Investigating the durability of reinforced concrete beams reinforced with GFRP under the alkaline reaction of aggregates

Master's thesis in the field of structural engineering

Alkaline reaction of aggregates[1] (AAR) is a chemical reaction that occurs in some concrete structures.  (AAR) is a reaction between alkaline fluids in the pores and silica aggregates. Concrete failure, including expansion and cracking due to the alkaline reaction of aggregates, is of particular importance. In addition, fiber-reinforced polymers in the form of plates or sheets can create many economic benefits in the construction industry. Recent advances in FRP indicate that they will play a huge role in building and repair applications in the future. In the last decade, FRPs have been widely used in the construction industry. The growing demand for the use of FRPs in concrete structures has created a great need to understand the short-term and long-term behavior of these composite materials under different environmental and loading conditions. Therefore, in this research, the durability of reinforced concrete beams reinforced with GFRP [2] under the alkaline reaction of aggregates has been investigated.

For this purpose, two types of concrete were made: one active concrete under the alkaline reaction of aggregates and the other passive concrete. A laboratory program includes the construction of 16 reinforced concrete beams, which are divided into eight groups. Also, in order to evaluate the effect of wrapping, two beams at the end of the sheet and near the supports were wrapped with a layer of GFRP sheet of 5 cm. In order to speed up the alkaline reaction, the alkalinity of active concrete reached 0.66% relative to the weight of cement to 1.75% alkalinity by adding sodium hydroxide to the mixing water. After 8 months, to evaluate the effect of GFRP in strengthening the beams and the effect of alkaline reaction of the aggregates, the bending strength and the midspan of the reinforced beams were compared with the control beams and the equivalent beams. The results showed that the bending strength of reinforced concrete beams increases by attaching GFRP plates to their tensile surface, and the alkaline reaction of aggregates reduces the bending strength of beams. Chapter 1 Generalities 1-1 Introduction In recent years, many advances have been made in the field of earthquake engineering and structural design. It has become such that today it is possible to design earthquake-resistant structures with more confidence. Many concrete structures are due to reasons 1- Calculation errors 2- Mistakes in construction and implementation 3- Weakness of old regulations 4- Change of use of the structure and operating loads on the structure 5- Corrosion and rusting of reinforcements and . . . does not satisfy the requirements of the new regulations, therefore strengthening the structures with fiber-reinforced polymers [1] (FRP) in the form of plates or sheets creates significant economic benefits in the construction industry. Recent developments in FRP indicate that in the future these materials will play a big role in construction applications and restoration of structures.

In the last decade, FRP has found many applications in civil engineering. The growing demand for the use of FRP in strengthening beams, columns, walls, slabs and concrete pipes has created a great need to understand the short-term and long-term behavior of the composite system under various loading conditions and environmental conditions. Composite materials may withstand a variety of service conditions, which are likely to include some truly aggressive conditions. For example, hot and humid weather, prolonged high temperature, sudden changes in ambient temperature and chemical corrosion can affect the durability of FRP. Adhesion and cohesion of composite materials may undergo environmental erosion and affect the bond between concrete and these materials. This may alter the performance and durability of the composite system. Another reason for such a lack of connection between composite and concrete is the temperature mismatch between the fibers and the matrix, which can create compressive stresses in the fibers. Another reason is the ability of composite materials to absorb moisture, which may affect the integrity between fibers and matrix.

Currently, the use of FRP in strengthening structures has increased significantly, while there is not enough information about the durability of FRP, one of which is the alkaline reaction of aggregates. In this research, polymer reinforced with glass fibers [2] (GFRP) of the E-glass [3] type, which is a good electrical insulator and has a relatively high mechanical resistance, and is cheap and affordable compared to other fibers, has been used..

Damage of concrete structures can happen as a result of the reaction between alkaline liquids in the holes (which mainly originate from Portland cement) and reactive minerals that are in some aggregates. This failure mechanism is known as aggregate alkaline reaction and occurs in different forms, the most common of which is the alkaline-silicate reaction. This reaction was first reported in 1940 (1319 solar years) in the United States[1]. Although damage due to this reaction has rarely been reported in our country, probably a large number of concrete structures in our country, including the Shahryar Dam located in East Azarbaijan province, where the aggregates were obtained from, are also affected by this reaction. This is when using cement with high alkalinity is more likely to cause this reaction. Therefore, it will be important to pay attention to this reaction.

The alkaline-silica reaction is the most common type of alkaline reaction of aggregates in the world and occurs when the reaction between the alkaline solution inside the cavities and the silica minerals in some aggregates occurs and forms an alkaline calcium silicate gel.  The aforementioned gel absorbs water and increases in volume, which results in concrete cracking. Another alkaline reaction of aggregates is the alkaline-carbonate reaction. This reaction occurs when cement alkalis react with dolomite limestone aggregates. The alkaline reaction of active aggregates takes place after concrete processing and causes the internal expansion and destruction of concrete, so the resistance of FRP and concrete to time and under adverse environmental conditions requires investigation and understanding of the factors affecting FRP when it is exposed to different environmental conditions. and engineering has been proposed. In this connection, depending on the type of structure and the purpose of retrofitting, various methods have been proposed by researchers. On the other hand, the alkaline reaction of active aggregates, which takes place after concrete processing and causes internal expansion and destruction of concrete, can cause damage and damage to concrete structures. One of the widely used and increasing methods for repairing and strengthening concrete members is the use of FRP sheets. Therefore, it is absolutely necessary to understand the short-term and long-term behavior of the composite system under various loading conditions and environmental conditions, and to recognize and detect the alkaline reaction in the members of reinforced concrete structures. Alkaline reaction of aggregates.

After carrying out alkaline reaction in the beams and strengthening them with GFRP, these beams are compared in terms of bearing the bending load with the beams in which alkaline reaction did not take place.

· Investigating the durability of reinforced concrete beams reinforced with GFRP sheets in alkaline solution.

The effect of a normal sodium hydroxide solution in carrying out alkaline reaction and the role of GFRP in improving bending resistance The beams are investigated.

· Investigating the effect of GFRP sheets on increasing the bending strength of reinforced concrete beams reinforced with these sheets.

The effect of GFRP sheets on improving the bending strength of reinforced concrete beams reinforced with these sheets is expressed in comparison with unreinforced beams. has been used Therefore, after collecting information about the strengthening of reinforced concrete beams with FRP and the durability of composite materials in different environments, as well as the effect of alkali addition on concrete and the description of the alkaline reaction of aggregates, the materials and materials used in this research are described, which are presented in the second and third chapters. Then, two series of reinforced concrete beams, one containing active sand and the other with inactive sand, were designed and built. Therefore, 16 reinforced concrete beams 400 x 80 x 80 mm were molded and concreted and after 28 days of processing, they were divided into eight groups and placed in two environmental conditions. These beams were kept in these environments for eight months and then tested under the single point bending test.

In the fourth chapter, the details of the construction and design of the samples and the testing process are fully presented. After carrying out the beam breaking load test and their corresponding displacement, the results were compared and subjected to further investigation.