The versatility of concrete as a construction material is unparalleled. Its basic constituents are readily available in most parts of the world, it can be made and formed with relative ease into various shapes and aesthetically-pleasing designs, and, in general, it has been reliable, durable, and sustainable as a construction material.
There have, however, been durability problems for reasons that include the effects of the aggressive macro- and micro-environments to which concrete is sometimes subjected, the use of poor quality materials, poor quality control, and a failure to adhere to good concreting practices. The primary durability issues for concrete in aggressive environments include corrosion of embedded reinforcement, chemical and sulfate chemical attack, alkali-aggregate reactivity, and deterioration due to repeated cycles of freezing and thawing under saturated conditions.
To address these challenges, there are durability-enhancing chemical admixtures and supplementary cementitious materials (SCMs) that—in addition to proper mixture proportioning techniques and knowledgeable structural design and construction—can ensure the durability of concrete in aggressive environments. The durability-enhancing admixtures include air entrainers, high-range water reducers (HRWRs, also commonly referred to as superplasticizers), corrosion inhibitors, lithium-based admixtures for mitigation of alkali-silica reaction (ASR), waterproofers, shrinkage reducers, and a crack-reducing admixture. The benefits and potential limitations of these specialty admixtures are discussed briefly in this article.
High-Range Water Reducers
The durability of concrete is significantly influenced by its permeability, which, in turn, is influenced by, among other things, the water-cementitious materials ratio (w/cm) of the concrete. As a result, the w/cm of concrete intended to have low permeability is generally limited by code and specifications to no more than 0.45 and, quite often, a maximum of 0.40 where corrosion protection of reinforcement is desired.
HRWRs provide significant water reduction that can range between 12% and 40%, and they facilitate the use of low water contents without compromising concrete workability. Consequently, HRWRs are an essential component in producing low-permeability concrete and flowing concrete which, by definition, should have a slump of 7.5 inches or greater.
In general, the low w/cm afforded by HRWRs results in higher early and ultimate strengths. As such, HRWRs enable more effective use of cementitious materials. HRWRs have evolved over the years and current formulations are based on engineered molecules such as polycarboxylate ether (PCE). Unlike first-generation products that had to be added at the jobsite because of short slump life, current products are formulated for addition at the batch plant, where greater control can be exercised. The high slump and workability obtained with HRWRs leads to faster discharge, pumping, and placement of concrete, while reducing the amount of effort needed to properly consolidate concrete. Furthermore, HRWRs are absolutely needed in the production of self-consolidating concrete (SCC), a highly flowable and stable concrete mixture that requires minimal, if any, mechanical effort for consolidation within formwork. In concrete structures with highly congested reinforcement, properly proportioned SCC will facilitate construction and help minimize consolidation-related defects. This helps protect the overall durability of the structure.
The disintegration of improperly air-entrained concrete by weathering is often due to the effects of freezing and thawing of water in the capillary pores of concrete. In critically saturated concrete, freezing of water in the capillary pores leads to an increase of approximately 9% in the volume of the water. If no relief is provided to accommodate the expansion, this can lead to the development of high tensile stresses within the concrete matrix. Repeated exposure to freezing and thawing cycles under these conditions will eventually lead to cracking and deterioration of the concrete. Therefore, concrete that will be exposed to cycles of freezing and thawing in a critically saturated state should be air-entrained. Entraining air in concrete provides an outlet and relief for water in the capillary pores during freezing, thereby protecting the concrete matrix from damage. Air-entraining admixtures are used primarily to create a stable air-void system in concrete and properly air-entrained concrete can withstand years of exposure to freezing and thawing without damage. Air-entraining admixture formulations are typically based on natural wood resins (vinsol resin) and rosins, and synthetic detergents. Due to significant raw material cost increases over the years, the use of vinsol resin-based air-entraining admixtures is on the decline.
A universal concrete durability problem is the corrosion of embedded steel reinforcement. If left unchecked, this leads to cracking, spalling, and delamination of the concrete; loss of steel area; and, ultimately, loss of structural integrity. Corrosion is often due to depassivation of the reinforcing steel by chlorides that may come from deicing salts, brackish water, seawater, or spray from these sources.
Reinforcement in concrete structures that will be exposed to chlorides in service should be protected against chloride-induced corrosion. Corrosion protection measures include good design practice (such as adequate concrete cover to protect the reinforcement), the use of corrosion-resistant concrete, and external measures (coatings and sealers) that require periodic maintenance. Corrosion-resistant concrete includes the use of low-permeability concrete and corrosion-inhibiting admixtures.
