Lawrence F. Kahn

Repair and strengthening of reinforced concrete beam-column joints: State of the art

The latest report by Joint ACI-ASCE Committee 352 (ACI 352R-02) states that joints in structures built before the development of current design guidelines need to be studied in detail to establish their adequacy and that methods of connection repair and strengthening need to be developed. Prior to developing new strengthening schemes, it is important that the findings from research previously conducted on other strengthening techniques be known. This paper presents a comprehensive up-to-date literature search pertaining to the performance of as well as to the repair and strengthening techniques for, nonseismically designed reinforced concrete beam-column joints, reported between 1975 and 2003. These techniques included: 1) epoxy repair; 2) removal and replacement; 3) concrete jacketing; 4) concrete masonry unit jacketing; 5) steel jacketing and addition of external steel elements; and 6) strengthening with fiber-reinforced polymeric (FRP) composite applications. Each method of repair or strengthening is reviewed with emphasis on its application details, required labor, range of applicability, and performance. Relative advantages and disadvantages of each method are discussed.

Shear Friction Tests with High-Strength Concrete

This paper reports on an experimental study that indicates the current ACI shear friction concept can be extended to high-strength concrete. 50 pushoff specimens were tested with uncracked, precracked, and cold-joint interfaces. These specimens mimicked those used by prior researchers in the development of shear friction. Concrete strengths ranged from 46.9-123.4 MPa, with transverse reinforcement ratios between 0.37-1.47%. An equation is proposed that more accurately predicts the shear friction strength of cold-joint and uncracked interfaces for high-strength concrete. Recommendations are provided.

Utilization of Savannah Harbor river sediment as the primary raw material in production of fired brick

A laboratory-scale study was conducted to assess the feasibility of the production of fired bricks from sediments dredged from the Savannah Harbor (Savannah, GA, USA). The dredged sediment was used as the sole raw material, or as a 50% replacement for natural brick-making clay. Sediment bricks were prepared using the stiff mud extrusion process from raw mixes consisted of 100% dredged sediment, or 50% dredged sediment and 50% brick clay. The bricks were fired at temperatures between 900 and 1000 °C. Physical and mechanical properties of the dredged sediment brick were found to generally comply with ASTM criteria for building brick. Water absorption of the dredged sediment bricks was in compliance with the criteria for brick graded for severe (SW) or moderate (MW) weathering. Compressive strength of 100% dredged sediment bricks ranged from 8.3 to 11.7 MPa; the bricks sintered at 1000 °C met the requirements for negligible weathering (NW) building brick. Mixing the dredged sediment with natural clay resulted in an increase of the compressive strength. The compressive strength of the sediment-clay bricks fired at 1000 °C was 29.4 MPa, thus meeting the ASTM requirements for the SW grade building brick. Results of this study demonstrate that production of fired bricks is a promising and achievable productive reuse alternative for Savannah Harbor dredged sediments.

Creep and Shrinkage of High-Performance Lightweight Concrete

Two high-performance lightweight concrete (HPLC) mixtures with average compressive strengths of 68.5 and 75.4 MPa (9950 and 10,950 psi) were developed. Their air-dry unit weights were 1875 and 1905 kg/cubic m, respectively. 26 creep specimens were loaded at 16 or 24 hrs to 40 or 60% of their initial strength. This preliminary investigation showed that expanded slate HPLC experienced less creep, but slightly more shrinkage than normalweight HPC of similar paste content, mixture proportions, and strength. The 620-day creep coefficients of the 68.5 and 75.4 MPa (9950 and 10,950 psi) HPLC were 1.66 and 1.29, respectively. Creep and shrinkage were compared with estimates from 12 models.

Effect of Internally Stored Water on Creep of High-Performance Concrete

The study examined internally stored waters effects on high-performance concretes (HPCs) long-term deformations on 150 compressive strength specimens and 130 creep and shrinkage specimens. Creep and shrinkage monitoring was performed on sealed and unsealed concrete specimens for 500 days using either lightweight or normalweight aggregate with different initial moisture conditions. Prewetted lightweight aggregate use, when compared with air-dried lightweight aggregate mixtures, decreased creep by approximately 45%. Prewetted lightweight aggregate HPC experienced 10% lower creep than that obtained in a similar normalweight granite aggregate HPC. It is proposed that prewetted lightweight aggregate creep reduction is due to inhibition of water seepage, expansion afforded by internal curing, and hydration enhancement.

Tests on deep I-shape pultruded beams

The paper describes the full-scale tests conducted to study the behavior and recommend design criteria for deep I-shape reinforced plastic (RP) pultruded beams subjected to transverse loads in the plane of the web. The beam utilized in these experiments was made of a vinylester matrix reinforced with E-glass, and their cross sections consisted of a 610 mm x 9.5 mm (24 in. x 3/8 in.) web and 190 mm x 19 mm (7 1/2 in. x 3/4 in.) flanges. Beams were instrumented at various locations using strain gages, LVDTs and potentiometers. Both finite element analysis and approximate beam theory for composite sections predicted deflections within 5 percent of those observed. Engineering beam theory may be used for design of such RP beams.

