Mechanical Properties of Polymer Concrete

Polymer concrete was introduced in the late 1950s and became well known in the 1970s for its use in repair, thin overlays and floors, and precast components. Because of its properties like high compressive strength, fast curing, high specific strength, and resistance to chemical attacks polymer concrete has found application in very specialized domains. Simultaneously these materials have been used inmachine construction also where the vibration damping property of polymer concrete has been exploited.This review deals with the efforts of various researchers in selection of ingredients, processing parameters, curing conditions, and their effects on the mechanical properties of the resulting material.


Introduction
Polymer concrete is a composite material which results from polymerization of a monomer/aggregate mixture. The polymerized monomer acts as binder for the aggregates and the resulting composite is called "Concrete. " The developments in the field of polymer concrete date back to the late 1950s when these materials were developed as replacement of cement concrete in some specific applications. Early usage of polymer concrete has been reported for building cladding and so forth. Later on because of rapid curing, excellent bond to cement concrete and steel reinforcement, high strength, and durability, it was extensively used as repair material [1]. Precast polymer concrete has been used to produce a variety of products like acid tanks, manholes, drains, highway median barriers, and so forth.
The properties of polymer concrete differ greatly depending on the conditions of preparation. For a given type of polymer concrete, the properties are dependent upon binder content, aggregate size distribution, nature and content of the microfiller, curing conditions, and so forth [2]. The most commonly used resins for polymer concrete are unsaturated polyester resin, methyl methacrylate, epoxy resins, furan resins, polyurethane resins, and urea formaldehyde resin [3]. Generally, more than 75-80% volume in polymer concrete is occupied by the aggregates and fillers. The aggregates are normally taken as inert materials dispersed throughout the polymer matrix. Normally aggregates are added in two size groups, that is, coarse aggregates comprising material of more than 5 mm size and fine aggregates having size less than 5 mm. The grading of aggregates in the case of polymer concrete is nonstandardized till date and varies widely from system to system. In addition to the coarse and fine aggregates, microfillers are also added sometimes to the polymer concrete system mainly with an aim to fill the microvoids. Similar to the conventional concrete, polymer concrete can also be reinforced for improving its mechanical properties with different kinds of fibres. The use of steel, glass, polypropylene, and nylon fibres has been reported in the literature.
The importance of research on polymer concrete materials has been recognized as early as in 1971 with the setting up of ACI Committee 548-Polymers in Concrete. The Committee has been responsible for developing a large database on properties of polymer concrete. The Committee has also issued state-of-the-art reports and user guides on polymer concrete. RILEM (International Union of Testing and Research Laboratories for Materials & Structures) with setting up of Technical Committee TC-105-CPC (Concrete Polymer Composites) and TC-113-CPT (Test Methods for Concrete Polymer Composites) has been instrumental in 2 Journal of Composites preparing various test methods for these materials. Society of Material Science Japan (JSMS) has also contributed towards the development of polymer concrete materials with the help of Synthetic-Resins-for-Concrete Committee. Society of Material Science Japan has also published design recommendations for polyester concrete structures as well as a mix design guide. Amongst the countries which are using polymer concrete composites, the standardization work on various test methods and applications has been taken up mainly by Japan, United States, United Kingdom, Germany, and erstwhile Soviet Union.
Owing to their superior properties like rapid curing, high compressive strength, high specific stiffness and strength, resistance to chemicals and corrosion, ability to form complex shapes, excellent vibration damping properties, and so forth, polymer concrete materials have also been extensively used for applications other than for which these were originally developed. Use of polymer concrete has been reported in electrical insulation systems [4,5] as well as machine tool applications since late 70s wherein these have been used to replace traditional materials like cast iron for machine tool bases [6][7][8][9][10][11][12][13][14]. Lot of research has been carried out in last few decades to develop promising applications of polymer concrete, that is, its use in machine tool structures [15][16][17][18][19][20][21][22]. However, before the potential of these materials as an alternative material can be fully harnessed, a methodology for assessment of the long term properties must be available.

