Admixtures are available today that can significantly increase durability.
By Charles Nmai
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.