Evaluation of Effects of Casting And Curing Conditions and Specimen Type on Concrete Strength and Permeability

Time and curing conditions may impact the strength and permeability of concrete. The strength and permeability of concrete with and without supplementary cementitious materials (SCMs) were evaluated as a function of specimen type, season during which construction occurred, and age. Three concrete mixtures were included in the study, a control mixture with 100% portland cement, a mixture with 35% replacement (by weight) of cement with slag cement, and a mixture with 25% replacement with slag and 15% replacement with fly ash. Pavement slabs containing each mixture were cast in the summer, fall, and spring, along with companion 4 × 8 in. cylinders, to determine the effect of seasonal variations in environmental conditions on the strength and permeability of concrete. Cylinders were cured in both the laboratory and the field, and cores were taken from each slab. Specimens were evaluated for compressive strength, void content using the boil test, and ionic conductivity using the rapid chloride permeability (RCP) test at ages of 28, 56, 90, 180, 360, and 720 days. Additional laboratory tests were performed to evaluate the correlation between diffusion coefficient obtained from ponding tests and void content, and ionic conductivity. The study demonstrated that cores and field-cured cylinders have lower compressive strength and greater permeability than lab-cured cylinders. Concrete cast during either hot or cold weather tends to exhibit lower strength and greater permeability than concrete cast closer to 70 degrees Farenheit; mixtures containing fly ash tend to be more affected by lower temperature at early ages. The use of SCMs, however, mitigates some of the effects of hot weather. The results of the boil test do not correlate well with diffusion coefficient or ionic conductivity

Variation of Concrete Strength, Permeability, and Porosity due to Specimen Type, Season, and Age

Time and curing conditions may impact the strength and permeability of concrete. The strength and permeability of concrete with and without supplementary cementitious materials (SCMs) were evaluated as a function of specimen type, season during which construction occurred, and age. Three concrete mixtures in which the cementitious material consisted of 100% portland cement, 65% portland cement and 35% slag cement, or 60% portland cement, 25% slag cement, and 15% Class C fly ash were evaluated. Pavement slabs containing each mixture were cast in the summer, fall, and spring, along with companion 4 × 8 in. cylinders, to determine the effect of seasonal variations in environmental conditions on the strength and permeability of the concrete. Cylinders were cured in both the laboratory and the field, and cores were taken from each slab. Specimens were evaluated for compressive strength, ionic conductivity using the rapid chloride permeability (RCP) test, and void content using the boil test at ages of 28, 56, 90, 180, 360, and 720 days. Equations are presented that characterize the change in strength, ionic conductivity, and porosity over time, and relationships between lab-cured cylinder values and values from field-cured cylinders and cores for compressive strength, RCP, and boil test were established. The study demonstrates that concrete cast in moderate temperatures exhibited greater compressive strength, lower charge passed in the RCP test, and a lower percentage of voids in the boil test than concrete cast in high or low temperatures; the use of slag cement or slag cement and Class C fly ash as partial replacements for portland cement lessened the negative impact of high temperatures on these properties, but was detrimental to the early age properties of concrete cast in cold temperatures. Cores and field-cured cylinders exhibited lower compressive strength and greater ionic conductivity and voids than lab-cured cylinders. The equations developed in this report reasonably predict the change in strength, charge passed, and percentage of voids over time. No correlation was found between results from the boil test and results from the RCP test

