Epoxy Resins as Adhesives for Prestressed Concrete

Transcription

1 PROCEEDINGS PAPER Epoxy Resins as Adhesives for Prestressed Concrete Presented at the Seventh Annual Convention of the Prestressed Concrete Institute by Harold G. Within the past few years some of the fastest growing applications for epoxy resins have involved concrete. These applications include repair, resurfacing, skidproofing, treatment for water and chemical resistance, and bonding of concrete to itself and other materials. One interesting discovery has been the use of an epoxy adhesive for bonding freshly placed concrete to old concrete. It appears that many connection problems of prestressed concrete structures can be solved with epoxy adhesives, and serious study along these lines is now under way. It is the purpose of this paper to discuss formulation, application techniques and properties of epoxy resin compositions which are of particular interest in concrete bonding applications. Epoxy resins are a relatively new "Jones-Dabney Company Division of Devoe & Raynolds Company Inc., Louisville, Ky. Cooke, Jr.* class of synthetic resins which contain within their structure more than one highly reactive group of atoms known as the epoxide group. A typical example is the diglycidyl ether of bisphenol which in various commercial forms is the most widely used epoxy resin for solvent-free applications. When an epoxy resin is mixed with a material containing two or more groups which react with the epoxy group, a chemical reaction takes place and an insoluble, infusible product is formed. The material that reacts with the epoxy resin is called a curing agent which, depending on its degree of reactivity, may cure the resin at room temperature or at some elevated temperature. The ratio in which the resin and curing agent are mixed is dependent on the epoxide content (epoxide equivalent) of the resin and the reactive group content Fig. 1-Typical example of diglycidyl ether of bisphenol structure. 24 PCI Journal

2 (equivalent weight) of the curing agent. An excessive concentration of some curing agents will have a plasticizing effect on the cured resin, and this method is sometimes used to impart flexibility to the cured resin. A deficiency of curing agent will result in an incomplete cure and will usually yield a brittle product. A number of factors should be considered in the formulation of epoxy resin compositions for concrete applications. Viscosity and thixotropy can be regulated to give the desired imparting flexibility and reactive diluents can be used to reduce viscosity. Inert fillers are used not only to reduce the cost of the formulation but also to modify the properties of both the cured and uncured composition. Since the use of fillers will prolong pot life, reduce exothermic properties, reduce cure shrinkage and coefficient of thermal expansion, and increase thermal conductivity, the success of an epoxy formulation may well depend on an intelligent use of fillers. A listing of liquid epoxy resins covering a broad viscosity range and TABLE I.-TYPICAL PROPERTIES O:f EPOXY RESINS Epoxy Resin (Epi-Rez 510)~ (Epi-Rez 5077) (Epi-Rex 504) Viscosity (cps. at 77o F.) 12, Epoxide Equivalent Remarks General purpose Medium viscosity Low viscosity unmodified epoxy epoxy resin epoxy resin resin TABLE 11.-TYPICAL PROPERTIES OF ROOM TEMPERATURE CURING AGENTS Curing Agent (Epi-Cure 87)~ (Epi-Cure 872) (Epi-Cure 855) Viscosity (cps. at 77 F.) 4, Equivalent Weight Color Remarks Very reactive Intermediate Moderate Short pot life reactivity reactivity Long pot life handling properties; curing rate can be adjusted within fairly wide limits to meet required gel times or curing schedules; and flexibility and strength can be regulated to meet the needs of specific applications. The curing rate and properties of the cured resin will depend largely on the selection of resin and curing agent. Modifiers are available for "Registrated U.S. Patent Office April, 1962 room temperature curing agents covering a range of reactivities are shown in Tables I and II. These are commercially available materials which have shown considerable promise in concrete applications. As indicated in Table II, the rate at which a system will cure at room temperature can be varied over a fairly wide range by selection of the curing agent. The short pot life of 25

3 ~~- TABLE 111.-TYPICAL PROPERTIES OF ROOM TEMPERATURE CURED EPOXY RESIN SYSTEMS Resin System A B c Composition (Parts by Weight) Epoxy Resin A (Epi-Rez 510) I 100 Curing Agent A (Epi-Cure 872) Curing Agent C (Epi-Cure 855) Original Viscosity (cps. at 77o F.) 4,000 1,800 1,000 Pot Life lib. at room temperature) 20 minutes I% hours ly4 hours Properties after Several Days at Room Temperature Hardness (Shore D) 86 I Tensile Strength (psi) 9,000 8,300 1,850 Percent Elongation Initial Modulus 0.44 X X X 10 6 Flexure Strength (psi) 16,000 12,900 - Compressive Yield (psi) 13,200 10,500 - Izod Impact (ft. lbs./in. notch) Heat Distortion Temperature CF.) TABLE IV.-TYPICAL PROPERTIES OF SAND FILLED ROOM TEMPERATURE CURED EPOXY RESIN SYSTEMS Resin System Composition (Parts by Weight) Resin System from Table III 325 Mesh Silica A' B' C' 135 (A) 150 (B) 150 (B) 100 Asbestine 3X (Talc) 85-1-~ ~~~-;-~~ Crystal Silica Sand ,000 Properties after Two Weeks at 77o F. Tensile Strength (psi) 2,600 5,800 Flexural Strength (psi) 5, Flexural Modulus (psi) 2.3 X X 10 6 Compressive Strength (psi) 8,500 12,600 Unnotched Izod Impact (ft. lbs./in.) ,200 2, X 106 4, PCI Journal

