By Brian Miller
Brian Miller was formerly a Technical Services Engineer with NPCA and a member of the NPCA TechTeam.
In this final article of the chemical admixtures series, we will look at specialty admixtures such as corrosion inhibiting, shrinkage reducing and alkali-silica reaction reducing admixtures. There are other admixtures on the market, and new ones are being developed every day. However, for the scope of this article we will focus on those most commonly used.
Admixtures, in general, are defined as a material other than water, aggregates, hydraulic cementitious material or fiber reinforcement that is used as an ingredient of a cementitious mixture to modify its freshly mixed, setting or hardened properties and is added to the batch before or during mixing. As noted in Part 1 of this series, chemical admixtures are usually further defined as nonpozzolanic (not requiring calcium hydroxide to react) in the form of a liquid, suspension or water-soluble solid.
CORROSION-INHIBITING ADMIXTURES (CIAs)
Corrosion of reinforcing steel in concrete has always been a problem. The related cost for corrosion damage of infrastructure in the United States alone exceeds billions of dollars each year. When reinforcing steel corrodes, one of the primary substances produced is ferric oxide, more commonly known as rust. Rust occupies several times the initial volume of the steel. This increase in volume creates stresses within the concrete surrounding the corroding reinforcing steel. These stresses eventually cause spalling of the protective concrete cover. While unsightly, this also may create the possibility of catastrophic structural failures. As the corrosion process accelerates, the cross-section of good reinforcing steel decreases. This increases stresses in the steel that can exceed safe limits. Therefore, several methods have been developed to reduce the corrosion rate of reinforcing steel and extend the service life of the structure. These include the use of corrosion-inhibiting admixtures (CIAs).
Corrosion-inhibiting admixtures are used in concrete that is exposed to external chloride sources. External chloride sources include deicing salts (commonly used to melt ice in winter) and marine environments where concrete is either submerged in or located close to seawater. Other applications include structures near chloride contaminated soils or concrete containing admixed chlorides, such as calcium chloride. Note that ACI 318 limits the concentration of admixed chlorides in concrete.
How they work
Corrosion-inhibiting admixtures work by elevating the chloride threshold. The chloride threshold is defined as the concentration of chloride ions necessary at the reinforcing steel surface to initiate active corrosion. This is usually done by reducing the anodic reaction or the availability of ferrous and ferric ions to react with chloride. Typically in the high pH environment of concrete, a passive (protective) layer (ferrous hydroxide) is formed and remains fairly stable, protecting the reinforcing steel from active corrosion. In the presence of chloride ions or carbonation, this passive layer breaks down and corrosion of the reinforcing steel is accelerated.
There are several materials used to make corrosion-inhibiting admixtures, including calcium nitrite, sodium nitrite, dimethyl ethanolamine, amines, phosphates and ester amines. Of these, calcium nitrite is the most widely used and is classified as an active, anodic corrosion inhibitor. Calcium nitrite reacts with the iron ions, increasing the stability of the passive layer. In essence, the iron is attracted to the nitrite more than the chloride. Since the efficiency ratio (the effective ratio of the nitrite to the chloride) of the nitrite is approximately equal, the concen¬tration of the chloride ion must exceed that of the nitrite ion in order to break down the passive layer and begin actively corroding.
Other corrosion-inhibiting admixtures have mechanisms that reduce the rate at which chloride gets into the concrete (chloride ingress). These are sometimes referred to as passive systems. Some of these are considered to be chloride screening admixtures in that they have a hydrophobic (repels water) component, which lines the concrete’s pore structure and reduces moisture ingress. Chloride ions are transported by moisture in the pore structure of concrete. By reducing the moisture ingress, they also reduce the chloride ingress. While chloride screening admixtures are not technically corrosion inhibitors, they are effective at increasing the time-to-corrosion initiation, which ultimately increases the service life of a structure.
Finally, some corrosion-inhibitors are cathodic-based. They interfere with the reduction of oxygen, which is necessary for the corrosion process. These types of admixtures are not commonly used. However, cathodic protection systems (designed for use after concrete has hardened or been exposed to chloride) are more commonly used to extract chlorides or reduce the rate of corrosion for existing structures.
Effects on concrete
Corrosion inhibiting admixtures, such as calcium nitrite, can increase the chloride threshold by five times. Calcium nitrite is also considered an accelerator. This may be of concern in warmer climates or with increased dosages.
Nitrite is water-soluble and therefore present in the pore solution, increasing its conductivity. As a result, concrete containing calcium nitrite will have increased Rapid Chloride Permeability (RCP) values (See ASTM C 1202, “Standard Test Method for Electrical Indication of Concrete’s Ability to Resist Chloride Ion Penetration.”) Calcium nitrite has very little effect on air-entrainment.
Some organic corrosion inhibitors or pore lining CIAs will require much greater dosages of air-entraining and water-reducing admixtures. They may also slightly reduce compressive strength results.
