Technical Information

Concrete

Concrete

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.

Partners:

This place is
reserved for
YOUR
Banner

Introduction

On-Site Concrete

Concrete cast in situ is site-mixed or ready-mixed concrete which remains in its final position in the structure after being placed in the formwork.

The ready-mix plant facilities from which the concrete is delivered have now become so widespread in many markets that the contractor can be supplied quickly and reliably.

A facility on site still offers economic and logistic advantages on large construction sites where concrete is required continuously.

Concrete cast in situ can be produced in different variations and must conform with a variety of specifications. Its application can be divided into the following stages:

Back to top

Precast Concrete

Precast concrete is used to form structures which are delivered after hardening. Long journeys in the fresh concrete state disappear, which changes the whole production sequence. Concrete used for the production of precast structures requires an industrialized production process, and a good concrete mix design with continuous optimization is essential. The following points are important through the different stages of the process:

Back to top

Applications

Forms of Concrete

Concrete is produced in four basic forms, each with unique applications and properties.

• Ready-mixed concrete is by far the most common form in mature markets. It is batched at local plants for delivery in trucks with revolving drums.
• Precast concrete products are cast in a factory setting. These products benefit from tight quality control achievable at a production plant. Precast products range from concrete bricks and paving stones to bridge girders, structural components, and panels for cladding.
• Concrete masonry, another type of manufactured concrete, may be best known for its conventional blocks. Today's masonry units can be molded into a wealth of shapes, configurations, colors, and textures to serve an infinite spectrum of building applications and architectural needs. Cement-based materials represent products that defy the label of "concrete," yet share many of its qualities. Conventional materials in this category include mortar, grout, and terrazzo.
• Soil-cement and roller-compacted concrete are used for pavements and dams. Other products in this category include flowable fill and cement-treated bases. A new generation of advanced products incorporates fibers and special aggregate to create roofing tiles, shake shingles, lap siding, and countertops. And an emerging market is the use of cement to treat and stabilize waste.

Back to top

Material Technology

Materials 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.

Supplementary Cementing Materials

Supplementary cementing materials, also called mineral admixtures, contribute to the properties of hardened concrete through hydraulic or pozzolanic activity. Typical examples are natural pozzolans, fly ash, ground granulated blast-furnace slag, and silica fume, which can be used individually with portland or blended cement or in different combinations. These materials react chemically with calcium hydroxide released from the hydration of portland cement to form cement compounds. These materials are often added to concrete to make concrete mixtures more economical, reduce permeability, increase strength, or influence other concrete properties.

Fly ash, the most commonly used pozzolan in concrete, is a finely divided residue that results from the combustion of pulverized coal and is carried from the combustion chamber of the furnace by exhaust gases. Commercially available fly ash is a by-product of thermal power generating stations.

Combined Use of Fly Ash Conformity

To ensure sufficient alkalinity of the pore solution in reinforced and prestressed concrete, the following requirements shall be met for the maximum amount of fly ash and Silicafume:

• Fly ash ≤ (0.66 × cement – 3 × Silicafume) by mass
• Silicafume/cement ≤ 0.11 by mass

Blast-furnace slag, or iron blast-furnace slag, is a nonmetallic product consisting essentially of silicates, aluminosilicates of calcium, and other compounds that are developed in a molten condition simultaneously with the iron in the blast-furnace.

Silica fume, also called condensed silica fume and microsilica, is a finely divided residue resulting from the production of elemental silicon or ferro-silicon alloys that is carried from the furnace by the exhaust gases. Silica fume, with or without fly ash or slag, is often used to make high-strength concrete.

Below is a summary of the specifications and classes of supplementary cementing materials:

• Ground granulated iron blast-furnace slag
• Slags with a low activity index
• Slags with a moderate activity index
• Slags with a high activity index
• Fly ash and natural pozzolans

Aggregates

Aggregates are classified 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 (often 4 mm.). 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 (4mm, respectively). The maximum size of the coarse aggregates is generally in the range of 9.5 to 37.5 mm (8 to 32 mm).

