Antoine E. Naaman

Ultra-High Performance Concrete with Compressive Strength Exceeding 150 MPa (22 ksi): A Simpler Way

Although intensive research related to ultra high-performance concrete (UHPC) and its composition has been conducted over the past 2 decades, attaining compressive strengths of over 150 MPa (22 ksi) without special treatment, such as heat curing, pressure, and/or extensive vibration, has been nearly out of reach. This paper describes the development of a UHPC with a compressive strength exceeding 200 MPa (30 ksi), obtained using materials commercially available in the U.S. market and without the use of any heat treatment, pressure, or special mixer. The influence of different variables such as type of cement, silica fume, sand, and high-range water reducer on compressive strength is evaluated. The test results show that the spread value, measured through a slump cone test on a flow table, is a good and quick indicator to optimize the mixture packing density and thus its compressive strength.

High Performance Fiber Reinforced Cement Composites

High Performance Fiber Reinforced Cement Composites (HPFRCC) are characterized by a stress-strain response in tension that exhibits strain-hardening behavior accompanied by multiple cracking, and related relatively large energy absorption capacity. This international workshop was the fourth in a series dealing with such composites. The first international workshop on HPFRCC was organized in Mainz, Germany, June 1991, under the auspices of RILEM and ACI. It was funded in part by US National Science Foundation (NSF) and by the Deutsche Forschungsgemeinschaft (the German NSF). Other cosponsors included ACBM, the University of Michigan, the University of Stuttgart and the Alexander von Humboldt foundation. The second workshop took place in Ann Arbor, Michigan, in June 1995, and the third in Mainz Germany, in June 1999. In each case hard-cover proceedings were published as a special RILEM publication. While the first workshop in 1991 included mostly US and German participants, subsequent workshops were opened to researchers from other countries. This last workshop assembled about sixty participants from eighteen countries, with essentially same sponsors and co-sponsors. Since the first workshop, continuous developments have occurred in new materials, processing, standardization, and in improved products for building and other structures. Also, enhanced theory and modeling of HPFRCC can now better describe their behavior and explain their reinforcing mechanisms. While the root definition of HPFRCC is simplest (that is to exhibit strain hardening and multiple cracking behavior in tension) to clearly differentiate them from other cement composites, this is not the only description of desirable performance. Durability, ductility, fire resistance, impact resistance, diffusion, imperviousness, and constructability at reasonable cost, are other important attributes that need to be further clarified. Additional sub-characterization for “deflection hardening” and “deflection softening” to reflect potential applications, has been defined. One feature of this last workshop, is that the organizers pre-selected lead topics they believed offer technical challenges at this time. The lead topics were: selfcompacting and self-consolidating FRC mixtures; fire resistance; impact and blast resistance; constitutive properties under reversed cyclic loading; modeling with discontinuous fibers; scale and size effects; and criteria for seismic applications. Other important topics included hybrid composites, modeling bond, and design recommendations by RILEM TC 162TDF.

Tensile stress-strain Properties of SIFCON,

Slurry-infiltrated fiber concrete (SIFCON) composites differ from conventional fiber reinforced concrete in at least two aspects: they contain a much larger volume fraction of fibers, and they use a matrix consisting of very fine particles. As such, they could be made simultaneously to exhibit outstanding strength and ductility properties. This research deals with the tensile stress-strain properties of SIFCON and comprises an experimental and an analytical program. Parameters investigated include the matrix composition and the fiber type where length, aspect ratio, surface characteristics, and overall fiber geometry vary. It is shown that SIFCON composites can exhibit tensile strength up to 4 ksi (28 MPa) at peak strains ranging from 1 to 2 %. A model is proposed to predict the ascending branch of the stress-strain curves of SIFCON from its compressive strength and its fiber-reinforcing parameters.

Stress-Strain Curves of Normal and Lightweight Concrete in Compression

A relatively simple experimental technique is used to obtain the stress-strain curves of concretes up to a strain of 0.006. The stress-strain curves of normal weight concretes of strenghts up to 11,000 psi and lightweight concrete of strengths up to 8000 psi are reported. An analytic expression for the stress-strain curve of concrete is developed to reflect experimental results. The analytic expression has four constants which depend on the properties of both the ascending and the descending portions of the stress-strain curve and can be evaluated from the knowledge of four key points of the curve. The coordinates of the four key points were expressed in function of the compressive strength of concrete so as to allow prediction of the entire curve solely from the knowledge of the compressive strength. /Author/

Stress at Ultimate in Unbonded Post-Tensioning Tendons: Part 2--Proposed Methodology

The first part of this study reviewed existing prediction equations for the stress f sub pm in unbonded tendons at nominal bending resistance, compared prediction equations with experimental results from 143 beam tests reported in the technical literature, and pointed out some of the drawbacks in existing prediction models for f sub ps. In the second part, the background for a new rational methodology for the analysis of beams prestressed with unbonded tendons is covered, and a new prediction equation for f sub ps at ultimate is developed. The equation is shown to account for most of the variables found important in the analysis, and to predict experimental results with much better accuracy than any of the prediction equations reviewed in the first part. The developed equation is proposed for adoption in the ACI Building Code.

