Frequently Asked Questions About Concrete

Are there different types of portland cement? 
Though all portland cement is basically the same, eight types of cement are manufactured to meet different physical and chemical requirements for specific applications: 

• Type I is a general purpose portland cement suitable for most uses. 
• Type II is used for structures in water or soil containing moderate amounts of sulfate, or when heat build-up is a concern. 
• Type III cement provides high strength at an early state, usually in a week or less. 
• Type IV moderates heat generated by hydration that is used for massive concrete structures such as dams. 
• Type V cement resists chemical attack by soil and water high in sulfates. 
• Types IA, IIA and IIIA are cements used to make air-entrained concrete. They have the same properties as types I, II, and III, except that they have small quantities of air-entrained materials combined with them. 
• White portland cement is made from raw materials containing little or no iron or manganese, the substances that give conventional cement its gray color.

How can you tell if you're getting the amount of concrete you're paying for? 
The real indicator is the yield, or the actual volume produced based on the actual batch quantities of cement, water and aggregates. The unit weight test can be used to determine the yield of a sample of the ready mixed concrete as delivered. It's a simple calculation that requires the unit weight of all materials batched. The total weight information may be shown on the delivery ticket or it can be provided by the producer. Many concrete producers actually over yield by about ½ percent to make sure they aren't short-changing their customers. But other producers may not even realize that a mix designed for one cubic yard might only produce 26.5 cubic feet or 98 percent of what they designed. 

How do you control the strength of concrete? 
The easiest way to add strength is to add cement. The factor that most predominantly influences concrete strength is the ratio of water to cement in the cement paste that binds the aggregates together. The higher this ratio is, the weaker the concrete will be and vice versa. Every desirable physical property that you can measure will be adversely affected by adding more water. 

How do you protect a concrete surface from aggressive materials like acids? 
Many materials have no effect on concrete. However, there are some aggressive materials, such as most acids, that can have a deteriorating effect on concrete. The first line of defense against chemical attack is to use quality concrete with maximum chemical resistance, followed by the application of protective treatments to keep corrosive substances from contacting the concrete. Principles and practices that improve the chemical resistance of concrete include using a low water-cement ratio, selecting a suitable cement type (such as sulfate-resistant cement to prevent sulfate attack), using suitable aggregates, water and air entrainment. A large number of chemical formulations are available as sealers and coatings to protect concrete from a variety of environments; detailed recommendations should be requested from manufacturers, formulators or material suppliers. 

How do you remove stains from concrete? 
Stains can be removed from concrete with dry or mechanical methods, or by wet methods using chemical or water. 
Common dry methods include sandblasting, flame cleaning and shotblasting, grinding, scabbing, planing and scouring. Steel-wire brushes should be used with care because they can leave metal particles on the surface that later may rust and stain the concrete. 
​Wet methods involve the application of water or specific chemicals according to the nature of the stain. The chemical treatment either dissolves the staining substance so it can be blotted up from the surface of the concrete or bleaches the staining substance so it will not show. 

How is portland cement made? 
Cement manufacturers mine materials such as limestone, shale, iron ore, and clay, crushed and screened the rock, and place it in a cement kiln. After being heated to extremely high temperatures, these materials form a small ball called “clinker” that is very finely grounded to produce portland cement.

Lime and silica make up about 85 percent of the ingredients of cement. Other elements include alumina and iron oxide. The rotating kiln that cooks the materials resembles a large horizontal pipe with a diameter of 10 to 15 feet and a length of 300 feet or more. One end is raised slightly. The raw mix is placed in the high end and as the kiln rotates the materials move slowly toward the lower end. Flame jets at the lower end heat all the materials in the kiln to high temperatures that range between 2,700 and 3,000 degrees Fahrenheit. This high heat drives off, or calcines, the chemically combined water and carbon dioxide from the raw materials and forms new compounds (tricalcium silicate, dicalcium silicate, tricalcium aluminate and tetracalcium aluminoferrite). For each ton of material that goes into the feed end of the kiln, two thirds of a ton comes out the discharge end, called clinker. This clinker is in the form of marble sized pellets. The clinker is very finely ground to produce portland cement. Manufacturers often add gypsum and/or limestone during the grinding process. 

Is there a universal international specification for portland cement? 
Each country has its own standard for portland cement, so there is no universal international standard. The United States uses the specification prepared by the American Society for Testing and Materials-ASTM C-150 Standard Specification for Portland Cement. There are a few other countries that also have adopted this as their standard, however, there are countless other specifications. Unfortunately, they do not use the same criteria for measuring properties and defining physical characteristics so they are virtually "non-translatable." The European Cement Association located in Brussels, Belgium, publishes a book titled "Cement Standards of the World." 

