Paul Zia

Flexural Strength Design of Concrete Beams Reinforced with High-Strength Steel Bars

Current design requirements limit the allowable design strength of high-strength steel reinforcements to 80 ksi (550 MPa), which limits engineers from fully using the enhanced strength characteristics of low-carbon and chromium high-strength steel. In this paper, a methodology is presented for the flexural strength design of concrete beams reinforced with high-strength reinforcing steel that conforms to the requirements of ASTM A1035-07. The design method is based on simple analysis techniques that satisfy fundamental principles of equilibrium and compatibility. Strain limits for tension-controlled sections and compression-controlled sections are proposed that are consistent with the approach of the current and past ACI 318 Codes. The proposed method is compared with previously-reported experimental results, and the application of the proposed method is demonstrated by a numerical design example. Findings indicate that flexural members designed using the proposed design methods and criteria have comparable flexural strength characteristics with members designed according to current ACI 318 requirements, using Grade 60 (400 MPa) and Grade 75 (520 MPa) reinforcing steel.

Bond Characteristics of ASTM A1035 Steel Reinforcing Bars

This paper presents results from a study of the bond characteristics of high-strength steel reinforcing bars that conform to ASTM A1035. In the study, a total of 69 large-scale beam-splice specimens were independently tested at 3 universities. Concrete with nominal strengths of 5000 and 8000 psi (35 and 55 MPa) were used. Maximum bar stresses were compared with predictions obtained using the bond equations in the ACI 318-05 code provisions and those proposed by ACI Committee 408. Maximum stress levels of 120, 110, and 96 ksi (830, 760, and 660 MPa) were developed in No. 5, No. 8, and No. 11 (No. 16, No. 25, and No. 36) bars, respectively, not confined by transverse reinforcement. The failure of beams with spliced bars not confined by transverse reinforcement was sudden and produced explosive spalling of the concrete cover over the entire splice length. Providing confinement for No. 8 and No. 11 (No. 25 and No. 36) spliced bars using transverse reinforcement allowed stresses of up to 150 ksi (1035 MPa) to be developed. The design equations in ACI 318 showed a large percentage of the developed/calculated strength ratios below 1.0, indicating they should not be used in their present form for development and splice design with high-strength reinforcing steel The ACI Committee 408 equation is more conservative and provides a reasonable estimate of the strength for both unconfined and confined splices using a strength reduction factor of 0.82 and design parameters (cover, spacing, and concrete strengths) similar to those used in this study.

Implementation of Self-Consolidating Concrete for Prestressed Concrete Girders

This report documents the first experience of using self-consolidating concrete for pretressed concrete bridge girders in North Carolina. Under construction in eastern North Carolina was a multi-span bridge which used one hundred thirty AASHTO Type III girders, each 54.8 ft (16.7 m) long. To demonstrate the full-scale field production of self-consolidating concrete, and for comparative purposes, three girders from one production line of five girders were selected for the experimentation. Two of the girders were cast with self-consolidating concrete and one with normal concrete as control. The plastic and hardened properties of both the self-consolidating concrete and the normal concrete were monitored and measured. The plastic properties of self-consolidating concrete included unit weight, air content, slump flow, visual stability index (VSI), and passing ability measured by J-ring and L-box. Hardened properties of the two concretes included temperature development during curing, compressive strength, elastic modulus, and flexural tensile strength, creep and shrinkage. The prestressing force was monitored by load cells . The transfer lengths of prestressing strands were determined by embedded strain gauges , and from the measured strand end-slips. Finally, the three girders were tested in flexure up to the design service load to determine and compare their load-deformation characteristics. Based on the satisfactory results of this study, the two prestressed SCC girders were installed in the bridge for service as other normal concrete girders.

Behavior of High-Performance Steel as Shear Reinforcement for Concrete Beams

High performance (HP) steel is characterized by enhanced corrosion resistance and higher strength in comparison to ASTM A615-06 Grade 60 steel, which may reduce the amount of required reinforcement. This paper investigates the behavior of HP steel as shear reinforcement for concrete beams, especially under overload condition with the steel being at high stress levels. Nine reinforced concrete beams were constructed using No. 9 longitudinal bars and No. 3 stirrup bars. The main variables considered in this study are the stirrup spacing and type of reinforcing steel material. Test results indicate that using HP steel reinforcement increases the shear capacity and enhances the serviceability in terms of strength gain and reduction of shear crack width. Current design codes can conservatively be used for the design of HP steel using a yield strength of 80 ksi (552 MPa). The maximum shear resistance of a concrete section recommended by the ACI Code should be maintained for HP steel reinforcement. This research could not fully use the strength of the HP steel stirrup because the failure was controlled by crushing of the concrete in the strut, suggesting that the HP steel should be paired with high-strength concrete for best results.

TRANSFER LENGTH OF EPOXY-COATED PRESTRESSING STRAND

To provide corossion protection for 7-wire prestressing strand in prestressed concrete used in adverse environments, a manufacturer has developed an epoxy-coated strand. The effect of the epoxy coatings and grit on transfer length for 3 different sizes of prestressing strand was investigated. The transfer length of 3 differenct densities of grit were also studied. Fifty-three specimens were cast to determine the transfer length of epoxy-coated and bare (uncoated) prestressing strand. The transfer length was determined by measuring concrete strains before and after release. Also studied were the effects of time on transfer length of epoxy-coated strand - referred to as transfer-length creep - the relationship between end slip and transfer length of prestressing strand, and the effects of elevated temperatures on the transfer length of epoxy-coated strand. The experimental and analytical results from this research were compared to the results in the literature, provisions in the ACI Building Code (ACI 318-83), and previously proposed bond provisions.

