Abstract & Authors

1. Introduction

In engineering construction, improving the durability of concrete structures is an important technical measure for guaranteeing safe operation, extending service life, conserving resources, saving energy and achieving sustainable development. The durability of a concrete structure means that during its service period the quality and performance of the concrete do not fall below the limits permitted by the design. The decline of concrete performance is called “deterioration,” and the factors that cause it — the various internal and external causes and elements that seriously weaken and damage the service function of a structure — are termed “deterioration factors.” They include external factors arising from the service environment and improper use such as overloading, as well as factors caused by internal defects.

2. Deterioration Factors and Protection of Concrete Structures

2.1 Deterioration factors of concrete structures

Concrete is a porous material containing a great many pores and micro-channels, and in service it is readily attacked by various deterioration factors. Aggressive media can be gaseous, liquid or solid. They include not only acids, alkalis, salts and various solvents but also gases such as SO2 and CO2 in the air, acid rain, industrial and domestic wastewater, de-icing salts and, importantly, water itself. Water is a low-viscosity, highly penetrating liquid and an excellent carrier for corrosive media; in defective reinforced concrete it causes corrosion of the steel. Below 0 C it is also the chief cause of frost-heave damage, as liquid water expands on freezing. Water is therefore one of the most important deterioration factors of concrete structures. According to their corrosion mechanisms and damage phenomena, aggressive media can broadly be classified as chemical dissolution, leaching attack, decomposing attack and expansive corrosion; corrosive media (acids, alkalis, salts and solvents) are thus the principal deterioration factors. Structures with different functions face their own specific factors. In the flood-discharge tunnel of a hydropower dam, high-velocity flow causes cavitation and abrasion of the lining; one discharge season can transform the surface from pockmarks to small pits, small holes and large holes, and if not detected and repaired the entire tunnel lining can be washed away. In 2005 more than 4,000 cubic meters of reinforced concrete were washed out of a flood-discharge tunnel at the Ertan hydropower dam. Water-conveyance channels are abraded by sediment-laden water, particularly below spillway and sluicing gates, where deep trenches and large pits are scoured every year. High-velocity and sediment-laden flows are thus the specific deterioration factors of spill tunnels and channels. Electroplating wastewater is extremely corrosive, and water temperatures as high as 80 C intensify attack on the tank concrete; high temperature is thus a deterioration factor for such tanks. For marine and coastal structures, seawater and salt spray are the deterioration factors; concrete under long-term radiation suffers radiation damage; and defective concrete — poorly compacted or honeycombed concrete, inadequately cured concrete, or concrete that develops cracks in service, followed by carbonation and cracking — loses strength and inevitably leaks, which accelerates structural damage. Existing defects are themselves deterioration factors.

2.2 Purpose and significance of concrete protection

Concrete structures with different functions and in different environments each have specific deterioration factors. Concrete protection means analyzing the deterioration caused by function and operating environment and applying targeted measures — waterproofing, anti-corrosion, freeze-thaw resistance, abrasion resistance, radiation resistance and defect restoration — so as to improve durability and extend service life.

2.3 Protection methods

There are three approaches: (1) strengthening the ability of concrete itself to resist deterioration factors (internal protection), for example through high-performance admixtures that increase density or use reinforcement potential differences to prevent steel corrosion; (2) using coatings and similar barriers to prevent deterioration factors from entering (external protection); and (3) treating inherently defective structures so they meet design requirements, for example through grouting reinforcement that raises the strength and durability of weak ground.

3. New Eco-Friendly Concrete Protection & Grouting Materials

Many materials are used across these three approaches; here we focus specifically on new eco-friendly modified epoxy materials. Their advantages lie in the high durability and environmental safety of the materials themselves, and they fall into three groups: (1) new eco-friendly chemical grouting materials; (2) penetrative-consolidation concrete protection materials (waterproof, anti-corrosion and surface-restoration coatings); and (3) new high-solids solvent-free epoxy protection materials.