What are recommended mix proportions for good concrete? 
Good concrete can be obtained by using a wide variety of mix proportions if proper mix design procedures are used. A good general rule to use is the rule of 6's: 

• A minimum cement content of six bags per cubic yard of concrete, 
• A maximum water content of 6 gallons per bag of cement, 
• A curing period (keeping concrete moist) a minimum of six days, and 
• An air content of 6 percent (if concrete will be subject to freezing and thawing). 

What are the decorative finishes that can be applied to concrete surfaces? 
Color may be added to concrete by adding pigments-before or after concrete is place-and using white cement rather than conventional gray cement, by using chemical stains, or by exposing colorful aggregates at the surface. Textured finishes can vary from a smooth polish to the roughness of gravel. Geometric patterns can be scored, stamped, rolled, or inlaid into the concrete to resemble stone, brick or tile paving. Other interesting patterns are obtained by using divider strips (commonly redwood) to form panels of various sizes and shapes ¬ rectangular, square, circular or diamond. Special techniques are available to make concrete slip-resistant and sparkling. 

What is 3,000 pound concrete? 
It is concrete that is strong enough to carry a compressive stress of 3,000 psi at 28 days. Concrete may be specified at other strengths as well. Conventional concrete has strengths of 7,000 psi or less; concrete with strengths between 7,000 and 14,500 psi is considered high-strength concrete.

What is alkali-silica reactivity (ASR)? 
Alkali-silica reactivity is an expansive reaction between reactive forms of silica in aggregates and potassium and sodium alkalis, mostly from cement, but also from aggregates, pozzolans, admixtures and mixing water. External sources of alkali from soil, deicers and industrial processes can also contribute to reactivity. The reaction forms an alkali-silica gel that swells as it draws water from the surrounding cement paste, thereby inducing pressure, expansion and cracking of the aggregate and surrounding paste. This often results in map-pattern cracks, sometimes referred to as alligator pattern cracking. ASR can be avoided through 1) proper aggregate selection, 2) use of blended cements, 3) use of proper pozzolanic materials and 4) contaminant-free mixing water.

What is the difference between cement and concrete? 
Although the terms cement and concrete often are used interchangeably, cement is actually an ingredient of concrete. Concrete is a mixture of aggregates and paste. The aggregates are sand and gravel or crushed stone; the paste is water and portland cement. 
Cement comprises from 10 to 15 percent of the concrete mix, by volume. Through a process called hydration, the cement and water harden and bind the aggregates into a rocklike mass. This hardening process continues for years meaning that concrete gets stronger as it gets older. 

Portland cement is not a brand name, but the generic term for the type of cement used in virtually all concrete, just as stainless is a type of steel and sterling a type of silver. Therefore, there is no such thing as a cement sidewalk, or a cement mixer; the proper terms are concrete sidewalk and concrete mixer.

Why do concrete surfaces flake and spall? 
Concrete surfaces can flake or spall for one or more of the following reasons: 

• In areas of the country that are subjected to freezing and thawing the concrete should be air-entrained to resist flaking and scaling of the surface. If air-entrained concrete is not used, there will be subsequent damage to the surface
• The water/cement ratio should be as low as possible to improve durability of the surface. Too much water in the mix will produce a weaker, less durable concrete that will contribute to early flaking and spalling of the surface.
• The finishing operations should not begin until the water sheen on the surface is gone and excess bleed water on the surface has had a chance to evaporate. If this excess water is worked into the concrete because the finishing operations are begun too soon, the concrete on the surface will have too high a water content and will be weaker and less durable.

Why does concrete crack? 
Concrete, like all other materials, will slightly change in volume when it dries out. In typical concrete this change amounts to about 500 millionths. Translated into dimensions-this is about 1/16 of an inch in 10 feet. The reason that contractors put joints in concrete pavements and floors is to allow the concrete to crack in a neat, straight line at the joint when the volume of the concrete changes due to shrinkage.
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Will concrete harden under water? 
Portland cement is a hydraulic cement which means that it sets and hardens due to a chemical reaction with water. Consequently, it will harden under water. 

How long will a concrete pavement typically last? 
It is common for concrete pavements to last 30 years or more1 before major rehabilitation. Even after the first major rehabilitation, many concrete pavements continue to remain in service for several more years2. In fact, the first concrete city street in Iowa was built in 1904 in LeMars (Eagle Street now First Ave NW), was not overlaid until the late 1960s, and is still in use today.

Concrete pavements allow great flexibility in design life by providing the designer with the broadest possible range. In the past, it was common to design concrete pavements for 30 years3. Today, concrete pavements are being designed for service lives of 30 to 50 years. The Iowa Department of Transportation (DOT), for example, has moved to a service life of 40 years4 while the Minnesota DOT uses 50 years5. The Statewide Urban Design and Specifications (SUDAS) also include a 50 year design life for concrete pavements6.

