CONCRETE WORKInspectionThe quality of concrete depends largely on workmanship in construction. The best of materials and design practice would not be effective unless the construction were well performed. Inspection is provided to assure satisfactory work, in accordance with the plans and specifications and with good practice. It also secures a record of the job for future reference. The cost of competent inspection is relatively little compared with the resulting insurance of quality of the structure. Often the cost may be more than offset be cause competent inspection prevents mistakes and permits more economical and workable mixes to be used. InspectorIt is the responsibility of the engineer or architect to select qualified men as inspectors, pay them adequately, train them well, encourage continued study, supply them with appropriate equipment and technical references, define their responsibility and authority clearly, and back them up on the job. The inspector should be informed of any special prob lems or conditions which are likely to be met on the job. He should be told at once of any special interpretations of the specifications or special agreements with the contrac tor; and whenever practicable he should be consulted beforehand to bring all possible information to bear on the decision. He should be given to understand what type of matters he is to decide on the job and what type he is to refer to his superior. His decisions, once made, should be consistently supported unless actual unfairness to the contractor would result. Inspection before ConcretingPreliminary StudyUpon first arrival at the work the inspector should familiarize himself as soon as possible with the plans and specifications, and with the essential requirements of any related specifications and building codes. If there is no separate engineering staff, the inspector should see that the various drawings conform with each other and with the specifications and any special instructions; examine any shop details and compare with the scale drawings; and examine any erection drawings. He should observe the general layout of the work, including the contractor's plant, equipment, and organization, with particular attention to the batching, mixing, transporting, and compacting devices. He should become familiar with the conditions at the site (lines, grades, foundations); the location of rights of way, roadways, streams, sewers, pipe lines, poles, or wires which might be affected by construction; regular and special traffic arrangements; and safety regulations.lnspection of preparations Before concrete is placed in a given section of the work, the specification requirements regarding excavation, forms, reinforcement, embedded fixtures, and joints must be fulfilled and the work inspected. To keep delays at a minimum, inspection should be continual through the three stages of the work, as follows: Preliminary When excavation has been completed or forms built. If dimensions and stability are satisfactory, the contractor may then clean the foundation or oil the forms and may install any reinforcement and fixtures. Semi-final or "clean-up" When everything is in place for concreting, a detailed inspection of foundations, forms, and all equipment or parts to be embedded in the concrete. If the installations are satisfactory, the work is ready for final clean-up. Final Immediately before concrete is placed. Forms and fixtures must not have been displaced. Surfaces must be clean and, if specified, must be wetted. Assuming that the requirements with regard to suit ability of materials, proportions, and working conditions (weather, time, lighting, equipment, etc.) have been met, the contractor may then proceed with concrete placing. Excavation and foundation Excavated surfaces upon or against which concrete is to be placed should conform to the specified location, dimensions, and shape. For foundations, excavation should be continued until sound material is reached; and the soil under footings should be the original undisturbed soil. Surfaces against which concrete is to be placed and with which bond is desired should be clean and moist. If concrete is to be supported by piles, the inspector should check the number, location, and penetration It is customary to keep a permanent record of all piles driven, including the following items: method of driving, type and weight of hammer, drop of hammer under last five blows, penetration of pile under last five blows, number of piles driven, number of piles specified, and length of pile under cut-off level. Forms Before concreting is permitted to progress, forms should be inspected to see that they are in the correct location and that they will result in concrete of the required dimensions. Forms should be checked as soon as possible so that errors can be corrected with minimum delay. Location and dimensions of forms after they are filled with concrete may not be the same as when they were built. The weight of concrete and of workmen and equipment employed in placing the concrete may cause the forms to settle, sag, or bulge. For this reason any supports, especially shores bearing on the ground, should have an adequate area of bearing. If settlement of supports or sagging of spans is to be expected, allowance should be made in the original construction of forms. Fresh concrete, when vibrated, exerts maximum pressures. Form surfaces should be smooth, mortar-tight, and free from holes and seams which would permit appreciable amounts of mortar to escape, particularly if the concrete is to be vibrated. Seams of forms should be tight to prevent accumulation of dirt before concreting or formation of fins of mortar. Before concrete is placed, form surfaces should be wetted, oiled, or coated with satisfactory materials. If specifications require oiling, the oil should be applied before steel reinforcement is placed or it will get on the reinforcement and destroy the bond. Coatings for forms should be nonstaining. All foreign material such as chips, blocks, sawdust, dried mortar, and ice should be removed. Foreign material is likely to accumulate in corners and places difficult of access. Placing reinforcement Reinforcement should be checked for size, bending, spacing, location, firmness