Aggregates are inert granular materials such as sand, gravel, or crushed stone that, along with water and portland cement, are an essential ingredient in concrete. For a good concrete mix, aggregates need to be clean, hard, strong particles free of absorbed chemicals or coatings of clay and other fine materials that could cause the deterioration of concrete. Aggregates, which account for 60 to 75 percent of the total volume of concrete, are divided into two distinct categories - fine and coarse.

• Fine aggregates generally consist of natural sand or crushed stone with most particles passing through a 9.5 mm
  (8 mm) sieve.  
• Coarse aggregates are any particles greater than 4.75 mm (4 mm), but generally range between 9.5 mm to
  37.5 mm (8 to 32 mm) in diameter. Gravels constitute the majority of coarse aggregate used in concrete with crushed stone making up most of the remainder

Natural gravel and sand are usually dug or dredged from a pit, river, lake, or seabed. Crushed aggregate is produced by crushing quarry rock, boulders, cobbles, or large-size gravel. Recycled concrete is a viable source of aggregate and has been satisfactorily used in granular subbases, soil-cement, and in new concrete. Aggregate processing consists of crushing, screening, and washing the aggregate to obtain proper cleanliness and gradation. If necessary, a benefaction process such as jigging or heavy media separation can be used to upgrade the quality

Once processed, the aggregates are handled and stored in a way that minimizes segregation and degradation and prevents contamination. Aggregates strongly influence concrete's freshly mixed and hardened properties, mixture proportions, and economy. Consequently, selection of aggregates is an important process. Although some variation in aggregate properties is expected, characteristics that are considered when selecting aggregate include:

• grading
• durability
• particle shape and surface texture
• abrasion and skid resistance
• unit weights and voids
• absorption and surface moisture

Grading refers to the determination of the particle-size distribution for aggregate. Grading limits and maximum aggregate size are specified because grading and size affect the amount of aggregate used as well as cement and water requirements, workability, pumpability, and durability of concrete. In general, if the water-cement ratio is chosen correctly, a wide range in grading can be used without a major effect on strength. When gap-graded aggregate are specified, certain particle sizes of aggregate are omitted from the size continuum. Gap-graded aggregate are used to obtain uniform textures in exposed aggregate concrete. Close control of mix proportions is necessary to avoid segregation

Shape and Size Matter

Particle shape and surface texture influence the properties of freshly mixed concrete more than the properties of hardened concrete. Rough-textured, angular, and elongated particles require more water to produce workable concrete than smooth, rounded compact aggregate. Consequently, the cement content must also be increased to maintain the water-cement ratio. Generally, flat and elongated particles are avoided or are limited to about 15 percent by weight of the total aggregate. Unit-weight measures the volume that graded aggregate and the voids between them will occupy in concrete. The void content between particles affects the amount of cement paste required for the mix. Angular aggregate increase the void content. Larger sizes of well-graded aggregate and improved grading decrease the void content. Absorption and surface moisture of aggregate are measured when selecting aggregate because the internal structure of aggregate is made up of solid material and voids that may or may not contain water. The amount of water in the concrete mixture must be adjusted to include the moisture conditions of the aggregate. Abrasion and skid resistance of an aggregate are essential when the aggregate is to be used in concrete constantly subject to abrasion as in heavy-duty floors or pavements. Different minerals in the aggregate wear and polish at different rates. Harder aggregate can be selected in highly abrasive conditions to minimize wear

Water

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

Chemical Admixtures

Chemical admixtures are the ingredients in concrete other than portland cement, water, and aggregate that are added to the mix immediately before or during mixing. Producers use admixtures primarily to reduce the cost of concrete construction; to modify the properties of hardened concrete; to ensure the quality of concrete during mixing, transporting, placing, and curing; and to overcome certain emergencies during concrete operations.

Successful use of admixtures depends on the use of appropriate methods of batching and concreting. Most admixtures are supplied in ready-to-use liquid form and are added to the concrete at the plant or at the jobsite. Certain admixtures, such as pigments, expansive agents, and pumping aids are used only in extremely small amounts and are usually batched by hand from premeasured containers.