Several corrosion-inhibiting admixtures are commercially available, but the predominantly used products are either inorganic 30% solutions of calcium nitrite or an amine-ester organic corrosion inhibitor. Calcium nitrite inhibitors provide protection to embedded reinforcement by forming a protective oxide layer at the surface of the steel and are used at dosages ranging from 2 to 6 gal/yd3 of concrete. Dosages of 3 to 4 gal/yd3 are typical in parking structures and upwards of 5 gal/yd3 in marine structures and some prestressed bridge girders. Dosages are determined by taking into account the design corrosion service life of the structure, the expected chloride exposure, the quality of the concrete, and other design details such as concrete cover over the reinforcement.
Because calcium nitrite accelerates the time-of-set of concrete, a retarding admixture is often needed in the concrete mixture to facilitate concrete placement, consolidation, and finishing. A novel workability-retaining admixture introduced into the marketplace in 2009 is also being used to maintain the workability and consistency of calcium nitrite-treated concretes, as was the case during construction of two cooling towers at Brayton Point in Fall River, Mass.
The amine-ester organic corrosion inhibitor provides protection to steel via a two-fold mechanism, by reducing chloride ingress and by forming a protective film at the steel surface. It is used at a fixed dosage of 1 gal/yd3 but, unlike calcium nitrite, it does not accelerate the time-of-set of concrete. In air-entrained concrete, however, it often requires an increase in the dosage of air-entraining admixture. It may also result in a slight decrease in compressive strength, depending on the proportions of the concrete mixture. The amine-ester organic inhibitor was used in the Denver International Airport parking structures.
Lithium for ASR Mitigation
Alkali-silica reactivity (ASR) is a potentially destructive phenomenon that can occur in concrete resulting from a reaction between dissolved alkalis and certain types of siliceous aggregates that contain reactive silica. The reaction produces an absorptive gel that can expand and lead to abnormal cracking and disintegration of concrete. The net result is a reduction in strength (load-carrying capacity) and reduced service life of the concrete structure.
Lithium compounds have been shown since the early 1950s to be effective in mitigating ASR in concrete, and lithium nitrate-based admixtures are currently available for use in concrete mixtures that may be susceptible to ASR. The lithium ion interferes with the ASR process by combining with reactive silica to form a lithium-silica gel that does not absorb water and, therefore, does not expand. Lithium-based admixtures can be used synergistically with some SCMs to provide cost-effective mitigation of ASR in concrete. Dosages are based on the alkali content of cement and the reactivity of the aggregate, and are typically established or verified by testing.
Watertightness is a key performance criterion for concrete structures designed for the containment, treatment, or transmission of water, wastewater, or other fluids. From a concrete perspective, watertightness can be achieved by using a well-proportioned, low permeability, workable concrete that is properly consolidated and cured. Also, cracking of the concrete must be minimized. Increasingly, waterproofing admixtures, in particular crystalline-based waterproofing admixtures, are being specified for concrete structures where there is a need to minimize water movement through the concrete.
Shrinkage-Reducing and Crack-Reducing Admixtures
The benefits of the durability-enhancing admixtures discussed up to this point can be compromised if the concrete cracks, because cracking will facilitate the ingress of aggressive agents or movement of water through the concrete. Cracking cannot be controlled 100%, but it can be minimized by designing for volume change and also proportioning the concrete for low shrinkage and cracking.
In addition to the traditional methods of reducing drying shrinkage (such as minimizing the total water and paste content of a concrete mixture), shrinkage-reducing admixtures (SRAs) can also be effective. SRAs can reduce drying shrinkage by about 30% to 50% in the long term and provide benefits with respect to the magnitude of curling in slabs-on-ground and cracking overall. Recommended dosages range from 0.5 to 1.5 gal/yd3 and dosages of 0.75 to 1.0 gal/yd3 are typical. In addition to reducing drying shrinkage, a novel crack-reducing admixture (CRA) now available in the marketplace has been shown to provide significantly better performance with respect to the magnitude of cracking and initial crack width, should cracking occur.
Commercially available durability-enhancing chemical admixtures are available today and are very effective. In combination with supplementary cementitious materials, proper concrete mixture proportioning, and good structural design and construction practices, these materials will improve the durability of concrete in aggressive environments.
Dr. Charles Nmai, P.E., is engineering associate/manager, engineering services with BASF Corp.–Admixture Systems, Beachwood, Ohio. He is an ACI Fellow.