H. Najm

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.

Effects of poly(vinyl alcohol) on fiber cement interfaces. Part I: Bond stress-slip response

Abstract This is the first part of a two-part article describing the effects of adding poly(vinyl alcohol) (PVA) to a cement based matrix to improve the bond at the fiber-matrix interface. Two types of fibers were used, steel and brass fibers (simulating brass-coated steel fibers) in a series of pull-out tests where the load versus global slip up to complete pull-out was recorded. The measured slips was that at the section where the fiber penetrates the matrix. The first article describes the mechanical effects of the addition of PVA, while the second article presents the microscopic observations. Correlation between the two studies is pointed out in the second part and conclusions are drawn. In particular, it is observed that the addition of PVA in the amount of 1.4% by weight of cement matrix leads to a significant improvement in the bond strength as well as in the frictional resistance, thus pull-out work, after the peak load.

ELASTIC MODULUS OF SIFCON IN TENSION AND COMPRESSION

A graphical evaluation of various analytical models predicting the modulus of elasticity of fiber reinforced composites is first presented. Upper and lower bound solutions and their applicability to fiber reinforced concrete and slurry-infiltrated fiber concrete (SIFCON) are discussed. An experimental investigation of the elastic modulus of SIFCON in tension and compression is then described. Two different types of fibers, two different aspect ratios, seven different levels of volume fractions, and 4 different matirix compositions were used. The value of the modulus of elasticity is found to be dependent upon not only the peak compressive strength, but also factors such as fiber volume fraction, fiber aspect ratio, fiber orientation and alignment, and testing procedure. Experimental data are analyzed and conclusions are drawn.

Effects of poly(vinyl alcohol) on fiber cement interfaces. Part II: Microstructures

Abstract Changes in the microstructure of hydrated cement paste around steel, brass, brass-coated steel, and polypropylene fibers induced by the addition of 1.4% poly(vinyl alcohol) (PVA) have been examined by scanning electron microscopy. The microstructures were examined both after the fibers had been carefully peeled from the matrix and after the fibers had been pulled from the matrix along their axes. The previously noted enhancement by PVA of the pull-out properties from ordinary Portland cement matrix of steel fibers is attributed to the formation of a ductile, fine-grained interfacial layer. The formation of this microstructure is suggusted to arise from the effect of PVA on the nucleation of CH and CSH at the fiber surface and on the presence of polymer around the fibers. The enhancement of the pull-out properties for brass and brass-coated steel fibers is attributed to the formation of a strong bond between the fiber and matrix that diverts failure to the porous zone surrounding the fiber. Interfacial structures against low surface energy fibers, like poly-propylene, showed minimal effect from the polymer.

Effect of Large-Diameter Polymeric Fibers on Shrinkage Cracking of Cement Composites

This paper presents the contribution of large-diameter polymeric fibers (LDPFs) to the reduction of crack width caused by plastic and drying shrinkage. These fibers, shown to improve mechanical properties, have been used for a number of field applications including highway pavements and whitetopping. Test results show that LDPFs provide substantial reduction in plastic and drying shrinkage cracking. LDPFs provide the same crack reduction as steel fibers at one-half the fiber volume fraction.