The effectiveness of an admixture depends on several factors including: type and amount of cement, water content, mixing time, slump, and temperatures of the concrete and air. Sometimes, effects similar to those achieved through the addition of admixtures can be achieved by altering the concrete mixture-reducing the water-cement ratio, adding additional cement, using a different type of cement, or changing the aggregate and aggregate gradation.

Five Functions

Admixtures are classed according to function. There are five distinct classes of chemical admixtures: air-entraining, water-reducing, retarding, accelerating, and plasticizers (superplasticizers). All other varieties of admixtures fall into the specialty category whose functions include corrosion inhibition, shrinkage reduction, alkali-silica reactivity reduction, workability enhancement, bonding, damp proofing, and coloring. Air-entraining admixtures, which are used to purposely place microscopic air bubbles into the concrete, are discussed more fully in "Air-Entrained Concrete."

• Water-reducing admixtures usually reduce the required water content for a concrete mixture by about 5 to 10 percent. Consequently, concrete containing a water-reducing admixture needs less water to reach a required slump than untreated concrete. The treated concrete can have a lower water-cement ratio. This usually indicates that a higher strength concrete can be produced without increasing the amount of cement. Recent advancements in admixture technology have led to the development of mid-range water reducers. These admixtures reduce water content by at least 8 percent and tend to be more stable over a wider range of temperatures. Mid-range water reducers provide more consistent setting times than standard water reducers.
• Retarding admixtures, which slow the setting rate of concrete, are used to counteract the accelerating effect of hot weather on concrete setting. High temperatures often cause an increased rate of hardening which makes placing and finishing difficult. Retarders keep concrete workable during placement and delay the initial set of concrete. Most retarders also function as water reducers and may entrain some air in concrete.
• Accelerating admixtures increase the rate of early strength development, reduce the time required for proper curing and protection, and speed up the start of finishing operations. Accelerating admixtures are especially useful for modifying the properties of concrete in cold weather.
• Superplasticizers, also known as plasticizers or high-range water reducers (HRWR), reduce water content by 12 to 30 percent and can be added to concrete with a low-to-normal slump and water-cement ratio to make high-slump flowing concrete. Flowing concrete is a highly fluid but workable concrete that can be placed with little or no vibration or compaction. The effect of superplasticizers lasts only 30 to 60 minutes, depending on the brand and dosage rate, and is followed by a rapid loss in workability. As a result of the slump loss, superplasticizers are usually added to concrete at the jobsite.
• Corrosion-inhibiting admixtures fall into the specialty admixture category and are used to slow corrosion of reinforcing steel in concrete. Corrosion inhibitors can be used as a defensive strategy for concrete structures, such as marine facilities, highway bridges, and parking garages, that will be exposed to high concentrations of chloride. Other specialty admixtures include shrinkage-reducing admixtures and alkali-silica reactivity inhibitors. The shrinkage reducers are used to control drying shrinkage and minimize cracking, while ASR inhibitors control durability problems associated with alkali-silica reactivity.

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.

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.

The k-Value

If type II additions are used (fly ash and Silicafume), the k-value permits them to be taken into account in the calculation of the water in the fresh concrete. (The k-value concept may differ from country to country.)

Use of:

The actual value of k depends on the specific addition.

k-Value Concept for Fly Ash Conformity

The maximum amount of fly ash to be taken into account for the k-value concept shall meet the requirement:

Fly ash/cement  ≤  0.33 by mass

If a greater amount of fly ash is used, the excess shall not be taken into account for the calculation of the water/(cement + k × fly ash) ratio and the minimum cement content.

