Concrete Basics

Concrete is a mixture of two components: aggregates and paste. The paste, comprised of cement and water, binds the aggregates (usually sand and gravel or crushed stone) into a rocklike mass as the paste hardens because of the chemical reaction of the cement and water. Supplementary cementitious materials and chemical admixtures may also be included in the paste.
For more on concrete basics, click here.

Materials for Use in Concrete

Cement

Cements set and harden by reacting chemically with water. During this reaction, called hydration, cement combines with water to form a stonelike mass, called paste. When the paste (cement and water) is added to aggregates (sand and gravel, crushed stone, or other granular material) it acts as an adhesive and binds the aggregates together to form concrete, the world’s most versatile and most widely used construction material. More.

Supplementary Cementitious Materials (SCM)

Supplementary cementitious materials are generally divided into:

• Pozzolans (fly ash, silica fume, and natural pozzolans, such as calcined shale, calcined clay or metakaolin)


• Slag cements

These materials when used in conjunction with portland or blended cement, contribute to the properties of the hardened concrete through hydraulic activity, pozzolanic activity, or both.

Additional references:
Supplementary Cementing Materials For Use in Concrete CD
Fly Ash, Slag, Silica Fume, and Natural Pozzolans
Design and Control of Concrete Mixtures
Supplementary Cementing Materials for Use in Blended Cements
Benefits of Ternary Mixtures

Aggregates

Aggregates are classified by ASTM C33 (AASHTO M 6/M 80) as fine or coarse. Fine aggregate consists of natural sand, manufactured sand, or a combination thereof with particles that are typically smaller than 5 mm (0.2 in.). Coarse aggregate consists of either (or a combination of) gravel, crushed gravel, crushed stone, air-cooled blast furnace slag, or crushed concrete, with particles generally larger than 5 mm (0.2 in.). The maximum size of the coarse aggregates is generally in the range of 9.5 to 37.5 mm 3/8 to 1 ½ in.).

More on why we use aggregates in concrete.

More on recycled aggregates, click here.

Water

Water Content: Why Less Is More
The quality of hardened concrete is greatly influenced by the amount of water used in relation to the amount of cement. Higher water contents dilute the cement paste (the glue of concrete). Here are some advantages of reducing water content:

• Increased compressive and flexural strength
• Lower permeability, thus increased watertightness and lower absorption
• Increased resistance to weathering
• Better bond between concrete and reinforcement
• Less volume change from wetting and drying
• Reduced shrinkage and cracking

ASTM Specification for Mixing Water.

More about Adding Water On-Site.

Chemical Admixtures

Admixtures are those ingredients in concrete other than portland cement, water, and aggregates that are added to the mixture immediately before or during mixing.

For basics on chemical admixtures, click here.

A wide range of admixtures are available. The table below provides a list of common types of chemical admixtures. The effectiveness of an admixture in concrete depends upon many factors including cementitious materials properties, water content, aggregate properties, concrete materials proportions, mixing time and intensity, and temperature.

Identifying Material Incompatibilities

The wide variety of materials options and mix proportions possible in concrete allows it to be customized for a wide range of applications and placement and service environments. However, the cementitious materials (cements, fly ashes, slag cements, etc.) and chemical admixtures (accelerators, retarders, water reducers, etc.) are all chemically complex and this complexity can lead to problems when they don’t work together properly. Even when all materials meet and exceed their specification requirements individually, problems can arise under field conditions.

Although these problems are relatively rare, the resulting construction delays, performance issues, and loss of confidence in concrete as the preferred construction material are unacceptable. FHWA and PCA co-sponsored research into these phenomena, with the goal of minimizing or preventing these problems in the field. The project developed relatively simple protocols for evaluating concrete material combinations both pre-construction and during construction. More on identifying material incompatibilties.

Early-Age Cracking

Early-age cracking can be a significant problem in concrete. Early age for concrete is the first seven days starting with final set, which is when the concrete has obtained a benchmark level of stiffness. During this time, concrete undergoes a significant amount of volume change caused by many variables, such as the hydration reaction (chemical shrinkage), water content (drying shrinkage and swelling), and temperature changes (thermal dilation).

Volume changes in concrete will drive tensile stress development when they are restrained, which is the case with most concrete. Tensile stresses are forces trying to pull apart the concrete and are opposite from compressive stresses. Cracks can develop when the tensile stress exceeds the tensile strength. While concrete is strong in compression, the tensile strength is generally only 10% of the compressive strength. At early ages, this strength is still developing while stresses are generated by volume changes. Controlling the variables that affect volume change can minimize cracking and create a higher quality concrete placement. More on early-age cracking.

Mass Concrete

Mass concrete is a hot topic. Owners desire long service lives so engineers design concrete mixtures for low permeability. These mixtures typically have high cementitious materials contents, which results in high temperatures within the concrete. To avoid cracking and other temperature related damage to the concrete, contractors must control the temperature and temperature difference in the concrete. This can pit the schedule against the service life.

When all involved parties work together, appropriate changes can be made to achieve the desired service life with minimal impacts to the schedule. The key is an understanding of mass concrete. Selection of an appropriate concrete mixture is the first step. More on mass concrete.

Self-Consolidating Concrete

Flow with Show: Self-Consolidating Concrete Offers New Opportunities for Architectural Concrete

What is Self-Consolidating Concrete (SCC) and how is it tested?

Self-Cleaning Concrete

Self-cleaning buildings and pollution-reducing roadways: These may sound like futuristic ideas, but they are realities of some of today’s concrete. Recently introduced formulations of cement are able to neutralize pollution. Harmful smog can be turned into harmless compounds and washed away. Anything made out of concrete is a potential application, because these cements are used in the same manner as regular portland cements. These products provide value through unique architectural and environmental performance capabilities.

Proprietary technology (based on particles of titanium dioxide) is what makes this cement special—capable of breaking down smog or other pollution that has attached itself to the concrete substrate, in a process known as photocatalysis. More on self-cleaning concrete.

Standards

Significance of Tests and Properties of Concrete and Concrete-Making Materials (STP169D) (LT205)
In a very real sense, specifications are the letter of the law. But for the professional that needs to know the how and why behind the development, this encyclopedic reference is the place to turn. Ever wonder how much strength a test cylinder will lose from rolling and bumping around in the back of a pickup truck or from being dropped from waist level? How about the effect of bearing strips on test cylinders used for splitting tensile strength? Which DOT performed the original research for strength determination using maturity? How much heat contribution should I expect from each of the four major cement compounds? This is the type of information that can help avoid headaches and avert disasters. This peer-reviewed work is a must have for the cement and concrete professional producing, using, or testing materials in conformance with ASTM specifications.
Read a review of the book.
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