By Martnez-Barrera, Gonzalo; Vigueras-Santiago, Enrique; Hernndez- Lpez, Susana; Brostow, Witold; Menchaca-Campos, Carmina

Fiber reinforced concrete (FRC) contains fibers physically mixed with gravel, sand, cement, and water. So far, adequate mechanical performance of FRC has been obtained at high cost and using complex technologies; important here is the geometry and surface characteristics of the polymers. We have modified polymeric-fiber surfaces by using gamma radiation. Irradiated polypropylene (PP) fibers were submitted to 0, 5,10, 50, and 100 kGy of gamma irradiation dosages. First, tensile strength of PP fibers was evaluated, and then fibers blended at 0, 1.0, 1.5, and 2.0% in volume with Portland cement, gravel, sand, and water. The highest values of compressive strength were obtained with irradiated-fibers at 10 kGy and 1.5% in volume of fiber. The result is 101 MPa, as compared to 35 MPa for simple concrete without fibers. POLYM. ENG. SCI., 45:1426-1431, 2005. 2005 Society of Plastics Engineers

INTRODUCTION

As argued by Castao and his colleagues [1, 2], ceramics were the first engineering materials known to mankind and they still constitute the most used class of materials in terms of weight. Hydraulic cements and cement-based composites including concretes are the main ceramic-based materials.

Concrete modification by using polymeric materials has been studied for the last four decades [3]. Nevertheless, in certain applications these kinds of materials fail and for a number of applications it is necessary to use other complex and expensive technologies. In general, the reinforcement with fibers or particles of brittle building materials has been known tor a long time, for example putting straw into the mud for housing walls [4] or reinforcing mortar using animal hair. In the case of concrete, many materials have been used for reinforcing it: jute, bamboo, coconut [5, 6], husk rice, cane bagasse, and sawdust [7], as well as synthetic materials such as poly(vinyl alcohol), polypropylene (PP), polyethylene, and polyamides [8].

The materials used so far to reinforce the concrete matrix are adhering only by physical interactions and not by pri- mary chemical bonds. Other procedures used, such as chemical attack or thermal treatment, are costly and time consuming. An alternative that we have decided to explore is the use of gamma radiation to modify the material surface and have at least a chance to make it compatible with the brittle ceramic matrix.

The aggregates in concrete take up to 75% of the total volume; clay, lime, organic matter, or chemical salts have to be absent. ASTM standards define the shape and size of aggregates [7, 8]. Concrete manufactured from Portland cement is the most utilized due its range of applications (structures, blocks, pavement, etc.) and the resulting properties including durability and plasticity.

Generally, simple concrete (SC) shows high compressive bul low tensile strength. Thus, it is necessary to add various aggregates. The problem of adhesion of added polymers has already been noted. The polymer is surrounded by sand and gravel, facilitating the development of cra/es when subjected to mechanical stress. Formation of cra/es along with alternative mechanisms of polymer response under load have been thoroughly reviewed by Donald [9].

Improvement of concrete using polymeric materials has been attempted before [10-14]. Given the properties of concrete without polymer reinforcement, the main objectives of adding a polymer are: high compressive and tensile strength, high impact and abrasive resistance, service in adverse environments (wind, moisture, etc.), lower weight, and lower costs [3, 12].

Early researchers reported that addition of recycled poly(ethylene terephthalate) (PET) aggregates improves the compressive strength up to 112 MPa as compared to 35 MPa for SC [12, 14]. The US National Institute of Standards and Technology (NIST) reported 150 MPa of compressive strength for a concrete reinforced with epoxies or polyesters; however, such technologies are costly and complex [13].

There were attempts to add polymeric fibers as aggregates to concrete; mechanical properties were improved when controlling content and length of the fibers [7]. Usually, the volume of the fibers was between 0.3 and 3.5% with respect to the total concrete volume; the fiber lengths were from 0.1 to 100 mm. A 1:10 ratio of the diameter to the fiber length is recommended by ASTM. Nevertheless, the sand and the gravel are very important players also; the final performance depends on local structures.

