CHAPTER XXXII

INSPECTION OF CONCRETE WORK-A SUMMARY OF ESSENTIAL RULES AND PRINCIPLES OF CONSTRUCTION, FOR SECURING GOOD CONCRETE WORK

The Work of the Inspector.-Inspection of the Cement, Sand, and Aggregates.—Proportioning and Mixing.-Inspection of Forms, Reinforcement and Placing Concrete.-Rules for Removing Forms.-Rules for Surface Finish.-Rules for Blocks, Piles, and Castings.

CAREFUL inspection is essential in all concrete work. The best design will come to naught unless it be carried out with the aid of careful and skilful workmanship and the use of good materials. Good construction can be assured only when the work is under the control of competent and conscientious inspectors.

The Work of the Inspector.-The work of the inspector may be divided into the following parts:

1. Inspection of the cement, sand, and aggregate; a. quality;

b. storage.

2. Proportioning, measuring, and mixing of the ingredients. 3. Inspection of forms, arch-centres, column moulds, etc.

4. Placing of the reinforcement.

5. Placing of the concrete.

(a) General rules.

(b) In reinforced work.

(c) In hot weather.
(d) In freezing weather.

6. Bonding new to old work.

7. Removal of the forms.

8. Surface finish.

9. Moulded blocks, piles, ornamental castings, etc.

Inspection of the Cement.-Cements are subjected to laboratory tests to determine their:

(a) Fineness.

(b) Time of set.

(c) Soundness.

(4) Specific gravity.

(e) Strength.

(a) Fineness is determined by passing the cement through sieves of various meshes and noting the percentages retained.

(b) Time of set is found by making pats of the cement and noting the time required to resist the penetration of wires of specified weight.

(c) Soundness is tested by noting the condition of the edges of the pats; also by subjecting pats to a steam bath and observing whether they blow, swell, or crack.

(d) Specific gravity is determined by weighing a given volume in air and noting the loss of weight when immersed in a liquid of known specific gravity, such as alcohol, which does not act on the cement.

(e) Strength is determined by moulding briquettes of 1 sq. in. sectional area, permitting them to remain in air and under water for specified periods, and then breaking in testing machines, and noting the breaking loads.

Cement should be stored in its original package, until ready for use. It should also be kept in a clean, dry place. If stored in a damp place, the cement will partially set and become valueless for construction purposes.

Inspection of Sand and Aggregates.-Sand for impervious concrete should be silicious in character, of graduated size, and with coarse, rounded grains. Sand should also be clean, but excessive cleanliness is not essential as an admixture of clay in amounts up to 10 per cent results in no material reduction in the strength of mortars. A small percentage of clay also tends to increase the imperviousness of the concrete.

When specifications call for sharp sand, the grains should be angular. Sharp sand was until quite recently always required by engineers, on account of its binding properties. Recent experiments, however, indicate that sand with rounded grains is less liable to fracture; and when graduated, so that the smaller grains fit between the larger ones without wedging them apart, is far more impervious when used in mortar or concrete.

The best aggregates for concrete are trap rock and gravel.

Hard limestones and granite are also good. Soft limestones, sandstones and schists are less durable, while slate, shale, and cinders are poor materials to use. The size of the aggregate is of importance. In massive work, the stone should pass through a 2 1/2 inch ring, in reinforced concrete beams, the diameter should not exceed 3/4 of an inch.

Rules for Proportioning, Measuring, and Mixing.-American engineers proportion concrete mixtures by measure, thus: a 1:2:3 concrete is one composed of 1 volume of cement, 3 volumes of sand, and 6 volumes of aggregate.

I

The duty of the inspector is to make certain that the specified proportions are accurately and uniformly adhered to. This requires: (a) That definite measuring units be employed.

(b) That the accuracy of the measure boxes, hoppers, etc., be verified.

(c) That the filling of the measuring boxes, hoppers, etc., be

exact.

(d) That when two or more boxes or hoppers, filled with sand or stone, go to make up a batch, the exact number be employed for each and every batch.