The following k-values are permitted for concrete containing cement type CEM I conforming to EN 197-1:

Minimum cement content for relevant exposure class:

• This may be reduced by a maximum amount of k × (minimum cement content – 200) kg/m³.
• Additionally, the amount of (cement + fly ash) shall not be less than the minimum cement content required.
• The k-value concept is not recommended for concrete containing a combination of fly ash and sulphate resisting CEM I cement in the case of exposure classes XA2 and XA3 when the aggressive substance is sulphate.

k-Value Concept for Silicafume Conformity

The maximum amount of Silicafume to be taken into account for the water/ cement ratio and the cement content shall meet the requirement

Silicafume/cement ≤ 0.11 by mass

If a greater amount of Silicafume is used, the excess shall not be taken into account for the k-value concept.

k-values permitted to be applied for concrete containing cement type CEM I conforming to EN 197-1:

Minimum cement content for relevant exposure class:

• This shall not be reduced by more than 30 kg/m³ in concrete for use in exposure classes for which the minimum cement content is ≤300 kg/m³.
• Additionally, the amount of (cement + k × Silicafume) shall be not less than the minimum cement content required for the relevant exposure class.

Back to top

Chlorides

The chloride content of a concrete, expressed as the percentage of chloride ions by mass of cement, shall not exceed the value for the selected class given in the table below.

Maximum chloride content of concrete

^a    For a specific concrete use, the class to be applied depends upon the provisions valid in the place of use of the concrete.

^b    Where type II additions are used and are taken into account for the cement content, the chloride content is expressed as the percentage chloride ion by mass of cement plus total mass of additions that are taken into account.

Back to top

Mix Designs

In its simplest form, concrete is a mixture of paste and aggregates. The paste, composed of portland cement and water, coats the surface of the fine and coarse aggregates. Through a chemical reaction called hydration, the paste hardens and gains strength to form the rock-like mass known as concrete.

Within this process lies the key to a remarkable trait of concrete: it's plastic and malleable when newly mixed, strong and durable when hardened. These qualities explain why one material, concrete, can build skyscrapers, bridges, sidewalks and superhighways, houses and dams.

Proportioning

The key to achieving a strong, durable concrete rests in the careful proportioning and mixing of the ingredients. A concrete mixture that does not have enough paste to fill all the voids between the aggregates will be difficult to place and will produce rough, honeycombed surfaces and porous concrete. A mixture with an excess of cement paste will be easy to place and will produce a smooth surface; however, the resulting concrete is likely to shrink more and be uneconomical.

A properly designed concrete mixture will possess the desired workability for the fresh concrete and the required durability and strength for the hardened concrete. Typically, a mix is about 10 to 15 percent cement, 60 to 75 percent aggregate and 15 to 20 percent water. Entrained air in many concrete mixes may also take up another 5 to 8 percent.

Portland cement's chemistry comes to life in the presence of water. Cement and water form a paste that coats each particle of stone and sand. Through a chemical reaction called hydration, the cement paste hardens and gains strength. The character of the concrete is determined by quality of the paste. The strength of the paste, in turn, depends on the ratio of water to cement. The water-cement ratio is the weight of the mixing water divided by the weight of the cement. High-quality concrete is produced by lowering the water-cement ratio as much as possible without sacrificing the workability of fresh concrete. Generally, using less water produces a higher quality concrete provided the concrete is properly placed, consolidated, and cured.

Other Ingredients

Although most drinking water is suitable for use in concrete, aggregates are chosen carefully. Aggregates comprise 60 to 75 percent of the total volume of concrete. The type and size of the aggregate mixture depends on the thickness and purpose of the final concrete product. Almost any natural water that is drinkable and has no pronounced taste or odor may be used as mixing water for concrete. However, some waters that are not fit for drinking may be suitable for concrete.

Excessive impurities in mixing water not only may affect setting time and concrete strength, but also may cause efflorescence, staining, corrosion of reinforcement, volume instability, and reduced durability. Specifications usually set limits on chlorides, sulfates, alkalis, and solids in mixing water unless tests can be performed to determine the effect the impurity has on various properties. Relatively thin building sections call for small coarse aggregate, though aggregates up to six inches (150 mm) in diameter have been used in large dams. A continuous gradation of particle sizes is desirable for efficient use of the paste. In addition, aggregates should be clean and free from any matter that might affect the quality of the concrete.