Factors Affecting Properties of Polymer Concrete
Polymer concrete is prepared by mixing a polymeric resin with aggregate mixture. Microfillers are also employed sometimes to fill the voids contained in the aggregate mixture. Polymeric resins that are commonly used in polymer concrete are methacrylate, polyester resin, epoxy resin, vinylester resin, and furan resins. Unsaturated polyester resins are the most commonly used resin systems for polymer concrete because of their low cost, easy availability, and good mechanical properties [23]. Furan resins are also used to a great extent in European countries. MMA has got a limited application because of its higher flammability and disagreeable odour; however, it has received some attention because of its good workability and low temperature curability [3]. The choice of particular type of resin depends upon factors like cost, desired properties, and chemical/weather resistance required. Epoxy resins are preferred over polyester because of their better mechanical properties as well as better durability when subjected to harsh environmental factors, but higher cost is a deterrent in their widespread acceptance. A comparative study on the properties of epoxy and polymer concrete states that traditionally epoxy concrete has better properties than polyester concrete, but the properties of polyester concrete can be enhanced up to the same level by addition of microfillers and silane coupling agents [24].
The resin dosage reported by various authors mostly lie in the range of 10 to 20% by weight of polymer concrete. Early studies on polyester resin concrete while taking resin content as a variable reported that compressive strength of polymer concrete is dependent upon the resin content [25]. Both the compressive strength and flexural strength increase with the increase in polymer content. After reaching the peak these either decrease or remain unchanged with further increase in the resin content. The lowest polymer content at which the properties are maximum will represent the optimum resin content for the system under study. It is observed that both flexural and compressive strength attain the maximum value between 14 and 16% resin content by weight. Further studies in this area have also provided similar results. Variation of compressive strength of polymer concrete for various types of resins and their dosage has been reported in the literature [26]. It was observed that the highest strength was obtained in all types of resins at a resin dosage of 12%. For two types of epoxy resins, the strength decreased by increasing the resin content to 15%, whereas, for polyester resin, it almost remained constant. The optimum resin content for a particular polymer concrete system is also dependent upon the nature of aggregate used in the system. Higher resin dosage is recommended when using fine aggregate, because of the large surface area of these materials [27][28][29].
Various types of aggregate materials have been used by the researchers, most of these based upon the choice of locally available materials to reduce the cost. River sand [30,31], foundry sand [27,32,33], crushed stone [34,35], quartz, granite [36][37][38], and gravel are some of the materials reported by various authors.
A large number of studies have been reported regarding the effect of reinforcement of polymer concrete by addition of various types of fibers. Steel fibers, glass fibers, carbon fibres, and polyester fibres have been added in polymer concrete in varying quantities for enhancement of its properties. Most of the studies have reported the addition of glass fibres in the range of 0 to 6% by weight of polymer concrete. It has been reported that addition of glass fibres improves the postpeak behaviour of polymer concrete. The strength and toughness of polymer concrete also increase with addition of fibres. Few studies on silane treatment of glass fibres before their use in polymer concrete report an enhancement in mechanical properties up to the extent of 25% [39]. Table 1 provides the details of the various types of reinforcements and their effect on the properties of polymer concrete as reported by various researchers.
A microfiller is also often added to polymer concrete mix to reduce the void content in aggregate mixture and thereby increase the strength of polymer concrete. The microfiller is a fine powder with a particle size less than 80 microns. Use of calcium carbonate, fly ash, and silica fume has been reported in literature. Fly ash is a by the product of the coal burning in power plants and is used as a filler because of its easy availability and because its usage in polymer concrete is reported to yield better mechanical properties as well as reduced water absorption [37]. Addition of fly ash also improves the workability of fresh polymer concrete mix resulting in products with excellent surface finish [40]. Studies have shown that small size of spherical particles also contributes to a better packing of the aggregate materials which reduces porosity and hinders the penetration of  Journal of Composites aggressive agents, thus considerably improving the chemical resistance of polymer concrete [23]. Addition of fly ash has been reported by a number of researchers which not only results in improvement in the workability of the polymer concrete mix but also has a significant effect on the mechanical properties. Enhancement in compressive strength up to 30% has been reported by addition of 15% fly ash in polymer concrete [41]. Addition of fly ash is also reported to have better performance enhancement when compared to addition of silica fume as a filler [42]. Heat assisted drying of the aggregates before mixing with resin has been suggested by most of the researchers. It has been reported that water content of the aggregate has a remarkable influence on the strength of polymer concrete and therefore the water content shall be limited to 0.1% [30]. It has been recommended by various researchers later on that the moisture content of the aggregate shall not exceed from 0.1% to 0.5% for better mechanical properties [41,[43][44][45]. Various curing regimes have been reported by researchers like room temperature curing, high temperature curing, water curing, and so forth. Curing time studies on polymer concrete have established that it achieves around 70-75% of its strength after a curing of one day at room temperature [31,45,46], whereas normal Portland cement concrete usually achieves about 20% of its 28-day strength in one day. The early strength gain is important in precast applications because it permits the structures to resist higher stresses early due to form-stripping, handling, transportation, and erection operations. It is observed that compressive strength of polymer concrete almost becomes constant after dry curing for a period of 7 days [47].
The influence of the aggregate grading on the properties of polymer concrete has been long known. The coarse and fine aggregate should be proportioned in such a way that aggregate mixture has minimum void content and maximum bulk density. This minimizes the amount of binder required to assure proper bonding of all the aggregate particles. Normally, the binder content ranges from 5% to 15% of the total weight but if the aggregate mix is fine, it may even require up to 20% binder. Very few studies have been reported in the literature regarding the proportioning of the aggregate mix in polymer concrete. Earlier studies in this regard have reported that polymer concrete made with aggregate grading according to Fuller's curve had the highest strength [30,48]. Further it was reported that use of gap graded aggregate resulted in minimum void content. An empirical relation has also been suggested in the literature, which can be used to determine the proportions of coarse and fine aggregates of least-void content [49]. Later studies suggest the optimum mix composition of aggregate for minimising the void content based on design of experiments approach [50]. The mix composition suggested was again based upon the use of gap graded aggregates.
Since, for the cost considerations, the binder content used in polymer concrete materials is quite low, the adhesion of aggregates takes place through a fine layer of resin around the aggregates. A larger contact area is, therefore, desirable which necessitates a proper space filling of the gaps by smaller aggregates or microfiller particles. Use of a silane coupling agent (which strengthens the adhesion between the resin and the aggregates) improves the adhesion and thus the ultimate strength of the polymer concrete. Adhesion at the interface, in absence of any chemical bonding, may be sufficiently good even when it is due to secondary forces between two phases. The use of silane coupling agents, which may provide chemical bonding between the two phases, considerably improves the interfacial adhesion and therefore enhances the mechanical properties of these materials. A few studies on the use of various types of silane coupling agents have been reported in the literature. Various methods of application of silane agents like integral blend method and surface treatment method have been compared [24,51,52]. It has been reported that when using integral blend method of silane addition, 1% silane by the weight of resin gives optimum results [53,54]. Compressive strength and flexural strength of polymer concrete containing silane coupling agents are 15 to 20% higher than those of normal polymer concrete [53].

