P. Balaguru

FIRE RESISTANT ALUMINOSILICATE COMPOSITES

The fire response of a potassium aluminosilicate (Geopolymer) matrix carbon fiber composite was measured and the results compared to organic matrix composites being used for transportation, militar y, and infrastructure applications . At irradiance levels of 50 kW/m 2 typi- cal of the heat flux in a well developed fire, glass- or carbon-reinforced polyeste r, vinylester, epoxy, bismaleimde, cyanate ester, polyimide, phenolic, and engineering thermoplastic laminates ignited readily and released appreciable heat and smoke, while carbon-fiber reinforced Geopolymer com- posites did not ignite, burn, or release any smoke even after extended heat flux exposure . The Geopolymer matrix carbon fiber composite retains sixty-seven percent of its original flexural strength after a simulated large fire exposure.

BOND BEHAVIOR OF CORRODED REINFORCEMENT BARS

Corrosion of reinforcement is an international problem, causing extensive damage to various types of structures. While considerable research has been carried out on measuring the amount of corrosion and methods to slow the progress of corrosion, estimating the strength of structural members with corroded reinforcement has not received sufficient attention. The two main mechanisms for the loss of strength are loss of cross section of reinforcement and loss of bond between concrete and reinforcement. This study focuses on the loss of bond strength. Reinforcing bars embedded in concrete were subjected to accelerated corrosion using external current source. The mass loss of reinforcement varied from 0% for the control to 5.91%. The average compressive strength was 4,000 psi (28 MPa). Results indicate that it is possible to induce predetermined accelerated corrosion by passing external current; low levels of corrosion (less than 1% mass loss) improves bond strength; corrosion levels resulting in more than 1% mass loss lead to reduction in bond strength; slip at failure reduces exponentially with corrosion, even at low levels; and even after excessive corrosion, measurable bond strength exists. The results obtained in this investigation are being used for the evaluation of beam and slab specimens.

Comparative study of high temperature composites

Abstract Two classes of composite made using either ceramic matrix with high temperature fibers or carbon/carbon have been used for various applications that require high temperature resistance, over three decades. However, their use has been limited to special applications because of the high costs associated with fabrication. Typically the composites are cured at more than 1000°C, and in most instances the heating has also to be carried out in controlled environments. In addition, because of the high processing temperature, only certain type of expensive fibers can be used with the ceramic matrices. A recently developed inorganic matrix, called polysialate can be cured at temperatures less than 150°C, making it possible to use carbon and glass fibers. Composites made using carbon, glass and combinations of carbon and glass fibers have been tested in bending and tension. This paper presents the comparison of processing requirements and mechanical properties of carbon/carbon composites, ceramic matrix composites made with silicon carbide, silicon nitride and alumina fibers and carbon/polysialate composites. The results indicate that carbon/polysialate composite has mechanical properties comparable to both carbon/carbon and ceramic matrix composites at room and high temperatures. Since the polysialate composites are much less expensive, the authors believe that it has excellent potential for more applications in aerospace, automobile and naval structures.

PROPERTIES OF FIBER REINFORCED STRUCTURAL LIGHTWEIGHT CONCRETE

The authors of this paper present the results of an experimental study of the behvaior of fiber reinforced structural lightweight concrete. Properties investigated include workability and behavior under compression, splitting tension, and flexure. The independent variables were composition of fine aggregate, fiber content, fiber length, and presence of silica fume. Fine aggregate consisted of all lightweight aggregate or combinations of lightweight aggregate and nautral sand. Two fiber lengths were investigated with various fiber contents. The lightweight aggregate used was made of expanded shale. High-range water-reducing admixture was used to improve workability. An air-entraining admixture was used to reduce the unit weight and improve workability. Results reveal that a compressive strength of approximately 42 MPa can be obtained for concretes with an equilibrium density of 1650 kg/m3. Fibers increase flexural and splitting tensile strengths significantly. The modulus of elasticity is enhanced by about 30 percent. Fiber reinforced concrete displays excellent ductility.