Hydration

Soon after the aggregates, water, and the cement are combined, the mixture starts to harden. All portland cements are hydraulic cements that set and harden through a chemical reaction with water. During this reaction, called hydration, a node forms on the surface of each cement particle. The node grows and expands until it links up with nodes from other cement particles or adheres to adjacent aggregates.

The building up process results in progressive stiffening, hardening, and strength development. Once the concrete is thoroughly mixed and workable it should be placed in forms before the mixture becomes too stiff.

During placement, the concrete is consolidated to compact it within the forms and to eliminate potential flaws, such as honeycombs and air pockets. For slabs, concrete is left to stand until the surface moisture film disappears. After the film disappears from the surface, a wood or metal handfloat is used to smooth off the concrete. Floating produces a relatively even, but slightly rough, texture that has good slip resistance and is frequently used as a final finish for exterior slabs. If a smooth, hard, dense surface is required, floating is followed by steel troweling.

Curing begins after the exposed surfaces of the concrete have hardened sufficiently to resist marring. Curing ensures the continued hydration of the cement and the strength gain of the concrete. Concrete surfaces are cured by sprinkling with water fog, or by using moisture-retaining fabrics such as burlap or cotton mats. Other curing methods prevent evaporation of the water by sealing the surface with plastic or special sprays (curing compounds).

Special techniques are used for curing concrete during extremely cold or hot weather to protect the concrete. The longer the concrete is kept moist, the stronger and more durable it will become. The rate of hardening depends upon the composition and fineness of the cement, the mix proportions, and the moisture and temperature conditions. Most of the hydration and strength gain take place within the first month of concrete's life cycle, but hydration continues at a slower rate for many years. Concrete continues to get stronger as it gets older.

Back to top

Classification by Consistence

The classes of consistence in the tables below are not directly related. For moist concrete, i.e. concrete with low water content, the consistence is not classified.

¹  Not in the recommended area of application

²  Not in the recommended area of application (but common for self-compacting concrete

Back to top

Conformity Control

This comprises the combination of actions and decisions to be taken in accordance with conformity rules adopted in advance to check the conformity of the concrete with the specification.

The conformity control distinguishes between designed concrete and prescribed concrete. Other variable controls are also involved depending on the type of concrete.

The conformity control may be performed either on individual concretes and/or concrete families.

Minimum rate of sampling for assessing compressive strength (to EN 206-1)

^a    Sampling shall be distributed throughout the production and should not be more than 1 sample within each 25 m³.

^b    Where the standard deviation of the last 15 test results exceeds 1.37 s, the sampling rate shall be increased to that required for initial production for the next 35 test results.

Conformity criteria for compressive strength: see EN 206-1.

Back to top

Proof of other Concrete Properties

Certificates of conformity according to EN 206-1 must be provided for other fresh and hardened concrete properties in addition to compressive strength.

A sampling and testing plan and conformity criteria are specified for tensile splitting strength, consistence (workability), density, cement content, air content, chloride content and w/c ratio (see the relevant sections in EN 206-1).

Back to top

Technical Properties

Standard EN 206

The European Concrete Standard EN 206-1:2000 was introduced in Europe with transition periods varying country by country. The name is abbreviated for simplicity to EN 206:1 below.

It applies to concrete for structures cast in situ, precast elements and structures, and structural precast products for buildings and civil engineering structures.

It applies to

• normalweight concrete
• heavyweight concrete
• lightweight concrete
• prestressed concrete

European Standards are under preparation for the following types of concrete:

• sprayed concrete
• concrete to be used in roads and other traffic areas

It does not apply to:

• aerated concrete
• foamed concrete
• concrete with open structure (“no-fines” concrete)
• mortar with maximum particle _ _ 4 mm
• concrete with density less than 800 kg/m³
• refractory concrete

Back to top

Definitions

Back to top

Exposure Classes

The environmental actions are classified as exposure classes. The exposure classes to be selected depend on the provisions valid in the place of use of the concrete. This exposure classification does not exclude consideration of special conditions existing in the place of use of the concrete or the application of protective measures such as the use of stainless steel or other corrosion resistant metal and the use of protective coatings for the concrete or the reinforcement.

