Crack injecting systems

Structural adhesives General-purpose adhesives Crack injecting systems Anchoring systems Grouts... 

Cracks occur in concrete structures for several reasons (ACTI, 1993a Concrete Society, 1982). The purpose of crack injecting systems is to fill in the cracks. Epoxy systems are a popular choice for non-moving cracks. In cracks where some movement is anticipated, polyurethane systems are used (Table 1.3). 

Whereas epoxy crack injecting systems are applied in static cracks, polyurethane systems are suitable for cracks where some movement is anticipated. There are other applications where polymeric materials are considered as competitors to each other. For example, neutral cure silicone sealants and polyurethane sealants are both suitable for use in construction joints. The former type offers greater durability and ageing resistance. However, polyurethane sealants are widely used because of their cost-effectiveness. Furthermore, some products are preferred over their counterparts for environmental reasons. 

Polysulphides. Polysulphides are used as curing agents for epoxies in the presence of tertiary amines which accelerate the cure such cured systems exhibit good flexibility and tensile strength at ambient temperature. In a real sense they are flexibihsers for epoxy systems. Specific applications are for the crack injection system (for moving cracks) and coatings (Chapter 10). 

Crack injecting systems Excellent adhesion to concrete in dry or damp conditions Ability to cure at low temperatures near freezing point Mechanical strength Static cracks in concrete structures... 

Epoxy crack injection systems are unfilled compositions, based on low-viscosity liquid epoxy resins and low-viscosity moisture-tolerant hardeners. The low viscosity of the system aids penetration into fine cracks and achieves the right balance of gel time (Anchor Chemicals/Air Products, undated a). Cracks as narrow as 0.05 mm can be filled with epoxy injection systems (CCA/, 1985). 

Leak sealer or crack-injecting systems, for cracks where movement is anticipated ... 

THERMOSETTING POLYMERS IN CONSTRUCTION APPLICATIONS 4.10 Polyurethane leak sealers and crack-injection systems... 

It has been mentioned in section 3.5 on epoxy crack injection systems that epoxies do not do well in wet conditions because they resist movement. Polyurethane resins are suitable for injection of cracks in concrete or masonry in dry or damp conditions. They can be successfully applied in cracks having a width in excess of 0.2 mm. On curing they form an elastic seal impermeable to water. 

Application of the product is made by standard resin-injection equipment, having a minimum injection pressure of 0.4N/mm (4 bar). The method of application is almost the same as that of an epoxy crack-injection system. 

Acrylic concretes and mortars are used in applications which require fast curing. Acrylic crack-injecting systems are suitable for wet cracks. Products such as adhesives, asphalt modifiers and floor-care polymers find only a small percentage of applications. 

An efficient feed injection system produces extremely small droplets that vaporize quickly. Rapid vaporization minimizes the amount of non-vaporized hydrocarbons that block the active sites. An effective feed nozzle system must instantaneously vaporize and crack asphaltenes and polynuclear aromatics to lower boiling entities. 

Consider the example in Fig. 26 of determining if there is a problem with the feed injection system of a fluidized catalytic cracking unit (Ramesh et al., 1992). In this example, there is a set of rules that relate combinations of process observations to establish or reject this possibility. 

In the DCC unit, the hydrocarbon feed is dispersed with steam and cracked using a hot solid catalyst in a riser, and enters a fluidized bed reactor. A known injection system is employed to achieve the desired temperature and catalyst-to-oil contacting. This maximizes the selective catalytic reactions. The vaporized oil and catalyst flow up the riser to the reactor where the reaction conditions can be varied to complete the cracking process. The cyclones that are located in the top of the reactor effect the separation of the catalyst and the hydrocarbon vapor products. The steam and reaction products are discharged from the reactor vapor line and enter the main fractionator where further processing ensure the separation of the stream into valuable products. 

Crack repair restores structural integrity, seals the cracks against moisture and restores appearance. Epoxy injection systems are employed to fill, fine or coarse cracks in horizontal or vertical surfaces (Figure 3.3) conventional equipment can be used. The benefits of epoxy injection systems are ... 

Residual fuel oils and heavy marine fuels are composed of high-boiling-petroleum fractions, gas oils and cracked components. Residual and clarified oil streams from the FCC process can contain degraded alumina/silica catalyst fines. These 20- to 70-micron-diameter fines are known to contribute to a variety of problems in fuel injection and combustion systems. In marine engines, excessive injector pump wear, piston ring wear, and cylinder wall wear can all be due to the abrasive action of catalyst fines on these fuel system parts. 

---