Concrete Quality Control

The term quality control in civil engineering parlance is used to mean that the work is done according to the specifications provided in a contract document. Concrete structures like multistoried buildings, skyscrapers, or dams are built to last many decades and hence, quality control of concrete during the construction of such important structures is a must and it has to be attended to with an intelligent and scientific approach. The specifications of work should be framed with extreme care so as to serve effectively as a guide to get the job done well. As a matter of fact, the specifications are as important as the design of the project.

The concrete as such is a heterogeneous material and hence, it would assume a wide range of properties, if produced without exercising any control. For preparing high-quality concrete, the field organization may broadly be divided into the following three divisions:

(i) Engineering division which provides lines and grades and makes the initial inspection of all forms, reinforcement, and installation of all embedded parts.

(ii) Manufacturing division which exercises control over concrete materials, batching, and mixing.

(iii) Placing division is concerned with the control of concrete placing and of related operations prior and subsequent thereto.

The concrete produced at the site should be the strongest, densest, most workable, and most economical for the job for which it is prepared. The amount of cement should be low and that of aggregates should be high. The most economical concrete with the highest possible density is obtained by observing the following general requirements:

(i) The air bubbles should be eliminated from the body of the concrete.

(ii) The cement particles should be of the smallest size.

(iii) The concrete should be compacted fully so as to remove the voids.

(iy) The concrete should be cured sufficiently and adequately, say for 28 days.

(v) The cubical particles of the aggregates should be used so that good interlocking is gained.

(vi) The water-cement ratio should be kept low.

The quality control of concrete demands a high degree of awareness among the personnel connected with the production of concrete, namely, supervisors, laborers, etc. regarding the factors which influence the quality of concrete such as adding of water, cleanliness of various ingredients, adequate curing, etc. It, therefore, becomes necessary to train such personnel for getting concrete of high quality. In a similar way, the engineer in charge of the work should have sufficient knowledge about proper working and maintenance of equipment used in producing concrete such as concrete mixers, vibrators, batching plants, etc.

The quality control aspect of concrete is a detailed topic by itself and extreme care is to be exercised at every stage of concrete production so as to obtain the desired results. The quality control, if properly achieved, grants the following advantages:

(i) It helps in improved utilization of scarce resources and in extended

utilization of low-grade materials.

(ii) It helps to minimize failures.

(iii) It results in lower costs of construction as higher stresses can be assumed.

(iv) The structure becomes durable with lower costs of maintenance.

It should be remembered that the road to achieving quality control is not easy and hence, if a uniform product is required, all the ingredients and procedures that make up this conglomerate must also be uniform within permissible limits, not once in a while, but every day until the structure is completed. Hence all suitable precautions must be taken to ensure proper inspection of the ingredients, batching, mixing, transporting, and placing and if found necessary, the standards applicable to all these processes must be improved so as to deliver to the owner a project of acceptable quality.

Following are some of the important facts associated with the Concrete

(1) It is proved that cement concrete is more economical than steel.

(2) The aggregates should be completely free from lumps of clay, organic and

vegetable matter, fine dust, etc.

(3) It is not advisable to use seawater for making prestressed concrete.

(4) The presence of moisture in sand increases the volume of sand.

(5) The admixtures are sometimes added to the concrete.

(6) The water makes the concrete workable.

(7) The minimum quantity of water should be used to have a reasonable degree of

workability.

(8) More water should not be added to attain the required degree of workability.

(9) After use, the concrete mixer should be thoroughly washed and cleaned.

(10) The seawater can be adopted for concrete structures that were finishing characteristics are not important.

(11). The theory of the formation of concrete is based on the phenomena of formation

of voids.

(12) When concrete is fresh, it should have enough workability.

(13) In the method of arbitrary volumetric proportions, the fine coarse ratio of 1:2 is usually adopted.

(14) The bulking of sand should be taken into account when volumetric proportioning of the aggregates is adopted.

(15) The large quantities of concrete should not be deposited on the formwork at a time.

(16) The internal vibrators should be operated continuously while they are being

withdrawn.

(17) The lightweight concrete should be made adequately dense when used for

R.C.C. work.

(18) The dry sand and the sand completely flooded with water have practically the

same volume.

(19) The inside surface of the formwork should be coated with crude oil or soft soap solution.

(20) For concrete structures, exceeding 12 m in length, the expansion and contraction

joints are provided.

(21) cement concrete has attained the status of major building material in all branches of modern construction.

(22) The concrete would assume a wide range of properties if produced without exercising any control.

(23) When no-fines concrete is used, the pressure on formwork is greatly reduced.

(24) The rendering of walls becomes essential when no-fines concrete is used.

(25) The internal vibrator can be placed vertically or at a slight inclination not

exceeding 10° to the vertical.

(26) The surface vibrators are found to be more effective for compacting very dry

concrete mixes.

 (27) The form vibrators require more power.

(28) The vibrating tables are invariably used in the preparation of precast structural

products in factories and test specimens in laboratories.

(29) The slump test is not a reliable test.

(30) The grades of concrete lower than M15 are not to be used in R.C.C. work.

(31) The popularity of various types of admixtures in concrete is increasing rapidly.

(32) The method of minimum voids of proportioning concrete does not give

satisfactory result.

(33) The method of maximum density of proportioning concrete is not very popular.

(34) The hand compaction of concrete should be preferred, wherever feasible.

(35) The concept of the early beginning of curing is accepted.

(36) The use of curing compounds permits early curing.

(37) It is necessary to determine the locations of the construction joints well in

advance.