Mechanical Behavior of BFRP Cable Rock Bolts: Experimental and Analytical Study
Publication: Journal of Composites for Construction
Volume 28, Issue 5
Abstract
In this study, the mechanical behavior of novel fully grouted cable rock bolts made of basalt fiber–reinforced polymer (BFRP) was investigated, which included pullout and double-shear joint behavior. Three BFRP cables were prepared: (1) single-tendon; (2) untwisted multiple-tendon; and (3) twisted multiple-tendon. In the pullout tests, the effects of cable type, borehole diameter, and encapsulated length were studied. However, the influence of the pretension load level was focused on in the double-shear joint tests. The results suggested that an overly thick grout annulus significantly reduced the pullout strength and stiffness. For the BFRP cable with multiple tendons, the initial pullout stiffness was enhanced compared with the BFRP cable with a single tendon due to better interfacial bonding with the grout. In the double-shear joint tests, the pretension of the cable significantly improved the shear capacity and stiffness. With an enhancement in the confinement from pretension, the partial failure of the grout at an early stage could be prevented. The parallel and twisted-tendon BFRP cable rock bolts had larger failure displacements than the single-tendon cable. This analytical study found that a pullout analytical model available in the literature was accurate when back-calculating the stress conditions along the rock bolt. The predicted shear capacity of the rock joint showed good agreement with the experimental values. A parametric study indicated that the joint shear capacity was sensitive to the tensile and shear strengths of the cable but was irrelevant to the cable moduli. Considering the effectiveness of pretension and creep safety for BFRP, a suitable range for the pretension load was suggested.
Practical Applications
This study attempts to promote the use of novel, fully grouted cable rock bolts made of basalt fiber–reinforced polymer (BFRP) materials in geotechnical engineering applications. Different types of cable rock bolts are produced, which include single-tendon, untwisted multiple-tendon, and twisted multiple-tendon cables. In the pullout tests, the untwisted multiple-tendon cable exhibited the highest pullout capacity, because of its better interfacial bonding with the grout. From the double-shear joint tests, the pretension of the cable could significantly improve the shear capacity and stiffness. In general, the BFRP cable rock bolts could have a competitive mechanical behavior compared with the steel rock bolts. The respective analytical models for pullout and double-shear joint specimens have proved to be accurate for predicting the mechanical responses of the cable rock bolts. These concise but reliable analytical models could allow researchers and designers to better utilize these novel cable rock bolts in potential applications.
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Data Availability Statement
All the data, models, and codes generated or used during the study appear in the published article.
Acknowledgments
The authors gratefully acknowledge the financial support that was provided by the National Key Research and Development Program of China (No. 2017YFC0703000) and the National Natural Science Foundation of China (NSFC, 51878149).
Notation
The following symbols are used in this paper:
- A
- cross-sectional area of the rock bolt;
- Aj0
- cross-sectional area of the joint surface;
- d0
- ineffective diameter of the rock bolt beyond which the effect of the bolt disappears;
- db
- diameter of the rock bolt;
- dg
- diameter of the borehole;
- Eb
- tensile elastic modulus of the rock bolt;
- Eg
- elastic modulus of the grout;
- Er
- elastic modulus of the concrete block;
- fu
- failure (ultimate) strength of the rock bolt;
- fy
- yielding strength of the rock bolt;
- Gb
- transverse shear modulus of the rock bolt;
- Gg
- shear modulus of the grout;
- Gr
- shear modulus of the concrete block;
- I
- moment of inertia of the rock bolt;
- i
- dilation angle of the joint;
- K
- coefficient related to the bolt properties in double-shear joint modeling;
- L
- encapsulated length of rock bolt;
- lp
- half of the plastic length in a rock bolt of a double-shear joint;
- NO and QO
- axial and shear forces of the rock bolt;
- Ny and Qy
- axial and shear capacities of the rock bolt under yielding/ultimate limit;
- P0
- axial load in the rock bolt;
- P0max
- maximum axial load in the rock bolt;
- RT, RN, and RQ
- contributions from the pretension force, additional axial force, and shear force on the joint shear capacity;
- St
- transverse shear strength of the rock bolt;
- sr and sp
- residual shear strength and peak shear strength at the bolt–grout interface;
- T
- pretension load of the rock bolt;
- Wur
- weight of the upper concrete block in a sheared joint;
- XO
- NO + T;
- x0
- fully decoupling length for Li and Stillborg's model of the bolt–grout interface;
- α
- bolting angle of the joint;
- β
- dimensionless parameter for Li and Stillborg's model;
- Δ
- ascending length for Li and Stillborg's model;
- νg
- Poisson's ratio of the grout;
- νr
- Poisson's ratio of the concrete block;
- σb
- axial stress in the rock bolt;
- σc
- unconfined uniaxial compressive strength of the concrete block;
- σg
- unconfined uniaxial compressive strength of the grout;
- σnW and σnT
- effective normal stresses from the rock weight and pretension load;
- τ
- experimental shear stress at the surface of rock bolt;
- τb
- shear stress at the bolt–grout interface;
- ϕr
- residual friction angle of the joint surface; and
- ω
- the ratio of residual strength to peak strength of the bolt–grout interface.
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Information & Authors
Information
Published In
Journal of Composites for Construction
Volume 28 • Issue 5 • October 2024
Copyright
© 2024 American Society of Civil Engineers.
History
Received: Nov 28, 2023
Accepted: May 17, 2024
Published online: Jul 26, 2024
Published in print: Oct 1, 2024
Discussion open until: Dec 26, 2024
ASCE Technical Topics:
- Cables
- Composite materials
- Construction engineering
- Construction methods
- Continuum mechanics
- Dynamics (solid mechanics)
- Engineering fundamentals
- Engineering materials (by type)
- Engineering mechanics
- Equipment and machinery
- Fiber reinforced composites
- Geomechanics
- Geotechnical engineering
- Grouting
- Joints
- Materials engineering
- Materials processing
- Plastics
- Pretensioning
- Pullout behavior
- Soil dynamics
- Soil mechanics
- Solid mechanics
- Structural dynamics
- Structural engineering
- Structural members
- Structural systems
- Synthetic materials
- Uplifting behavior
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