3.1 Eco-friendly chemical grouting materials

Ten years ago, Director Jiang Shuozhong put forward the concept of green grouting on behalf of the chemical grouting profession, giving forward-looking guidance for the development of grouting materials. At that time, most epoxy grouts used epoxy–furfural–acetone systems with amine curing agents. Low-molecular-weight aliphatic amines are toxic and furfural is moderately toxic; in some developed countries it has been restricted or banned. Research institutes subsequently began developing furfural-free green grouts, but new green materials that retain excellent penetration and high mechanical performance after removing furfural, are productized, and are used in large quantities to consolidate low-permeability weak ground beneath hydropower dams, have rarely been reported. Together with South China University of Technology we developed three products:

(1) Eco-friendly high-penetration modified epoxy grout. We screened low- or slightly toxic ketone and aldehyde diluents to replace furfural but found that both penetration and mechanical performance fell sharply without it; the furan structure of furfural strongly influences mechanical performance, acting rather like coarse aggregate in concrete. Guided by our earlier studies of penetration and reaction mechanisms, we investigated the influence of the diluents on penetration and of the reaction intermediates on mechanical behavior, and by increasing the room-temperature reaction activity of the diluents and changing the curing agent, preserved both penetration and strength. The resulting slightly toxic, low-odor epoxy grout can both consolidate low-permeability weak ground in dam foundations and grout fine cracks in concrete.

(2) Solvent-free toughened epoxy grout. Using a low-viscosity long-chain reactive diluent terminated by epoxy groups and a long-chain modified amine curing agent, we developed a green grout whose hardened body has appreciable toughness while its tensile, shear and compressive strengths all meet standard JC/T 1041-2007. Although its viscosity makes it unsuitable for weak dam foundations, it performs better on concrete cracks, especially active cracks treated by chemical grouting. In high-altitude cold regions the large seasonal temperature range produces large crack-width changes; non-toughened epoxy often creates new cracks beside the original one, whereas tough grout withstands the thermal movement and is more durable.

(3) Fast-curing epoxy grout for direct water stopping and strengthening. Cracks that seep or discharge pressurized water are common in tunnels and underground works. Polyurethane, long used for water-stop injection, initially performs well but re-leaks within weeks to a year or two because of low bond strength to concrete, and it cannot strengthen the structure; metro authorities have consequently banned polyurethane for tunnel water-stop grouting. Ordinary epoxy, with its long initial-setting time, cannot stop active water, so projects first inject fast-setting inorganic material and then epoxy — a complex procedure beyond many contractors, creating demand for an epoxy that stops water and strengthens directly. We approached this through fast-reacting curing agents and active fillers; the key is controlling initial set and cure speed and achieving non-dispersion of the grout in pressurized flowing water — too fast and the pump and lines block, too slow and the water is not stopped. The resulting grout can be injected directly against low-pressure flowing water to stop leaks and strengthen cracks, as demonstrated successfully on a leaking basement crack in a building in Dongguan. Construction procedures and a family of formulations for different water pressures are being completed and commercialized.

3.2 Penetrative-consolidation concrete protection materials

Building on our pioneering high-penetration modified epoxy chemical grouts, and responding to the porous nature of concrete, we developed the penetrative-consolidation epoxy concrete protection material (waterproofing, anti-corrosion and surface-restoration coating). When applied to concrete it penetrates and fills from outside inward along fine pores, capillaries and micro-cracks invisible to the eye. At a dosage of 0.4-0.6 kg/m2 it penetrates 1-2.5 mm, and the lower the concrete grade the deeper the penetration. After curing, the consolidated material is far stronger than the concrete itself in compression, shear, tension and bending, so penetration–filling–consolidation creates a strengthened layer — a protective “armor” on the surface. The armor protects concrete in five ways: (1) density and strength increase; unlike non-penetrating coatings it creates an integrated inside-and-outside protection instead of a single surface film, making it difficult for water, SO2, CO2 and other aggressive media to penetrate and greatly raising waterproof and anti-corrosion capacity; (2) because of the heterogeneity of concrete the penetrated interface is interlocking, eliminating stress concentration between the consolidated layer and the original concrete, while the rooted surface film is monolithic with it and cannot delaminate or peel; (3) existing fine cracks and micro-cracks are restored through penetration and strengthening, and weak surface laitance gains strength; (4) the surface film formed outside the consolidated layer acts as a bonding primer before it fully cures, improving the bond of flexible coatings, membranes or render applied over it; and (5) the strengthened layer has real thickness: at 0.5 kg/m2 and 2 mm penetration it is equivalent to a 2,000-micrometer heavy-duty anti-corrosion coating, but being integral with the concrete it bonds far better and is more durable. Completed projects show that the consolidated layer not only waterproofs and prevents corrosion but greatly improves freeze-thaw resistance, abrasion resistance and carbonation resistance, combining multiple protective functions and significantly extending service life. In twelve years of application the high-penetration modified epoxy has performed successfully in high-speed rail, metro, highways, hydropower dams, ports and harbors, bridges, tunnels, sewage tanks, heritage conservation and civil buildings, and has stimulated domestic research and production and the development of corresponding standards; the waterproofing and anti-corrosion of metro shield segments now uses this material almost universally. It is described in detail in Waterproofing Design and Construction of Tunnel Works by Zhu Zuxi and Lu Ming. However, because it contained moderately toxic furfural, the original material was not environmentally friendly and produced strong odors indoors and underground, harming sensitive individuals; restrictions within China were only a matter of time. Guided by our reaction and penetration mechanisms, at the end of last year we solved the sharp strength loss of the furfural-free system and developed the new-generation eco-friendly penetrative-consolidation epoxy protection material. Mechanical performance and penetration are undiminished, but the product is environmentally friendly with very low odor, achieving product renewal and meeting European environmental requirements for export.

