samples may be taken from the batch immediately after depositing it on the subgrade. At least five samples should be taken at different times, and these samples should be thoroughly mixed to form the test specimen.
When making a slump test, dampen the cone and place it on a flat, moist, nonabsorbent surface, From the sample of concrete obtained, immediately fill the cone in three layers, each approximately one-third the volume of the cone. In placing each scoop full of concrete in the cone, move the scoop around the edge of the cone as the concrete slides from the scoop. This ensures symmetrical distribution of concrete within the cone. Each layer is then "rodded in" with 25 strokes. The strokes should be distributed uniformly over the cross section of the cone and penetrate into the underlying layer. The bottom layer should be rodded throughout its depth.
If the cone becomes overfilled, use a straightedge to strike off the excess concrete flush with the top. The cone should be immediately removed from the concrete by raising it carefully in a vertical direction. The slump should be measured immediately after removing the cone. You determine the slump by measuring the difference between the height of the cone and the height of the specimen (figure 6-4). The slump should be recorded in terms of inches of subsidence of the specimen during the test.
After completing the slump measurement, gently tap the side of the mix with the tamping rod. The behavior of the concrete under this treatment is a valuable indication of the cohesiveness, workability, and placability of the mix. In a well-proportioned mix, tapping only causes it to slump lower. It doesn't crumble apart or segregate by the dropping of larger aggregate particles to a lower level in the mix. If the concrete crumbles apart, it is oversanded. If it segregates, it is undersanded.
A mix must be workable enough to fill the form spaces completely, with the assistance of a reasonable amount of shoveling, spading, and vibrating. Since a fluid or "runny" mix does this more readily than a dry or "stiff' mix, you can see that workability varies directly with fluidity. The workability of a mix is determined by the slump test. The amount of the slump, in inches, is the measure of the concrete's workability - the more the slump, the higher the workability.
The slump can be controlled by a change in any one or all of the following: gradation of aggregates, proportion of aggregates, or moisture content. If the moisture content is too high, you should add more cement to maintain the proper water-cement ratio.
The desired degree of workability is attained by running a series of trial batches, using various amounts of fine to coarse aggregate, until a batch is produced that has the desired slump. Once the amount of increase or decrease in fines required to produce the desired slump is determined, the aggregate proportions, not the water proportion, should be altered in the field mix to conform. If the water proportion were changed, the water-cement ratio would be upset.
Never yield to the temptation to add more water without making the corresponding adjustment in the cement content. Also, make sure that crewmembers who are spreading a stiff mix by hand do not ease their labors by this method without telling you.
As you gain experience, you will discover that adjustments in workability can be made by making very minor changes in the amount of fine or coarse aggregate. Generally, everything else remaining equal, an increase in the proportion of fines stiffens a mix, whereas an increase in the proportion of coarse loosens a mix.
Before you alter the proportions set forth in a specification, you must find out from higher authority whether you are allowed to make any such alterations and, if you are, the permissible limits beyond which you must not go.
As previously mentioned, concrete consists of four essential ingredients: water, cement, sand, and coarse aggregate. The same mixture without aggregate is mortar. Mortar, which is used chiefly for bonding masonry units together, is discussed in a later chapter. Grout refers to a water-cement mixture called neat-cement grout and to a water-sand-cement mixture called sand-cement grout. Both mixtures are used to plug holes or cracks in concrete, to seal joints, to fill spaces between machinery bedplates and concrete foundations, and for similar plugging or sealing purposes. The consistency of grout may range from stiff (about 4 gallons of water per sack ofContinue Reading