Section 7: Concrete Materials
Anchor: #i10142707.1 Hydraulic Cements
Hydraulic cements are defined as cements that not only harden by reacting with water but also form a water-resistant product. Hydraulic cements set and harden by reacting chemically with water. During this reaction, called hydration, cement combines with water to form calcium-silica-hydrates, which are the materials providing cementing actions, calcium hydroxide, and a few other compounds.
The primary constituents of hydraulic cements are calcium and silica. Small amounts of iron, alumina, and sulfates also exist. Sources of raw materials for the manufacture of cements include limestone (for calcium), clay (for silica, alumina, and iron) and gypsum (sulfate). These raw materials are crushed, milled, and proportioned in such a way that the resulting mixture has the desired chemical composition and is then fed into a kiln where temperatures of 2,600 to 3,000°F change the raw material chemically into cement clinker, grayish-black pellets about the size of 1/2-in. diameter marbles.
The clinker is cooled and then pulverized, resulting in hydraulic cement. In the pulverization process, a small amount of gypsum is added to control the hydration of aluminates.
The four principal compounds of hydraulic cement are:
Tricalcium silicate (alite: C3S) hydrates and hardens rapidly and is largely responsible for initial set and early strength. In general, the early strength of hydraulic cement concrete is higher with increased percentages of tricalcium silicate.
Dicalcium silicate (belite: C2S) hydrates and hardens slowly and contributes largely to strength increase at ages beyond one week.
Tricalcium aluminate (C3A) liberates a large amount of heat during the first few days of hydration and hardening. Gypsum, which is added to cement during final grinding, slows down the hydration to control the heat of hydration. Without gypsum, cement sets rapidly, called flash set. Large amounts of C3A make cement vulnerable to external sulfate attack and, for sulfate resistant cement, its amount is limited to a maximum of 8% for Type II and 15% for Type III.
Tetracalcium aluminoferrite (C4AF) reduces the temperature required to change the raw material chemically into cement clinker, thereby making the cement manufacturing process more energy efficient. It hydrates rather rapidly but contributes very little to strength. Most color effects in concrete are due to tetracalcium aluminoferrite and its hydrates.
ASTM C150, Standard Specification for Portland Cement, provides for eight types of hydraulic cements; the most commonly used cements are listed below.
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- Type I: A general-purpose cement suitable for all uses where the special properties of other types are not required. This is the most widely used cement type for pavement concrete. Anchor: #MOKPYABS
- Type II: When moderate sulfate resistance or moderate heat of hydration is desired. Anchor: #IWSHMDKY
- Type III: For use when high early strength is desired. For TxDOT paving concrete, this cement type is allowed only for Class HES concrete. Anchor: #QGUKLOAG
- Type V: For use when high sulfate resistance is desired.
Most cement producers in Texas produce a Type I/II cement. This type of cement is widely used in TxDOT projects. This cement meets the requirements for both Type I and Type II cements, and can be used in concrete where either Type I or Type II cement is required.
Anchor: #i10531737.2 Blended Cements
Blended hydraulic cements are produced by intimately and uniformly blending hydraulic cement with other types of fine materials at the cement plant during the clinker grinding process. The primary blending materials are fly ash, slag cement, and limestone. Other pozzolans, such as silica fume, can also be used in blended cements. In Item 421, four types of blended cements are allowed:
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- Type IP consists essentially of an intimate and uniform blend of Portland cement and a pozzolan. DMS-4600 requires Type IP cements used for TxDOT projects to contain 20% to 40% of a Class F fly ash. Anchor: #FTHWBVNS
- Type IS consists essentially of an intimate and uniform blend of Portland cement and slag cement. DMS-4600 requires Type IS cements used for TxDOT projects to contain greater than 35% of slag cement. Anchor: #BAPPWGXK
- Type IT consists of an intimate and uniform blend of Portland cement and a combination of two other materials such as fly ash and silica fume, or fly ash and limestone. Anchor: #VLSNSUIE
- Type IL consists essentially of an intimate and uniform blend of Portland cement and a maximum of 15% limestone.
7.3 Fly Ash
Fly Ash is the most used Supplementary Cementing Material (SCM) in Texas due to its availability. Fly ash is a byproduct of the coal burning electric power generating plants. After the ignition of coal in the furnace, the ash residue is carried away by the exhaust gases, and is then collected with electrostatic precipitators or in filter bag houses. As the fly ash travels to the collectors, the material cools and forms spherical glassy particles.
There are two classes of fly ash, Class F and Class C fly ash. Fly ashes are categorized into these classes based on their chemical composition. Class F fly ashes have higher amounts of silica and lower amount of calcium, while Class C fly ashes tend to have lower amounts of silica and higher amount of calcium. Both classes of fly ashes are acceptable for use in concrete pavement provided the total cementitious content of concrete is 520 lb/CY or less.
Class F fly ashes are generally pozzolanic which means they possess little to no cementing properties. However, in the presence of water and calcium hydroxide, they will react and form compounds having cementing properties. Class F fly ashes are excellent in improving the long-term durability of concrete. Class F fly ashes are effective in reducing permeability and mitigating alkali-silica reactions (ASR), delayed ettringite formation (DEF) and external sulfate attack at relatively low replacement rates.
