At its core, concrete hasn’t changed very much since its discovery – cement, water, aggregate. What has changed, remarkably, is our understanding and use of the admixtures that help concrete retain its strength.
An admixture is anything you add to a concrete mix other than cement, water, aggregate, or fiber, with the purpose of altering its (fresh, setting, or hardened) properties. Admixtures themselves aren’t a new development, by any means; the Romans experimented with everything from horse blood to rendered animal fat (for cold weather applications), while the Chinese employed rice gluten, molasses, elm extract, and various other refined chemicals in their design mixes.
Modern chemists and engineers, approaching the problem with a more evolved tool set, developed a range of mineral and chemical admixtures over the course of the twentieth century. These allow engineers to tailor concrete to very specific applications. Admixtures exist which accelerate or reduce set times, increase flex and compressive strength, extend working time, improve the efficiency of cementation, alter aesthetics, prevent cracking or corrosion… There is an admixture, or cocktail of admixtures, for almost any application.
ASTM International (formerly the American Society for Testing and Materials) publishes standards for a number of chemical admixtures. For detailed information, their standards sheets are a good place to start; for this article, we’re going to restrict ourselves to discussing air-entraining agents, water-reducing set controllers, high-range water-reducers, and corrosion-inhibiting admixtures. A complete treatment of the topic would fill a hefty book or three… and be outdated faster than you could read it.
Air-Entraining Concrete Admixtures
Freeze and thaw cycles are brutal. When concrete sets, a network of capillary pores is left behind where the mixing water used to be. Water that works its way into these pores and freezes creates a lot of stress – the volume of frozen water is 9% greater than liquid.
Pavements, bridges, and other structures which are exposed to the elements in temperate climates simply won’t last under the repeated strain. That’s where air entrainment comes in; today, somewhere around 80% of cement pavements in the United States contain air entrainment agents.
(Interesting historical note: modern air-entraining admixtures were accidentally rediscovered in the 1930s, when it was noted that concrete from mills using beef tallow as a lubricating agent produced pavement that was less dense and had far greater resistance to the elements. The Romans got there first, of course, but we’re used to that in the West.)
By deliberately creating small (0.2 mm) air voids, equally distributed throughout the mix, air entrainment builds expansion resistance into concrete. The voids provide room for water to expand without pushing against the capillary walls, which would ordinarily lead to scaling, cracking, or spalling.
This resistance comes at a cost, of course. Reducing the density of concrete affects compressive strength. Roughly, compressive strength will be reduced by 5% for every 1% increase in air content.
Water-reducing Set Controllers
For a given density (read: air content), the final strength of concrete varies with the ratio of water to cementitious material. Reducing the need for water in the design mix through the addition of water-reducing concrete admixtures allows engineers to achieve greater flex and compressive strengths without sacrificing workability. These same agents enable control over set times, accelerating or retarding hardening as needed.
In order to be considered a water-reducing agent, an admixture must lower the amount of required water for a given workability and slump by at least 5%, though reduction of 12% or greater is possible.
Typically, the reducing agents are organic, which slows set times and increases workability. Many manufacturers dampen this effect through the addition of accelerating agents, like calcium chloride, which creates problems of its own in reinforced or prestressed concrete (hugely corrosive, best avoided) and precipitates many air-entraining agents.
Non-chloride accelerating agents are reasonably available – nitrates, nitrites, formate salts, and thiocyanates – but their efficacy varies with on-site temperature. Some are most effective at temperatures below freezing, for example, but will set poorly on more hospitable job sites.
High Range Water Reducers
These agents are in a category of their own for several reasons. First, they can reduce water requirements by upwards of 30%, producing self-leveling design mixes at standard water/cement ratios, or normal workability with dramatically increased strength at lower ratios. Second, the resulting concrete is less permeable, stronger, and gains strength much faster than unaltered design mixes.
Third – and one of the main reasons behind the popularity of high range water reducing admixtures – these agents produce highly workable, self-consolidating (ie, requiring no vibration) concretes with less shrinkage, making them easy to deploy in tricky on-site applications.
The most effective high range water reducing admixtures are made from polyether-polycarboxylates, which grant a degree of freeze/thaw resistance without air entrainment and more efficiently disperse cement and mineral components in the design mix.
Corrosion-Inhibiting Concrete Admixtures
Reinforced concrete is amazing stuff, but leaves structures vulnerable to loss of integrity from corrosion of enclosed steel elements. To combat this, engineers developed concrete admixtures which protect reinforcements from exposure to common corrosive agents.
If a reinforced concrete structure is exposed to seawater, deicing salts (as would parking structures, pavements, or roadways), or contains chloride-based accelerants, its steel elements will degrade rapidly. Corrosion-inhibiting admixtures act to slow the onset of corrosion and retard the process, once begun.
These agents are available in organic formulations (esters and amines), as well as inorganic, calcium nitrate-based mixtures. Organic formulations will retard set time somewhat and may not work in all applications.
Continued admixture development gives modern engineers incredible flexibility in the strength, endurance, and workability of their concrete. With the correct balance of admixtures, there’s very little we can’t make it do.