In this article we will discuss about:- 1. Introduction to Polymer Impregnated Concrete 2. Process of Impregnation 3. Polymers Used for Impregnation 4. Uses of Polymer Impregnated Concrete 5. Properties of Polymer Impregnated Concrete.
Introduction to Polymer Impregnated Concrete:
Polymer impregnated concrete is one of the widely used polymers composite. It is nothing but a conventional pre-cast concrete, cured and dried in oven or by dielectric heating from which the air in open cells is removed by vacuum process.
Then a low viscosity liquid monomer or pre polymer partially or fully is impregnated or diffused into the pore system of the hardened cement composites or cement concrete and then polymerised using radiation or by the application of heat or by chemical initiation. The partial or surface impregnation improves the durability and chemical resistance, but the overall improvement in the structural properties is modest. On the other hand in depth or full impregnation improves structural properties considerably.
Hardened concrete, after a period of moist curing contains a considerable amount of free water in voids. The water filled voids form a significant component of the total volume of concrete ranging from 5% in dense concrete to 15% in gap graded concrete. In polymer impregnated concrete these water filled voids are filled with polymers. The air and moisture in voids affect the monomer loading.
Process of Impregnation:
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1. Concrete specimens of well-designed and adequately moist cured of optimum strength, are taken.
2. The moisture of the specimens is removed by drying the concrete by heating at a temperature of 120°C to 150°C. The small specimens can be heated in an air oven. For large cast in-situ surfaces a thick blanket of sand usually 10 mm thick can be used to check a steep thermal gradient. Infra-red heaters may also be used. To expel the large part of the free water in the concrete about 6 to 8 hour heating is required.
3. To avoid flammability, the concrete surface is cooled to safe levels (about 35°C).
4. The air from the dry concrete specimens is removed by vacuum process. The degree of vacuum and its duration have been-found to have a significant influence on the quantity of monomer that can be impregnated i.e. on the depth of impregnation.
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5. To achieve the desired depth of penetration of the monomer, the specimens can be soaked in the monomer. The soaking duration in the monomer depends on the viscosity of monomer, characteristics of concrete and preparation of specimens prior to soaking. The soaking duration for the desired depth of penetration can be reduced by exerting external pressure by the use of nitrogen gas. Generally air is used for this purpose.
6. To prevent the evaporation of the monomer, the surface should be covered with a plastic sheet.
7. Now the polymerization of the monomer is initiated. Polymerization can be effected by thermal catalytic technique or by ionizing radiation. For polymerization by thermal catalytic technique the catalyzed monomer is heated to a temperature between 60°C to 150°C depending upon the type of monomer. The heating can be done under water or by low pressure steam injection or by infra-red heaters or in an air oven. The duration of heating may vary from 2 to 6 hours depending upon the polymer used.
The heating decomposes the catalyst and initiates the polymerization reaction. This reaction is called a thermal catalytic reaction. When monomer has penetrated into the concrete, polymerization can also be initiated using ionizing radiation such as gamma rays.
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Fully polymerized or cross linked polymers become solids and occupy the full volume in which they have been impregnated. At the impregnation stage, the polymer has to be in a pre-polymer liquid form, which is generally called monomer. The state of polymerization of monomers or of pre-polymer resins is brought about also by adding initiators and cross linking agents.
Polymers Used for Impregnation:
Broadly following polymers are used for impregnation:
1. Thermoplastics:
Usually these polymers soften between 100°C to 150°C called glass transition temperature. Thus the advantage of using thermoplastic impregnated concrete is lost at such temperatures. Thermoplastic monomers have low viscosity and can penetrate well into the hardened concrete and fill large part of the pores. Their polymerization is achieved by addition reactions not leading to low molecular weight byproducts.
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2. Thermosetting Resins:
These polymers are more viscous and difficult to impregnate into the concrete. They can withstand higher temperatures without softening. But the condensation reactions which occur may lead to the formation of low molecular weight byproducts, which would occupy some of the space.
Thus it is necessary that a monomer or its polymer should be chemically compatible with the compounds of cement and the constituents of hydrated cement paste to prevent their adverse effects.
Monomers/resins used for polymer impregnated concrete are as follows:
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1. Styrene
2. Methyl methacrylate (MMA)
3. Butylacrylate
4. Acrilonitrite
5. Epoxies and their copolymer combinations, and
6. Polyster.
The amount of monomer that can be loaded into the concrete specimen depends on the amount of water and air voids that occupy the total void space.