Class C fly ashes have both pozzolanic and cementing properties. Due to the higher calcium content, Class C fly ashes will react with water and harden. Class C fly ashes are effective in reducing permeability, but not as effective in mitigating alkali-silica reactions (ASR), delayed ettringite formation (DEF) and external sulfate attack. High replacement rate of Class C fly ash are needed to mitigate ASR and DEF, and due to the chemistry of the glass phase, Class C fly ashes are susceptible to external sulfate attack and should not be used in sulfate environments. However, due to the low cementitious content of paving concrete mix designs, both ASR and DEF are not major concerns, and since concrete pavements are not in direct contact with natural subgrades, external sulfate attack is also not of concern; therefore, Class C fly ashes are allowed in paving concrete.
Anchor: #i10517297.4 Aggregates
Coarse and fine aggregates make up 60% to 75% of the volume of the concrete mixtures and can have a strong influence on the fresh and hardened properties of the concrete. Almost any aggregate can be used to produce quality concrete provided the aggregates are durable and clean. Item 421 lists the aggregate requirements.
The particle size distribution or gradation of the aggregate is an important characteristic. Coarse aggregate Grades 2 and 3 listed in Item 421 are typically used in concrete pavements and have large nominal maximum aggregate size. Aggregates are typically gap graded, meaning they are missing sizes throughout the distribution. Gap graded aggregates tend to need more paste (cementitious materials and water) to fill the voids between aggregate particles. As the paste content increases, the amount of potential shrinkage also increases. One method to minimize paste content is to optimize the gradation of the aggregates.
7.4.1 Optimizing Aggregate Gradations
There are several methods that can be utilized to analyze aggregate gradation, and each method has its own pros and cons. The department uses a recently developed percent retained method commonly called the “Tarantula Curve.” In this method, the combined percent retained on every standard sieve for coarse and fine aggregate is used to analyze the gradation. If the percent retained on each sieve is within the limits of the Tarantula Curve, then the gradation is considered optimized. Procedures for analyzing aggregate gradations are outline in Tex-470-A, “Optimized Aggregate Gradation for Hydraulic Cement Concrete Mix Designs.” It is important to note that if an effort is not made to reduce paste content in the mix design, then there are no benefits to utilizing optimized gradation.
Figure 3-6. Percent Retained Chart.
7.4.2 Coefficient of Thermal Expansion of Coarse Aggregates
Since the coarse aggregate makes up the majority of the volume of the concrete, it has a large impact on the coefficient of thermal expansion (CTE) of the overall concrete. The CTE of the concrete influences the long-term performance of CRCP. CRCP sections constructed with concrete having a high CTE value result in shallow spalling at every transverse crack location causing rough ride of the pavement. Through several years of research, it has been determined that if the CTE of the concrete is limited to not more than 5.5 microstrain/°F, the shallow spalling problem was virtually eliminated. Item 360 limits the CTE of concrete used for CRCP to not more than 5.5 microstrain/°F.
Anchor: #i10527667.5 Water
Any municipal water source approved by the Department of Health can be used in concrete with no additional testing. Water sources or blends of concrete wash water not approved by the Department of Health can still be used provided they do not contain deleterious amounts of ions such as alkali, chloride, and sulfates, or total solids. Item 421 lists the requirements that water from non-municipal sources must meet. In addition to the chemical compositions of the water, the effect that non-municipal water sources have on concrete setting time and strength must also be evaluated.
Anchor: #i10528137.6 Chemical Admixtures
Chemical admixtures are used to enhance both the fresh and hardened properties of concrete. The department pre-approves several types of chemical admixtures for use. The pre-approved types are listed below.
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- Type A – Water Reducing Admixture: These admixtures reduce the quantity of mixing water required to produce concrete of a given consistency. Anchor: #FAAUGMFK
- Type B – Retarding Admixture: These admixtures retard the setting of concrete. Anchor: #QCVLRHGT
- Type C – Accelerating Admixture: These admixtures accelerate the setting and early strength development of concrete. Anchor: #RAWQCKUH
- Type D – Water-Reducing and Retarding Admixture: These admixtures reduce the quantity of mixing water required to produce concrete of a given consistency and retard the setting of concrete. Anchor: #CXEVCAKF
- Type G – High Range Water Reducing Admixture: These admixtures reduce the quantity of mixing water required to produce concrete of a given consistency by 12% or greater. Anchor: #GYAISVPR
- Type F – High Range Water Reducing and Retarding: These admixtures reduce the quantity of mixing water required to produce concrete of a given consistency by 12% or greater and retard the setting of concrete. Anchor: #XLPKNNLA
- Air Entraining Admixture: These admixtures are used to stabilize air mixed into the concrete during mixing and create a system of small, closely spaced air voids within the concrete.
The common types of chemical admixtures used in paving concrete are Type A, Type B, Type D, and air entraining admixtures. Since most concrete pavement are placed with a slip form paver, a high degree of workability is not necessary or wanted; therefore, high range water reducers are not used. Some of the recent admixtures are called mid-range water reducers. There is not an official ASTM classification for these admixtures, but they are usually approved as Type A or Type F admixture. These mid-range admixtures provide slightly more water reduction compared to normal range water reducers, reduce stickiness, and improve finishing, pumping, and placing properties.