Characterization of Mechanical Properties of Polymer
Concrete. There have been a lot of studies reported on characterization of mechanical properties of polymer concrete since early 1970s. Table 2 summarizes the efforts of various authors and the major conclusions drawn based upon these studies.

Fatigue Studies on Polymer Concrete (PC).
Studies on fatigue behaviour of polymer concrete are very scarce in the literature. The two million cycle fatigue endurance limit has been reported as a stress level of 59%, very similar to that of cement concrete [68]. A study to evaluate the effect of frequency of testing concluded that frequency of testing shall be taken as a parameter for fatigue testing of polymer concrete. Fatigue behaviour of polymer concrete has been described based upon S-N relationships. These relationships are based upon the basic power law functions. The research has shown that the empirical equations used to predict fatigue behaviour of plain concrete fit well for polymer concrete also [69]. Equation (1) as described for cement concrete was applied to fatigue data of polymer concrete [70]: where is the probability of survival, is the stress level, is number of cycles to failure, and , , and are experimental constants. Failure probability has been incorporated in the S-N relationships for polymer concrete to take care of the stochastic nature of fatigue [71].

Discussion
Polymer concrete has initially been developed as an alternative material in the domain of civil engineering but over a period of time, owing to its superior properties, has found favour as a replacement material in machine building applications. Rapid curing, high compressive strength, high specific stiffness and strength, resistance to chemicals and corrosion, ability to mould into complex shapes, and excellent vibration damping properties are mainly responsible for its use in these applications. It has been observed that the properties of polymer concrete depend upon various parameters like type and amount of resin/polymer used, type and mix proportioning of aggregate, moisture content of aggregate, nature and content of reinforcing fibers, addition of microfillers, curing conditions, use of silane coupling agents, and so forth.
Epoxy resins provide better mechanical and durability properties than polyester, vinylester, furan, and methacrylate resins, but there is inherent high cost associated with these materials. The properties of polyester concrete can also be enhanced to the level of epoxy concrete by addition of microfillers and silane coupling agents. The resin dosage reported by various authors mostly lies in the range of 10 to 20% by weight of polymer concrete. Higher resin dosage is recommended when using fine aggregate, because of the large surface area of these materials. The studies to find the optimum resin dosage for maximizing the mechanical properties have yielded different results depending upon the specific type of resin and aggregate used. It is observed that initially strength increases with increase in resin dosage, but, after reaching the peak, the same either decreases or remains unchanged with further increase in the resin content. Most of the researchers have reported maximum strength for resin dosage in the range of 12-16% by weight of polymer concrete.
Addition of various types of fibers like glass fibers, steel fibers, and carbon fibers in polymer concrete enhances its mechanical properties such as toughness, compressive strength, flexural strength, and fatigue strength. The usual range of fiber addition in polymer concrete is up to 6% by weight of polymer concrete. It was observed that silane treatment of fibers before addition into polymer concrete further enhances its mechanical properties. Addition of microfillers like fly ash, silica fume, calcium carbonate, and so forth in polymer concrete has been reported not only to enhance the mechanical properties but also to improve the workability of mix. Enhancement in compressive strength up to 30% has been reported with addition of 15% fly ash in polymer concrete.
Various types of aggregate materials have been used by the researchers, most of them based upon the choice of locally available materials to reduce the cost. The use of river sand, foundry sand, crushed stone, quartz, granite, and gravel has been reported. Till date no standard mix proportion and aggregate grading criterion are available for polymer concrete and, therefore, a number of optimized mix proportions are reported in the literature. These mixes are based upon various optimization criteria like Fuller's curve, and maximum bulk density, and minimum void content and have been developed for various types of locally available aggregates. Almost all the studies are in agreement that use of gap graded aggregate results in better mechanical properties. A few empirical relations are provided in the literature to determine the proportion of coarse and fine aggregates for obtaining least void content, but their application in various other aggregate types is still to be evaluated. It is