Basics of Concrete
ConcreteScience© , Technical Article # 1
Author: Ashok Kakade, P.E.; Fellow of ACI, ICRI, ASCE, ICI; A Licensed Civil Engineer, Concrete Science®, Inc.
What is concrete?
In simple terms, concrete can be defined as a man-made rock manufactured by mixing cement, aggregates (sand and rock), and water. This is just the beginning! Concrete can quickly become a complex mixture of different types of cement, sands, coarse aggregates, fibers, chemical admixtures, and mineral admixtures.
Concrete supplied by a ready-mix concrete supplier is governed by ASTM C 94.
This article is written with “simplicity” in mind to provide clear and basic understanding of the concrete and concrete-science without technical jargons.to provide clear and basic understanding of the concrete and concrete-science without technical jargon crafted with the utmost simplicity in mind, ensuring a clear and basic understanding of concrete and concrete-science without overwhelming technical jargon. Subsequent blogs will delve deeper into the fascinating world of providing a clear and basic understanding of concrete and concrete science without technical jargon. Subsequent blogs will provide more in-depth information on concrete science.
What is cement?
The cement used in the typical concrete construction, white or gray in color, is called hydraulic cement. It consists of Portland cement and has several modifications. Hydraulic cement is defined as cement that not only hardens by reacting with water but also forms a water-resistant product. The cement manufactured from gypsum or lime are non-hydraulic because their hydrated products are not resistant to water. In ancient structures, lime mortar were used, but water resistance characteristic was achieved by mixing pozzolanic materials with it.
There are primarily five different types of cement: Type I & II are most common in general construction. Type III is used when high early strength is desired-especially in cold climates. Type IV is used when low heat of hydration is desired. For example, it is used for the construction of a massive structure such as a Dam. Type V cement is used when high sulfate resistance is desired, typically in a foundation in a soil that has high sulfates contents.The high sulfate limits are defined in the Building Code, ACI 318.
There are more variations in the cement when pozzolanic mineral admixtures are added. The basic five types of cement are governed by ASTM C 150. ASTM is the abbreviation of the American Society for Testing & Materials.
What are aggregates?
The aggregates are primarily divided into two sizes: fine aggregates (sand) and coarse aggregates (rock). Fine aggregates are less than 4.75 mm in size, and coarse aggregates are greater than 4.75 mm in size.
The aggregates can be regular weight (e.g., granite, sandstone, limestones), lightweight (e.g., Pumice or synthetic aggregates), or heavyweight (barium minerals, iron ores, and titanium ores). The regular-weight aggregates weigh about 100 pounds per cubic foot, the light-weight aggregates weigh less than 70 pounds per cubic foot, and the heavy aggregates weigh 180 to 380 pounds per cubic foot.
Most residential construction uses regular-weight aggregates; commercial and high-rise residential buildings use both regular and light-weight aggregates. The heavy-weight aggregates are generally used in nuclear power plant-type construction where radiation shielding is required.
Both natural and manmade aggregates can potentially cause issues within concrete. These aggregate particles can react with the cement paste, leading to an alkali-silica gel or alkali-carbonate reaction-related expansion and cracking.
Metallic or sulfur-rich aggregate particles can cause staining or expansion within the concrete. These are just a few examples of problems that can occur.
The gradation of aggregates is governed by ASTM C 33.
It is best to evaluate the aggregates before they are used in concrete.
The microscopic examination of aggregates and concrete specimens is a critical step in ensuring the quality and durability of construction materials. This testing is governed by ASTM C 295.
What is the role of water?
Typically, potable (drinkable) water is considered suitable for mixing concrete. Drinking water seldom contains dissolved solids more than 1,000 ppm (parts per million). However, water unsuitable for drinking may also be used in mixing concrete. This is because recycled water from mining and many other industrial operations may be safely used in mixing water for concrete. Even seawater, which contains over 30,000 ppm dissolved salts, can be used in mixing concrete, provided the concrete does not have mild steel reinforcement.
It is essential to determine the suitability of water for mixing concrete by making a test specimen and testing it to determine the concrete's strength and setting time.
The high impurities in mixing water may affect the concrete strength and its setting time. It can also cause efflorescence (white powdery salt deposits) on the concrete surface and corrosion of the embedded reinforcing steel.
What is the role of air content?
Air is an integral part of the mixed and hydrated concrete. Some air is introduced during the concrete mixing, some is trapped in concrete during concrete placement and compaction, and some air, called entrained air, is purposefully included in concrete during concrete mixing.
The entrained air content in concrete is especially important where the concrete is exposed to freezing and thawing conditions.
The concrete exposed to free water will absorb it. If the weather gets below freezing, the water absorbed in the concrete will start freezing. When water freezes, it occupies approximately 9 percent more volume. The correct volume and spacing of the entrained air provide room for the water to expand without causing tensile strain on the concrete.
This allows water to freeze and thaw without causing a crack in the concrete.
Understanding and implementing this knowledge can significantly enhance the durability and performance of concrete structures in such conditions.
Standards Used for Concrete-Making Materials
ASTM standards used for testing the construction material are:
ASTM C 150 - Standard Specification for Portland Cement
ASTM C 33 - Standard Specification for Concrete Aggregates
ASTM C 94 : Standard Specification for Ready-Mixed Concrete
ASTM C 295 - Petrographic Examination of Aggregate for Concrete
ASTM C 457 - Microscopical Determination of Parameters of the Air-Void System in Hardened Concrete
ASTM C 856 - Petrographic Examination of Hardened Concrete
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