4 the more reactive systems may present a handling problem, particularly in cases where the handling of fairly large volumes is necessary. Even though the moderately reactive curing agents will produce a satisfactory cure at temperatures above 60 F. in a reasonable length of time, it is often desirable to enhance the rate of cure by the application of heat. For example, a system that will reach its ultimate strength in two to four days at room temperature will cure to the same degree in twenty to thirty minutes at 200 F. The properties of the cured epoxy resin can be varied over a wide range as shown in Table III which shows some typical properties of room temperature cured systems exhibiting varying degrees of flexibility. For most applications involving concrete, inert fillers such as sand, silica flour, alumina, talc, asbestos fibers and others are used in conjunction with the resin and curing agent. The effect of high loadings of inert fillers on the physical properties of two of the systems shown in Table III are given in Table IV. A comparison of the properties shown in Tables III and IV show that the cured epoxy resins retain their high order of strength properties even when compounded with appreciable concentrations of fillers. Other outstanding properties of cured epoxy resins are as follows: 1 Adhesion The adhesion of cured epoxy resins to practically all substrates is excellent. Of particular interest from an adhesive standpoint is the bonding of old concrete to old concrete, and of freshly poured concrete to old concrete. The bond of a properly formulated adhesive is invariably stronger than the concrete itself. Epoxy resins are also being used to bond other materials such as wood and steel to concrete. 2. Low Cure Shrinkage Compared with other thermosetting resins, the shrinkage which takes place during the cure of an epoxy resin is quite low and by incorporating filler into the composition the shrinkage is even further reduced. A room-temperature-cured sand-filled epoxy composition similar to composition C of Table IV has a linear cure shrinkage of less than one mil per ten inches. This property is extremely important in bonding applications, for shrinkage during cure creates a strain on the adhesive which may break the adhesive bond or fracture the substrate. 3. Low Coefficient of Thermal Expansion The coefficient of thermal expansion of most unfilled epoxy compositions is in the vicinity of 80 X I0-6 BOND SHEAR STRENGTHS OF EPOXY BONDED SUBSTRATES ALUMINUM TO ALUMINUM COPPER TO COPPER STEEL TO STEEL CONCRETE TO CONCRETE CREOSOTED WOOD TO CREOSOTED WOOD CREOSOTED WOOD TO STEEL SHEAR STRENGTH (PSI) 2, ,739 FAILURE IN CONCRETE 1,610 1,744 April,

5 in./in./"c. By the incorporation of high concentrations of silica or alumina it is possible to reduce this value to less than 20 X 10-6 in./in./ C. which is close to that of concrete and most metals. The low coefficient of expansion is desirable to prevent failures resulting from normal temperature cycles. 4. Chemical Resistance Cured epoxy resins are particularly noted for their resistance to common solvents and chemicals, particularly those of an alkaline nature that are present in concrete. 5. Weatherability The exterior durability of cured epoxy resins as measured by retention of physical properties is outstanding. Long term testing over a period of several years has shown very little loss in adhesive strength to various metal substrates. 6. Ease of Formulation and Application In addition to the above properties, versatility and convenience of handling epoxy resin compositions are important factors for the present and projected growth of epoxy resins. For those who do not have the time or equipment to evaluate the many formulation variables, epoxy resin compositions are available from formulators throughout the country for many different types of applications. For applications where automation is indicated, a variety of automatic dispensing equipment is available. The use of epoxy resins in concrete applications is not a new development. As early as 1953 the Materials and Research Laboratory of the California State Division of Highways began to investigate the possibilities of using epoxy resins in the repair and maintenance of highways and concrete structures. For 28 the past eight years epoxy-based adhesives have been undergoing tests as bonding agents for concrete in highways, bridges, aircraft runways, buildings, streets and sidewalks. A review of some of these application studies is given in an article, "Epoxy Adhesives in Concrete Construction", in the November, 1959, issue of Civil Engineering. Most of these studies have been made with a flexible blend of an epoxy resin and a polysul de resin cured with a tertiary amine catalyst. After several years study on the use of epoxy resins for the repair of concrete structures, the U. S. Army Corps of Engineers in 1959 issued purchase descriptions for both filled and refilled flexible epoxy resin base adhesives. Instructions for the use of these materials were published in Technical Memorandum No by the Rigid Pavement Laboratory, U. S. Army Engineer Division, Ohio River Division Laboratories, Cincinnati 27, Ohio. An excellent report on test procedures and evaluation studies on three epoxy resin systems is given in General Report No. 28, "Epoxy Resins for Concrete Construction and Repair - Interim Report". This report, which is dated April17, 1961, is based on studies made at the Denver, Colorado, Concrete Laboratory Branch, Division of Engineering Laboratories, Bureau of Reclamation, U. S. Department of the Interior. The importance of concrete surface preparation to obtain maximum bonding strength is emphasized in all of these studies. A dilute muriatic acid etch, followed by thorough rinsing and drying is recommended for clean concrete. Concrete surfaces contaminated with oils or greases PCI Journal