Uses of Polymer Impregnated Concrete:
The polymer impregnated concrete can be used as follows:
1. Surface Impregnation of Bridge Decks:
The impregnation of bridge decks renders them impervious to the intrusion of moisture, deicing chemicals and chloride ions etc.
2. Application in Irrigation Structures:
The effect of cavitation and erosion in dams and other hydraulic structures is catastrophic. Conventional repairs of such damages are very expensive and time consuming, resulting in huge losses due to loss of benefits from irrigation, flood control and power generation. In such situations the polymer impregnated treatment may prove to be cost effective. The concrete may be removed from the damaged portion and the damaged area patched up, dried and treated by impregnation.
3. Use as Structural Members:
Polymer-impregnated concrete has a bright future to be used as a structural material. Polymer-impregnated pre-stressed concrete beams have shown remarkable high performance over conventional concrete. The maximum tendon force in case of impregnated concrete could be upto 4 times that of ordinary concrete. The creep deflection was found of the order of 1/9 to 1/16 that of static deflection. The shear strength also improves by the same factor as that of compressive strength. The compressive strength of PIC being of the order of 100 to 140 MPa, it can be used for heavier loads and longer span bridges and pre-fabricated sections.
4. Marine and Under Water Applications:
Greatly improved structural properties and low water absorption and permeability make the polymer impregnated concrete an excellent material for marine and under water application such as sea floor structures, desalination plants etc. It has also been observed that even partial impregnation of concrete piles in sea water reduced the corrosion of reinforcing bars by 24 times i.e. by the use of partial impregnation reinforcement corrosion reduced to 1/24 that of ordinary R.C.C. work.
The materials to be used in the construction of flash distillation plants (vessels) have to withstand the corrosive effects of distilled water, brine and vapours at a temperature upto 143°C. The carbon steel vessels used at present for desalination are costly and deteriorate after prolonged use. The use of PIC will prove economical over the conventional carbon steel vessels.
5. Nuclear Power Plants:
To meet the power requirement demand for industrial purposes, most of the countries have resorted to nuclear power generation. For the generation of nuclear power, pressure vessels are required to withstand the high temperature and at the same time to be able to provide shield against radiation.
To avoid radiation hazards nuclear power generation also needs the containment of spent fuel rods which remain radioactive for a long time. The present high density concrete shield is not very effective. PIC having high strength and durability coupled with high impermeability can be used to solve these problems.
6. Sewage Disposal Works:
Sewer pipes when buried under sulphate infested soils deteriorate due to the attack of effluents. The sewage treatment works made of concrete are also attacked severely from corrosive gases. Polymer impregnated concrete being highly resistant to shalphate and acids may prove to be a most suitable material for this purpose.
7. Impregnation of Ferro Cement Products:
Ferro cement products being thin, generally 1 to 4 cms thick are liable to corrode. The impregnation of polymer will improve the functional efficiency of ferro cement products.
8. Water Proofing of Structures:
Seepage and leakage of water through structural elements such as roof and slabs is a perpetual problem and has not been fully solved by the use of conventional water proofing methods. The use of polymer impregnated mortar may solve this problem.
9. Industrial Use:
To withstand the chemical attack generally concrete has been used for flooring in dairy farm product buildings, tanneries and chemical factories etc. The performance of conventional concrete has not been found very satisfactory. It is hoped that polymer impregnated concrete will provide durable flooring in such situations.
Properties of Polymer Impregnated Concrete (PIC):
1. Compressive Strength:
Loading with a 6.4% polymer and using methylemethacrylate as monomer, compressive strength was found of the order of 144 MPa using radiation technique of polymerisation. When the thermal catalytic process of polymerisation was applied on the same type of specimen, they gave compressive strength as 130 MPa whereas control (un-impregnated) specimens gave compressive strength of 38 MPa only.
Styrene impregnated specimens also showed similar trend, but with somewhat lower strength. The polymerisation by radiation method produced concrete of higher strength than that produced by thermal catalytic method.
Perlite lightweight aggregate concrete impregnated with MMA and polyester styrene also has shown considerable increase in compressive strength. However MMA impregnated specimens gave higher strength than polyester styrene impregnated specimens. The average compressive strength of a 1:8 mix non air entrained perlite concrete specimens impregnated with MMA was of the order of 56 MPa for polymer loading 6.3% whereas un-impregnated (controlled) specimens gave only compressive strength as 1.2 MPa.