The concrete may be subject to more than one of the actions described. The environmental conditions to which it is subjected may thus need to be expressed as a combination of exposure classes.

Limiting values for exposure classes for chemical attack from natural soil and ground water

Back to top

Strength Classes

The characteristic compressive strength of either 150 mm diameter by 300 mm cylinders or of 150 mm cubes may be used for classification.

Compressive strength classes for normalweight and heavyweight concrete:

Compressive strength classes for lightweight concrete:

Density classes for lightweight concrete:

Back to top

Specification of Concrete

The concrete grade designations have changed due to the introduction of EN 206-1 (e.g. for a tender).

Example: Pumped concrete for ground slab in ground water area

Back to top

Construction

Mixing, transporting, and handling of concrete should be carefully coordinated with placing and finishing operations. Concrete should not be deposited more rapidly than it can be spread, struck off, consolidated, and bullfloated. Concrete should be deposited continuously as near as possible to its final position. In slab construction, placing should be started along the perimeter at one end of the work with each batch placed against previously dispatched concrete. Concrete should not be dumped in separate piles and then leveled and worked together; nor should the concrete be deposited in large piles and moved horizontally into final position.

Consolidation

In some types of construction, the concrete is placed in forms, then consolidated. Consolidation compacts fresh concrete to mold it within the forms and around embedded items and reinforcement and to eliminate stone pockets, honeycomb, and entrapped air. It should not remove significant amounts of intentionally entrained air. Vibration, either internal or external, is the most widely used method for consolidating concrete. When concrete is vibrated, the internal friction between the aggregate particles is temporarily destroyed and the concrete behaves like a liquid; it settles in the forms under the action of gravity and the large entrapped air voids rise more easily to the surface. Internal friction is reestablished as soon as vibration stops

Finishing

Concrete that will be visible, such as slabs like driveways, highways, or patios, often needs finishing. Concrete slabs can be finished in many ways, depending on the intended service use. Options include various colors and textures, such as exposed aggregate or a patterned-stamped surface. Some surfaces may require only strikeoff and screeding to proper contour and elevation, while for other surfaces a broomed, floated, or troweled finish may be specified. In slab construction, screeding or strikeoff is the process of cutting off excess concrete to bring the top surface of the slab to proper grade. A straight edge is moved across the concrete with a sawing motion and advanced forward a short distance with each movement.

Bullfloating eliminates high and low spots and embeds large aggregate particles immediately after strikeoff. This looks like a long-handled straight edge pulled across the concrete. Jointing is required to eliminate unsightly random cracks. Contraction joints are made with a hand groover or by inserting strips of plastic, wood, metal, or preformed joint material into the unhardened concrete. Sawcut joints can be made after the concrete is sufficiently hard or strong enough to prevent raveling. After the concrete has been jointed, it should be floated with a wood or metal hand float or with a finishing machine using float blades. This embeds aggregate particles just beneath the surface; removes slight imperfections, humps, and voids; and compacts the mortar at the surface in preparation for additional finishing operations. Where a smooth, hard, dense surface is desired, floating should be followed by steel troweling. Troweling should not be done on a surface that has not been floated; troweling after only bullfloating is not an adequate finish procedure. A slip-resistant surface can be produced by brooming before the concrete has thoroughly hardened, but it should be sufficiently hard to retain the scoring impression

Curing

After concrete is placed, curing is the process in which the concrete is protected from loss of moisture and kept within a reasonable temperature range. Curing has a strong influence on the properties of hardened concrete such as durability, strength, watertightness, abrasion resistance, volume stability, and resistance to freezing and thawing and deicer salts. Exposed slab surfaces are especially sensitive to curing. Surface strength development can be reduced significantly when curing is defective. Therefore, curing is a key player in mitigating cracks in the concrete, which severely impacts durability.