BOND-SLIP MECHANISMS OF STEEL FIBERS IN CONCRETE

A comprehensive experimental program on pullout and pullthorugh tests of steel fibers from a cement-based matrix is described. Emphasis is placed on the accurate measurements of the pullout load versus end-slip response. Parameters included 3 different type of fibers, three different mortar matrixes with low medium, and high strengths, one cement-based slurry matrix, and additives such as latex, fly ash, and microsilica. The medium from which the fiber was pulled out included a control mortar mix without fibers, mortar mixes with 1, 2, and 3 % fibers by volume, and a SIFCON matrix containing about 11 % fibers by volume. For smooth fibers, 5 different diameters and 3 different embedment lengths were investigated. Experimental pullout load versus slip curves are needed to derive typical bond shear stress versus slip curves considered to be a property of the interface.

On the shear behavior of engineered cementitious composites

Abstract Results of an experimental investigation of structural response of shear beams made of a special class of cementitious composites, referred to as engineered cementitious composites (ECCs), are reported. ECCs are designed with tailored material structure and have been shown to exhibit pseudo strain-hardening tensile behavior. The improved performance in shear over conventional plain, fiber-reinforced, and wire mesh reinforced concrete is demonstrated. It is suggested that ECCs can be utilized for structural applications where superior ductility and durability performance are desired.

Loading Rate Effect on Pullout Behavior of Deformed Steel Fibers

Single-fiber pullout test results under loading rates ranging from seismic to static level are described in this paper. A basis for better understanding of the effect of strain rate on fiber-reinforced cement composite tensile properties is provided through loading rate effect investigation on single-fiber pullout behavior. There is evaluation of two high-strength deformed steel fiber types (twisted and hooked fibers) known, under static pullout loading, to have slip-hardening behavior. That twisted steel fiber pullout response shows rate sensitivity that is matrix compressive strength dependent is revealed by experimental results. Rate sensitivity under pullout for various matrixes tested was not shown, on the other hand, by high-strength hooked fibers. That twisted fiber pullout energy can be up to five times that of hooked fibers and increases with the compressive strength matrix was also shown in the test results.

Structure and properties of poly(vinyl alcohol)-modified mortar and concrete

The structure and properties of mortars and concretes containing up to 2 wt% (based on cement) of poly(vinyl alcohol) (PVA) were examined and compared with those without PVA. Among changes occurring with the addition of PVA were increases in air void content and apparent fluidity and a reduction in the bleeding of fresh mortar and concrete. The increased fluidity caused increased slump for fresh concrete. The microstructure was examined by polarizing optical microscopy and scanning electron microscopy in backscattered mode of cut surfaces after hardening. The porous interfacial transition zones around sand grains and coarse aggregate were significantly reduced, and the cement particles were uniformly distributed without significant depletion near aggregate surfaces. For mortars, using a prewetting mixing technique, the compressive strength was decreased moderately, but the flexural strength was unchanged. For concretes, with the same mixing technique, the compressive strengths after 28 days of hydration were relatively unchanged, but the postpeak area of the compression stress-strain curve was reduced, accompanying a change in fracture behavior from debonding to cohesive failure of the coarse aggregate. When concrete having the same air void content with PVA as without was made, the compressive strength was moderately increased.

Stress at Ultimate in Unbonded Post-Tensioning Tendons: Part 1--Evaluation of the State of the Art

This study deals with the stress at ultimate fpm in unbonded tendons of prestresed and partially prestressed beams for the purpose of computing their nominal bending moment resistances. The first part comprises mainly: (1) some background information followed by a comprehensive review of existing experimental and analytical investigations dealing with the stress at ultimate in unbonded tendons, (2) a summary of prediction equations for fpm suggested by different investigators in various North American and European codes, and (3) an evaluation of typical prediction equations achieved by comparing predicted with experimentally observed results. The experimental results were obtained from 9 different investigations totaling 143 beam tests described in the technical literature and carried out since 1960 in various parts of the world. In the second part of this study, the background for a new rational methodology for analysis of beams prestressed with unbonded tendons is covered and a new prediction equation for fpm at ultimate is developed.