The 2024 International Code Council model building codes will reference two additional American Concrete Institute (ACI) concrete design and construction standards. ACI CODE 440.11-22 Building Code Requirements for Structural Concrete Reinforced with Glass Fiber-Reinforced Polymer (GFRP) Bars (440.11) will be referenced by the International Building Code (IBC) and ACI CODE 562-21 Assessment, Repair, and Rehabilitation of Existing Concrete Structures (562) will be referenced by the International Existing Building Code (IEBC). Although adoption by authorities having jurisdiction (AHJ) will dictate when provisions become explicitly adopted in a particular area, these standards represent current state of the practice and can be used in the interim through an appropriate alternative means and methods process.
GFRP Reinforced Concrete
ACI Committee 440 developed CODE 440.11 to establish minimum requirements for the strength, stability, serviceability, durability, and integrity of GFRP-reinforced concrete structures. The standard is modelled after ACI CODE 318 Building Code Requirements for Structural Concrete and currently limited to the design and construction of cast-in-place reinforced concretes with GFRP reinforcing bars meeting ASTM D7957 Standard Specification for Solid Round Glass Fiber Reinforced Polymer Bars for Concrete Reinforcement shown in Figure 1.
Benefits and Uses of GFRP
Compared to steel reinforcing bars, the lighter weight of GRFP reinforcing bars are easier to install and may reduce the overall structure’s weight. The bars are non-metallic, so they are not susceptible to electrochemical corrosion that may result from chloride exposure in coastal areas or where chloride deicing salts are used. Thus, properly designed structural concrete with GFRP reinforcement may have improved performance in corrosive environments, such as coastal, marine, and parking garage structures. The non-magnetic nature of GFRP bars is beneficial in facilities housing electromagnetic-sensitive equipment, such as Magnetic Resonance Imaging (MRI) machines.
Limitations
ACI CODE 440.11 permits the use of structural concrete reinforced with GFRP reinforcement for all structural elements in Seismic Design Category (SDC) A and elements not part of the seismic lateral force-resisting system in structures assigned to SDC B and C. However, the proposed language in the 2024 edition of the IBC limits applications to structures assigned to SDC A. GFRP bars are not permitted to be used in any structural member in a structure assigned to SDC D, E, or F. These restrictions are applied to the use of GRFP bars because they are not ductile. Although GRFP is linearly elastic to failure, beyond the failure point there is no ductility and moment redistribution is limited. Inelastic analysis is not permitted in design of GRFP structures.
Due to the lower resistance to elevated temperatures in comparison to conventional steel reinforcing, ACI CODE 440.11 does not permit the use of structural concrete reinforced with GFRP bars in fire-rated construction, except where structural fire resistance has been shown by calculations or tests and approved by the building official. ACI CODE 440.11 additionally requires that bars not be used in environments with a service temperature higher than 27oF below the glass transition temperature of the bar. ASTM D7957 requires bars to have a minimum mean glass transition temperature of 212oF. GFRP bars with glass transition temperatures over 250oF are commercially available.
The Commentary in CODE 440.11 and information in ACI PRC 440.1R-15 provide additional context on elevated temperature performance, implications, and recent research. In Type IIIB construction and except for exterior walls in Types IIB and VB construction structural elements are not required to have fire resistance ratings. Thus according to types of construction and occupancy and use in Chapters 5 and 6 of the International Building Code there are many acceptable applications of GFRP bars. For instance, schools, four stories or less; businesses four stories or less; and hotels, motels, apartment buildings, and condominium buildings five stories or less are permitted as Type IIB construction for fully sprinklered buildings. Refer to the specific requirements of the IBC for an individual project.
Differences in Detailing and Design
Concrete reinforced with GFRP reinforcement behaves differently than concrete reinforced with steel reinforcement, resulting in different detailing requirements, serviceability considerations, and strength calculations. When compared to steel bars, GRFP bars have a higher transverse coefficient of thermal expansion, larger strains at service loads, and exhibit different mechanical and bond behavior resulting in the following differences in detailing requirements:
- Different skin, secondary side face, reinforcement spacing,
- Closer tie spacing for compression members,
- Greater cover for large bar diameters in structures that experience considerable thermal cycling,
- Different stirrup anchorage requirements, and
- Different development lengths.
Serviceability
GFRP bars have a higher tensile strength, but a lower modulus of elasticity than steel bars. Thus, GFRP flexural members are typically governed by serviceability, rather than by ultimate strength. Deflections must always be calculated and checked against limits for flexural members. At code-prescribed reinforcing levels, crack widths at tension faces may be greater in GRFP-reinforced members than in steel-reinforced members. Thus, additional reinforcing may be required or desired to reduce crack widths. Crack control may not be as critical where GFRP is used because of its corrosion resistance.
