A concrete mix additive refers to any number of concrete mix-additives that can be provided from batch or worked with at time of pour. These products vary in feasability depending on your concrete project’s requirements, and some may only really be useful for more commercial/larger-scale concrete projects. Generally speaking, when looking for higher performance in any number of categories/functions with concrete, mix additives play a vital role.
Concrete Mix Design Schedules
A concrete mix design schedule or concrete mix design properties table outlines the requirements for various additives and concrete strengths for various portions of a building. Concrete mix design schedules incorporate such details as strength of concrete(MPa), water content, aggregate size, air range, exposure classification, and other notes.
Strength of Concrete: This is an indicator of the minimum MPa a concrete element must achieve after a given period of time. Various times can be provided, such as 14 days, 28 days, and even 56 days. Compressive strength of concrete samples taken on site is provided through real life testing, typically done by contracted testing companies. Achieving the correct strength in the prescribed time period is required, otherwise the services of a structural engineer need be consulted.
Aggregate Size: This refers to the general maximum size of aggregate permitted within the mixture designated for each concrete element.
Water Content: The ratio of water to cement product in a concrete mix design.
Air Range: This specification denotes air entrainment properties for various concrete mix designs.
Exposure Classification: Identification of the concrete mix design utilization in relation to resistance to sulfates, freeze-thaw, corrosion etc.
Specifications for Admixtures
Generally speaking concrete admixtures for concrete projects and mixes can be referenced by specification by back to ASTM C260 or ASTM C494/C494M.
These specifications are typically standard through Canada/USA for air entrainment or chemical admixtures.
Technical data sheets from suppliers, referenced in conjunction with these standards can allow for a more detailed/accurate interpretation of the exact characteristics.
Concrete Air Entrainment (Air Range)
Concrete air entrainment(air range % modification) is one of the most popular concrete mix additives for concrete. Concrete air entrainment comes in a variety of percentages and is provided as a range typically in mix design schedules.
The result of air entrainment in concrete is the provision of bubbles in the air during the curing process. While very small in nature, these allow for the migration of liquid through the concrete, or at least, enough as to provide for the reduction in the capacity for concrete to crack (freeze-thaw).
When ordering concrete, the percentage of air entrainment can be specified with the plant/batch. For interior slabs, concrete air entrainment is generally undesirable as it reduces the ability to finish the slab properly, losing out on a smooth finish so critical for warehouses or basements and other structures with exposed interior concrete slabs.
Photo Above: A concrete interior slab such as above can benefit from reduced air range for increased level of finish. Concrete grinding and polishing is still essential in meeting final satisfactory finish for day-to-day usage typically.
Concrete Surface Inhibitors / Curing Inhibitors
Concrete surface inhibitors or curing inhibitors refer to products that impede the curing time of concrete. This terminology can apply to surface characteristics and properties of concrete admixtures.
Concrete curing surface inhibitors are additives for concrete that slow the curing time of a concrete. This can be useful for concrete elements like concrete toppers or exposed aggregate concrete sidewalks, walkways, patios, etc.
Superplasticizers are products that will inhibit the curing process of concrete also. In this sense, if for some reason curing inhibition was necessitated, specific admixtures could be used which inherently have concrete curing inhibitor properties.
Superplasticizers
A superplasticizer provides increased fluidity to concrete, theoretically making it easier to handle. This can be advantageous for a concrete that is difficult to handle, such as high performance low water/cement ratio concrete for high performance requirement structural elements and pours.
Superplasticizers also can increase the strength of concrete by several MPa in some cases. MPa are megapascals, a unit of measurement in determination of the strength of the concrete.
There are various levels of superplasticizer on the market, varrying in range with strength of chemicals, from low performance to high performance. The selection of the appropriate product comes down to dictation by the structural engineer or concrete contractor, depending on the project. You might see a high performance superplasticizer on larger scale high performance concrete projects.
Fiber Reinforcements
Fiber reinforcement is provided to strengthen concrete in the way that reinforcement(rebar) increases the strength of concrete elements. Concrete reinforcement is done with fibers such as plastic or steel fibres. They are added at set amounts for the concrete utilized to meet the strength requirements dictated by the structural engineer or architect etc.
This is an alternative to traditional concrete reinforcements like rebar, epoxy coated rebar, or galvanized rebar.
Non-steel fiber reinforcements refer to macro-fibres / micro-fibres products that can be added to a number of concrete elements, typically concrete slabs, and even as standards in municipal/city sidewalk construction.
Photo Above: Concrete sidewalks can commonly implement fiber reinforcements.
Crystalline Waterproofing
Crystalline waterproofing is an additive which increases impermeability of water through concrete. It is an additive that is used sometimes in lieu of roll-on waterproofing products. Crystalline waterproofing products can also increase the MPa of the concrete, similar to a plasticizer. They are used in areas like elevator pits, parkade slabs, sump pits, or other areas which require increased resistance to intrusion of water.
Curing Accelerators / Set Accelerators
Curing accelerators increase the rate to which concrete cures. They may sometimes be noted as, high-early, fast-cure or rapid-set concretes. These mixes are perfect for elements of a building or outdoor area that need be poured and set quickly.
