David Darwin

DEVELOPMENT LENGTH CRITERIA FOR CONVENTIONAL AND HIGH RELATIVE RIB AREA REINFORCING BARS

Statistical analyses of 133 splice and development specimens in which the bars are not confined by transverse reinforcement and 166 specimens in which the bars are confined by transverse reinforcement are used to develop an expression for the bond force at failure as a function of concrete strength, cover, bar spacing, development/splice length, transverse reinforcement, and the geometric properties of the developed/spliced bars. Results are used to formulate design criteria that incorporate a reliability-based strength reduction factor that allows the calculation of a single value for both development and splice length for given material properties and member geometry. As with earlier studies, the analyses demonstrate that the relationship between bond force and development or splice length is linear but not proportional. The most accurate representation of the effect of transverse reinforcement on bond strength obtained in the current analysis includes parameters that account for the number of transverse reinforcing bars that cross the developed/spliced bar, the area of the transverse reinforcement, the number of bars developed or spliced at one location, the relative rib area of the developed/spliced bar, and the size of the developed/spliced bar. The yield strength of transverse reinforcement does not play a role in the effectiveness of the transverse reinforcement in improving development/splice strength. Depending on the design expression selected, for conventional and high relative rib area bars that are not confined by transverse reinforcement, development lengths average 2 to 14 percent higher and splice lengths 12 to 22 percent lower than those obtained using American Concrete Institute (ACI) 318-95. For conventional reinforcing bars confined by transverse reinforcement, development lengths average 5 percent lower to 16 percent higher than those obtained using ACI 318-95. For high relative rib area reinforcing bars confined by transverse reinforcement, development lengths average 3 to 17 percent lower than those obtained using ACI 318-95, while splice lengths average 25 to 36 percent lower than those obtained using ACI 318-95. When confined by transverse reinforcement, high relative rib area bars require development and splice lengths that are 13 to 16 percent lower than required by conventional bars.

Effect of Deformation Height and Spacing on Bond Strength of Reinforcing Bars

The details are described of a study of the effect of deformation pattern on bond strength usign 1 inch diameter machined bars with deformation heights of 0.05, 0.075, and 0.10 inch and deformation spacings ranging from 0.26 to 2.2 inch. The study found that the bond force-slip response of reinforcing bars is a function of the relative rib area of the bars, independent of the specific combination of rib height and rib spacing. Under all conditions of bar confinement, the inititial stiffness of load-slip curves increases with an increase in the relative rib area. Under conditions of relatively low confinement, in which bond strength is governed by splitting of the concrete, bond strngth is independent of deformation pattern. Under conditions in which additional bar confinement is provided by transverse reinforcement or higher cover, bond strength increases, compared to the bond strength of bars with less confinement. The magnitude of the increase in bond strength increases with an increase in the relative rib area.

Splice Strength of High Relative Rib Area Reinforcing Bars

This paper describes the testing and analysis of 83 beam-splice specimens containing 16, 25, and 36 mm bars with relative rib areas ranging from 0.065 to 0.140. Concrete containing two different coarse aggregates were used to evaluate the effect of aggregate properties on bond strength. Sixty specimens contained uncoated bars with confining transverse reinforcement. Thirteen specimens contained uncoated bars without confining reinforcement, and 10 specimens contained epoxy-coated bars, nine without confining reinforcement, and one with confining reinforcement. The tests are analyzed to determine the effect of relative rib area and bar diameter on the increase in bond strength provided by confining reinforcement. The tests also provide a preliminary indication of the effect of high relative rib area on the splice strength of epoxy-coated bars. The splice strength of uncoated reinforcement confined by transverse reinforcement increases with an increase in the relative rib area and the bar diameter of the spliced bars. The increase in splice strength provided by transverse reinforcement increases as the strength of the coarse aggregate increases. The use of reinforcing bars with an average relative rib area of 0.1275, an increase from the average value for conventional bars of 0.0727, can provide up to a 26 percent decrease in splice length compared to conventional reinforcement when confining reinforcement is used. The savings obtainable with high relative rib area bars is highest for low covers and bar spacings. Epoxy coating seems to have a less detrimental effect on splice strength for high relative rib area bars than for conventional bars. The results indicate that the maximum development length modification factor used for epoxy-coated reinforcement may be reduced by 20 percent.

Quantitative backscattered electron analysis of cement paste

Abstract Procedures are developed to obtain reproducible quantitative data from polished specimens of cement paste using automated backscattered electron imaging. Signal production, contrast, image resolution, and imaging techniques are discussed. Preparation of a silicon-magnesium standard is described. Typical results and a statistical basis for establishing the number of frames required to provide confidence in the results are presented. The silicon-magnesium standard provides an objective method for setting both a scanning electron microscope and an image analysis system for quantitative backscattered electron analysis of phases within cement paste. The number of frames required for a selected degree of confidence decreases, but the total area required increases as magnification decreases. The number of frames is also a function of the specific phase, being greatest for unhydrated cement particles and least for inner product and calcium silicate hydrate.

Development Length Criteria: Bars Not Confined by Transverse Reinforcement

An expression that accurately represents development and splice strength as a function of concrete cover and bar spacing is developed. The Expression is then used to establish and evaluate modifications to the bond and development provisions of the ACI Building Code (ACI 318-89) for bars in concrete members that are not confined by transverse renforcement. The expression provides a more accurate representation of development and splice strength than do the earlier expressions, and it provides better guidance when there is a significant difference between the concrete cover and one-half of the clear spacing between bars. Proposals for new design criteria, including one under study by ACI Committee 318, are compared. The criteria differ in terms of relative safety economy, and ease of application. Side-by-side comparisons in design offices are recommended. In all cases, an additional development length modification factor of 1.1 is recommended for reinforcing steels with specified yield strength in excess of 60,000 psi (414 MPa).