of installation, and surface condition. Steel should not be bent or straightened in a manner that would injure the material. Heating of reinforcement for bending should be resorted to only when the entire operation is approved by the engineer, as heating may change the characteristics of the steel. Storage and handling Storage conditions which might cause excessive rusting of the steel should be avoided. Before reinforcement is placed, the surface should be free from objectionable coatings, particularly heavy corrosion caused by out-of-door storage Reinforcement should be properly spaced, spliced, and anchored; embedded a given minimum distance from the surface; and accurately located and firmly held. Particular attention should be paid to clearance at stirrups and column ties, which project beyond the remainder of the reinforcement; and to clearance under horizontal bars midway between supports, to detect sagging. Support All steel reinforcement should be accurately located in the forms and firmly held in place before and during the casting of concrete, by means of built-in concrete blocks, metallic supports, spacer bars, wires, or other devices adequate to insure against displacement during construction and to keep the steel at the proper distance from the forms. Bar supports and spacers should be sufficient in number and sufficiently heavy to carry properly the steel they support, even when subject to construction loads.Embedded fixtures Anchor bolts, inserts, pipe sleeves, pipes, conduits, wiring, flashings, manhole-cover frames, instruments, and other embedded fixtures and mechanical equipment should in general be fixed firmly in position before concrete is placed. Preparation of contact surfaces depends upon whether or not bond with the concrete is desired. Built-in fixtures should not be permitted to affect the position of metal reinforcement except as shown on the plans, nor to be so located as to reduce appreciably the strength of the construction. Wood inserts should be well soaked in advance of concreting; otherwise they tend to swell and cause the concrete to split. Openings Where conditions appear to require additional openings not shown on the plans, proposed openings should be approved by the engineer. And all except very small openings should be protected by short diagonal bars so placed as to carry the stresses around the opening.Expansion joints Joints and dowels should be carefully aligned, and any fins or wedges of concrete which may form during construction should be re moved. Sliding joints should be free to move, and may require some form of lubrication such as asphalt or tar. The inspector should be on the lookout for construction conditions which might later interfere with movement of joints or with proper drainage. Preparation for mixing and placing concrete As the work approaches actual placement of concrete, the inspector should see that all elements of this operation are in readiness and in proper condition to give the required performance. The adequacy of supply and suitability of materials should be checked. The accuracy of batching equipment should be confirmed. Methods to be used for transporting, transferring, handling, and placing the concrete should be reviewed for avoidance of whatever may cause poor consistency control or segregation. Facilities for curing, rain protection, and cold-weather protection should be at hand as needed.

Inspection of ConcretingThe essential requirements for satisfactory concreting are as follows: 1. Concrete should be uniform and should satisfy specification and job requirements as to workability, water-cement ratio, and quality. 2. Equivalent basic quantities of materials for a given mix should be uniform from batch to batch. It follows that the materials themselves should be uniform and of satisfactory quality. 3. Mixing should be thorough enough to distribute the materials uniformly and spread the cement-water paste evenly onto surfaces of the aggregate. 4. Concrete should be conveyed, placed, and vibrated without segregation; and vibration should be so thorough as to fill all parts of the form, eliminate air and rock pockets, and form a bond with adjacent steel or concrete. 5. The concrete should be well cured. Working conditions Concreting should proceed only after specification requirements have been met with regard to preparation of site; location and condition of forms and reinforcement; condition and sufficiency of equipment for mixing, transporting, placing, finishing, and curing the entire unit of placement; suitability of materials; and sufficiency of personnel. Some specifications prohibit concreting at night (to insure visibility) or during periods of extreme heat, extreme cold, or rain unless stated conditions of protection of the work are fulfilled. Batching of materials The essential requirement for proper batching is that the required proportions of materials be obtained uniformly from batch to batch, regardless of fluctuations in the supply. Mixing Essential requirements of satisfactory mixing of concrete are that the materials be uniformly distributed and that the cement-water paste be well plastered on surfaces of the aggregate. The mixer should be in good condition, properly designed particularly as to blading, not overloaded, fed efficiently, and operated at the optimum speed as recommended by the manufacturer. If concrete is to be mixed by hand, the inspector should insist on thorough and long-continued mixing. The dry materials should be mixed first, then water added and the entire mass turned at least three times or until a homogeneous mixture of the required consistency is obtained.Control of consistency Usually the consistency is regulated by varying the amount of water added at the mixer, based on observed consistency of previous batches. If aggregates are uniform in moisture content and grading, there will be small need for varying the amount of added water Tests for consistency should be made to insure compliance with specification requirements, but workability of the concrete and suitability of its consistency should be judged by how the concrete is responding to working and vibration in the forms, not by the test alone. The usual test for record of consistency