Know the differences between beneficial air and detrimental air.
By Eric Carleton, P.E.
Unless you’re an anaerobic bacterium, you may need air to survive. You may know that, but did you know that air is also an important factor in determining concrete’s durability? This is especially true for concrete that is exposed to freezing and thawing. Air in concrete is defined as air voids within the concrete matrix. But not all air is good air, and it’s important for precasters to understand the difference.
During the concrete mixing process, mixer blades can introduce pockets of air into the batched concrete. These randomly spaced, large-diameter air pockets can be detrimental to concrete’s quality by reducing available strength and creating surface defects (bug holes). Entrapped air voids also provide minimal durability benefit from internal water freezing and expansion cracking. Proper concrete placement and consolidation techniques reduce large entrapped air pockets.
During the hydration process, the concrete matrix will produce a capillary pore system. These openings are very small (10 nanometers to 10 micrometers) and are defined as “the interconnected spaces between hydration products of hardened concrete.”1 These capillary openings go through a series of changes from dry batching and the interfacing of cement particles to the introduction of water, cement reactions and the consequential plugging of most of the pores with hydration constituents. Capillary pores do not provide beneficial air openings to reduce water freeze/thaw destruction and are linked to permeability. Consequently, modern concrete mixes incorporate fine secondary cementitious materials or, when specified, integral water sealant admixtures to further reduce hardened concrete capillary pores.
Creation of air-entraining admixtures
While entrapped air can be harmful to concrete, entrained air in the concrete matrix through chemical means creates beneficial bubbles. These bubbles are spaced not more than 1/5 millimeter apart and are about 1/10 millimeter in size. Entraining air creates billions of small bubbles within a cubic meter or yard of concrete, providing a perfect space to relieve stresses when water freezes within hardened concrete. This entrained air has proven very beneficial for concrete since its introduction to the industry in the 1940s. Like many great discoveries, it came about by accident. Individuals in New York began to notice some concrete mixes experienced little or no scaling or degradation, while others fared much worse. This led to the discovery that some mixes contained cement that inadvertently included petroleum products due to a bearing failure in the mill during the grinding process while other cement inadvertently contained animal fat. Both of these obscure additions added a surfactant to the mix and produced large amounts of small, uniformly spaced dense bubble configurations. Many decades of research and scientific advancements by academics and members of the private industry has led to the creation of modern admixtures for air entrainment, which are a staple in most concrete mixes batched in cold climates.
Even manufacturers in warm climates have found value in air entrainment admixtures, including improvement in fresh concrete properties such as workability, reduced bleed water and aggregate segregation. Though today’s modern air-entraining admixtures still provide benefits, additional specific water-reducing admixtures are now available to precast producers. A potential detrimental effect of using an air-entraining admixture is a small reduction in long-term compressive strength. However, careful attention in trial batching with modifications to the quanity of cementitious materials and/or the addition of water-reducing admixtures negates these issues.
Air entrainment and dry-cast concrete
Although wet-cast concrete benefits greatly from air entrainment, dry-cast or zero-slump precast concrete is an anomaly. Since dry-cast concrete has a very low water-cement ratio and requires a vigorous consolidation process, any air-entraining admixture and corresponding entrained air structure is eliminated. However, defying traditional wet-cast concrete performance, thousands of precast concrete pipes, box culverts and other drainage structures exposed to harsh freeze/thaw conditions have not experienced degradation, even in areas where road salt or tidal conditions occur. Until recently, no high-level research has been devoted to this phenomenon. It is speculated that the dry mix process produces capillary pores and unhydrated cement that are slightly larger than normal capillary pores produced within a wet-cast concrete paste. These openings may provide an avenue for the expanding water and allow for the corresponding stress relief.
Though air-entraining admixtures have been used within the concrete industry for more than 75 years, they remain one of the most challenging and temperamental admixtures.
Some items to consider are:
- An increase in cement fineness will decrease air content.
- High-cement content mixes will reduce air content compared with low-cement content mixes.
- Round fine aggregate is favorable to air entrainment.
- It is important to add air-entraining admixtures with initial mix water or directly to fine aggregate.