It is well known that gamma irradiation causes three main processes when applied to polymers-scission, crosslinking, and grafting of chains [15-21]-and these processes involve generation of free radicals [13]. In certain cases, crystallization of parts of amorphous /ones can occur. Improvement of tensile strength and impact resistance of polystyrene and nylon 6,6 by using gamma irradiation has been reported [18-22]. A structural modification of recycled HDPE by irradiation improved the compressive strength of Portland cement [23].

In the present work we have studied the gamma radiation effect on polypropylene fibers and their use for the mechanical improvement of Portland concrete. On this basis we propose a novel technology for manufacturing polymer reinforced concrete. The present work constitutes a part of a larger project on effects of irradiation on properties of polymers and composites [18, 21, 22, 24, 25].

EXPERIMENTAL

Specimen Preparation

For preparing the specimens, natural silica as sand (according to ASTM C-778) and gravel of a local company were used, as well as Portland cement (Cruz Azul(TM), Mexico) and gamma irradiated PP alaclic libers whose diameter varies from 0.2 to 0.6 mm, with 6 mm length on the average. We have followed C-1154 ASTM. Five different lots, identified as A, B, C, D, and E, were prepared separately on different days.

The proportions of eomponents in the eonerele were 1/2.75 for eemenl/aggregates. The water/cement ratio of 0.485 was used aeeording to ASTM C-305. The fiber contents were 1.5, 2.0, and 2.5% in volume. After mixing, the specimens (2" diameter and 4" long) were placed in a controlled temperature room at 23.0 3.0C for a period time of 20 to 72 hr, with the surface exposed to moisture in air, providing no less than 50% humidity according to ASTM C-511.

Mechanical Tests

The tensile tests were carried out in a Zwick dynamometer according to the ASTM D638 standard with the test speed of 10 mm/ min.

The compressive tests were carried out in an Instron Universal Testing machine Model 1125 according to the ASTM C-109M standard. The testing allowed tolerance for the specimens was 28 days 12 hr and the charge speed was between 91 and 184 kg/s, holding the charge until reaching the maximum value to assure the reliability of the test.

Morphological Characterization

Taking account the length and diameter of the fibers as well as expected gamma irradiation effects, the following contrasting process was used. First the fibers were submerged in OsO^sub 4^ for 48 hr and cooled in liquid nitrogen for 0.5 hr. Then specimens were cut with a RMC model MT 6000-XL micrometer that produces very thin and uniform cuts with diamond knives, and then vacuum-coated with carbon (thickness between 3 and 10 nm) in a vacuum pump (E.F. Fullam) at 50 mTorr. Finally, the surfaces were analyzed by scanning electron microscopy (SEM) in a JEOL model JSM-5200 machine in the secondary-electron mode.

Irradiation Procedure

The polypropylene fibers were subjected to gamma irradiation in air at the room temperature applying dosages of 5, 10, 50, and 100 kGy at the dose rate of 6.1 kGy/h. The irradiation was provided by a 651 PT Gammabcam Irradiator manufactured by the Atomic Energy of Canada Limited (AECL, Chalk River, Ontario), and located at the Institute of Nuclear Sciences of the National Autonomous University of Mexico.

RESULTS

In Fig. 1, we present values of tensile stress at yield points as a function of the irradiation dose. We see that at the 5 kGy dose the tensile stress at yield amounts to 92.1 MPa-that is only 14.0% higher than for fibers that were not irradiated. The irradiation energy causes structural modifications of the fibers. Above 5 kGy the tensile stress at yield point values decrease to 47.9 MPa for 100 kGy, approximately 40.8% less with respect to raw fibers.

FlG. 1. Tensile stress at yield point of the polypropylene fibers.

FIG. 3. SEM micrograph of raw polypropylene fibers.

Another important mechanical parameter characterizing effects of addition of fibers into concrete is the tensile strain; its behavior is shown in Fig. 2. For raw fibers 21.0% of strain value is obtained when the tensile stress is applied. When increasing the radiation dose, the tensile strain at yield point decreases, down to 49.2% with respect to raw fibers for 100 kGy. Thus, values of both stress and strain at yield point decrease at least 40% when applying up to 1OO kGy of radiation. Such mechanical behavior is clearly a consequence of morphological changes.

