International Concrete Abstracts Portal

This CD-ROM contains 10 papers that were presented at sessions sponsored by ACI Committee 544 at the Spring 2011 ACI Convention in Tampa, FL. The topics of the papers cover durability aspects of fiber-reinforced concrete, ranging from permeability, shrinkage cracking, long-term behavior in chloride environment and resistance to chloride penetration, as well as applications of fiber-reinforced concrete for coupling beams for highrise core-wall structures, beams for bridges, panels and suspended foundation slabs. Note: The individual papers are also available. Please click on the following link to view the papers available, or call 248.848.3800 to order. SP-280

Early-age cracks on concrete surfaces are one of the main routes through which aggressive agents penetrate into the concrete and threaten the long-term durability of structures. A test method capable of simulating early-age shrinkage cracks in 2-D cement paste samples under low-pressure condition is presented. The method is capable of characterizing evaporation parameters in two distinct stages of drying while monitoring sequential formation of shrinkage cracks. Using a gravimetric approach, the mass loss of the specimen is monitored continuously throughout the test as the sample is subjected to low pressure environment. Formation of microcracks are documented simultaneously using digital time lapse photography. The mass loss data is used in conjunction with specimen, size, and thickness to compute the evaporation rate from the surface. These results are used in turn using a diffusion based model to compute the effective moisture diffusivity as a function of fiber dosages. Effects of AR-glass fibers on evaporation parameters and shrinkage cracks are studied. Image analysis results indicate significant effects of fiber on controlling early-age shrinkage cracks.

To evaluate the influence of self-fibrillating macro-synthetic fiber reinforcement on the chloride penetration resistance of normal and self consolidating concrete mixtures, a total of 20 non-air entrained self consolidating concrete (SCC) mixtures with water to binder ratios between 0.4 and 0.45 (made using a ternary blend cement) and self-fibrillating macro-synthetic fiber dosages from 0.2 to 0.4% were evaluated. The chloride penetration resistance of all mixtures were evaluated using the Rapid Migration Test allowing the chloride diffusion coefficient to be calculated from the depth of chloride penetration which is determined visually. To evaluate the influence of cracking, which is typically more pronounced in SCC mixtures, shrinkage cracking resistance testing and chloride penetration testing on a cracked specimen were also conducted. At 28 days the chloride migration coefficient of all fiber reinforced self consolidating concrete (FRSCC) mixtures were slightly higher than the corresponding plain SCC mixtures; however at 120 days the FRSCC mixtures were slightly lower than the corresponding SCC mixtures. The superior performance of the FRSCC mixtures beyond 80-90 days is potentially due to reduced internal cracking due to the reinforcing effect of the fibers used in this study. The presence of plastic shrinkage cracking was shown to significantly influence the rate of chloride ingress locally around the crack. The influence of plastic shrinkage cracking was most appropriately modeled as an effective reduction in concrete cover equal to the crack depth. The influence of self-fibrillating macro-synthetic fiber addition on the service life of real structures was evaluated by incorporating the reduction in cover associated with plastic shrinkage cracks and chloride migration properties into corrosion modeling software (Life 365) to estimate the time to corrosion initiation in SCC with and without fibers with a reinforcement cover depth of 75mm which it typical for marine structures. The time to corrosion initiation for a non fiber reinforced mixture was calculated to be 18.6 years while the time to corrosion initiation for a mixture containing self-fibrillating macro-synthetic fiber at a modest dosage 3.7kg/m3(6.2lbs/yd3) was calculated to be 34.6 years, which represents a 85% improvement in service life.

The design of Steel–Fiber–Reinforced–Concrete (SFRC) pavements is mainly controlled by requirements in terms of serviceability limit states (short and long–term deformation, cracking, durability) and only marginally by requirements in terms of ultimate limit states. Despite that, experimental data on the behavior of SFRC under serviceability loads, especially as far as creep and durability are concerned, is nowadays still very limited. This paper describes the first results of an experimental campaign aimed at investigating the long–term behavior of cracked SFRC beams—with dimensions defined in order to be representative of a pavement strip—exposed to chloride solutions. In order to represent serviceability conditions where environmental actions act while structural elements are under loading a specific test procedure was developed. In the first stage of the procedure the specimens (two beams) were pre–cracked up to a chosen crack–opening value, then they were unloaded and subsequently reloaded with a fraction of the load corresponding to the flexural tensile–strength of the cracked cross section (previously reached). This reduced load was then kept constant, in a four–point bending scheme. During this stage of the test one of the beams was exposed to drying–wetting cycles in a 5% NaCl solution while the other beam was left unexposed. This long–term test lasted for about 8 months and was performed in a climate–controlled room. The mid–span deflection and the crack opening of the beams were monitored for the entire length of the test. At the end of the long–term test the beams were loaded up to failure. The results obtained show that the effects of the exposure to the aggressive environment are limited on both the long–term behavior and the residual flexural tensile–strength of the SFRC beams.

Durability is nowadays a key-parameter in Reinforced Concrete (RC) structures. Several codes require that structures have a defined service life during which the structural performance must satisfy minimum requirements by scheduling only ordinary maintenance. Durability can be associated to permeability, defined as the movement of fluid through a porous medium under an applied pressure load, which is considered one of the most important property of concrete. Permeability of concrete is strictly related to the material porosity but also to cracking. The former is basically controlled by the water/cement (w/c) ratio while microcracks and cracks are related to internal and external strains or deformations experienced by the RC structures. Shrinkage, thermal gradients and any factor determining volumetric instability, as well as the loads acting on a structure, lead to both microcraking and visible cracking. It is well known that, after cracking, tensile stresses are induced in the concrete between cracks and, hence, stiffen the response of a Reinforced Concrete (RC) member under tension; this stiffening effect is usually referred to as “tension stiffening”. After the formation of the first crack, the average stress in the concrete diminishes and, as further cracks develop, the average stress will be further reduced. When considering Fiber Reinforced Concrete (FRC), an additional significant mechanism influences the transmission of tensile stresses across cracks, arising from the bridging effect provided by the fibers between the crack faces; this phenomenon is referred to as “tension softening”. Fibers also significantly improve bond between concrete and rebars and act to reduce crack widths. The combination of these two mechanisms results in a different crack pattern, concerning both the crack spacing and the crack width. The present paper describes results from a collaborative experimental program currently ongoing at the University of Brescia and at the University of Toronto, aimed at studying crack formation and development in FRC structures. A set of tensile tests (52 experiments) were carried out on tensile members by varying the concrete strength, the reinforcement ratio, the fiber volume fraction and the fiber geometry.