Introduction
 

Concrete Basics

Concrete walls/foundations, driveways and walks can greatly enhance a property's value and appearance. Like any other significant purchase, buying concrete requires a basic understanding of the products being considered.

Teamwork and Planning
Healthy, handsome, long-lasting concrete takes thorough planning, a quality mix, professional placement and proper curing and maintenance. The time to think about what you want in appearance, performance and maintenance is before the concrete is placed.

Concrete construction is best completed by professionals with an extensive understanding of concrete and significant experience working with it. Usually several parties are involved -- the contractor/builder, the ready-mix producer, and the owner. The technical aspects such as planning, preparation, mix specification, placing and finishing are the responsibility of the builder, ready-mix producer and, largely, the concrete contractor.

Mix Design
Concrete is a combination of portland cement, crushed stone, sand and water. Admixtures, which are ingredients added before or during the mixing of concrete, are also used. Admixtures are used to strengthen concrete, to speed up or slow down the set-up time, and to help protect concrete against the effects of temperature changes and exposure to chemicals such as deicers.

Because concrete is a blend of natural materials, it may have some natural imperfections.

The performance of exterior concrete slabs is greatly influenced by the introduction, or entrainment, of microscopic air bubbles into the concrete. Air entrainment helps protect concrete that will be exposed to freezing and thawing and deicers. An air entrainment admixture causes microscopic air bubbles to form throughout the concrete. These tiny bubbles function as relief valves when water in the concrete freezes, helping to prevent surface deterioration. The typical air entrainment for exterior flatwork is in the five to seven percent range.

Slump is the term used to describe the consistency, stiffness and workability of fresh concrete. The results of a slump test are stated in inches. It is influenced by the amount of water in fresh concrete. More water means higher slump, but water is not the only influence. Admixtures can be used to increase slump without increasing the water in the concrete. The type of aggregate, the air content, the admixtures, temperature and the proportions of all the ingredients affect slump. The typical slump used for external flatwork is four inches, plus or minus one inch. For a concrete foundation the preferred slump is five inches, plus or minus one inch.

A pound per square inch (psi) is the unit of measurement used to describe the compressive strength of concrete. The most desirable strength for external concrete slabs varies based on climatic conditions. In broad terms, the colder the climate the higher the desired psi. Areas which experience large numbers of freeze and thaw cycles also require a higher psi concrete mix design. The typical concrete strength used for a driveway in southeastern Michigan or northwestern Ohio is 4,000 psi. The concrete used on a highway bridge may have a compressive strength of 5,000 psi or more.

When ordering ready mixed concrete, customers should advise the ready mixed producer of the intended use for the concrete. Kuhlman Concrete can mix hundreds of different concrete formulations to meet the requirements of specific projects.

Selecting a Contractor
Selecting an experienced and qualified contractor is one of the most important steps in assuring a long service life for ready mixed concrete. A good source of information on contractors is your local concrete producer. The producer will have had contact with dozens of contractors and will be glad to give you a list of the most qualified for a specific type of job. Be sure to ask prospective bidders for a reference list of both recently completed jobs and projects completed in years past. When checking a contractor's references, ask if the work was completed on schedule, within budget, was of good quality, and if the customer would use the contractor again.

 

 Composition

 The two major components of concrete are a cement paste and inert materials. The cement paste consists of portland cement, water, and some air either in the form of naturally entrapped air voids or minute, intentionally entrained air bubbles. The inert materials are usually composed of fine aggregate, which is a material such as sand, and coarse aggregate, which is a material such as gravel, crushed stone, or slag. In general, fine aggregate particles are smaller than 6.4 mm (.25 in) in size, and coarse aggregate particles are larger than 6.4 mm (.25 in). Depending on the thickness of the structure to be built, the size of course aggregate particles used can vary widely. In building relatively thin sections, a small size of coarse aggregate, with particles about 6.4 mm (.25 in) in size, is used. At the other extreme, aggregates up to 15 cm (6 in) or more in diameter are used in large dams. In general, the maximum size of coarse aggregates should not be larger than one-fifth of the narrowest dimensions of the concrete member in which it is used.