For the raw materials we see in the SEM results in Fig. 3 the homogeneous surfaces (the dark line separates two fibers). Application of the ionizing energy at 5 kGy results in formation of small pieces of scrap of material forming a "brick-wall" structure (Fig. 4) that corresponds to the increment of 14.0% for tensile stress. Finally, poor mechanical values are due to modified surfaces of the fibers when irradiating with high energies (100 kGy); extensive damage of the surface is seen in Fig. 5.

FIG. 2. Tensile strain at yield point of the polypropylene fibers.

FIG. 4. SEM micrograph of polypropylene fiber irradiated at 5 kGy.

The compressive strength results for all concrete specimens are shown in Table 1; the averages are displayed in Fig. 6. It is important to observe in Table 1 the variations of values according to the lots (A to E). The differences are small, corroborating adequate blending in each case.

In Fig. 6, we see compressive strength values varying from 35.9 to 41.7 MPa for concrete with non-irradiated fibers; these are higher than 35 MPa for simple concrete. Moreover, the compressive strength increases when the fiber content increases; the highest values are obtained when adding 2.5% of non-irradiated fibers. The initial bond between the fibers and the concrete can be attributed to physical adhesion and static friction caused by the surface finish of the fiber. There is a noticeable tendency ol" the fibers to exhibit resistance to compression. The fibers hold on to the concrete matrix, and fiber pull-out is not observed. The eventual failure of the fibers as well as of concrete is brittle; concrete disintegrates into pieces in a rather sudden way, while the fibers largely still preserve their original size.

FIG. 5. SEM micrograph of polypropylene fiber irradiated at 100 kGy.

FIG. 6. Compressive strength of concretes reinforced with irradiated polypropylene fibers.

In general terms, the compressive strength values of the concrete are increasing when increasing the radiation dose in the fibers. However, a detrimental behavior is observed for higher dosages (50 and 100 kGy), for concrete containing 1.5 and 2.0%; thus, an optimum exists.

TABLE 1. Compressive strength for concrete reinforced with gamma- irradiated polypropylene fibers.

With increasing irradiation dose, the compressive strength of the concrete is increasing while tensile strain of the irradiated fibers goes down; we have seen this in Fig. 2. This suggests hardening of the fibers, as reflected in changes in the tensile modulus (Fig. 7). There is a maximum value al 10 kGy. One infers the existence of a mechanism of load transfer between the concrete and fibers when external load is applied; in a extremal case the strain in the fibers has the highest value when the strain in concrete is zero.

The ionizing energy generates more contact points and in consequence a larger contact area between the fibers and the concrete phase. Eventually, the concrete will split parallel to the fibers and the resulting crack will propagate out to the surface. An increasing number of contact points with the concrete resist the load by inclined forces, oriented at some angle relative to the longitudinal axis of the fiber. In consequence, the splitting cracks follow the reinforcing fibers, and the bond transfer drops rapidly unless reinforcement is provided to restrain the opening of the splitting crack. The "brick-wall" structure necessarily affects the fracture pattern of the concrete. Another structure that can be called "sphere-formation" is along with the "brick-wall" structure responsible for the decrease of the tensile strength. Figure 8 shows formation of small spheres on the fiber surfaces.

FIG. 7. Tensile modulus of polypropylene fibers.

The highest values of compressive strength are found for concrete containing 1.5% in volume of irradiated fibers at 10 kGy. Thus, there is a compressive strength improvement of 189% with respect to simple concrete. Above 10 kGy, the values go down; nevertheless, an improvement of 133% remains.

For concrete containing 2.5%, the highest compressive strength is achieved for irradiated fibers at 50 kGy. We conclude that, when adding only 1.0% more of fibers, it is necessary to apply much higher ioni/ing energy, going from 10 kGy to 50 kGy. Al the high dose of 100 RGy, a deteriorated "brick-wall" structure (see Fig. 5) is obtained, reducing the compressive strength. In spite of these results, the comprcssive strength improvement resulting from addition of irradiated PP fibers remains in the range between 120.8 and 189.4%.

FIG. 8. SEM micrograph of a polypropylene fiber irradiated at 50 kGy.