Cement differs in volume when measured loose, and when packed in the barrel; cement barrels also vary in capacity. Hence the engineer, contractor, and inspector should reach an agreement as

to:

(a) Whether the cement is to be measured loose or packed.

(b) What the cubic contents of a barrel or bag of cement shall be called.

The measures used should be tested to make sure that each holds the amount intended. This can be very simply done by using a known measure to fill the measuring box employed, or the volume of the box can be mathematically computed.

When automatic measuring devices are used to proportion the cement, the inspector should see:

(a) That they are regulated to give the proper proportions. (b) That the materials do not clog, choke, or arch in the feed hoppers.

(c) That the feed hoppers are kept amply supplied with materials.

2.

Concrete is mixed by: 1. Hand turning with shovels and hoes; Machine mixing.

Rules for Hand Mixing.-Rule 1. The batches should be of such size that they can be proportioned without using fractions of mea

sures.

Rule 2. Mix the cement and sand dry with hoes or shovels.

Rule 3. Over the dry sand and cement mixture spread the broken stone which has been previously wetted and on top of the stone apply water evenly.

Rule 4. Finally turn the whole the specified number of times with shovels.

Rule 5. The quantity of concrete in each batch should be not greater than can be mixed and deposited before the cement begins to

set.

Rules for Machine Mixing.-Concrete mixers are of three types: (a) Batch mixers.

(b) Continuous mixers.

(c) Gravity mixers.

In batch mixers the materials are charged, mixed, and discharged in batch units; in continuous mixers the materials are discharged in a continuous stream; and in gravity mixers the materials are caused to mingle by falling through specially constructed troughs, tubes, or hoppers.

Rule 1. The mixer should be of an approved type, and operated in such a manner as to mix the materials uniformly and efficiently. Rule 2. If a batch mixer is used, the batch should be (a) composed of the proper proportions, (b) thoroughly mixed, and (c) completely dumped out as a unit.

Rule 3. When a continuous mixer is used, the materials must be (a) fed evenly into the mixer in the proper proportions; (b) the automatic measuring devices must work accurately, and (c) the material must not "bridge" or "choke," and so cease to feed into the mixer drum.

Rule 4. The mixer must be given the requisite number of turns for each batch, as determined by trial.

Rule 5. The concrete in discharging from the mixer should not drop any considerable distance.

Rule 6. The mixer should be cleaned of all adhering mortar or

concrete when work is discontinued, as such cakes are liable to break or jar loose and be discharged as an inert body into the next batch.

Inspection of Forms.Forms are the moulds in which concrete is shaped, and it is the duty of the inspector to see that they are of ample strength, efficiently braced and in proper alignment. The following rules should also be observed:

Rule 1. The lumber should be of such quality, size, and finish as to promise absolute stability and reasonably perfect work under the conditions.

Rule 2. Forms should be oiled or wetted just before the concrete is deposited to prevent sticking. Oil should be used where a smooth surface is desired. It should not be used where the concrete is to be plastered or whitewashed, as the grease will discolor the work and weaken the bond.

Rule 3. White pine, yeilow pine, spruce, Oregon pine, and redwood are suitable for forms; hemlock is unreliable.

Rule 4. Forms should be thoroughly cleaned of shavings, chips, sawdust, dirt, or other accumulations just before the concrete is placed.

Rule 5. The construction of the forms should be such that they can be removed without injury to the concrete.

Rule 6. All forms must be erected in exact alignment, both vertically and horizontally; column and wall forms should be plumb; girder boxes and wall forms without winds or twists; arch and slab centres level: the alignment must be watched during the placing of the concrete, as the loading may distort the forms.

Rule 7. Forms should be (a) of ample strength; (b) of sufficient rigidity not to deflect unduly under load, and (c) horizontal forms should be given a camber to prevent them from deflecting below the horizontal. A common camber is 1/2 inch for every 10 feet of span.

Rule 8. The carpenter work should be accurate, the lines true and square, the joints close and the finish neat. All forms must be planed where required to produce a smooth surface finish.

Rule 9. All joints in forms should be tight enough to prevent leakage of the grout from the liquid mass.

Rule 10. Column moulds must be accurately spaced in all directions and set square with the lines laid down on the plans.