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Concrete The Reinforced Design Manual - ACI - A... UPD


The purpose of this Design Guide is to assist in the proper application of the design and detailing requirements. The many design aids and worked-out examples are provided to make designing and detailing reinforced concrete members simpler and faster. The goal is to acquire an understanding of the code requirements and to apply them properly and efficiently.




Concrete The Reinforced Design Manual - ACI - A...



BFRP Rebar is an alternative to galvanized or epoxy coated rebar, stainless steel rebar or GFRP rebar. BFRP rebar should be considered for any concrete member susceptible to corrosion of steel reinforcement by water, salt water, deicing salts or other corrosive agents as BFRP Rebar DOES NOT CORRODE. Other advantages of specifying BFRP Rebar: 4.5 times lighter than steel rebar, tensile strength is 2.5 times stronger than steel rebar, up to 30% less concrete coverage required versus steel rebar (subject to design), transparent to radio frequencies, non-conductive, non-magnetic, same thermal coefficient as concrete, dielectric, UV resistant, BFRP Rebar can withstand temperatures from -50 C to 300 C, less shipping costs, easier to handle, easier to cut, less cost than stainless steel rebar. Life span is 100 plus years in comparison to 50 years for steel rebar.


Once the size of the cross-section and the required area of longitudinal reinforcement have been determined for a reinforced concrete column based on strength requirements, the size and number of longitudinal reinforcing bars must be chosen to 1) provide an area of reinforcement equal to or greater than the amount that is required, and 2) satisfy the minimum and maximum spacing requirements in ACI 318-14, Building Code Requirements for Structural Concrete and Commentary. Columns that have longitudinal reinforcement ratios in the range of 1 to 2% are usually the most economical because concrete resists axial compression forces more cost-effectively than reinforcing steel. It is usually more economical to use larger column sizes with less longitudinal reinforcement.


Requirements for columns with tie reinforcement are given in ACI 318 Sections 10.7.6 and 25.7.2, and standard hook dimensions for ties are given in ACI Section 25.3.2. Tie spacing requirements for reinforced concrete columns in buildings assigned to SDC A and B are given in Figure 3. The clear spacing between ties must be at least (4/3)dagg. Depending on the shear strength requirements, the required tie spacing may be less than that in the figure.


Replacing steel with corrosion-resistant reinforcement such as stainless steel or fiber-reinforced polymer (FRP) bars in concrete structures can be beneficial. Strong fibers, durability, and lightweight are the attractive features of FRP bars. The initial cost, brittleness, low stiffness of some fibers, and inflexibility once produced were found to be the major disadvantages [5]. Although FRP materials have been used extensively to strengthen existing structures, their application as an internal reinforcing material in concrete have not gained much appreciation due to the perceived drawbacks such as larger crack widths, higher deflection resulting from lower modulus of elasticity, and catastrophic failure due to FRP rupture [6]. Nevertheless, continuous research on the application of FRP as a primary reinforcement has helped to develop current design guidelines and codes such as ACI 440.1R-15 [7], the IStructE interim guidelines [8], and the Canadian Highway Bridge Design Code [9].


The application of FRP reinforcement in concrete structures has been studied extensively. In the early 1990s, studies [10,11,12] focused on evaluating the behavior of FRP-reinforced beams and slabs to examine their conformity to the then-existing design codes. Further experimental investigation by Michaluk et al. [13], Hassan et al. [14], El-Salakawy & Benmokrane [15], Zhang et al. [16], and Benmokrane et al. [17] on GFRP, CFRP, steel, and hybrid-reinforced simply supported slabs and beams showed that FRP-reinforced elements demonstrate higher deflection and larger crack widths than steel-reinforced members. This was attributed to the lower modulus of elasticity of FRP bars, particularly the commonly used GFRP. Increasing the amount of reinforcement to enhance the axial rigidity and reduce the deflection in FRP-reinforced concrete structures was the general recommendation in the design codes. Although this is appropriate for simply supported slabs, the approach can be conservative for in-plane restrained slabs. Most bridge deck slabs are in-plane restrained (e.g., Y-beam and W-beam bridge decks). Therefore, it is vital that the benefits of compressive membrane action be considered in bridge deck design.


When a slab is restrained for in-plane expansion, the compressive membrane action phenomenon influences the serviceability and ultimate limit state behavior of the concrete slab. Compressive membrane action (CMA) in short-span-reinforced concrete slabs has been extensively discussed in the literature [18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33]. Research based on laboratory experiments and field applications have demonstrated [20,31,32,34] that these slabs require a considerably lower amount of steel than simply supported slabs. However, investigation on the influence of compressive membrane action in FRP-reinforced concrete beams and slabs is limited due to lesser number of research. The benefits of CMA in steel-reinforced restrained slabs have been investigated extensively and these can be found elsewhere [18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,35,36]. CMA can benefit the design in several ways.


