Concrete exposure designs range in nomenclature using letters and numbers to distinguish various properties associated with varying types of concrete. Different concrete exposure types will coincide with set locations throughout a concrete design properties tables or mix-design schedule, coinciding with industry standard nomenclature to identify design properties. The exposure type is then utilized by a concrete supply company to distinguish the characteristics of the mix provided for structural requirements.
Various exposure characteristics have to be accounted for such as interactions and use-case like: chloride, freeze-thaw cycles, interior finishes, effluent/waste, sulfates, and residential type concretes.
In this post we outline some design characteristics inherent in concrete designs that can promote general good engineering for concrete mixes depending on the project, soil conditions, and exposure to conditions.
Table of Contents
Sulphate Resistance in Concrete Designs
Chloride Resistance in Concrete Designs
Freeze and Thaw Design Considerations
Concrete for Interior Finishes
Effluent/waste Exposed Concrete
Physical Concrete Cover
Standards with Concrete Exposure
Summary
General methodologies for various types of concretes and mix designs with specific resistances may vary from region to region, from supplier to supplier, and may not be uniformly agreed upon in all respects. Services of engineers and experienced concrete suppliers / contractors help to alleviate concerns with mix design selections.
Sulphate Resistance
Sulphates are salts of acids, that are naturally found in soils. If soils are found to contain sulphates engineers may request precautions be taken in concrete mix design properties for residential, commercial and other types of concrete construction elements.
Sulphate (sulfate) is found in soils, and is harmful to concrete, causing an effect appropriately named sulphate attack. Sulphate attack can lead to deterioration of concrete over time. The process by which this occurs is through sulfate (in water) that reacts with tricalcium aluminate (C3 A) or other materials in found in certain concrete/cement mixes. The reactions can cause concrete to expand, inducing cracking and other issues with structural integrity over time.
Cracks can cause ever more entry of sulphate attack, leading to even more issues with concrete structures.
This is one of the reasons that crack repair services are offered.
Reduce Sulphate Attack
Sulphate resistant concretes may have reduced tricalcium aluminate (C3 A). This type of concrete can also be known as high sulphate resisting type.
Sulphate resistance in concrete is commonly found in elements such as: foundation walls, grade beams, footings, pile caps, and other structural foundation elements. Sulphate resistance can be found as a property in concretes like sewage treatment, water storage, and general commercial / residential construction where sulphates are prevalent in soils. This means that actual soil sampling is an important task in identify the risk factor for sulphates at the site of construction, and so is review of the overall design/situation of concrete elements.
Concrete waterproofing means and methods may also provide substantial protection against water intrusion, and therefor reduce the negative consequences of sulphate attack. Suitable drainage around building foundations is another good practice, both for reduction in general hydrostatic pressure but in allowing for proper drainage of waters with sulphates in them, ideally away from foundational concrete elements.
Also, lower water-cement ratios can result in a ‘higher performance’ concrete mix design. Lower water-cement ratios can achieve a smoother and ‘denser’ appearance to the concrete, generally with higher MPa (strength), which may help in reducing negative effects of sulphate attack.
Chloride Resistance
Chlorides are salts found in things like road salts, de-icing chemicals, and found naturally in soils. Chlorides can leach into concrete and are generally detrimental to the rebar/concrete reinforcement. This can happens when scaling/flaking of concrete occurs in extreme circumstances. But the big negative consequence of the chlorides is potential corrosion of rebar/steel-reinforcement or welded-mesh for instance. Deterioration of rebar can cause deterioration of structure. The time period for chloride to negatively reduce the cross-section of rebar embedded in concrete can be substantial, years sometimes.
Some design characteristics of chloride resistant concrete design employ corrosion resistant rebar. This includes types of rebar like galvanized rebar, epoxy coated rebar, and stainless steel rebar. Chlorides affect what is know as the ‘passive layer’ surrounding rebar, which is the concrete close to rebar, but generally affect the rebar first. Specialized rebar as noted reduce the affects of chloride on metal, and slow the process deterioration of the passive layer, and subsequent structural concrete deterioration. With water and oxygen, and chlorides, corrosion processes for rebar/reinforcement can begin, which then cause rebar to change physical dimensions and cause concrete cracking/scaling/flaking etc.