is the slump test, made in accordance with ASTM C 143. Preferably tests for consistency and for air content of concrete should be made at the same time, as variations in air content greatly affect consistency.Control of air content This is important primarily because on one hand too much air detracts unnecessarily from strength without a sufficiently compensating improvement in durability, whereas too little will fail to provide workability and durability as desired. If the sand and water content are at a minimum for a certain amount of air entrainment, a drop in air content may cause a serious loss of workability. Conveying Methods of conveying concrete from mixer to forms should be such as to result in uniform concrete at the forms, with a minimum of slump loss. The method of conveying should not require a wetter consistency than that required for placing, nor should it induce segregation or result in excessive drying and stiffening. Wherever concrete is dumped or dropped, the direction of fall should be vertical; otherwise segregation of coarse aggregate will invariably occur. Placing The methods of placing and vibrating the concrete should be such as to keep it uniform and free from imperfections. This stage of the work is the key point of the whole operation of concreting. Proper methods of placing will not only prevent segregation and porous or honey combed areas but will also avoid displacement of forms and reinforcement, secure a firm bond between layers, minimize shrinkage cracking, and produce a structure of good appearance. To avoid segregation, the direction of drop should be vertical. Consolidation As concrete is being placed it should be consolidated thoroughly and uniformly by means of hand tools, vibrators (preferably), or finishing machines to secure a dense structure, close bond with reinforcement, and smooth surfaces. Construction joints Construction joints, as distinguished from expansion joints, are not intended primarily to permit movement between sections of the structure, but occur because of the requirements of construction. They should be properly located, neat, and well bonded. Finishing; Repairing Surface DefectsThe efficiency of the inspection will be judged largely by the condition of the finished surface. Also, the surface is subjected to the most severe conditions of wetting and drying, temperature change, and wear. In addition to meeting the requirements of appearance, the surface should be dense and free from excessive shrinkage, which causes surface cracking. Unformed surfaces While the surface is still fairly soft, the inspector should check the surface for alignment by means of a straightedge or template. The final troweling to secure a smooth surface should be delayed as long as possible. The proper time varies with the cement, weather, and other conditions; roughly, it is the time at which the surface can no longer be dented with the finger. Shallow cracks indicate excessive evaporation due to wind or sun; deeper cracks indicate excessive absorption by aggregates or perhaps the subgrade. If the concrete is still sufficiently plastic at the time the checks are dis covered, the cracks can be closed by reworking or refinishing the concrete. Finishing of unformed concrete cannot be performed in the rain, and rain will quickly damage newly finished work. Hence, either fully effective shelter must be provided or the work discontinued until the rain is definitely over. Formed surfaces Specifications should state clearly what finish is required or will be permitted on formed surfaces. These finishes may range from merely knocking off the fins and repairing imperfections to one of several decorative finishes. By the time the forms are removed, usually the concrete has acquired a considerable degree of hardness; therefore, any special finishes must be applied by some process such as brushing, scrubbing, floating, rubbing, or plastering. If it is desired to grind, chip, bush-hammer, or sandblast the surface, the concrete must be thoroughly hardened. The inspector should be on hand when forms are re moved, to observe the condition of the concrete surface before finishing or repairing is begun. Holes left by tie rods should be hammer-packed with stiff dry mortar of the same materials as, but somewhat leaner than, that in the concrete.

Inspection After ConcretingInspection does not end with the actual casting of concrete; it is still necessary to see that the concrete is protected from damage and is properly cured. Observation of finished parts of the work should be continued throughout the construction period. Curing By "curing" of concrete is meant keeping it moist and, if necessary, warm so that hydration of the cement can continue. Curing improves greatly the surface condition and general quality of. Most specifications require that exposed surfaces of concrete containing normal cement be kept continuously moist for at least 7 days, and many specifications require 14 days of curing. Protection from damage Construction operations may injure concrete already in place by overloading, jarring, or surface marring. Continued jarring or vibration, if not severe, is not generally detrimental; but heavy impact on green concrete is likely to disrupt the mass and should not be permitted. Floors over which construction activities are carried on should be covered, and projecting ornament should be protected from falling material. Nothing should be attached to projecting reinforcement. Underpinning and protective sheet piling should be left in place until the concrete has hardened. Backfilling against concrete should be done only when the concrete is strong enough to carry the load, and only if performed with care to avoid impact. Damage to waterproof coatings should be prevented by excluding sharp and heavy material from the backfill near the concrete, and by puddling the backfill to prevent future settlement. Maintenance and repair This is the work needed to preserve the intended load carrying capacity to ensure correct functioning, aesthetic appearance and continued safety. It consists of the prevention of deterioration by normal causes, wear, damage through accidents, vandalism and built-in imperfections.

Reference:ACI Manual of Concrete Inspection