- Dry coarse aggregates can soak up air-entraining admixtures, reducing dispersion and effectiveness.
- Mix changes from crushed stone to gravel or vice versa will affect your air entrainment.
- Dusty coarse aggregate will decrease air content.
- Water softeners can increase or decrease water content depending on the air entrainment composition.
- Hot concrete temperatures may reduce air content by 25%, while cool concrete temperatures less than 75 degrees Fahrenheit can increase air content up to 40%.
- Increasing the use of fly ash with high carbon content will decrease the amount of entrained air.
- Oil and grease inadvertently added to the mix may either increase or decrease air entrainment depending on the admixture’s composition.
- Other admixtures, particularly superplasticizers, can either increase or decrease the amount of air entrained.2
Beneficial air entrainment involves everyone
The list above establishes the importance of developing good working relationships with concrete admixture suppliers. However, precast manufacturers must still understand their mix constituents and processes if they expect to achieve consistency with the air content of their batched concrete. Quality control and production should consider forming teams to consistently test and identify changes in air entrainment and concrete mixtures and be prepared with a plan to minimize the variables and causes leading to any needed mix adjustment.
For more information on air entrainment or air-entraining admixtures, contact NPCA Technical Services at (800) 366-7731 or visit precast.org/technical-services.
Eric Carleton, P.E., is NPCA’s director of codes and standards. He is also an ASTM Award Merit recipient and currently serves as vice-chairman of ASTM C13, Concrete Pipe.
Air Entrainment Requirements
It is vital to satisfy all precast concrete air entrainment requirements expected by the specifying agency or owner. The NPCA Quality Control Manual for Precast Concrete Plants, Section 5.3.4, states precasters must test concrete production a minimum of once per day or once per 150 cubic yards produced, whichever comes first. During mix design revisions or batch modifications due to existing production conditions, the frequency of air testing may increase.
The manual also states, “Air content shall be determined by either the pressure method, ASTM C231, ‘Standard Test Method for Air Content of Freshly Mixed Concrete by the Pressure Method,’ or the volumetric method, ASTM C173, ‘Standard Test Method for Air Content of Freshly Mixed Concrete by the Volumetric Method.’”
Due to the relative ease of conducting the air test in accordance with C231 as compared to C173, most plants implement the C231 procedure. But both procedures provide accurate, reproducible results in evaluating the volume of air within the concrete mix.
One shortcoming when using the two air entrainment tests is they only provide the total volume of air. The total volume of air does not reveal the size of the bubbles. The bubble size is important for freeze/thaw durability, minimizing strength loss, minimizing bug holes and maintaining consistent air volumes between batches. The only means to quantify air entrainment quality (void size and spacing) is by cutting and polishing a hardened concrete sample and physically counting and measuring the air void matrix. The process procedures are detailed in ASTM C457, “Standard Test Method for Microscopical Determination of Parameters of the Air-Void System in Hardened Concrete.” This test can take days or weeks to determine a result, and as such, cannot provide a real-time analysis.
This problem provided a research opportunity for Tyler Ley, associate professor of civil and environmental engineering at Oklahoma State University. His team developed the Super Air Meter (SAM) as a way to enhance the C231 Type B test to not only measure the total air volume percentage in fresh concrete, but to also measure the size and spacing of the air-void system used. This test is outlined in AASHTO 118, “Standard Method of Test for Characterization of the Air-Void System of Freshly Mixed Concrete by the Sequential Pressure Method.” The SAM testing apparatus is a C231 testing machine, but is modified to withstand higher internal testing pressures required for the test.
First, the concrete sample is placed, prepared and tested using the existing standard. In the next phase, multiple pressure testing is conducted similar to C231, but pressure is increased to 14.5, 30 and 45 psi. Concrete mix deformations are measured between the different pressure conditions and then a SAM number is calculated.
“A SAM number of 0.20 has been shown to correctly determine over 90% of the time whether the spacing between bubbles meets the recommendations of the ACI 201 Concrete Durability Committee.”3
In 2013, the SAM was approved and is currently being evaluated by a Pooled Fund Study, which includes 16 different departments of transportation. Additionally, it is being used in 32 states, two Canadian provinces and three foreign countries.