Matthew O’Reilly

Effect of Corrosion Inhibitors on Concrete Pore Solution Composition and Corrosion Resistance

Three commercially available corrosion inhibitors—calcium nitrite, a solution of amines and esters, and an alkenyl substituted succinic acid salt—are evaluated in conjunction with conventional reinforcement in concrete based on corrosion rate, metal loss, the critical chloride corrosion threshold (CCCT), pore solution analyses, and concrete compressive strength. All three inhibitors increase time to corrosion initiation and decrease corrosion rate, but are less effective in cracked concrete than in uncracked concrete. Of the three inhibitors, the alkenyl-substituted succinic acid salt results in the greatest decrease in corrosion rate, but exhibits the lowest CCCT—below that measured in concrete with no inhibitor. The compressive strengths of concretes containing the amine-ester inhibitor and the alkenyl-substituted succinic acid salt were 15% and 60% lower, respectively, than concrete without an inhibitor. For the latter inhibitor, pore solution analyses indicated elevated sulfate contents at 1 and 7 days, which may explain the low CCCT and strength. Paste containing the amine-ester inhibitor had an elevated sulfate content at 7 days.

Conventional and high-strength hooked bars-Part 2: Data analysis

Empirical equations are developed to characterize the anchorage strength of hooked bars. The equations are based on tests of 245 simulated beam-column joint specimens with two hooked bars: 146 with confining reinforcement and 99 without. Bar stresses at failure for specimens used in the analysis ranged from 30,800 to 144,100 psi (212 to 994 MPa), and concrete compressive strengths ranged from 2570 to 16,200 psi (17.7 to 112 MPa). For the specimens analyzed, hooked bar anchorage strength was proportional to concrete compressive strength raised to the 0.29 power. For confining reinforcement parallel to and located within eight or 10 bar diameters of the straight portion of the hooked bar, the contribution to anchorage strength was proportional to the area of confining reinforcement; for confining reinforcement perpendicular to the straight portion of the bar, more legs of the confining reinforcement contributed to anchor strength, but each leg made a smaller contribution.

Conventional and High-Strength Hooked Bars—Part 1: Anchorage Tests

This paper presents the results of an experimental study on the anchorage strength of conventional and high-strength steel hooked bars. Three hundred and thirty-seven exterior beam-column joint specimens were tested with compressive strengths ranging from 4300 to 16,500 psi (30 to 114 MPa). Parameters investigated included the number of hooked bars per specimen, bar diameter, side cover, amount of confining reinforcement, hooked bar spacing, hook bend angle, hook placement, and embedment length. Bar stresses at failure ranged from 22,800 to 144,100 psi (157 and 994 MPa). The majority of the hooked bars failed by a combination of front and side failure, with front failure being the dominant failure mode. Test results show that development lengths of hooked bars calculated based on ACI 318-14 are very conservative for No. 5 (No. 16) bars and become progressively less conservative with increasing bar size and concrete compressive strength.

Anchorage Strength of Closely Spaced Hooked Bars

The effect of close spacing on the anchorage strength of standard hooks is investigated. Sixty-seven simulated beam-column joint specimens were tested, each containing three, four, or six No. 5, 8, or 11 (No. 16, 25, or 36) hooked bars arranged in one or two layers with center-to-center spacing ranging from two to six bar diameters. Anchorage strengths are compared with those of specimens containing two hooked bars with spacings of six to 12 bar diameters. The results demonstrate that the provisions in ACI 318-14 tend to overestimate the anchorage strength of hooked bars as concrete compressive strength and bar size increase and as spacing between bars decreases. Decreasing center-to-center spacing below six bar diameters results in lower anchorage strengths than for hooked bars with wider spacing. The anchorage strength of hooked bars can be represented by considering the minimum of the horizontal and vertical spacing between bars.

Conventional and high-strength steel hooked bars: Detailing effects

Findings from a study on the effect of hook bend angle, concrete clear cover, and orientation of confining reinforcement on hook anchorage strength are presented. The range of test parameters was much broader than in previous studies. Bar stress at anchorage failure ranged from 33,000 to 137,400 psi (228 to 947 MPa) and concrete compressive strengths ranged from 4300 to 16,500 psi (30 to 114 MPa). Anchorage strength of hooked bars was insensitive to bend angle (90 or 180 degrees) and side cover (between 2.5 and 3.5 in. [65 and 90 mm]). Confining reinforcement was found to increase anchorage strength for 180-degree hooked bars regardless of orientation (parallel or perpendicular to the embedment length). For 90-degree hooked bars, reinforcement oriented parallel to the embedment length had a greater effect on anchorage strength than reinforcement oriented perpendicular to the embedment length.