The utilization of adequate concrete cover is also essential in reducing the effects of chlorides on concrete structures. Concrete cover is the distance from the rebar/steel-reinforcement to the outside of the concrete. Corrosion inhibitors too are concrete mix additives advertised for reduction of corrosion of steel reinforcement.
Freeze and Thaw
Naturally concrete can be exposed to water from the surrounding property. If water leaches into concrete, through pores in the surface, or hydro-static pressure, the effects of such can migrate or trap water in concrete. Freeze/thaw is a concrete exposure design characteristic based on this fact, to mitigate the potentially deleterious effects of freeze-thaw cycles. Freeze-thaw cycles reffers to the fact that water, when it freezes, expands. If water expands in concrete, it can cause cracking, and damage concrete. This is obviously detrimental to the structural integrity of elements such as foundations walls, pile caps, footings, and grade beams, or even outdoor slabs.
You will find air entrainment as often purported in mitigating some risk posed by freeeze-thaw cycles. More specifically to serious mitigation, waterproofing means and methods are intended to halt water from even entering concrete in the first place.
Other elements in design methodology for things like grade beams include the utilization of void form for mitigating soil heave effects.
Interior Finishes
Interior slab concrete is often negated from much harmful effects that typical structural concrete might be exposed to. As such design characteristics for these types of concrete can be separated from more stringent requirements of structural concrete elements exposed to chlorides or sulphates for instance.
In addition, interior concrete slabs require a higher level of finish. This means that properties like lower-air-entrainment, sealing, hardeners and the like should be considered in providing an optimal slab finish, and final aesthetic flooring finish.
Effluent/waste Exposed Concrete
Certain structures that deal with waste water(effluent) may be considered for additional protection to harmful effects of waste water treatment.
Not only that, waste water facilities treat a variety of issues like chlorides, sulfates, and other issues that can damage concrete.
Some structures in concrete construction in waste water treatment plants require specialized coatings to promote protection against chemicals and processes. These coatings are typically specially formulated for such applications. Additionally concrete mix-additives and specialized concrete sealers/hardeners can be noted for a variety of serious damage mitigation, extending the lifespan of various concrete elements.
Waste water treatment plans utilize the movement of waters and fluids through various process, which can cause abrasion and physical erosion of concrete.
Physical Concrete Cover
Physical concrete cover is essential to general adequate concrete structural design. The distance from the rebar to the edge of the concrete, known as the concrete cover, provides a protective layer for reinforcement, from the effects of chloride or sulphates.
Most concrete elements will have their concrete cover requirements specified through structural concrete details or structural concrete section cuts.
Positioning of reinforcement during installation (tying/placement) is therefor critical to get correct, as concrete cover is measured in finite distance, like millimeters, sometimes being 50mm (~2″).
Standards
Standards like CSA A23.1:24 provide actual definitions of concrete classes of exposure, which, are often used on project mix design schedules or design property schedules for instance.
The utilization of the classifications in themselves provides enough information to concrete suppliers for the correct mix design to be supplied/used at various elements of the building’s construction. Concrete suppliers and contractors also rely on years on hands-on industry experience.
This method of classification reference serves to help with outlining requirements and design/performance characteristics required at bare minimum to achieve successful project conditions dictated by an engineer. It also takes guess work out of standardizing various exposure types and conditions for concrete through a classification system that is broadly recognized.
Summary
Concrete exposure design and concrete exposure classifications are key determinations in the selection for concrete mix designs through a variety of structural or architectural concrete projects. Providing the right mix design, as specified through structural drawings or engineer instructions, provides protection to a variety of exposure conditions that can potentially harm concrete integrity.
Other strategies could use removal of soils which can negatively affect the concrete. These soils can be replaced with engineered fills that do not have high concentrations of potentially damaging solids in them.
Understanding some of the basic exposure design characteristics that concrete is provided for helps to ensure quality concrete structures last for years. Failure to use the right mix design can result in failures of concrete elements far before they would typically fail under normal conditions.
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