Curing the concrete aids the chemical reaction called hydration. Most freshly mixed concrete contains considerably more water than is required for complete hydration of the cement; however, any appreciable loss of water by evaporation or otherwise will delay or prevent hydration. If temperatures are favorable, hydration is relatively rapid the first few days after concrete is placed; retaining water during this period is important. Good curing means evaporation should be prevented or reduced.

It is also necessary that the concrete be allowed to properly mature to provide the full service life. It is this service life, which can extend to hundreds or thousands of years, that provides concrete with the inherent ability to span the needs of many generations.

Curing methods:

• Liquid membrane-forming compounds sprayed onto the surface are effective, economical moisture barriers for moist-curing concrete.
• Polyethylene sheets are effective, economical moisture barriers for moist-curing concrete.
• Burlap kept saturated with water is an effective medium for moist-curing concrete.
• Straw or hay is still used to insulate fresh concrete in freezing weather.

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.

Building Concrete Slabs

Concrete is the material of choice for driveways, sidewalks, patios, steps, and for garages, basements, and industrial floors. It is relatively inexpensive to install and provides an attractive, durable surface that is easy to maintain. Proper attention to the standard practices and procedures for constructing exterior or interior concrete can yield a concrete surface that will provide long-lasting, superior performance.

Placing Contraction/Control Joints in Concrete Flatwork

The most widely used method to control random cracking in concrete slabs is to place contraction/control joints in the concrete surface. As concrete hardens, there is a reduction in volume, often resulting in cracking of concrete. Joints produce an aesthetically pleasing appearance since the crack takes place below the finished concrete surface. The concrete has still cracked, which is normal behavior, but the absence of random cracks at the concrete surface gives the appearance of an un-cracked section.

Contraction joints should be placed to produce panels that are as square as possible and never exceeding a length to width ratio of 1 ½ to 1. Joints are commonly spaced at distances equal to 24 to 30 times the slab thickness and established to a depth of ¼ the slab thickness. Joints should be sawed as soon as the concrete will withstand the energy of sawing without raveling or dislodging aggregate particles. For most concrete mixtures, this means sawing should be completed within the first 6 to 18 hours and never delay more than 24 hours.

Isolation/Expansion Joints

Isolation joints are used to relieve flexural stresses due to vertical movement of slab-on-grade applications that adjoin fixed foundation elements such as columns, building or machinery foundations, bridge abutments, light standards, drop inlets, and so on. The isolation joint material allows the slab-on-grade to move up or down with the changes of soil support conditions. Heaving/settling of moist soils due to freeze/thaw cycles and long-term settlement are the primary causes of changes in soil support conditions. In addition, an isolation joint may be used in slabs that require a change in contraction joint layout, which would create T intersections. The isolation joint would be considered a free edge allowing the termination of a contraction joint at a T intersection.

Expansion joints are used primarily to relieve stress due to confinement of a slab. If the slab is placed adjacent to structures on more than one face of the slab, an expansion joint should be placed to relieve stress. For example, if a slab were placed between two buildings, an expansion joint should be placed adjacent to the face of at least one of the buildings. Confinement on three faces would normally be handled by placing expansion joints on all three faces. Confinement on four faces should be isolated on all faces. This allows for thermal expansion and contraction without inducing stress into the system.

Safety Measures for Concrete Construction

Concrete construction is no exception to the importance of construction safety. Although claiming one of the lower jobsite-injury rates, dangers associated with both the material aspects and construction practices of concrete construction must be addressed to continue the industry’s focus on safety. Heightened awareness, improved safety training programs, and diligent enforcement are the keys to improving safety on the jobsite.

Bugholes

One of the primary influences affecting the surface aesthetics of concrete is bugholes. Bugholes, pinholes, blowholes, surface voids – they are recognized by various names, but all refer to a common problem that contractors want to minimize. Bugholes, are small, regular or irregular cavities (usually not exceeding 15 mm) resulting from entrapment of air bubbles on the surface of vertically formed concrete structures during placement and consolidation.

Hot Weather Concreting

When the temperature of freshly mixed concrete approaches approximately 25°C adverse site conditions can adversely impact the quality of concrete. Ambient temperatures above 32°C and the lack of a protected environment for concrete placement and finishing (enclosed building) can contribute to difficulty in producing quality concrete.