 When portland cement is mixed with water, the compounds of the cement react to form a cementing medium. In properly mixed concrete, each particle of sand and coarse aggregate is completely surrounded and coated by this paste, and all spaces between the particles are filled with it. As the cement paste sets and hardens, it binds the aggregates into a solid mass.

 Under normal conditions, concrete grows stronger as it grows older. The chemical reactions between cement and water that cause the paste to harden and bind the aggregates together require time. The reactions take place very rapidly at first and then more slowly over a long period of time. In the presence of moisture, concrete continues to gain strength for years. For instance, the strength of just-poured concrete may be about 70,307 g/sq cm (1000 lb/sq in) after drying for a day, 316,382 g/sq cm (4500 lb/sq in) in 7 days, 421,842 g/sq cm (6000 lb/sq in) in 28 days, and 597,610 q/sq cm (8500 lb/sq in) after 5 years.

 Concrete mixtures are usually specified in terms of the dry-volume ratios of cement, sand, and coarse aggregates used. A 1:2:3 mixture, for instance, consists of one part by volume of cement, two parts of sand, and three parts of coarse aggregate. Depending on the applications, the proportions of the ingredients in the concrete can be altered to produce specific changes in its properties, particularly strength and durability. The ratios can vary from 1:2:3 to 1:2:4 and 1:3:5. The amount of water added to these mixtures is about 1 to 1.5 times the volume of the cement. For high-strength concrete, the water content is kept low, with just enough water added to wet the entire mixture. In general, the more water in a concrete mix, the easier it is to work with, but the weaker the hardened concrete becomes.

 Concrete can be made to have any degree of water tightness. It can be made to hold water and resist the penetration of wind-driven rains. On the other hand, for purposes such as constructing filter beds, concrete can be made porous and highly permeable. Concrete can also be given a polished surface that is as smooth as glass. By using heavy aggregates, including steel fragments, dense concrete mixtures can be made that weigh 4005 or more kg/cu m (250 or more lb/cu ft). Concrete that weighs only 481 kg/cu m (30 lb/cu ft) can be made by using special lightweight aggregates and foaming techniques. Forms consisting of such lightweight aggregates can be floated on water, sawed into pieces, or nailed to another surface.

 For small jobs and minor repairs, concrete can be mixed by hand, but machine mixing ensures more uniform batches and, therefore, superior performance. For most home repairs and improvements—for example, floors, walks, driveways, patios, and garden pools—the recommended proportion is a 1:2:3 mix.

 After exposed surfaces of concrete have hardened sufficiently to resist marring, they should be cured by sprinkling or flooding  (covering) with water or by using moisture-retaining materials such as waterproof paper, plastic sheets, wet burlap, or sand. Special curing sprays are available. The longer concrete is kept moist, the stronger and more durable it will become. In hot weather, it should be kept moist for at least three days. In cold weather, drying concrete must not be allowed to freeze. This can be accomplished by covering the cement with a tarpaulin or some other material that helps trap the heat generated by the chemical reactions within the concrete that cause it to harden.


  Construction Techniques


 Concrete is poured into place in a number of ways. For the footings of small buildings, the wet concrete is poured directly into trenches dug into the earth below frost level. Concrete for foundations and certain types of walls is placed between supporting wood or metal forms, which are removed after the concrete has hardened. In lift-slab construction, floors and roof slabs are cast at ground level and then raised by hydraulic jacks and fastened to columns at the desired elevation. Slip forms are used to produce vertical shafts for silos and the cores of buildings. They are moved upward at a rate of 15 to 38 cm (6 to 15 in) per hour while concrete and reinforcements are put in place. The tilt-up method of construction is frequently used for one- and two-story buildings. Walls are cast in place on the ground or on the previously laid concrete floor and tilted into position by cranes. The walls are joined at the corners or between panels with cast-in-place concrete columns. To pave a highway or road with concrete, a slip-form paver is used. Two metal side forms are connected to a slip-form paver. A layer of concrete is poured between the side forms as the paver slowly moves forward on its treads; the side forms keep the concrete in position as it dries. Slip-form pavers can lay continuous strips of one or two lanes of concrete pavement.

