Examining the shear strength of concrete surfaces with adhesive bonding

Dissertation for Master Degree

Structural Civil Engineering

Abstract

Good bond strength between the old and new concrete surface is a key factor in the performance of concrete repairs. The purpose of this thesis is to investigate and evaluate the bond strength between the surface of the old concrete and the added concrete. In this thesis, a laboratory research was conducted to measure the shear strength of concrete joint surfaces that were concreted at different times. These surfaces were prepared by sandblasting, scraping with a metal brush, drilling by a drill, and smoothing. To connect these surfaces, Latex, Epoxy IV and Epoxy V adhesives available in the domestic market of the country were prepared and used from the companies of concrete designers, namikaran, abadgaran and Khatam Shimi concrete. Also, for comparison with the normal state, an experiment was done without using glue and only by pouring fresh concrete directly on the old surfaces. In addition to that, concrete with resistances of 20, 30 and 40 MPa were used to investigate the effect of concrete strength on bond strength.

ASTM C 882 and ASTM C 1042 regulations were used to conduct the test. The results of the tests showed that increasing the strength of concrete in the presence of a bonding agent will increase the bonding strength with a gentle slope, while in the case of direct bonding, it will have a significant effect on the bonding strength. Also, increasing the surface roughness in the presence of the bonding agent did not cause a noticeable change in the bonding strength, but in the absence of the bonding agent, sandblasting can provide the best conditions for surface preparation. In general, the use of appropriate adhesive agents are more economical and have better results than expensive and troublesome preparations.

Key words: friction cutting, slant shear, concrete repair, concrete surface roughness

Chapter one:

General

Introduction

Adding fresh concrete to the layer Old concrete is a common way to repair or strengthen a structure. Concrete repair involves removing weak concrete and replacing it with new concrete, and one of the most important factors in this process is the existence of good bond strength between the added concrete and the old surface during the useful life of the structure. When the repair operation is carried out, many factors such as surface roughness, the presence of small cracks, concrete density and its workability, as well as differences in the characteristics of materials such as modulus of elasticity, thermal movements and creep are effective in resistance and stress distribution. This system can be considered to include three phases: initial concrete surface, added concrete and bonding medium. The connection environment means the connection plate and its surroundings. This environment must have the ability to cope with the incoming tensions. This environment is usually by adding an adhesive agent or increasing the roughness and sometimes both of them. However, these methods are experimental and the effectiveness of the adhesive agent has not yet been proven. From the result of this study, designers can choose the strength of the concrete used for connection, the type of roughness created and the design resistance for an economical design. Epoxy resins are processed by generating internal heat. These systems include two parts, resin and hardener, which are activated and hardened after mixing with each other.

Epoxy resins can be used on surfaces such as styrofoam, red wood, hardwood, some plastic surfaces, and metal and concrete surfaces. Epoxy resins have good bending, shear and tensile strength, have very low water absorption and very high hardness, and have a setting time between 5 and 7 days.

Epoxy resins are low molecular weight prepolymers or high molecular weight polymers that usually have at least 2 separate parts (Figure 1-1) that must be combined and then used. These two parts are usually glycidyl[1] or oxirane[2]. A large part of epoxies are produced in industries and their raw materials are obtained from petroleum derivatives. As with other heat-curable polymer materials, it is common to combine different grades of epoxy resins or add additives, plastic materials, or fillers to achieve a desired process or end result or to reduce production cost. This manipulation operation is also called in the formula, which is commonly done in epoxy production factories in Iran.

1-2-1-1- types of epoxy [12]

Type 1) for non-load-bearing cases to connect hardened concrete to hardened concrete or other materials

Type 2) for non-load-bearing cases to connect fresh concrete to hardened concrete

Type 3) to connect anti-slip materials to hardened concrete and as a connector in epoxy concrete[4] that is in  (heat or mechanical movements) are used.

Type 4) for load-bearing cases to connect hardened concrete to hardened concrete or other materials

Type 5) for load-bearing cases to connect fresh concrete to hardened concrete

Type 6) to connect and cover prefabricated or prestressed sections and to connect pieces when temporary pre-tensioning is done.

Type 7) for non-load-bearing covering Prefabricated element parts when temporary retraction is not applied in the part-to-part connection.

1-2-1-2-grade[5]

We have three different grades according to the flow characteristics of epoxies:

Low viscosity (1 grade

Medium viscosity (2 grade

Non-flow [6] (3 grade

1-2-1-3- Epoxy classes

Epoxy classes specify the appropriate temperature for the adhesive action.

Class A) for use at a temperature below 4 degrees (the lowest possible temperature for epoxy operation)*

A: T < 4        

Class B) for use between a temperature of 4 to 15 Grade B: 4 < T < 15 Class C) for use at temperatures over 15 degrees C: T > 15 Class D) for temperatures between 4 and 18 degrees D: 4 < T < 18 Class E) for temperatures between 15 and 30 degrees E: 15 < T < 30

Class F) for temperature between 25 and 30 degrees F: 25 < T < 30          

*The indicated temperature is related to the desired concrete surface temperature, not the ambient temperature. For example, class A glue sticks well at room temperature. 1-2-2-latex [7]

Latex can be both natural and synthetic. Natural latex is a milky fluid (Figures 2-1 and 2-2) that is present in 10% of all flowering plants. It is a mixture of protein emulsion, pseudoalkali, starch, sugar, oil, resin, mazde ink, and gum that hardens on exposure to air and is usually secreted by the destruction of the plant shell. Synthetic latex is created by polymerizing a monomer such as styrene that is emulsified with surface-active substances[8].

2-1- Types of latex[15][

Type 1) Dispersible[9]- Limited for internal use and unusable in wet conditions

Type 2) Undispersible[10]- Usable in wet conditions

1-2-2-2- Adhesive selection criteria

1) Conditions when applying adhesive

Surface contamination - Contact surface temperature - Surface moisture - Surface access

2) Load type and magnitude

Direction (pressure, cutting tension, image change), duration, rate (static, dynamic) - Load frequency

1-3- Repairing structures Concrete

- The old method of repair was to remove the old concrete and replace it if there were cracks or defects. But today there are many ways to repair. The term concrete repair includes any reconstruction, maintenance and replacement. Regular inspections are an important factor in maintaining concrete and it is more economical from an economic point of view. 1-3-1- Types of concrete repair Coverage issues happen. Simple tools are usually used in this situation. Crack repair is done through the following process.

1) Cleaning the surface using detergent, if the surface is oily.