2. Tensile Strength:
The increase in tensile strength of a PIC has been found as high as 3.9 times that of control (un-impregnated) specimens for a polymer loading of 6.4%. MMA i.e. the tensile strength of impregnated concrete was of the order of 11.7 MPa as compared to the strength of un-impregnated (control). Specimen of 3 MPa using radiation process of polymerisation. Thermal catalytically initiated polymerisation produced concrete of 3.6 times that of control (un-impregnated) specimen and 7.3% less than radiation polymerisation process. The relation between polymer loading and compressive and tensile strength has been shown in Fig. 24.1.
3. Flexure Strength:
Polymer impregnated concrete with polymer loading of 5.6% MMA and polymerised by radiation showed flexural strength of the order of 18.8 MPA while control concrete showed flexural strength as 5.2 MPA i.e. the flexural strength of PIC was found 3.6 times more than un impregnated concrete.
4. Polymer Concrete (PC):
The polyester resin concrete has been found to produce flexural strength of the order of 15 MPa at 7 days.
5. Creep:
The compressive creep deformation of MMA and styrene impregnated concretes have been observed to be in the opposite direction of the applied load i.e., negative creep. During loading application after initial movement these concretes expand under sustained compression.
6. Shrinkage due Polymerisation:
The shrinkage takes place through two stages of impregnation treatment i.e., (a) through initial drying, (b) through polymerisation. Shrinkage through polymerisation is peculiar to PIC. It is several times higher than the normal drying shrinkage.
7. Durability:
The durability of PIC (polymer impregnated concrete) has been found much higher than un-impregnated concrete due to the saturation of the hydrated cement with corrosion resistant polymer.
8. Freezing and Thawing Resistance:
Polymer impregnated concrete has shown excellent resistance to freezing and thawing. MMA impregnated and radiation polymerised concrete withstood 8110 cycles of freezing and thawing whereas un-impregnated concrete withstood only 740 cycles. Partially impregnated concrete withstood 2310 cycles.
9. Resistance to Sulphate Attack:
Keeping failure criteria of 0.5% expansion, the improvement in the sulphate resistance in polymer impregnated concrete has been found at least 200% and 89% in partially impregnated concrete.
10. Acid Resistance:
In PIC concrete the acid resistance has been found to improve by 1200% when exposed to 15% HCl for 1395 days compared to un-impregnated concrete. 15% sulphuric acid and 5% hydrochloric acid effect on PIC has been shown in Fig. 24.2.
11. Resistance of Abrasion:
A 5.5% MMA impregnated concrete has been found 50 to 89% more resistant to abrasion than the un-impregnated concrete. Surface impregnated concrete slab surfaces have shown 20 to 50% improvement in abrasion resistance.
12. Water Absorption:
5.9% Polymer loaded concrete has shown a maximum reduction of 95% water absorption.
13. Coefficient of Thermal Expansion:
5.5% MMA impregnated and radiation polymerised concrete has a coefficient of thermal expansion of 5.63 x 10-6 and styrene impregnated concrete 5.10 x 10-6 whereas the coefficient of thermal expansion of un-impregnated concrete is 4.02 x 10-6. Hence the coefficient of thermal expansion of polymer impregnated concrete is higher than un-impregnated concrete.
14. Stress-Strain Relationship:
The flexural strength of polymer impregnated concrete has been found about 3.6 times higher than un-impregnated concrete due its higher modulus of elasticity. The stress-strain relationship of PIC has been found nearly linear upto failure. There is little departure from the linearity upto 90% ultimate strength. The stress-strain curve for MMA impregnated concrete, MMA and Butyla Crylate (BA) and Cement concrete stress-strain relationship is shown in Fig. 24.3 (a), whereas Fig. 24.3 (b) shows the stress-strain relationship for polymer concrete (PC) for the same polymers (MMA and MMa-BA).
The stress-strain curve for styrene TMPTMA impregnated concrete also showed the same characteristics as that of MMA impregnated concrete. The modulus of elasticity for MMA impregnated concrete was found as 49 GPa whereas for un impregnated concrete it was observed as 27 GPa. Thus the modulus of elasticity increased by 1.8 times of the MMA impregnated concrete.
