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.
A properly proportioned concrete mix possesses acceptable workability of the freshly mixed concrete and durability, strength, and uniform appearance of the hardened concrete while being economical.
Materials for Use in Concrete
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 0.2 inches. 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 0.2 inches. The maximum size of the coarse aggregates is generally in the range of 3/8 to 1 ½ inches. Read more on why we use aggregates in concrete.
Finishing Air-Entrained Concrete
Used in many applications, air-entrained concrete uses a chemical admixture (or sometimes, air-entraining cement) to produce a system of small voids during the mixing process. These voids are stabilized by the air-entraining admixture and remain in the hardened concrete paste. The primary use of air-entraining concrete is for freeze-thaw resistance. Read more on air-entrained concrete.
Admixtures are those ingredients in concrete other than portland cement, water, and aggregates that are added to the mixture immediately before or during mixing.
Read more on chemical admixtures.
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.
Click here for table - Admixtures, Standards and Effect.
Concrete as a Carbon Sink
The topic of global climate change is frequently in the news. The International Panel on Climate Change (IPCC) reports that the increase in the concentration of many compounds in the atmosphere will impact global climate. The most notable of the long-lived greenhouse gases are carbon dioxide and methane. Using concrete for building structures and infrastructure can contribute to the emission of carbon dioxide. Almost all construction processes from manufacturing, through transportation of materials and installation use energy, and much of this energy may come from the burning of fossil fuels.
What most people do not realize is that the release of CO2 from calcination in the manufacture of portland cement may be part of a cyclic process and is partially carbon neutral in smaller timeframes such as decades. It may be fully carbon neutral in longer timeframes. Concrete can absorb carbon dioxide and store it in a process commonly referred to as carbonation. This may be viewed simply as an additional, alternative loop of the complex carbon cycle. Carbon dioxide may be absorbed by concrete in its many forms such as buildings, bridges and pavements. Concrete does not even necessarily have to be directly exposed to the atmosphere for this process to occur. Underground concrete piping and foundations can absorb CO2 from air in the soil, and underground and underwater applications might absorb dissolved carbon dioxide (carbonates) present in groundwater, freshwaters and saltwaters. Read more on carbon dioxide and concrete.
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 percent 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. Read more on evaluating concrete defects.
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. Read more on mass concrete.
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 incompatibilities.
Mixing Water for Concrete
ASTM C1602, Standard Specification for Mixing Water Used in the Production of Hydraulic Cement Concrete, defines sources of mixing water as Batch water, Ice, Water added by the truck operator, Free-moisture on aggregate and water contained in admixtures. Read more on mixing water for concrete.
Perils of Power Washing
Power washing concrete surfaces can cause real problems. Relatively inexpensive high-pressure power washing units are commonly available. Some of those units can deliver water at pressures well in excess of 6,000 psi! Read more on power washing.
Construction materials are increasingly judged by their ecological characteristics. Concrete recycling gains importance because it protects natural resources and eliminates the need for disposal by using the readily available concrete as an aggregate source for new concrete or other applications.
Click here for more on recycled aggregates.
Self-Consolidating concrete (SCC) is a high-performance concrete that can flow easily into tight and constricted spaces without segregating and without requiring vibration. The key to creating self-consolidating concrete (SCC), also referred to as self-compacting, self-leveling, or self-placing concrete, is a mixture that is fluid, but also, stable, to prevent segregation.
Self-Consolidating Concrete Offers New Opportunities for Architectural Concrete
What is Self-Consolidating Concrete (SCC) and how is it tested?
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. Read more on self-cleaning concrete.
Ultra High Performance Concrete
Ultra High Performance Concrete (UHPC), also known as reactive powder concrete (RPC), is a high-strength, ductile material formulated by combining portland cement, silica fume, quartz flour, fine silica sand, high-range water reducer, water, and steel or organic fibers. The material provides compressive strengths up to 29,000 psi and flexural strengths up to 7,000 psi.
The material's unique combination of superior properties and design flexibility facilitated the architect's ability to create the attractive, off-white, curved canopies. Overall, this material offers solutions with advantages such as speed of construction, improved aesthetics, superior durability, and impermeability against corrosion, abrasion and impact—which translates to reduced maintenance and a longer life span for the structure. Read more on ultra-high performance concrete.
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 water tightness 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
Read more on adding water on-site.