The precautions required to ensure a quality end product will vary depending on the actual conditions during concrete placement and the specific application for which the concrete will be used. In general, if the temperature at the time of concrete placement will exceed 25°C a plan should be developed to negate the effects of high temperatures.

Cold Weather Concreting

Cold weather concreting is a common and necessary practice, and every cold weather application must be considered carefully to accommodate its unique requirements. A period of cold weather is defined when for more than 3 successive days the average daily air temperature drops below 5°C and stays below 10°C for more than one-half of any 24 hour period.” This definition can potentially lead to problems with freezing of the concrete at an early age.

Rule number one is that ALL concrete must be protected from freezing until it has reached a minimum strength of 3.5 MPa, which typically happens within the first 24 hours. In addition, whenever air temperature at the time of concrete placement is below 5°C and freezing temperatures within the first 24 hours after placement are expected, the following general issues should be considered:

• Initial concrete temperature as delivered;
• Protection while the concrete is placed, consolidated, and finished, and
• Curing temperatures to produce quality concrete.

Drying of Concrete

Unwanted moisture in concrete floors routinely causes millions of dollars in damage to buildings. Problems from excessive moisture include deterioration and de-bonding of floor coverings, trip-and-fall hazards, microbial growth leading to reduced indoor air quality, staining, and deterioration of building finishes.

The terms curing and drying are frequently used interchangeably with regard to the moisture condition of new concrete slabs.

The term “concrete moisture” is understood to mean the total water used in the concrete batch, plus curing water, minus the water bound in hardened cement due to hydration. Drying begins when water is no longer available at the exposed surface.

The moisture content of concrete must be viewed from the context of total water content of the fresh concrete mixture and the available moisture content of the hardened concrete. The total water content of a fresh concrete mixture is a function of the total cementitious materials and water cement ratio (w/cm).

Identifying and Evaluating Concrete Defects

Concrete structures are regularly constructed without complications. However, defects can occur that can be traced to problems related to environmental conditions during construction or with the concreting procedures used. In order to determine a repair method, it is necessary to identify what caused the defect . Evaluation of deficiencies helps ensure that repairs will be effective and the defect will not extend into the surrounding concrete.

Many concrete defects are immediately recognized and others are not. Concrete defects can be broken down into four broad groups based on visual observation: deformation of the surface, cracking of the surface, disintegration of the surface, and other defects.

Visual examination typically does not provide enough information to determine the cause or causes of the defect. In some cases, it may not provide evidence of a defect at all. In order to narrow the scope of an investigation to probable causes and suitable repair methods, the appropriate information factors and the proper evaluation methods need to be identified. More on concrete defects.

Back to top

Decorative Aspects

Back to top

Durability

The Perils of Power Washing

It is springtime and that means spring cleaning. Building owners and property managers will be looking at every exposed surface on their property and then deciding how to clean those surfaces. The cleaning method chosen for concrete requires careful consideration.

High-pressure power washing units are commonly available. Some of those units can deliver water at pressures well in excess of 400 bar. Moreover, it is not just high-pressure water that’s the problem. The water exits the nozzle at both a high pressure and a high velocity. The resulting momentum is great enough to dislodge not only dirt and debris but also to create flakes, popouts, and even concrete spalls. Good quality concrete will also experience accelerated wear from high-pressure power washing.

Back to top

Sustainability

Concrete Shines as Solar Reflectance Material

Concrete does a very good job of reflecting solar energy. The solar reflectance of 135 concrete specimens from 45 mixes representing exterior concrete flatwork was measured.

Solar reflectance index (SRI), a calculated value based on solar reflectance, SR, is one way to determine how much light energy a material reflects: stated another way, comparing SRI or SR of different materials tells which ones absorb less solar radiation. This is useful because darker materials absorb more heat, which is generally considered undesirable for its effect on the environment. This may have an immediate, local effect, like heat gain in urban areas (heat island).

Back to top