Anchorage Strength of Conventional and High-Strength Hooked Bars in Concrete

Key factors affecting the anchorage strength of hooked bars are investigated and design guidelines for the development length of hooked bars that apply to both conventional and highstrength steel and concrete are presented. In this study, 337 beam column joint specimens were tested. Parameters included number of hooks (2, 3, or 4), concrete compressive strength (4,300 to 16,510 psi [30 to 114 MPa]), bar stress at failure (22,800 to 141,600 psi [157 to 976 MPa]), bar diameter (No. 5, 8, and 11 [No. 19, 25, and 36]), concrete side cover (1.5 to 4 in. [38 to 102 mm]), quantity of confining reinforcement in the joint region, hooked bar spacing (3 to 11 bar diameters measured center-to-center), hook bend angle (90° or 180°), placement of the hook (inside or outside the column core, and inside or outside of the column compressive region), and embedment length. Using a subset of 214 simulated exterior beam-column joints, expressions are developed to characterize the anchorage capacity of hooked bars as a function of embedment length, concrete compressive strength, bar diameter, and amount and orientation of confining reinforcement. The results of this study show that front failure plays an important role in the behavior of hooked bars, which contrasts with the findings of previous studies. The provisions in the 2014 ACI Building Code become less conservative as the concrete compressive strength and bar diameter increase. The contribution of concrete compressive strength to the anchorage capacity of hooked bars can be represented by the concrete compressive strength to the 0.29 power, in contrast to the 0.5 power currently used in the ACI 318-14 Code. Confining reinforcement, expressed as the area of confining reinforcement per confined hooked bar, provides in an incremental rather than percentage increase in the anchorage capacity of hooked bars. Confining reinforcement parallel to the straight portion of the hooked bars contributes to the anchorage capacity of both 90° and 180° hooked bars. The contribution of confining reinforcement oriented perpendicular to the straight portion of the hooked bar differs from that of confining reinforcement parallel to the straight portion of the hooked bar and may be similar to the contribution of confining reinforcement to the development and splice strength of straight bars. Hooked bars with 90° and 180° bend angles produce similar anchorage capacities and can be used interchangeably. Increasing concrete side cover from 2.5 to 3.5 in. (64 to 89 mm) does not increase the anchorage capacity of hooked bars. These observations are incorporated into a new design equation that allows for the conservative design of hooked bars at concrete strengths up to 16,000 psi and steel stresses up to 120 ksi, well above current Code limits

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

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

Stainless Steel Reinforcement as a Replacement for Epoxy Coasted Steel in Bridge Decks

The use of deicing salts in the United States has resulted in the steady deterioration of roadway bridge decks due to the corrosion of reinforcing steel. Since the middle 1970s, the principal corrosion protection techniques for bridge decks have involved the use of epoxy-coated reinforcement (ECR) and increased cover over the reinforcing bars. The combination has greatly lengthened the life of bridge decks, but does not represent a perfect solution. The higher cover increases the bridge dead load and the cost of construction. Epoxy-coated reinforcement adds only slightly to the cost of bridge construction, but there are a number of well-documented cases in both the field and laboratory in which poorly adhering epoxy coatings have actually increased corrosion problems, and there is evidence that all epoxy coatings will eventually be susceptible to those shortcomings. As a result of these concerns, a number of other protective measures have been developed or are under development. These include the use of denser concretes, corrosion inhibitors, and corrosion-resistant steel alloys. Among the latter are various types of stainless steel, including 2304 duplex stainless steel and stainless steel clad reinforcing bars. Based on earlier studies, stainless steel reinforcement is generally less susceptible to corrosion than conventional and epoxy-coated reinforcement, but all stainless steels do not provide the same level of protection and their superiority to ECR has not been clearly demonstrated in all cases. 2304 duplex reinforcing bars and NX-SCR™ stainless steel clad bars (the only stainless steel clad reinforcement that was commercially available in the U.S. at the initiation of this study) have not undergone the same level of testing as other solid stainless steels and prototype clad bars in environments similar to those found in bridge decks. Combined with the additional initial cost of stainless steel compared to epoxy-coated reinforcement, there is a need to quantify the costs and benefits of using stainless steel reinforcement as a replacement for epoxy-coated steel in bridge decks

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 of High-Strength Reinforcing Bars with Standard Hooks