CIAs are typically dosed by gallons per cubic yard (gal/yd3) of concrete with the mix water. Calcium nitrite is commonly dosed at 3 to 6 gal/yd3 of concrete. As always, follow the manufacturer’s recommendations. Typically there is a direct relationship between increasing the dosage of CIA and increased corrosion protection.
As of this writing, ASTM International was preparing a specification for corrosion-inhibiting admixtures. However, there are other existing test methods available to determine whether admixtures have a negative effect on the reinforcing steel in concrete. These tests may also be used to show the benefit of CIAs and make relative comparisons of concrete or paste with and without CIAs. These test methods include ASTM G 109, “Standard Test Method for Determining the Effects of Chemical Admixtures on the Corrosion of Embedded Steel Reinforcement in Concrete Exposed to Chloride Environments,” and ASTM G 180, “Standard Test Method for Initial Screening of Corrosion Inhibiting Admixtures for Steel in Concrete.”
SHRINKAGE-REDUCING ¬ADMIXTURES (SRAs)
Concrete undergoes small but significant volumetric changes due to internal and external factors. A reduction in volume is typically referred to as shrinkage. If the concrete were unrestrained, shrinkage would essentially be of little concern. However, most concrete is restrained in some manner (either internally or externally), and therefore shrinkage results in internal stresses that cause cracking. Shrinkage occurs over time and in several stages. Here are some definitions of the common types of shrinkage.
Chemical shrinkage – occurs due to the hydration process because the cement and water occupy a greater volume than the hydration products.
Autogenous shrinkage – occurs when external moisture is not available. The hydration process then utilizes pore water, resulting in self-desiccation of the paste.
Subsidence – refers to shrinkage that occurs vertically due to sedimentation of solids. As the solids settle, bleed water rises to the top surface and evaporates. Air voids also rise to the top and contribute to subsidence.
Plastic shrinkage – refers to shrinkage that occurs during the plastic (wet) state of placement and results in tear-like cracks at the surface. It is caused by a combination of chemical and autogenous shrinkage when the evaporation of water at the surface is greater than the bleeding rate. Essentially, the concrete does not have enough strength developed to resist the stress caused from shrinkage.
Drying shrinkage – occurs after concrete has set and as it becomes dry. Concrete experiences increases and decreases in volume based on the temperature and amount of moisture present in the concrete. Essentially, cold temperatures cause concrete to contract and loss of moisture causes concrete
How they work
Shrinkage reducing admixtures have two approaches to reducing shrinkage. Some cause expansion of the concrete to offset the shrinkage. These include finely divided iron and calcium sulpho-aluminate. Care must be exercised in that the expansion caused should not be disruptive to the concrete component.
Others line the pore structure or reduce the surface tension of the water. This reduces the stresses in small capillaries, thereby reducing self-desiccation caused by shrinkage.
Effects on concrete
SRAs have been shown to reduce early age and ultimate shrinkage. Shrinkage reducing admixtures have little effect on most other concrete properties. However, some research has shown slight reduction in early age compressive strengths. This reduction is most relevant if the companion concrete has been properly moist-cured for a long duration (28 days). Studies have shown that for most common field applications, compressive strength has been the same or slightly increased.
Shrinkage reducing admixtures are typically dosed by gal/yd3 of concrete.
As of this writing, ASTM International had no specification for shrinkage reducing admixtures. The effects of SRAs
can be measured by ASTM C 1581, “Standard Test Method for Determining Age at Cracking and Induced Tensile
Stress Characteristics of Mortar and Concrete under Restrained Shrinkage.”
ALKALI-SILICA REACTIVITY (ASR)
Certain aggregates contain siliceous rocks or minerals, such as chert, opal, tridymite and cristobalite. This silica reacts with alkalis from the hydrated cement to form gel. When this gel is in contact with sufficient moisture, it expands. This expansive gel can cause cracking and pop-outs on the concrete’s surface. These cracks are typically seen as ‘Y’ shaped. ASR admixtures can be used to reduce this problem.
How it works
ASR admixtures are made from barium salts or lithium compounds of which the latter is the most popular. Lithium is used to form alkali-silica gels that are less soluble and do not absorb or bind as much water. Therefore, they are much less expansive.
Effects on concrete
ASR admixtures reduce the expansion associated with reactive aggregates and alkalis. They generally have minimal effects of other properties of concrete.
ASR admixtures are typically dosed by gal/yd3.
No specifications for ASR admixtures exist at the time this document was prepared.
There are several other chemical admixtures available or in the works. Some of these include defoaming agents used to reduce high air contents of concrete, water-repellent admixtures which are used to reduce the permeability of concrete, pigments and new liquid dispersion coloring admixtures for creating colored concrete, which will be addressed in a future article. Whatever the problem, goal or situation, there is probably an admixture available or being developed to assist you.
Consult your admixture supplier for the latest news on future products. A more in-depth look at admixtures and their usage is available in NPCA’s new Production and Quality School Level II (PQS II) course.