Concrete reinforced with GFRP may have different time-dependent deformation characteristics than conventional reinforced concrete. Per the Commentary, GFRP in the compression zone can not be considered in ACI CODE 440.11 equations for reducing creep due to the limited effect. Time-dependent deflection, when considered as a multiple of the instantaneous deflection, is lower; and the creep multiplier is calculated differently.
Strength
Because GFRP-reinforced concrete sections do not exhibit ductile behavior, there are lower resistance factors are used to calculate moment, axial, and combined moment/axial behavior (0.55 for tension-controlled transitioning to 0.65 for compression-controlled).
Since GFRP has a lower transverse strength and modulus than reinforcing steel, dowel action (the shear strength of cracked reinforced concrete) is assumed to be lower for sections containing the same area of reinforcing steel making shear capacity less on that basis.
Slenderness limits for compression members are reduced, and the effective stiffness of members is generally lower. Compared to concrete reinforced with steel bars, the required amount of GFRP for equivalent force capacity for temperature and shrinkage reinforcement is generally larger; the axial stiffness of GFRP reinforcement significantly impacts punching shear response; axial capacity of walls is reduced; and longitudinal torsional reinforcement diameter in comparison to transverse reinforcement spacing must be greater.
Research is still limited in some areas; there are currently no provisions in ACI CODE 440.11 for:
- Earthquake-resistant structures
- Deep beams
- Two-way joist systems
- Connection of precast members
- Use of strut-and-tie models
- Use of GFRP reinforcement with lightweight concrete
- Prestressing or post-tensioning using GFRP.
- GFRP-reinforced diaphragms
- Use of GFRP for shear-friction
Assessment, Repair, and Rehabilitation of Existing Concrete Structures – Uses and Limitations of ACI CODE 562
ACI CODE 562 was originally published in 2013, and most recently updated in 2021. It was developed to provide provisions for structural concrete repair, such as that represented in Figure 2.
The Code provides minimum requirements for assessing, repairing, and restoring structural concrete elements in existing structures. It is also intended for use for non-structural or aesthetic improvements if failure of such repairs would result in a dangerous condition. In all cases, repairs and restoration in accordance with the current building code adopted by the AHJ are permitted. ACI CODE 562 is intended as a compliance alternative to the existing building code. ACI CODE 562 is structured for use either as a stand-alone code or for use as a supplement to an existing building code. As with all reference standards, where there are conflicts, the jurisdictional code prevails. Where used with an existing building code, ACI 562 provides the methods to determine loads, evaluate/assess the structure with reduction factors, and addresses design, durability, construction, and quality assurance. Where used as a stand-alone code, ACI 562 also addresses preliminary assessment, general provisions, and load factors.
Seismic Force-Resisting Systems
ACI CODE 562 is applicable for repairs that do not change the strength, stiffness, or ductility of the seismic-force-resisting system. It is not applicable for strengthening those elements. Where such conditions exist, assessment, repair, or restoration should be in accordance with the provisions of the building code adopted by the AHJ (based on the International Existing Building Code or Chapter 34 of the IBC) and the American Society of Civil Engineers/Structural Engineering Institute 41 (ASCE/SEI 41) Seismic Evaluation and Retrofit of Existing Buildings.
ACI CODE 562 provides maximum strength reduction factors for restoration design and assessment. Load combinations are included for external strengthening subject to damage, vandalism, and fire. Any loads and deformations associated with damage, deterioration, load removal, construction sequencing and temporary actions such as jacking in post-tensioned anchorage zones require consideration.
Assessment, evaluation, and analysis are to be conducted to establish conditions, define causes of distress, and develop appropriate rehabilitation strategies. Investigation and evaluation are required where insufficient information is available or there is reason by the design professional to question capacity compared to design demands. Where required, the analysis is to consider in-place structural dimensions, structural alterations, loads or occupancy, and in-place material properties. Concrete compressive strength and steel yield strength may be determined from the original construction documents, historical properties indicated in ACI CODE 562, or physical testing. If material properties are determined through sampling and testing, increased resistance factors are permitted. Equivalent concrete compressive strength and reinforcing steel strength can be based on simplified equations for statistical evaluation or nondestructive testing where valid correlations with destructive testing are developed.
The structural analysis must consider the maximum effects due to gravity and lateral loads and consider conditions such as material properties, geometry, deformations, lateral drift, load duration, time-dependent deformations, interactions with foundations, material deterioration, bond loss, redistribution of forces (in members and the structural system), load paths, previous repairs, and previous structural modifications. Serviceability issues, if identified, are to be evaluated. The structural analysis must also consider the repair process, including sequencing and material removal, and load testing permitted as part of the evaluation.