Hooked bars are used to anchor reinforcing steel where member dimensions prevent straight bars from developing their full yield strength. Prior to the current study, the quantity of data has been limited with regards to the capacity of hooked bars–particularly when high-strength steel or concrete is used. As a result, current design provisions in ACI 318-14 limit yield strength and concrete compressive strength to 80,000 psi and 10,000 psi, respectively, for the purpose of determining the development length of hooked bars. The purpose of this study was to determine the critical factors that affect the anchorage strength of hooked bars in concrete and to develop new design guidelines for development length allowing for the use of high-strength reinforcing steel and concrete. In this study, a total of 337 beam-column joint specimens were tested. Parameters included number of hooks (2, 3, or 4), concrete compressive strength (4,300 to 16,510 psi), bar diameter (No. 5, No. 8, and No. 11), concrete side cover (1.5 to 4 in.), amount of transverse reinforcement in the joint region, hooked bar spacing (3db to 11db center-to-center), hook bend angle (90° or 180°), placement of the hook (inside or outside the column core, and inside or outside of the column compressive region), and embedment length. The results of this study show that current ACI 318-14 code provisions are unconservative for larger hooked bars and higher compressive strength concrete. The effect of concrete compressive strength on the anchorage capacity of hooked bars is less than represented by the 0.5 power currently used in ACI provisions; the 0.25 power provides a more realistic estimate of capacity. The addition of confining transverse reinforcement in the hook region increases the anchorage capacity of hooked bars–the value of the increase depends on the quantity of confining reinforcement per hooked bar. Hooked bars with 90° and 180° bend angles exhibit similar capacities, and no increase in capacity was observed when increasing side cover from 2.5 to 3.5 in. Anchoring a hooked bar outside the column core or outside the compressive region of a column provides less capacity than anchoring the hooks at the far side of a beam-column joint or in a wall with a high side cover. Hooked bars also exhibit a reduction in capacity if the center-to-center spacing is less than seven bar diameters. These observations are used to develop a new design equation that allows for the conservative design of hooked bars

STAINLESS STEEL REINFORCEMENT AS A REPLACEMENT FOR EPOXY COATED STEEL IN BRIDGE DECKS (FHWA-OK-13-08 2231)

The corrosion resistance of 2304 stainless steel reinforcement and stainless steel clad reinforcement was compared to conventional and epoxy-coated reinforcement (ECR). 2304 stainless steel was tested in both the as-received condition (dark mottled finish) and repickled to a bright finish. Specimens were evaluated using rapid macrocell, Southern Exposure, and cracked beam tests. ECR and stainless steel clad specimens were evaluated with the coating having no intentional damage and with the coating or cladding penetrated. ECR with the coating penetrated is used to represent ECR that has undergone damage during construction. Clad bars were also bent to evaluate the corrosion resistance of the cladding after fabrication. Bars were tested for corrosion loss and chloride content at corrosion initiation. The critical chloride corrosion threshold for each system was established, as was an average corrosion rate after initiation. Results obtained from the southern exposure and cracked beam tests are used to the estimate cost effectiveness for each system under a 75-year and 100-year service life. Epoxy-coated reinforcement and stainless steel clad bars with and without intentional penetrations in the coating, as well as 2304 stainless steel in the as-received and repickled conditions exhibit a significant increase in corrosion resistance and critical chloride corrosion threshold compared to conventional steel, with undamaged epoxy-coated specimens exhibiting the lowest corrosion rate. In the as-received condition, 2304 stainless steel did not satisfy the requirements of ASTM A955, while repickled 2304 did. The undamaged stainless steel clad bars satisfied the requirements of the rapid macrocell test in ASTM A955; however, some cracked beam specimens containing stainless steel clad bars exhibited corrosion rates greater than the maximum allowable value permitted by ASTM A955. Conventional reinforcing steel is the least cost-effective form of reinforcement, with 2304 stainless steel in the as-received condition, ECR with penetrations through the epoxy, correctly pickled 2304 stainless steel, and stainless steel clad reinforcement representing progressively more cost-effective materials. Stainless steel clad reinforcement, however, is not currently available, and its failure to pass ASTM A955 calls its long-term performance into question. Increasing the cover over the top mat of steel and considering partial-deck replacements, where applicable, are methods that should be considered to decrease life cycle costs.Final Report, October 2010-July 2013N