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Changes in the microstructure of hydrated cement paste around the pull-out characteristics of brass or brass-coated steel, brass, brass-coated steel, and polypropylenefibers induced by steel fibers. The properties of a fiber composite are the addition of 1.4% poly(vinyl alcohol) (PVA) have been examined sensitive to the interfacial structure between the reinby scanning electron microscopy. The microstructures were exam-forcement and the matrix. This leads to the question of ined both after the fibers had been carefully peeled from the matrix how had the interfacial microstructure been modified and after the fibers had been pulled from the matrix along their axes. by PVA. The previously noted enhancement by PVA of the pull-out proper-The interfacial zones adjacent to the fiber in OPC ties from ordinary Portland cement matrix of steel fibers is attrib-matrices have been studied for various fibers that have uted to the formation of a ductile, fine-grained interracial layer. The included steel [2,3], glass [4], and polypropylene [5]. formation of this microstructure is suggested to arise from the effect The interfacial structures were found to be substanof PVA on the nucleation of CH and C-S-H at the fiber surface tially different from those in the bulk paste [6]. The and on the presence of polymer around the fibers. The enhancement typical interfacial microstructure against steel fiber has of the pull-out properties for brass and brass-coated steel fibers is been described to be the following [7]: Adjacent to the attributed to the formation of a strong bond between the fiber and interface is a thin "duplex" film consisting first of a matrix that diverts failure to the porous zone surrounding the fiber, calcium hydroxide (CH) layer and then a calcium sili-Interfacial structures against low surface energy fibers, like poly-cate hydrate (C-S-H) layer. Between this duplex propylene, showed minimal effect from the polymer. ADVANCED layer and the bulk is a porous zone consisting of clus-CEMENT BASED MATERIALS 1994, 1, 122-130 ters of C-S-H gel, some ettringite fibrils, and large KEY WORDS: Adhesion, Bond, Bond strength, Brass fibers, CH crystals in various orientations. Both the interfacial Cement matrix, Debonding, Friction, Poly(viny! alcohol), and porous regions constitute weaknesses in the struc-Pull-out work, Slip, Steel fibers ture. The purpose of this article is to describe the interfacial structure formation in the presence of PVA and how this modified structure enhances the mechanical A s described in the accompanying article [1], properties. poly(vinyl alcohol) (PVA) added to ordinary __ Portland cement (OPC) is capable of enhancing the bond properties between the cement and fi

used, steel and brass fibers (simulating brass-coated steel fibers) in ple, Wei et al. [7,8] found that the pull-out force of a a series of pull-out tests where the load versus global slip up to fiber was almost quadrupled by adding 15% by weight complete pull-out was recorded. The measured slip was that at the of cement of a water-dispersion of acrylic polymer parsection where the fiber penetrates the matrix. The first article de-ticles. This improvement was suggested to arise from scribes the mechanical effects of the addition of PVA, while the the small size (50 to 100 nm) of the acrylic polymer second article presents the microscopic observations. Correlation be-particles which, being smaller than the cement partitween the two studies is pointed out in the second part and conclu-cles, are able to fill in the porous zone that typically sions are drawn. In particular, it is observed that the addition of surrounds the fibers. In the work described here, a PVA in the amount of 1.4% by weight of cement matrix leads to a water-soluble polymer, poly(vinyl alcohol) (PVA), was significant improvement in the bond strength as well as in the used. This polymer is dispersed as molecules which frictional resistance, thus pull-out work, after the peak load. AD-are orders of magnitude smaller than particles, and the VANCED CEMENT BASED MATERIALS 1994, 1, 115-121 question to be answered is whether a much smaller KEY WORDS: Adhesion, Bond, Bond strength, Brass fibers, amount of this polymer could be as effective as the Cement matrix, Debonding, Friction, Poly(vinyl alcohol), acrylic particles, such as used by Wei et al. Pull-out work, Slip, Steel fibers In the first part of this study pull-out tests are carried out to investigate the effects of adding PVA to the cement matrix on the pull-out load versus slip response of plain smooth steel and brass

The viscoelastic behaviour of macro-defect-free (MDF) cement was studied by dynamic mechanical analysis. MDF specimens with various moisture contents in the range 1.70%-3.20% moisture, were measured at 1 Hz as a function of temperature from 34-96 ~ and as a function of frequency from 0.05-5 Hz at 34~ The viscoelasticity of MDF was found to be affected by moisture content through its plasticizing effect on the PVA binder. Time-moisture and temperature-moisture superposition of the shear moduli were found to be possible for MDF, and linear relationships between log time and linear moisture and between temperature and moisture were found. How the microstructure of MDF affects the viscoelastic response is also discussed through mechanical models. A comparison of the models with known experimental data suggests that the viscoelastic response arises from both direct connections between the inorganic particles and from connections through the polymer binder. Inorganic links are estimated to connect 45% of the inorganic phase.