Corrosion-Induced Concrete Cracking for Uncoated and Galvanized Reinforcing Bars

United States Department of Transportation Federal Highway Administration under Contract No. DTFH61-03-C-0013,

Corrosion Performance of Poorly Pickled Stainless Steel Reinforcement

XM-28 (UNS S24100) and 2304 (UNS S32304) stainless steel reinforcing bars with different levels of pickling were evaluated for corrosion resistance using the rapid macrocell and cracked beam tests outlined in ASTM A955. Two heats of XM-28 from the same producer were evaluated using the rapid macrocell test. A single heat of 2304 was evaluated in two conditions; as-received from the manufacturer and re-pickled using both ASTM A955 tests. The poorly pickled heat of XM-28 reinforcement failed the rapid macrocell test with a peak individual corrosion rate exceeding 16 μm/y, while the properly pickled heat passed with no significant corrosion measured. The poorly pickled 2304 reinforcing steel failed the macrocell and cracked beam tests, with peak corrosion rates of 1.07 and 6.48 μm/y, respectively, while upon re-pickling, the same heat of steel passed both tests. These results suggest the need for a method to verify that the pickling process has been performed properly. Performance during the first week of the rapid macrocell tests or requiring that the bars exhibit a bright, shiny, uniformly light surface represent two potential methods for establishing the adequacy of pickling.

Effect of preexisting cracks on lap splice strength of reinforcing bars

The effect of preexisting subsurface cracks on the strength of lap splices was investigated. Ten full-scale beams with No. 11 (No. 36) bars and lap splice lengths of 33, 79, and 120 in. (838, 2007, and 3048 mm) were tested. The beams had mitigating features that prevented catastrophic failure upon propagation of the preexisting cracks, such as staggered splices and the presence of some reinforcement crossing the plane of the cracks. The effect of preexisting cracks on the bar stress at failure was found to be most severe for the shortest splices and not significant for the two other splice lengths evaluated. The effect was found to be dependent on the amount of reinforcement crossing the plane of the cracks. Splice strength was unaffected in beams with the largest amount of reinforcement, and reduced on the order of 50% in beams without any reinforcement crossing the plane of the cracks.

Corrosion Performance of Prestressing Strands in Contact with Dissimilar Grouts

To improve the corrosion protection provided to prestressing strands, anti-bleed grouts are used to fill voids in post-tensioning ducts that result from bleeding and shrinkage of older Portland Cement grouts. Environmental differences caused by exposing the strands to dissimilar grouts, however, have the potential to cause rapid corrosion. Portland Cement grout, gypsum grout, and four commercially available prepackaged grouts were analyzed to determine the chemical composition of the resulting pore solutions and tested to determine if using a second grout will provide improved corrosion protection for prestressing strands or result in accelerated corrosion. The potential consequences of leaving the voids unfilled were also evaluated. Pore solutions were analyzed for pH and sodium, potassium, fluoride, chloride, nitrite, sulfate, carbonate, nitrate, and phosphate ion content. The analyses were used to develop simulated pore solutions. Selected grouts and simulated pore solutions were paired to evaluate their potential to cause corrosion of, respectively, grout-wrapped and bare stress-relieved seven-wire prestressing strands using the rapid macrocell test. Strands were also evaluated in simulated pore solutions containing chlorides and in deionized water. Because exposure of strands to water or chlorides has the potential to cause rapid corrosion, filling voids in post-tensioning ducts with an anti-bleed grout is recommended. Gypsum grout, with its low pH and high sulfate content, will cause accelerated corrosion of strands when used in conjunction with Portland Cement grout or any of the commercially prepackaged grouts tested. When paired with Portland Cement grout, the prepackaged anti-bleed grouts evaluated in this study resulted in corrosion losses significantly below those observed for strands exposed to salt or water. The highest corrosion measured for a prepackaged grout occurred for the grout with the highest pore solution sulfate content.

Case for Changing Reinforcing Bar Deformation Spacing Requirements

The bond strength of four sets of reinforcing bars is evaluated, two each with No. 5 and No. 10 (No. 16 and No. 32) bars, which have, respectively, nominal diameters of 0.625 and 1.27 in. (15.9 and 32.3 mm). One bar of each size satisfies the criterion for maximum deformation spacing in ASTM reinforcing bar specifications, while the other has deformations that exceed the maximum spacing. All bars exceed the requirements for minimum deformation height. Research related to the effect of deformation properties on bond strength, including the research used to establish the requirements for deformations in ASTM reinforcing bar specifications, is also reviewed. The test results match earlier research and demonstrate that (1) bond strength is not governed by the specific value of deformation height or spacing, but by the combination of the two as represented by the relative rib area of the bars and (2) the bond strength of the bars with deformation spacings that exceed those in ASTM reinforcing bar specifications is similar to the bond strength of the bars that meet the specification. Based on this and prior research, it is recommended that ASTM reinforcing bar specifications be modified to allow for deformation spacing up to 90 % (currently a maximum of 70 %) of the bar diameter provided the ratio of deformation height to deformation spacing is greater than or equal to the minimum ratio for bar deformations meeting the current requirements in ASTM reinforcing bar specifications.