How does the service life of concrete compare to that of asphalt? 
Typical comparisons of service lives between asphalt and concrete show a ratio of about 2:1, with concrete having the higher of the two7. Iowa DOT, and others, use this ratio in their pavement type selection process8. A similar ratio is also reflected in the SUDAS pavement design guidance which uses a 50 year design for concrete and a 30 year design for asphalt9.
Iowa's primary highway system further demonstrates the greater longevity of concrete pavements10. Our primary system includes about 1,300 miles of concrete pavements over 30 years old that have never been overlaid. There are no asphalt pavements on today's primary system in Iowa that have made it over 30 years without overlay and only about 1% (25 miles) of Iowa's asphalt pavements have made it to 20 years without overlay.

How closely does concrete compare to asphalt on initial cost?
In Iowa, the cost to construct a concrete pavement is typically within 10% of the cost to construct an equivalently designed asphalt pavement11. We sometimes see comparisons that do not use equivalent designs and these should not be used as a true comparison of construction costs. For example, comparing the cost to construct an 8" asphalt pavement to the cost to construct an 8" concrete pavement would not give accurate results because 8" of concrete will carry far more load than 8" of asphalt12. This would be like comparing the cost to construct an 8" concrete pavement to the cost to construct a 10" concrete pavement.

Which type of pavement costs less to maintain?
Studies conducted in the Midwest indicate that concrete pavements cost significantly less to maintain13. In support of these results, many agencies include relatively few maintenance activities for concrete in their pavement type selection processes14. Specifically, the only maintenance activity reflected in Iowa DOT's analysis is joint resealing.

Which type of pavement costs less to own?
​The Iowa Concrete Paving Association recommends using Life Cycle Cost Analysis (LCCA) as a means to evaluate the true cost of owning a pavement. 15LCCA takes into account the construction costs, maintenance costs, and the service life of the pavement. In its simplest form, LCCA would be the sum of the two costs divided by the number of years in the evaluation period. An accurate LCCA will include equivalently designed pavement sections and an evaluation period equal to the longest of the service lives for all pavement options. It is important to note that equivalently designed concrete and asphalt pavement sections will not perform equivalently. For example, Iowa DOT's pavement type selection process uses equivalent designs, but the asphalt pavement is projected to require major rehabilitation in half the time as the concrete pavement16.
Under long-term analysis like properly conducted LCCAs, concrete pavements are commonly found to be the most cost effective options. Studies across the country report significant savings (upwards of 72% in one Iowa study17) as a result of selecting concrete over asphalt18.
There is probably no agency in Iowa that has more experience with pavement construction and maintenance than the Iowa DOT. Each year, the Iowa DOT performs life cycle cost analysis for each of their upcoming paving projects. After performing LCCAs, the Iowa DOT selects concrete over 90% of the time19.
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(1) City of West Des Moines, 2004 Street Management System Report and ERES Studies of: 
      Interstate 15 - Utah; 
      Interstate 40 - Oklahoma; 
      Interstate 40 - Tennessee
(2) Quick Guide to Transportation System Facts
(3) Iowa DOT's 30 Year Design Pavement Type Selection Process
(4) Iowa DOT's 40 Year Design Pavement Type Selection Process
(5) MnDOT's Pavement Selection Life Cycle Cost Analysis
(6) Statewide Urban Design and Specifications (SUDAS)
7) ERES Studies and City of West Des Moines, 2004 Street Management System Report
(8) Iowa DOT's 40 Year Design Pavement Type Selection Process and MnDOT's Pavement Selection Life Cycle Cost Analysis
(9) SUDAS
(10) Quick Guide to Transportation System Facts
(11) Alternate Bidding – Bid Tabs
(12) Equivalency Chart for Concrete and Asphalt Pavements
(13) Impact of Pavement Type on County Road Systems and A Comparison of the Life-Cycle Costs of Asphalt Concrete and Portland Cement Concrete Pavements in Olmsted County, Minnesota
(14) Iowa DOT's 40 Year Design Pavement Type Selection Process, MnDOT's Pavement Selection Life Cycle Cost Analysis, examples of maintenance projections from City of Cedar Falls and City of Carroll (pavement type selection processes)
(15) Life Cycle Cost Analysis: A Guide for Comparing Alternate Pavement Designs
(16) Iowa DOT's 40 Year Design Pavement Type Selection Process
(17) Impact of Pavement Type on County Road Systems
(18) Impact of Pavement Type on County Road Systems; A Comparison of the Life-Cycle Costs of Asphalt Concrete and Portland Cement Concrete Pavements in Olmsted County, Minnesota; ERES' studies from Utah, Oklahoma, and Tennessee
(19) Iowa DOT Full-Depth Paving Projects by Pavement Type (2000-2004)

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