3.3 High-solids solvent-free epoxy protection materials

This series uses bisphenol-A epoxy resin combined with a low-viscosity epoxy reactive diluent and a modified amine curing agent, with solids content above 95%; the products are non-toxic, odorless and highly durable. They are strong and tough and, although they do not penetrate, when combined with the high-penetration epoxy primer their pull-off strength on damp or wet substrates reaches 4-6 MPa. Four products make up the series.

(1) Solvent-free epoxy anti-corrosion intermediate coat. Applied by spray or brush without pinholes or bubbles and outperforming comparable foreign products, it is an ideal middle coat for ports, harbors and bay bridges; it serves electroplating wastewater tanks at water temperatures up to 80 C with a service life several times longer than epoxy FRP; it resists freeze-thaw and de-icing salts in northern cold regions; and as an anti-corrosion waterproof coating in the seawater-conveyance tunnel of a nuclear power plant it outperformed a U.S. product, while field tests on a bay bridge outperformed American and Swiss products. Because the cured epoxy is highly water-resistant it can seal the water face of hydropower dams and can waterproof fish-fry ponds and potable-water tanks, being non-toxic, odorless and durable.

(2) High-abrasion-resistant solvent-free epoxy mortar. It resists abrasion and cavitation by high-velocity sediment-laden flow above 50 m/s and replaces solvent-based epoxy mortar in dam spill tunnels and plunge pools. It not only abrades less but is fully non-toxic and odorless in the enclosed tunnel environment. Old water-conveyance channels whose surfaces are carbonated, cracked and broken need not be demolished and recast: after surface preparation, a render of solvent-free epoxy mortar suffices, greatly cutting repair costs; on large seeping surfaces in tunnels and basements it outlasts polymer cement mortar.

(3) Solvent-free epoxy flooring. Since 2000, solvent-based epoxy floors were popular in parking garages, factory floors and offices, but low-price competition steadily reduced quality, with severe peeling and blistering and a sharp fall in use. We developed solvent-free self-leveling and thin-coat floors meeting national standards in adjustable colors; combined with the eco-friendly high-penetration epoxy primer they do not blister or peel, are non-toxic and odorless, can be applied on damp or wet substrates, and reach pull-off strength above 4 MPa.

(4) Flexible solvent-free epoxy waterproof putty. This putty is designed for render on brick masonry. Brickwork is water-permeable and its cement render cracks readily; conventional waterproof putty, lacking flexibility, cracks with the render and leaks. The solvent-free epoxy putty is very flexible and does not crack with the render, giving a durable, environmentally friendly waterproof putty. It can also treat seeping concrete in basements, underground works and tunnels and is especially suitable for home use.

4. Conclusion

As the state pays increasing attention to the quality and durability of construction, requirements for concrete protection rise; as environmental requirements rise, the environmental standards demanded of materials become stricter. This is a challenge and an opportunity for companies developing, producing and selling concrete protection materials. Our experience shows that scientific and technological innovation must deliver not only high-performance products but also continually advancing high-performance environmentally friendly products. We are ready to cooperate with quality-conscious, honest material suppliers, distributors and contractors for mutual benefit, promoting durable eco-friendly concrete protection materials within China and abroad and bringing credit to the brand of Chinese manufacture in competition with foreign brands.

First author: Ye Linhong (1942–), born in Rongxian, Sichuan, former Vice President of the Guangzhou Branch of the Chinese Academy of Sciences and research professor, a State-Council special-allowance expert. His career has been devoted to modified epoxy adhesives, chemical grouts, waterproof and anti-corrosion coatings and grouting and reaction mechanisms, with numerous national, provincial and ministerial awards, two national invention patents, six utility-model patents and more than fifty published papers. He is currently Chairman and Chief Engineer of Guangzhou Yongke New Materials Technology Co., Ltd.