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What is Concrete?  
   

An excerpt from ACI document SP 1 - Concrete Primer

1. INTRODUCTION

1. Q. What are concrete, hydraulic cement, mortar, and grout?

A. Concrete is a composite material that consists of a binding medium (Section 4.1) embedded with fine aggregate (typically sand) and coarse aggregate (typically gravel) (Section 4.3). Essential concrete ingredients are shown in Fig. 1. In hydraulic cement concrete, the binder is cement

paste, a mixture of hydraulic cement and water (Section 4.2),and possibly one or more admixtures (Section 4.4).

Hydraulic cement is cement that sets and hardens by chemical reaction with water (hydration) and is capable of doing so under water (ACI 225R).* The hydration reactions result in the formation of a hard solid mass. The most widely used hydraulic cement is portland cement. Other kinds of hydraulic cement include blended cements and ground granulated blast-furnace slag (ACI 233R). Pozzolans, both natural (ACI 232.1R) and artificial (fly ash, ACI 232.2R, and silica fume, ACI 234R) are often used as a cementitious ingredient of concrete. Mortar is a mixture of cement paste and fine aggregate. Grout is a mixture of cementitious material and water, with or without fine aggregate, proportioned to produce a pourable consistency without segregation of the constituents.

 

sp01

Fig. 1—Essential ingredients of concrete. (Photo courtesy of PCA.)

2. Q. What causes hardening of hydraulic cement (or concrete)?

A. When hydraulic cement is mixed with water to form a paste, the phases of the cement react with the water (hydration) to form a slowly developing cementitious structure that adheres to and binds together the fine-and coarse-aggregate particles to form hardened concrete. The most abundant hydration product is calcium-silicate hydrate. As long as moisture and unhydrated cement particles are present, the hydration products continue to form, increasing the strength of the concrete.

3. Q. Are the properties of concrete well enough known to enable safe and durable structures to be built or is further research necessary?

A. The answer to both questions is yes. The principles of producing concrete and understanding the laws of concrete behavior are well enough established through long experience and extensive research to make it possible to design and erect structures that meet the recognized requirements of engineering use and safety. There is still a need for continued research, however. New questions are constantly arising, and new methods and machines for construction operations are being developed. If concrete is to meet increasingly higher expectations with regard to durability and structural efficiency, and continue in the forefront as a building material, the new requirements need to be met by ever-increasing knowledge obtained from research and experience.

4. Q. What are the requirements for a successful concrete structure?

A. In the words of the first Concrete Primer: “The concrete must have sufficient strength to carry the loads imposed. The concrete must be able to endure under the conditions of exposure to which it will be subjected. The concrete must be economically produced in comparison with other materials equally strong and durable, which might be used. Thus the requirements may be briefly stated as strength, resistance to degradation, and economy.” These words are equally applicable today.

5. Q. Assuming that concrete is made from the correct ingredients and in the correct proportions, what other requirements must be met to ensure a durable structure, that is, a structure with long life? 

A.The important overall requirements are related to measuring,

mixing, transporting, placing, curing, and inspection (ACI 304R):

(1)

All materials should comply with specifications.

(2)

The methods of storing, handling, and measuring all ingredients should be such that the selected mixture can be accurately obtained at all times (ACI 213, 221).

(3) The concrete should be adequately mixed, and it should be transported and placed by methods that will avoid segregation and loss of ingredients. The consolidated mass should be uniform without rock pockets or

honeycombed areas (ACI 309).

(4) The arrangement of joints and methods for bonding successive lifts of concrete are important details that can vitally affect the performance of the structure even though the concrete itself is durable. Provisions should be made in the structural plans for drainage to avoid areas of constant saturation that would be more susceptible to damage by freezing than other portions of the structure (ACI 210.2R,

325.9R).

(5) Curing of the concrete should not be neglected. This includes protection against extremes of temperature as well as provision for ensuring availability of moisture during the critical early period. No detail of concrete construction

offers such possibilities for increased strength and durability at so low a cost as is offered by the possibilities of better curing (ACI 308, 305, 306).

(6) Careful inspection should be enforced in all of the above operations (ACI Manual of Concrete Inspection, SP-2). After a dissertation on correct practices in the maintenance of the Roman aqueduct in AD 97, Julius Frontinius noted that “all these the workmen know but few observe.”

6. Q. What types of tests are performed to evaluate the properties of the hardened concrete for its suitability for a given purpose?

A. Specimens for strength tests in compression (or in flexure, if required) should be made from all trial batches and from several batches after a satisfactory mixture has been established to determine if the strengths are within the range intended. Also, if concrete is exposed to the environment and resistance to degradation is of concern, tests for chloride penetration, shrinkage, and the air-void system or resistance to freezing and thawing would be desirable. If wholly unfamiliar materials are used, the test program before starting the work will need to be considerably expanded, commensurate with the magnitude of the work. On large operations, major economies can be produced by ample preliminary studies and adequate control tests (see Chapter 8).

7. Q. What is the effect of aging on concrete?

A. Aging, if one means merely the effect caused by the passage of time, has no effect on concrete. Of course concrete sets, hardens, gains strength, and exhibits reduced permeability with the passage of time, but it is not the passage of time alone that causes these things to happen. If the concrete is kept very cold, none of this will happen. If all moisture is removed, none of this will happen. Many or even most concretes are confronted with potential deteriorative service conditions. If the concrete has not been provided with immunity against these influences, it may well slowly deteriorate as time passes, but not simply because time passes. Concrete need not deteriorate.

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