STRAIN-RATE SENSITIVE BEHAVIOR OF CEMENT PASTE AND MORTAR IN COMPRESSION

The strain-rate sensitivity of the cement paste and mortar constituents of concrete is studied experimentally. Saturated cement paste and mortar specimens are loaded in compression to 15,000 microstrains, 27 to 29 days after casting, using strain rates ranging from 0.3 to 300,000 microstrains/sec. Water-cement ratios of 0.3, 0.4, and 0.5 are used. Strain-rate sensitivity of the material is measured in terms of the initial elastic moduli, maximum stress, and corresponding strain. The initial elastic moduli and the strength of cement paste and mortar increase by 7% and 15%, respectively, with each order of magnitude increase in strain rate. The strain at the maximum stress is the greatest for the lowest strain rate. With an increase in strain rate, the strain at the maximum stress first decreases and then increases.

Role of Silica Fume in Compressive Strength of Cement Paste, Mortar, and Concrete

Controversy exists as to why silica fume increases the strength of concrete when it is used as a partial replacement for cement. Some evidence supports the view that the increase in strength is due to an increase in the strength of the cement paste constituent of concrete. However, contradictory evidence exists that shows no increase in the strength of cement paste, but substantial increases in concrete strength, when silica fume is used. The latter evidence is used to support the theory that silica fume strengthens concrete by strengthening the bond between cement paste and aggregate. This study is designed to explain the contradictory evidence and establish the role played by silica fume in controlling the strength of concrete and its constituent materials. These goals are accomplished using cement pastes, mortars, and concretes with water-cementitious material ratios ranging from 0.30 to 0.39. Mixtures incorporate no admixtures, a superplasticizer only, or silica fume and a superplasticizer. The research demonstrates that replacement of cement by silica fume and the addition of a superplasticizer increases the strength of cement paste. It also demonstrates that cement paste specimens, with or without silica fume, can exhibit reduced strength compared to other specimens with the same water-cementitious material ratio if the material segregates during fabrication, thus explaining some earlier experimental observations. The segregation of cement paste is caused by high superplasticizer dosages that do not cause segregation of concrete with the same water-cementitious material ratio. Concrete containing silica fume as a partial replacement for cement exhibits an increased compressive strength because of the improved strength of its cement paste constituent. Changes in the paste-aggregate interface caused by silica fume appear to have little effect on the uniaxial compressive strength of concrete.

BOND OF EPOXY-COATED REINFORCEMENT: BAR PARAMETERS

The effects of coating thickness, deformation pattern, and bar size on the reduction in bond strength between reinforcing bars and concrete caused by epoxy coating are described. Tests included beam end and splice specimen testing. The results are compared with the splice tests that were used to establish the epoxy coated bar provisions in the 1989 ACI Building Code and 1989 AASHTO Bridge Specifications. Epoxy coating are found to reduce bond strength significantly, but the extent of the reduction is less than that used to select the development length modification factors in the ACI Building Code and AASHTO Bridge Sepcifications. Coating thickness has little effect on the amount of bond strength reduction for No. 6 bars and larger. However, the thicker the coating, the greater the reduction in bond strength for No. 5 bars. In general, the reduction in bond strength caused by an epoxy coating increases with bar size. The magnitude of the reduction depends on the deformation pattern; bars with relatively larger rib-bearing areas with respect to the bar cross section are affected less by the coating than bars with smaller bearing areas.

Effect of Cracking on Chloride Content in Concrete Bridge Decks

Field survey studies measuring bridge deck cracking and chloride contents of both cracked and uncracked concrete are presented in this paper, as part of a larger overall evaluation of bridge deck performance. The authors study three deck types, including monolithic decks, decks that have a conventional high density concrete overlay, and decks with high density concrete overlay that also contain either a 5% or 7% cement replacement by silica fume. Field survey results show that chloride content is not significantly affected by bridge deck type. Regarding chloride concentrations, samples taken away from cracks show the average concentration at the top of transverse reinforcement rarely exceeds even the most conservative estimates of the corrosion threshold for conventional reinforcements. However, at crack locations, chloride concentrations often are shown to exceed the corrosion threshold of conventional reinforcement in less than one year.

CONTROL OF CRACKING IN BRIDGE DECKS: OBSERVATIONS FROM THE FIELD

Crack surveys of bridge decks, performed over a 10-year period in northeast Kansas as part of three studies, provide strong guidance in identifying the parameters that control cracking in these structures. The surveys involve steel girder bridges—bridges that are generally agreed to exhibit the greatest amount of cracking in the concrete decks. The surveys include monolithic decks and decks with silica fume and conventional concrete overlays. The study demonstrates that crack density increases as a function of cement and water content, and concrete strength. In addition, crack density is higher in the end spans of decks that are integral with the abutments than decks with pin-ended supports. Most cracking occurs early in the life of a bridge deck, but continues to increase over time. This is true for bridges cast in both the 1980s and the 1990s. A key observation, however, is that bridge decks cast in the 1980s exhibit less cracking than those in the 1990s, even with the increase in crack density over time. Changes in materials, primarily cement fineness, and construction procedures over the past 20 years, are discussed in light of these observations. A major bright spot has been the positive effect of efforts to limit early evaporation, suggesting that the early initiation of curing procedures will help reduce cracking in bridge decks.