Chloride-Induced Corrosion of Prestressing Steels Considering Crevice Effects and Surface Imperfections

Abstract Cyclic potentiodynamic polarization (CPP) techniques were used to evaluate the influence of crevices present in stranded prestressing steels on the resistance to chloride-induced corrosion. Prestressing wire and strand specimens were exposed to simulated concrete pore solutions with chlorides added as sodium chloride (NaCl) up to 1.0 M. Stranding resulted in a 67% reduction in measured Cl−-induced corrosion resistance when compared with CPP experiments conducted on single prestressing wires. Forensic investigation of tested specimens showed that imperfections in as-received zinc phosphate (ZnPO4) surface coatings also played a role in corrosion initiation. Based on these data, a model has been developed to describe the influence of crevice corrosion mechanisms and surface imperfections on corrosion initiation in prestressing strand. When these reductions in corrosion resistance were applied to service life estimates of model concrete systems, reductions in time-to-corrosion were 28 to 36%.

High-strength self-curing low-shrinkage concrete for pavement applications

High-performance concretes (HPC) that are designed to possess high early strength, self-curing capabilities and low shrinkage may potentially improve rigid pavement performance. A multi-scale experimental programme examined mechanical properties and shrinkage behaviour of a low water-to-cementitious materials ratio concrete containing pre-wetted lightweight aggregate for self-curing; results were compared with companion HPC prepared with air-dry lightweight aggregate and with normal weight, normal strength concrete. The use of pre-wetted lightweight aggregate enhanced hydration and strength development during the first year, and limited autogenous shrinkage and substantially reduced drying shrinkage. Digital image analysis revealed that shrinkage was concentrated in the paste rather than in the aggregate in both the normal strength concrete and HPC. The image analysis also showed that strain concentrations at the aggregate/paste interface were less apparent in the HPC, presumably due to the reduced strain mismatch with the use of the lower modulus lightweight aggregate. These observations suggest that the use of HPC containing pre-wetted lightweight aggregate can potentially reduce microcracking and enhance overall pavement performance.

Performance of an RC Corner Beam-Column Joint Severely Damaged under Bidirectional Loading and Rehabilitated with FRP Composites

This paper presents the performance of a full-scale reinforced concrete corner beam column-slab specimen that was severely damaged under bidirectional quasi-static loading. The specimen was then rehabilitated and retested. The specimen was built using the pre-1970s construction practices including the use of low-strength materials (3000 psi [21 MPa], Grade 40 reinforcing bars) and deficiencies in reinforcement detailing. The rehabilitation process consisted of: (1) epoxy injection, (2) addition of a bar within the clear cover of the column at the inside corner, and (3) external application of a multilayer composite system made of unidirectional carbon-epoxy layers placed at different orientations. The carbon fiber-reinforced polymeric system was heat-cured at a temperature of 80°±10°C (176°±18°F) for 6 hours. The performance was evaluated both before and after rehabilitation based on the progression of damage and the hysteretic behavior including the changes in the strength, stiffness, and energy dissipation characteristics. The results indicated that even a severely damaged corner joint can be effectively rehabilitated using CFRP to achieve a ductile beam failure mechanism. The joint was upgraded to withstand story drift ratios of up to 3.7% applied simultaneously in both directions.

Micro- and Nanoscale Characterization of Effect of Interfacial Transition Zone on Tensile Creep of Ultra-High-Performance Concrete

Ultra-high-performance concretes (UHPCs) are nano- to microstructurally optimized construction materials whose use presents significant opportunities for improving the performance of prestressed bridge girders. In UHPC girders, transverse shear reinforcement may be eliminated because of the high tensile strength of the material achieved through the use of short dispersed steel fibers as part of the mix. Use of the concretes tensile strength requires that the long-term tensile performance be understood to avoid brittle shear failure in service. The scope of the present study was characterization of the tensile creep of UHPCs under different thermal treatment regimens, with complementary assessment of the underlying mechanisms by characterization by nanoindentation and scanning electron microscopy. In this study, tensile-creep tests were conducted for a period of 1 year with UHPCs subjected to three different moist thermal curing regimes (i.e., early curing at 90°C, early curing at 60°C, and curing at 23°C). The effects of the curing conditions were further examined by nanoindentation and scanning electron microscopy, with particular emphasis being placed on the influence of thermal curing on the fiber–matrix interface. On the basis of the findings of this multiscale study, it is proposed that an enhanced fiber–cementitious matrix interfacial region, created by thermal curing, contributes significantly to the observed reductions in tensile-creep deformation.