 For certain applications, such as the construction of swimming pools, canal linings, and curved surfaces, concrete may be applied by the shot crete method. In shot creting, concrete is sprayed under pneumatic pressure rather than placed between forms. Often the use of shot crete eliminates the need for formwork and permits placement of concrete in confined areas where conventional forms would be difficult or impossible to construct.

 Air-entrained concrete is concrete in which minute air bubbles are intentionally trapped by the addition of an admixture to the cement, either during its manufacture or during the batching and mixing of the concrete. The presence of a properly distributed amount of these bubbles imparts desirable properties to both freshly mixed and hardened concrete. In freshly mixed concrete, entrained air acts as a lubricant, improving the workability of the mix, thereby reducing the amount of water that needs to be added. Entrained air also reduces the need for fine material (sand).

 Entrained air in hardened concrete dramatically reduces the scaling that might otherwise result from the use of chemicals to melt ice on roads and streets. It also prevents damage to pavements caused by freezing and thawing. The air bubbles function as minute safety valves by providing room for the free water in concrete to expand harmlessly as freezing occurs.


 Concrete Masonry


 Concrete masonry is block and brick building units molded of concrete and used in all types of masonry construction. Concrete masonry is used for load-bearing and non-load-bearing walls; piers; partitions; fire walls; backup for walls of brick, stone, and stucco facing materials; fireproofing over steel structural members; fire safe walls around stairwells, elevators, and other enclosures; retaining walls and garden walls; chimneys and fireplaces; concrete floors; and many other purposes.

 About 60 percent of all concrete masonry units, such as cinder blocks, are made with lightweight aggregates. Processed clays, blast-furnace slag, shales , natural volcanic aggregates, and cinders are the lightweight aggregates most commonly used. The size of the masonry unit most commonly used for walls, both below and above ground, is 20 by 20 by 40 cm (8 by 8 by 16 in). Masonry units are laid horizontally, and are cored to reduce weight and to provide an insulating air space within the block. New types of concrete masonry, such as split and slump block, are being used as facing in homes, commercial buildings, schools, churches, and municipal facilities.

 Basic block types are fairly well standardized today. Specific types can usually be supplied for any construction without cutting or fitting. Special molds are available for the production of patterned shadow effects on exterior and interior block walls. It is possible to supply virtually any color or type of texture.


 Reinforced Concrete

 Concrete used in most construction work is reinforced with steel. When concrete structural members must resist extreme tensile stresses, steel supplies the necessary strength. Steel is embedded in the concrete in the form of a mesh, or roughened or twisted bars. A bond forms between the steel and the concrete, and stresses can be transferred between both components.

 Pre-stressing concrete has removed many limitations on the spans and loads for which a concrete structure can be economically designed. The basic function of pre-stressing is to greatly reduce the tensile stresses to which crucial areas of concrete structures are subjected. Pre-stressing is accomplished by stretching high-strength steel to induce compressive stresses in concrete. The strengthening effect of compression in concrete acts like horizontally squeezing a row of books. When you apply sufficient pressure to the books at each end, you induce compressive stresses throughout the entire row; thus, although the center volumes are unsupported, you can lift the books and carry them horizontally.

 Compressive stresses are induced in pre-stressed concrete by either pre-tensioning or post-tensioning the steel reinforcement. In the pre-tensioning process, the steel is stretched before the concrete is placed. After the concrete has hardened around the tensioned reinforcement, the stretching forces are released. The steel shortens somewhat, and because of the bond between the steel and concrete, the compressive stress in the concrete increases. In post-tensioning, the concrete is cast around, but not in contact with, unstretched steel. The steel is stretched after the concrete has hardened by anchoring one end against the concrete and using hydraulic jacks to pull the other. After stretching, the second end is also anchored, compressing the concrete.