They can reduce the time required for necessary supports(formwork / shoring) for some concrete elements. This sort of concrete can also help with tight schedules on projects that have many concrete elements that need to be poured in succession or with adverse weather conditions. Concrete projects that may be exposed to the elements, or where pour times are constrained by weather, can benefit from high-early mix designs also.
The early curing strength (such as 28 days) is increased, which is generally the goal of a curing accelerator. Longer term curing strength are generally unaffected.
Hardening Accelerators
A hardening accelerator can decrease the time to where concrete is unworkable (hardened). However, the intention of the product is not necessarily to increase long term(final) strength of the concrete. This can be beneficial where short term hard concrete is required either for foot traffic, or for equipment traffic, to both protect the concrete from unnecessary damage, but also promote adjacent construction. Surface protection for fresh concrete may still be warranted though regardless.
Concrete Water Reducers
Concrete water reducers, as the name implies, reduce the amount of water in the concrete mixture in set / cured. Concrete water reducers can increase ‘workability’ with less water consumed in the process for high performance concrete with lower water contents.
A water reducer allows smoother mixes of concrete with less water. On larger concrete pours, such as foundation walls, or pilasters, the utilization of high performance concrete mix designs may necessitate the utilization of concrete water reducers for improved work-ability. The utilization of concrete vibration with large formworks for example, for tall foundation walls, or especially thick slabs, in addition to concrete water reducers, will aid in a structurally and architecturally sound pour!
Improved aesthetic outward appearance is another benefit of concrete water reducers. An experienced concrete contractor will understand under what circumstances water reducers would/could be introduced.
Water reducing properties may extend curing times, but also increase strengths, the balance of which is thought through by engineers/concrete-suppliers.
Consult your concrete supplier or engineer to determine the subjective work-ability of any given mix design, according to your project’s specifications.
Photos Above: Concrete foundations for a home and a larger residential building slab. Larger structural concrete projects, such as commercial buildings, can benefit from products like concrete water reducers, in implementations for high performance concrete.
Color Mix-ins / Additives
Color mix-ins are additives used to change the aesthetic or architectural appearance of concrete. They are typical in landscape concrete elements like sidewalks, stairs, curbs, etc.
Bags of mix-ins are added with the mixture to produce permanent vibrant colors! This is as opposed to shake-on color hardening materials, which are actually applied post-pour of the concrete.
The natural color of concrete/cement mix-ins for colors can take a significant amount of time to show the final color. Base layers of concrete, un-colored can be placed close to the sub-grade, and layers above the un-colored concrete can be colored, saving on mix-in.
Alternatively color placed on surfaces like stamped concrete patios or sidewalks are utilized as surface treatments. Surface staining gives the ability for more creativity to get unique concrete designs.
Various Key Concrete Terms
Concrete Slump
Concrete slump refers to the viscosity and workability of a concrete mixture, and how the concrete might fall or be measured in a concrete slump test. The higher the slump value, the easier the concrete is to work with. This test is done with a metal cone, where the concrete is poured through the cone and allowed to settle for measurements. A product like a water reducer admixture can increase slump of concrete.
Concrete Core Testing
Concrete core testing involves taking samples of the concrete used in projects, structural concrete elements, and architectural concrete elements to evaluate its strength (MPa). Concrete is tested at various points during the phases of the curing process. Typically from 7, to 14, to 28 days and so on and so forth. Each strength measurement has to correspond with a specific MPa for the set time period of testing, dictated by the requirements of the structural engineer to correspond with a desired strength after a certain time span.
One such standard important for concrete testing is C31/C31M.
Field testing
Field testing of concrete involves literally taking samples of concrete and curing them in controlled environments, sometimes, totally done on the field, with coordination with the general contractor.
Shotcrete
Shotcrete is a terminology used for more liquid concrete used to spray against surfaces, trenches, walls etc. It provides temporary stabilization of an area where traditional concrete methods cannot be utilized or are not cost effective.
Shotcrete is applied in lifts at maximum thicknesses prescribed by the manufacturer. Shotcrete is a versatile product that is applied through pressurized pumping nozzles. High MPa is achievable with some shotcrete products. Infact, some custom blends can incorporate fiber reinforcements.
Concrete Exposure Classification Overview
Exposure Classification refers to structural determination characteristics dictated by a structural engineer. This sort of information is applicable to the mix-design of the concrete, tabulated, and associated with different use cases, like outdoor concrete, indoor, and at various MPa requirements. Through the process of tabulation of the structural determination characteristics a general contractor or concrete contractor can provide the right mix design for the right areas throughout the project and achieve the correct result.
Summary
Concrete mix additives are essential components of modern concrete construction. From everything from residential concrete, to commercial concrete, mix additives can improve work-ability, structural characteristics, waterproofing characteristics, and general aesthetic characteristics.
An experienced concrete contractor will understand where and when a specific concrete mix-additive is warranted.
Concrete mix-additives provide specialty characteristics for any number of utilizations for structural and architectural concrete.
On larger projects, where concrete mix design properties are set forth by engineers, the utilization of special mix-additives comes down to the consideration of the engineer and contractor. Meeting schedule, weather, or construct-ability constraints is one of the essential performance characteristics of most additives.
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