6 should be given a detergent wash followed by the acid etch treatment. More recently the Research Department of the Association of American Railroads has initiated a study of the use of epoxy resins in applications of interest to the railroad industry. A report of the progress of this study is given in three articles in the June, 1961 issue of "Railway Track and Structures". Of particular interest at this time are applications concerning the use of epoxy resins with prestressed concrete. During 1960, under the sponsorship of the Association of American Railroads, two prestressed concrete railroad bridges were built with epoxy shear keys. The first was a 29 foot span on the Burlington Line in Rochelle, Illinois, where four prestressed concrete girders were bonded together with an epoxy formulation very similar to composition B in Table IV. Approximately thirty minutes prior to positioning the beams on the bridge abutments the inner faces of the key way cavities were brush coated with an unfilled formulation (Composition B of Table III). The purpose of the brush coat was to insure good wetting of the concrete surface. Pressure sensitive foam-rubber tape was applied as a gasket below the key way, the beams were placed in position and the epoxy grouting compound was poured into the cavity. A total of seventy two gallons of epoxy grout was used in this construction. In order to accelerate the cure of the epoxy formulation, each of the key ways was provided with a % inch copper tubing running the length of the beam and connected to a common manifold on one side. Live steam was passed through the tubes for thirty five minutes after the epoxy grout was poured into the key ways. Following gelation of the April, 1962 epoxy resin, the conventional tar composition waterproofing membrane was applied. The rock road bed and wooden ties were placed under the rails and spiked into position. In replacing this bridge the railroad line was closed for a period of six hours. The second bridge replacement was on the Rock Island line near Letts, Iowa. The new structure is comprised of two 33-ft. precast prestressed concrete spans with a ballasted deck. Each of the spans consists of four hollow-box prestressed concrete girders. The girder sections are 3 ft. wide by 2 ft. 9 in. deep and 33 ft. 2 in. long, cast with shear key slots in the adjacent sides. Since it appeared that only one span could be replaced per day because of train traffic, it was decided to pre-assemble each span including application of the shear keys. They could then be handled as complete units and both erected in one day. Before bonding the girders with the epoxy grout, they were waterproofed with two coats of an epoxy waterproofing formulation. The first coat was a low viscosity primer and the second coat corresponded to composition B of Table III with the addition of a small amount of colloidal silica to give the composition a slightly thixotronic character. The girders were placed on steel beams to insure that they would form a level plane and the epoxy grout was applied as described for the Burlington bridge. Electric heating cables were used in place of the steam line, although they were not necessary because sufficient time for curing was afforded by bonding the girders out of track. After complete curing of the epoxy grout the bonded spans (each weighing 50 tons) were loaded on flat cars and moved to the bridge site. The old bridge 29

7 Fig. 2-Piacement of a box girder on Rochelle, Illinois bridge. Fig. 3-Epoxy resin being poured in keyway. Fig. 4-Steam heating process used to accelerate curing of epoxy resin.

8 Fig. 5-Waterproofed girders. Note electric heating cables at right foreground. April, 1962 Fig. 6-Piacement of 50-ton pre-assembled girders. 31

9 spans were removed, the new spans installed and the ballast, ties and rail were replaced. Because the girders had been waterproofed with an epoxy composition the conventional membrane type waterproofing was omitted. In replacing these two spans the track was out of service for seven hours. After approximately one year of service no signs of failure have been reported on these structures. Mr. R. J. Williams, Rock Island's Assistant Engineer for Bridges, gives these reasons for using the epoxy grout. "First, it gives us a higher strength and better bond than any other material we now know. Also, it should be more durable than a cement-grout shear key. Finally, it is the only satisfactory material that can be used to bond prestressed concrete girders together in the track in so short a time". Several bonding applications to prestressed concrete track ties are being studied at the Research Center of the Association of American Railroads. Laboratory testing indicates that either tie plates or the rail itself can be securely bonded to the concrete tie with an epoxy adhesive. Test specimens have withstood over 4,000,000 cycles of loading which simulate track conditions without failure. At the conclusion of the test the samples were broken with a sledge and in every case the break occurred in the concrete. The bonding of inserts in the concrete tie has also been investigated. Although static load tests invariably led to failure in the concrete, long term loading of the inserts indicated some tendency of this particular epoxy adhesive to creep. This is a subject which will require considerably more attention, particularly in structures where the epoxy resin is the principal stress bearing medium. Waterproofing and repair of concrete structures with epoxy resins are also included in this study. Field tests to determine the effect of weathering and repeated loading on epoxy connections to prestressed concrete are now in progress. Another interesting application is underway at the University of Arizona where Professor Gene M. Nordby, working with the Arizona Highway Department, is investigating the feasibility of bonding concrete bridge decks to steel beams in composite construction. These projects indicate the tremendous potential of epoxy resins in various concrete applications, particularly in structupl bonding of prestressed concrete. 32 PCI Journal