The specific gravities and weights per cubic foot of materials commonly used for aggregates are approximately as follows: Determination of voids in coarse material, such as gravel or broken stone not containing sand or other fine material, may be obtained by filling a measure of known volume with the material, and pouring in water until the measure is full.
The volume of water Then, the percentage of voids= —X100.
The total volume When the specific gravity of the material is known, the voids may be obtained by weighing a measured volume of the broken stone, subtracting this weight from the weight of an equal volume of the solid material, and dividing by the solid weight.
If the aggregate contains fine material, the methods used for sand as given in Art. 7 must be used.
It is evident that the percentage of voids in a mass of broken material is not a fixed quantity, but varies with the arrangement of the pieces. If the material were composed of equal cubes, it would be possible to place them side by side so as to leave no voids which could be filled by smaller material. Poured loosely into a measure, such cubes would probably show at least 45 per cent of voids, which would be somewhat modified by shaking down and compacting the mass.
When the aggregate contains small pieces which may lie in the voids of the ones, the tendency to variation in results according to arrangement is greatly reduced, but the method of filling the measure, and amount of shaking that is given, will somewhat affect the results. Commonly, the material is shoveled into the measure and lightly shaken to get what may be a fair estimate of the voids in the material as it is to be used.
When fine material is introduced into a coarse aggregate to fill the voids, particles of the fine material get between the larger pieces and hold them apart so that the voids to he filled in the larger material are increased, and cannot be completely filled. This is shown by the fact that the volume of the mixture is greater than that of the coarse aggregate even though the volume of fine aggregate used is much less than the volume of voids in the larger material.
Selection of Joint Committee of the Engineer ing Societies on Concrete and Reinforced Concrete makes the follow ing recommendations concerning the selection of aggregates in its 1917 report.
Extreme care should he used in selecting the aggregates for mortar and con crete, and careful tests made of the materials for the purpose of determining the quality and grading necessary to secure maximum density or a minimum percentage of voids. Bank gravel should be separated by screening into fine and coarse aggregates and then used in the proportions to be determined by density tests.
(a) Fine aggregate should consist of sand, or the screenings of gravel or crushed stone, graded from fine to coarse, and passing when dry a screen having 4 in. diameter holes; it preferably should be of siliceous material, and not more than 30 per cent by weight, should pass a sieve having 50 meshes per linear inch; it should he clean, and free from soft particles, lumps of clay, vegetable loam, or other organic matter.
Fine aggregate should always be tested for strength. It should he of such quality that mortar composed of 1 part Portland cement and 3 parts fine aggre gate by weight when made into briquettes, prisms or_cylinders will show a tensile or compressive strength, at an age of not less than seven days, at least equal to the strength of 1:3 mortar of the same consistency made with the same cement and standard Ottawa sand. If the aggregate he of poorer quality, the propor tion of cement should he increased to secure the desired strength. If the strength developed by the aggregate in the 1 : 3 mortar is less than 70 per cent of the strength of the Ottawa sand mortar, the material should he rejected. In testing aggregates care should be exercised to avoid the removal of any coating on the grains which may affect the strength; hank sands should not he dried before being made into mortar, but should contain natural moisture. The percentage of moisture may he determined upon a separate sample for correcting weight. From 10 to 40 per cent may be required in mixing bank or artificial sands than for standard Ottawa sand to produce the same consistency.
Coarse aggregate should consist of gravel or crushed stone which is retained on a screen having ; in. diameter holes, and should be graded from the smallest to the largest particles; it should be clean, hard, durable, and free from all dele terious matter. Aggregates containing dust and soft, flat, or elongated particles should be excluded. The Committee does not feel waranted in recommending the use of blast-furnace slag as an aggregate, in the absence of adequate data as to its value, especially in reinforced concrete construction. No satisfactory specifications or methods of inspection have been developed that will control its uniformity and ensure the durability of the concrete in which it is used.
The aggregate must be small enough to produce with the mortar a homo geneous concrete of sluggish consistency which will readily pass between and easily surround the reinforcement and fill all parts of the forms. The maximum size of particles is variously determined for different types of construction from that which will pass a 1-in. ring to that which will pass a 11-in. ring.
For concrete in large masses the size of the coarse aggregate may be increased, as a larger aggregate produces a stronger concrete than a fine one; however, it should be noted that the danger of separation from the mortar becomes greater as the size of the coarse aggregate increases.
Cinder concrete should not be used for reinforced concrete structures except in floor slabs not exceeding 8-foot span. It also may be used for fire protection purposes when not required to carry loads. The cinders should be composed of hard, clean, vitreous clinker, free from sulphides, unburned coal or ashes.