Behavior of Concrete Beams Reinforced with ASTM A1035 Grade 100 Stirrups under Shear

This paper investigates the behavior of concrete beams reinforced with different reinforcement ratios of high-strength steel stirrups. A yield strength of up to 100 ksi (690 MPa) is examined. The serviceability and effectiveness of using high-strength steel as transverse reinforcement in flexural members is also investigated. Nine large-scale reinforced concrete (RC) beams were tested twice under static loading up to failure. The beams were classified into three groups based on their shear resistance, with varied spacing of their shear reinforcement. The performance of these beams is compared to that of similar beams reinforced with ASTM A615 Grade 60 bars. The results show that the tested beams achieved similar shear strengths as the beams reinforced with Grade 60 bars when a higher yield strength of ASTM A1035 bars with a reduced reinforcement ratio was used. Cracking and deflection under service load of the beams with a reduced reinforcement ratio were also within acceptable limits. An examination of current design codes indicates that the American Concrete Institute, Canadian Standards Association and American Association of State Highway and Transportation Officials load and resistance factor design codes can accurately predict the shear strength of concrete beams reinforced with high-strength steel stirrups.

Shear Behavior of Large Concrete Beams Reinforced with High-Strength Steel

This study seeks to quantify the benefits of using high-strength steel for concrete reinforcement and provides experimental evidence of its high strength capabilities. Test results are presented of six large-size concrete beams reinforced with either conventional- or high-strength steel and tested up to failure. The beams were constructed without web reinforcement to evaluate the nominal shear strength provided by the concrete. The shear behavior, ultimate load-carrying capacity, and mode of failure are presented. The applicability of the current ACI design code to large-size concrete beams constructed without web reinforcement is discussed. The influence of the shear span-depth ratio, concrete compressive strength, as well as the type and the amount of longitudinal steel reinforcement is investigated. Findings indicate that using high-strength steel alters the mode of failure from diagonal tension to shear compression failure and results in higher shear strength compared with using conventional steel. Findings also suggest that the current ACI shear design provisions are unconservative for large-size concrete beams without web reinforcement. The expression needs to account for the size effect and reinforcement characteristics.

Characteristics of Compressive Stress Distribution in High-Strength Concrete

Concrete in the compression zone is subjected to a stress distribution, called the stress block, that follows the stress-strain relationship of a concrete cylinder tested in axial compression. This paper describes fundamental characteristics of the compressive stress distribution in the compression zone of flexural members with concrete compressive strengths up to 18 ksi (124 MPa). A total of 21 plain concrete specimens were subjected to combined flexure and axial compression up to failure. The main variable considered was the strength of concrete that ranged from 10.4 to 16 ksi (71.7 to 110.3 MPa). Each specimen was subjected to two independent loads with a specific configuration to induce maximum compressive strain at one face and zero strain at the opposite. The measured stress-strain curves and stress block parameters were compiled with the data found in the literature. The results were used to develop recommended revisions for the load and resistance factor design specifications to extend their current limitation of 10 ksi (69 MPa) for concrete compressive strength up to 18 ksi (124 MPa). The assumption that plane sections remain plane after deformation is found to be valid for concrete compressive strengths up to 18 ksi (124 MPa). The ultimate concrete compressive strain values of 0.003 for design by the current code provision and a Poisson ratio of 0.2, as used in the current code provision, is acceptable for concrete compressive strengths up to 18 ksi. The test results indicate that the stress block parameter should be reduced when concrete compressive strength exceeds 10 ksi (69 MPa).

EVALUATION OF NEW AIR PERMEABILITY TEST DEVICE FOR CONCRETE

This paper presents results of an evaluation of a device with 2 concentric chambers for nondestructive measurement of the air permeability of concrete. The Zia-Guth device is attached to the concrete surface by vacuum and measures the rate of pressure increase in the inner chamber as air flows through the outer chamber and concrete into the inner chamber. A computer program was developed to model the air flow through the concrete. The program produced a series of pressure-time curves that were combined with the experimental data to calculate the permeability constant of the concrete. The test procedure was validated by conducting an experimental program in the lab, with variables including concrete age, the use of silica fume, different water-cementitious material ratios, and curing conditions.

Short-Term Mechanical Properties of High-Strength Concrete

A comprehensive experimental program was undertaken to determine the short-term mechanical properties of high-strength concrete (HSC). Modulus of rupture beams and two different sizes of concrete cylinders with three different target compressive strengths ranging from 10 to 18 ksi (69 to 124 MPa) were subjected to three different curing methods and durations. Test results were combined with data from the literature to improve predictive equations for the elastic modulus and modulus of rupture of HSC. Of the three different curing methods, cylinders moist-cured for 7 days exhibited the highest compressive strengths at ages of 28 and 56 days. In contrast, 1-day heat curing generally resulted in the lowest compressive strength. The study shows that a Poissons ratio of 0.2 can be adequately used for HSC.