Flexural response of inorganic hybrid composites with E-glass and carbon fibers

By far, carbon and glass fibers are the most popular fiber reinforcements for composites. Traditional carbon composites are relatively expensive since the manufacturing process requires significant heat and pressure, while the carbon fibers themselves are inherently expensive to produce. In addition, they are often flammable and their use is restricted when fire is a critical design parameter. Glass fabrics are approximately one order of magnitude less expensive than similar carbon fabrics. However, they lack the stiffness and the durability needed for many high performance applications. By combining these two types of fibers, hybrid composites can be fabricated that are strong, yet relatively inexpensive to produce. The primary objective of this study was to experimentally investigate the effects of bonding high strength carbon fibers to E-glass composite cores using a high temperature, inorganic matrix known as geopolymer. Carbon fibers were bonded to E-glass cores (i) on only the tension face, (ii) on both the tension and compression faces, or (iii) dispersed throughout the core in alternating layers to obtain a strong, yet economical, hybrid composite laminate. For each response measured (flexural capacity, stiffness, and ductility), at least one hybrid configuration displayed mechanical properties comparable to all carbon composite laminates. The results indicate that hybrid composite plates manufactured using 3k unidirectional carbon tape exhibit increases in flexural capacity of approximately 700% over those manufactured using E-glass fibers alone. In general, as the relative amount of carbon fibers increased, the likelihood of precipitating a compression failure also increased. For 92% of the specimens tested, the threshold for obtaining a compression failure was utilizing 30% carbon fibers. The results presented herein can dictate future studies to optimize hybrid performance and to achieve economical configurations for a given set of design requirements.

Strength retention of fire resistant aluminosilicate–carbon composites under wet–dry conditions

Abstract The results of an experimental investigation of the durability of inorganic matrix–carbon composites are reported. The matrix, which can sustain temperatures up to 1000°C, is being evaluated for applications that require fire resistance, such as the interior of aircraft. The original matrix formulation, which had a high ratio of silica to alumina, was found to weaken when subjected to wet–dry cycles. Preliminary tests indicated that an efficient way to increase water-stability was to increase the amount of alumina in the matrix. Therefore, a systematic evaluation was carried out to obtain the optimum silica/alumina ratio for improving stability of composites in water. In-plane shear strengths were used as an indicator of strength retention after the specimens were subjected to 50 wet–dry cycles. The results indicate that silica/alumina ratio between 18 and 20 provide the best results. In this range, strength loss is negligible.

FRP composites for reinforced and prestressed concrete structures : a guide to fundamentals and design for repair and retrofit

Chapter 1: Introduction Chapter 2: Constituent Materials Chapter 3: Fabrication Techniques Chapter 4: Common Repair Systems Chapter 5: Flexure: Reinforced Concrete Chapter 6: Flexure: Prestressed Concrete Chapter 7: Shear in beams Chapter 8: Columns Chapter 9: Load Testing

Effects of Freeze-Thaw Exposure on Performance of Concrete Columns Strengthened with Advanced Composites

All research results to date have indicated that the use of fiber reinforced polymers (FRPs) in repair and retrofitting provide deteriorated infrastructure with new life and longevity. This paper presents the results of an experimental study on the performance of concrete columns wrapped with carbon and glass FRP composite sheets subjected to freeze-thaw conditions. The wrapped concrete specimens were conditioned in two different environments: room temperature (23 deg C) and 300 freeze-thaw cycles. The stress-strain behavior in compression of the freeze-thaw exposed specimens was obtained to evaluate their strength, stiffness, and ductility, which were then compared with the performance of unconditioned samples (room temperature). Two sets of tests were conducted using the exposed specimens. In the first set, in which the entire length of the samples was subjected to compression, failure occurred at the top and bottom. It was hypothesized that failure was due to the deterioration of concrete that was exposed rather than failure of the wrapping system. Therefore, the top and bottom parts (75 mm each) were sawed off and the tests were repeated using the shorter specimens. The shorter specimens retained 95% of the strength for the carbon composite wraps and approximately 88% of the strength of the glass composite wraps. Strength values of the wrapped systems before and after exposure were predicted using two different damage factors: one for glass fiber and one for carbon fiber. The predicted strength values compared satisfactorily with the experimental values.

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.

Analysis of Reinforced Concrete Beams Strengthened with Composites Subjected to Fatigue Loading

Synopsis: Use of high strength composites for repair and rehabilitation of bridges and parking decks is steadily increasing. Since these structures are subjected to fatigue loading, the performance of strengthened beams under this type of loading needs to be evaluated. An analytical procedure that incorporates cyclic creep of concrete and degradation of flexural stiffness is presented. The method is verified by computing cycle dependent deflections and comparing them with experimental results. The results presented in this paper also provide a summary of an experimental investigation, in which reinforced concrete beams were strengthened with glass fabrics (sheets) and subjected to fatigue loading. The comparison shows that the analytical model provides reasonably accurate prediction of deflections, for both reinforced beams and reinforced concrete beams reinforced with composites. Although Glass fiber composites were used for the evaluation, the model is also applicable to other types of fibers.