recommended that aggregate mix having maximum bulk density and having least void content shall be used along with optimum polymer content for achieving maximum strength. Moisture content in the aggregate has a deleterious effect on 6 Journal of Composites    the mechanical properties of polymer concrete and, therefore, it is recommended that moisture content in aggregate shall not exceed 0.5%. Curing conditions play an important role in the final properties of polymer concrete. For field use and ease of operation, room temperature curing is desirable and advantageous. Fast curing is one of the biggest advantages of the polymer concrete systems, with results showing almost 70% strength development after one day of curing at room temperature. Normal Portland cement concrete, on the other hand, usually achieves about 20% of its 28-day strength in one day. This early strength development is very useful in precast applications of polymer concrete. Although curing at elevated temperatures is observed to accelerate the strength development, it is almost universally accepted that 7-day room temperature curing is optimum period for polymer concrete.
In addition to the above parameters, adhesion at binder aggregate interface also has an influence on the properties of polymer concrete. Adhesion at the interface, in absence of any chemical bonding, may be sufficiently good even when it is due to secondary forces between two phases. Silane coupling agents by providing chemical bonding between the two phases considerably improve the interfacial adhesion and therefore enhance the mechanical properties of these materials. From the research available till date, it can be concluded that integral blend method of adding the silane agent in the polymer concrete mix is easy to implement and yields better mechanical properties. The compressive strength and flexural strength of polymer concrete containing silane coupling agents are 15 to 20% higher than those of normal polymer concrete.
Polymer concrete displays higher compressive strength and flexural strength when compared to Portland cement concrete. Compressive strength ranging from 70 to 120 MPa has been reported by various authors. The discussion in the preceding paragraphs states the governing parameters for the mechanical properties of any particular polymer concrete system and thus explains the large variation in the properties reported.
The study of fatigue behaviour of any material is of immense importance if the same has to be utilised for structures, machine tool parts, and so forth, wherein the cyclic loading is predominant. Unfortunately, fatigue behaviour of polymer concrete has not been studied to a great extent and there have been a few studies in this context and the same has been reported in this paper.

Concluding Remarks
Research on characterization of mechanical properties of polymer concrete has been carried out by number of researchers and sufficient data has been generated regarding the effect of various parameters like resin type and content, fiber reinforcements, microfillers, curing conditions, aggregate type and grading, and silane coupling agents on the properties of polymer concrete. Based on the critical review of the available literature on polymer concrete, the following conclusions can be drawn.
(1) Comparative studies between epoxy and polyester resins report that epoxy polymer concrete has far superior mechanical properties and durability.
(2) Various types of aggregate materials have been used by the researchers most of them based upon the choice of locally available materials to reduce the cost.
(3) The resin dosage reported by various authors mostly lies in the range of 10 to 20% by weight of polymer concrete. Higher resin dosage is recommended when using fine aggregate.
(4) It has been reported that addition of glass fibers improves the post peak behaviour of polymer concrete. The strength and toughness of polymer concrete also increase with addition of fibers.
(5) Seven-day room temperature curing criterion has found widespread usage by researchers in their research work and has been almost universally accepted.
(6) Enhancement in compressive strength up to 30% has been reported for addition of 15% fly ash (microfiller) in polymer concrete.
(7) It has been recommended that the moisture content of the aggregate shall not exceed 0.5% for better mechanical properties.
(8) It is recommended that aggregate mix having maximum bulk density and having least void content shall be used along with optimum polymer content for achieving maximum strength.
(9) Use of silane coupling agents further enhances the mechanical properties of polymer concrete.
It is well known that polymer concrete exhibits far better mechanical properties and durability than ordinary Portland cement concrete. Polymer concrete has proven itself to be a material which holds much promise due to its better mechanical properties and durability. It would be in the interest of polymer concrete industry/researchers if the material is categorised and promoted as a polymer composite.