The study presented in this paper experimentally and numerically investigate the behavior of in-plane restrained GFRP-reinforced slabs and provides a basis for consideration of CMA in design approach of FRP-reinforced flexural elements.


The test specimens were compared for their serviceability and ultimate limit state behavior changing various reinforcement parameters. The test slabs used one type of concrete mix design to keep the concrete strength parameter unchanged. Position of the reinforcement, namely either conventional two layers or single mid depth reinforcement; amount of reinforcement, spacing between reinforcement bars are the main reinforcement parameters considered.


The test variables were reinforcement percentage, spacing between bars, effective depth, and bar diameter, as detailed in Table 1. A steel frame with a tested linear stiffness of 855 kN/mm was used to provide an in-plane restraint (Figure 1 and Figure 2). The stiffness was obtained by measuring the axial deformation of the frame for applied axial loads. A similar frame has previously been used by Ruddle [36] and Taylor [41] to investigate arching action in steel-reinforced concrete structures. It was estimated from the relationship developed by Rankin and Long [32] that the frame would provide 70% of a rigid restraint based on the ratio of the in-plane restraint stiffness to the arching stiffness of the slab. Flexural test on GFRP-reinforced slabs for service and ultimate limit state were carried out by fixing the slabs into the steel frame.


Once a crack is formed in the tension zone, the behavior of the concrete slabs was influenced either by the position and the stiffness of the reinforcement or the stiffness of the membrane arch due to the in-plane restraints. All the double-reinforced slabs, independent of the amount of reinforcement, showed similar load versus deflection behavior up to 200 kN. The deflection values of the test slabs are compared in Table 2 for the EC1 [47] maximum tandem system load model 1 (TSLM1) service wheel load of 150 kN and also at failure load. This is an onerous comparison as the slab strip would have only a portion of the whole wheel load acting on it. The two slabs with single mid-depth reinforcement showed higher deflection exceeding the span/250 limit. The allowable deflection for flexural members is limited to span/250 for steel-reinforced concrete slabs and span/500 if the slab deformation would cause any damages to the structure underneath. Therefore, span/250 is considered for bridge decks. Similar limits can be found in ACI-318-19 [48] for steel-reinforced slabs. Considering an allowable deflection of 5.7 mm for a span of 1425 mm, it can be seen from Figure 4 that two-layer GFRP-reinforced slabs satisfy the deflection criteria for a service load of 150 kN.


Design codes such as Eurocode restrict the crack widths to less than 0.3 mm for steel-reinforced concrete structures. However, considering the corrosion-resistant nature of GFRP bars, the Canadian code [9] recommends up to 0.5 mm crack width for GFRP-reinforced structures exposed to extreme environmental conditions, and the Japanese guidelines [49] limit the crack width to 0.5 mm based on aesthetic concerns.


Due to the creep rupture failure of FRP materials, the service load level stress on GFRP bars is limited to 20% of the design tensile stress, as given in Table 7.4.1 of ACI 440.1R [7]. If the effect of creep due to sustained and cyclic loads is considered, the stress on GFRP bars must be maintained below 95.48 N/mm2. Among the three slabs reinforced with the conventional two layers of 0.6% GFRP reinforcement, test slabs with 50 mm spacing (G-0.6%-8 mm-50_T&B) demonstrate stress below 95.48 N/mm2. The stress on GFRP bars is influenced by the crack width, as is discussed in the literature [50]. A linear response can be observed when plotting the crack width against the reinforcement stress, as shown in Figure 7.


Lahlouh and Waldron [26] investigated three one-way slab strips where the amount of lateral restraint was changed by using different-sized supporting walls. Three slab and wall models were tested while keeping the reinforcement percentage and concrete strength constant. The slabs were supported by walls of 100, 200, and 300 mm thickness, respectively. The reinforcement ratio was unchanged for the slab (0.54%), and the reinforcement ratios in the walls were 1.34, 0.38, and 0.24% for 100, 200, and 300 mm wide walls, respectively. The side walls were of the same height, and the slabs had the same clear span. Experimental results published by Taylor et al. [19] were also used to validate the proposed nonlinear model. The original experiments were carried out to investigate the strength and behavior of steel-reinforced in-plane restrained slabs with various concrete strengths, reinforcement percentages, in-plane stiffnesses, and positions of reinforcement. The concrete strength varied between 30 N/mm2 and 100 N/mm2. Two different stiffness values were used, and two different reinforcement positions were investigated. 041b061a72


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