Design must meet the strength requirements of the design-basis code and consider loading, internal forces, and deformations to the existing and repaired structure during the repair process. Repairs must be integrated with the existing structure, including necessary load transfer and load sharing. Design must consider stresses at the bond line between the repair material and the existing concrete. Bond integrity testing is required where the interface shear stress is less than or equal to 30 psi; qualitative bond testing is necessary if greater than 30 psi unless interface reinforcement is designed per ACI CODE 318. Interface reinforcement is required if shear stresses are greater than 60 psi.
Existing materials may remain if performing satisfactorily; however, stresses are not to exceed those in the original basis of design. Stress concentrations and cracking must be considered with respect to repair geometry and configuration. The design of external fiber reinforced polymer wraps (FRP) for strengthening must have adequate unrepaired strength and FRP materials should comply with ACI SPEC 440.6 and ACI SPEC 440.8. Where determined by required consideration of elevated temperatures, supplemental protection is permitted.
ACI CODE 562 considers the properties and installation of repair systems. Damaged or corroded reinforcement is permitted to remain if the effective remaining cross-sectional area is used in the design and the impact on development and damage to deformations is considered. Any post-installed anchors shall comply with ACI CODE 318. Both new and existing reinforcement must be adequately developed. The effects of prestressing on the structure, if applicable, must be considered, including damage, geometry, losses, and repair sequence. Design with supplemental post-tensioning is required to consider stresses, limitations of the original design basis, load transfer, impact on adjoining construction, losses, deflections, adequate unrepaired strength for externally post-tensioned repairs, and other conditions.
Durability
Design must consider the causes of current conditions, defects, and potential deterioration of repairs, as well as serviceability. The impact of cracks on the durability of the repair and structure must be considered in the design. Causes of cracking, movement, size, orientation, width, pattern; substrate, location and water penetration must be assessed.
Corrosives and the presence of corrosion products, if not removed from reinforcement during repair, must be considered with respect to impact on the structure design life. The quality of the existing concrete and its capability to protect the reinforcement from corrosion, fire, and other damage must be evaluated. The cover must be sufficient to provide adequate fire protection, corrosion protection and reinforcement anchorage/development. The code has provisions for consideration of galvanic corrosion; however, protection with electrochemical methods must consider interaction with repaired elements, the structure, and the environment. Materials and reinforcement used in the design must be compatible with the structure and environment and may not adversely affect the durability of other materials.
Construction Documents
Additional items identified in ACI CODE 562 that are to be including in construction documents are:
- Contractor quality control requirements.
- Stability and temporary shoring design and requirements, including:
- portions requiring shoring,
- design loads and spacing requirements, and
- contractor responsibilities for providing and maintaining shoring.
- Considerations for shoring, including provisions for shoring being designed by a licensed design professional, and global stability.
- Temporary conditions and environmental issues, including hazardous materials.
- Post-tensioning repair sequence, if applicable.
- Inspection, testing, and construction requirements including:
- Requirements for inspection of existing reinforcement,
- Requirements for inspection of existing conditions prior to concealment from repair,
- Frequency of tests and reporting requirements, and
- Requirements for record retention.
Closing
ACI committees have developed both standards following strict American National Standards Institute (ANSI) accredited procedures. While both standards represent the current state of practice in the industry for minimum requirements for safe design and are maintained by industry experts. They provide flexibility for implementation by design professionals and introduce additional options and concepts.
Additional Resources
Additional ACI codes, specifications, and guides relevant to the design and construction of structural concrete reinforced with GFRP rebar and for the repair of existing structural concrete elements include:
ASCE/SEI 41 references:
- ACI 214.4R, Guide for Obtaining Cores and Interpreting Compressive Strength Results
- ACI 228.2 Report on Nondestructive Test Methods for Evaluation of Concrete in Structures
ACI CODE 562 references:
- ACI CODE 369.1 Standard Requirements for Seismic Evaluation and Retrofit of Existing Concrete Buildings
- ACI CODE 437.2 Code Requirements for Load Testing of Existing Concrete Structures
- ACI SPEC 440.6 Specification for Carbon Fiber-Reinforced Polymer Bar Material for Concrete Reinforcement
- ACI SPEC 440.8 Specification for Carbon and Glass Fiber-Reinforced Polymer Materials Made by Wet Layup for External Strengthen
ACI CODE 440.11 references:
- ACI SPEC 440.5 Construction with Glass Fiber-Reinforced Polymer Reinforcing Bars – Specification
- ACI 440.1 Guide for the Design and Construction of Structural Concrete Reinforced with Fiber-Reinforced Polymer Bars■