to incorporate time-dependent effects into the crack width equation. The work presented herein includes the results from 8 beams composed of four rectangular and T-shaped FRP-reinforced concrete beams tested for one year under four-point bending. Over a one year period, the crack widths increased as much as 2.6~3.0 times in GFRP and AFRP-reinforced specimens and 1.1~1.4 times in the CFRP-reinforced specimens compared to steel-reinforced specimens. In addition, the average multiple for crack width at one year relative to the instantaneous crack width upon the application of the sustained load was 2.4 in the specimens with a rectangular section and 3.1 in the specimens with a T-shaped section. As a result, it is recommended conservatively that the time-dependent coefficient be taken as 2.5 for the rectangular beams and 3.5 for T-beams.

FRP-보강근 콘크리트 부재들은 FRP-보강근이 철근에 비해 낮은 탄성계수와 부착강도를 갖고 있어 철근콘크리트 부재에 비해 과도한 균열폭의 가능성이 클 수 있다. 따라서 외국의 기준들에서는 FRP-보강근 콘크리트 부재의 균열제어를 위하여 허용균열폭의 제한규정을 두고 있는데, ACI 440.1R-15 설계지침에서는 최대 보강근 간격으로 제어하는 간접적인 방법으로 제안하고 있다. 그러나 제안식은 아직까지 장기하중이 균열폭에 미치는 시간종속적인 효과를 반영하지 못하고 있다. 이에 본 연구에서는 장방형단면뿐만 아니라 T형단면의 FRP-보강근 콘크리트 보를 대상으로 장기실험을 통하여 얻어진 실험결과를 바탕으로 단면형태별 균열폭 특성을 구분하여 파악하므로 써 장기균열폭 예측모델을 제안하는데 필요한 기초자료를 제공하고자 하였다. 따라서 단면형태별로 각각 한 개씩 의 철근콘크리트 비교시험체를 포함한 4개의 장방형보와 4개의 T형 보로 구성된 총 8개의 시험체를 제작하여 시공하중의 영향을 고려한 1년간 4점 가력 장기휨실험을 수행하였다. 결과로서 시간종속적인 영향을 받는 순수장기균열폭은 철근 시험체에 비해 보강근의 탄성계수가 낮은 GFRP나 AFRP-보강근 시험체에서는 2.6~3.0배 증가하였으나 CFRP-보강근 시험체에서는 1.1~1.4배 증가에 그쳤다. 또한 즉시균열폭을 포함한 총장기균열폭은 장방형단면과 T형단면 시험체에서 평균적으로 각각 즉시균열폭의 약 2.4와 3.1배 증가를 보여주어 보수적으로 각각 2.5와 3.5의 시간종속계수를 구분하여 제안하였다.

Keywords

SHGSCZ_2018_v19n12_694_f0001.png 이미지

Fig. 1. Concrete dimensions and reinforcing details for beam specimens

SHGSCZ_2018_v19n12_694_f0002.png 이미지

Fig. 2. Measured crack widths for test specimens(a) rectangular section (b) T-shaped section

SHGSCZ_2018_v19n12_694_f0003.png 이미지

Fig. 3. Crack width increase over 1 year of sustained loading

SHGSCZ_2018_v19n12_694_f0004.png 이미지

Fig. 4. Crack width increase over 1 year of sustained loading as a multiple of immediate crack width

SHGSCZ_2018_v19n12_694_f0005.png 이미지

Fig. 5. Comparison of calculated to experimental results under sustained loading

Table 1. Details of test specimens

SHGSCZ_2018_v19n12_694_t0001.png 이미지

Table 2. Mechanical properties of reinforcing bars

SHGSCZ_2018_v19n12_694_t0002.png 이미지

Table 3. Applied moments and relative ratios

SHGSCZ_2018_v19n12_694_t0003.png 이미지

Table 4. Comparison of measured crack widths with calculated crack widths

SHGSCZ_2018_v19n12_694_t0004.png 이미지

Table 5. Total long-term crack widths at several time stages

SHGSCZ_2018_v19n12_694_t0005.png 이미지

References

  1. ACI Committee 440, "Guide for the Design and Construction of Concrete Reinforced with FRP Bar (ACI 440.1R-15)", p. 83, American Concrete Institute, Farmington Hills, Michigan, 2015.
  2. Canadian Standards Association, "Canadian Highway Bridge Design Code(CAN/CSA-S6-06)", p. 28, Fiber Reinforced Structures, Section 16, 2006.
  3. Japan Society of Civil Engineers, "Recommendation for Design and Construction of Concrete Structures Using Continuous Fiber Reinforcing Materials", p. 325, Concrete Engineering Series No. 23, 1997b.
  4. ACI Committee 318-99, "Building Code Requirements for Structural Concrete (ACI 318-99) and Commentary (ACI 318R-99)", p. 391, American Concrete Institute, Farmington Hills, Michigan 48333-9094, 1999.
  5. P. Gergely and L. A. Lutz, "Maximum Crack Width in Reinforced Concrete Flexural Members", Causes, Mechanism and Control of Cracking In Concrete, SP-20, American Concrete Institute, pp. 87-117, 1968.
  6. R. J. Frosch, "Another Look at Cracking and Crack Control in Reinforced Concrete", ACI Structural Journal, V. 96, No. 3, pp. 437-442, 1999.
  7. ACI Committee 440, "Guide for the Design and Construction of Concrete Reinforced with FRP Bar (ACI 440.1R-06)", p. 44, American Concrete Institute, Farmington Hills, Michigan, 2006.
  8. S. P. Gross, J. R. Yost and D. J. Stefanski, "Effect of Sustained Loads on Flexural Crack Width in Concrete Beams Reinforced with Internal FRP Reinforcement", SP-264-2, American Concrete Institute, Farmington Hills, Michigan, pp. 13-31, 2009.
  9. R. H. Scott and A. W. Beeby, "Log-Term Tension Stiffening Effects in Concrete", ACI Structural Journal, V. 102, No. 1, pp. 31-39, 2005.
  10. R. I. Gilbert, "Tension Stiffening in Lightly Reinforced Concrete Slabs", Journal of Structural Engineering, V. 133, No. 6, pp. 899-903, 2007. DOI: https://dx.doi.org/10.1061/(ASCE)0733-9445(2007)133:6(899)