The major difference between mortar and concrete is that, in addition to cement (and fly ash), fine aggregate (sand), and water, concrete also contains coarse aggregates such as crushed limestone or gravel, that are usually less than 1.5 inches in size. Concrete also contains chemical admixtures to achieve desirable properties such as good workability and freeze-thaw durability.
As described above, the workability of mortar is gauged using the flow test. In concrete the workability is evaluated using the slump test. When concrete is to be used for a construction project, the slump is usually specified. It is measured by compacting a wet concrete mix into a metal cone that is 12 inches high, with openings of 4 inches at the top and 8 inches at the bottom.
Excess concrete is made level with the top of the cone which is then slowly lifted up and thus away from the concrete mix. If the concrete is prepared using a relatively small amount of water, then the unsupported cone will collapse or slump only slightly. If more water is used, then the concrete slump will be higher.
The higher slump can also be achieved using special water-reducing admixtures that facilitate the use of less water to produce a very workable concrete which will have a higher strength than concrete with similar workability but higher water content.
The entrained air content in concrete can be measured using 3 methods: gravimetric (which is similar to the mortar method), volumetric, and pressure. The pressure method is based on Boyles Law and is the most commonly used. It can be conducted using two types of meters: a "Type A" and "Type B" meter. The Type B meter is the most popular and is the one described here. The test is conducted by first placing and compacting fresh concrete into a metal bucket. During concrete mixing and placement, air bubbles are introduced into the concrete. There are generally two types of bubbles: entrapped and entrained. Entrapped air bubbles are generally larger and have a small surface area, whereas entrained air bubbles are small (i.e. less than 1 mm) and dispersed throughout the cement paste. The entrained air therefore provides a more effective protection against freeze-thaw damage than the larger, poorly dispersed entrapped air voids. Large entrapped air voids are removed from the fresh concrete in the bucket using vibration or compaction with a steel rod (termed "rodding"). Remaining voids are removed by striking the bucket repeatedly with a rubber mallet.
After the bucket has been filled, the top of the concrete is made level with the top of the bucket by striking-off with a metal bar. The lid of the apparatus is secured to the bucket and water is introduced to eliminate air voids above the concrete. Air is then pumped into a cylinder of known volume until a specified pressure is achieved. The pressurized air is then released into the vessel; the resultant pressure drop is related to the entrained air content in the concrete and the gauge has been designed such that the air content of the concrete can be read directly from the gauge.
The initial set time of concrete is based on the same concept as for paste and mortar, that is, the resistance to penetration by a small steel rod or needle. However, in paste and mortar the Vicat needle penetrates the material using the weight of the needle assembly only, whereas for concrete a force is applied to a steel rod in order to penetrate into the concrete. The instrument used for concrete is called a "penetrometer". A sample of freshly mixed concrete is first screened to remove aggregate greater than 1/4 inch in size. The screened concrete is then placed into a cylindrical container (a large coffee can works well). Periodically, a steel rod of a known cross-sectional area is forced into the concrete to a depth of 1 inch in 10 seconds. As the concrete sets, a narrower rod size is selected in order to decrease the cross-sectional area and thus the force required to penetrate the concrete. When the resistance exceeds 4,000 pounds per square inch (PSI), then the concrete has achieved initial set.
When a concrete mix has been prepared with the desired slump and air content, specimens are prepared for compressive strength testing. Instead of cube specimens, the concrete is placed and compacted into cylindrical molds, which are usually made of plastic, that have a diameter to length ratio of 1:2 (e.g. 4 inch diameter and 8 inch length). The concrete is then struck-off so that it is level with the top of the mold. A plastic bag or lid is placed over the exposed concrete, which is then cured in the mold for 24 hours. The concrete is "stripped" from the mold using a special tool designed for this purpose, then placed into a mist room or tank of water for curing.
When a concrete cylinder is to be tested for compressive strength, it is important that the cylinder end surfaces be very smooth and flat, and each end surface needs to be approximately 90 degrees to the long axis of the cylinder. In other words, when looking at the cylinder, it should not appear as if the top and bottom surfaces are slanted. To meet these strict requirements, the cylinder ends must be precisely cut and honed (which is rarely done) or "capped". There are several capping methods that are commonly used. The first method uses machined steel caps that contain a hard rubber liner to provide smooth and level end surfaces for each cylinder. Another method uses a molten sulfur-based capping compound that is applied to the surfaces using a special mold. The molten sulfur compound hardens very rapidly and provides a very hard, smooth surface.
The strength of the concrete cylinders is tested using a compression tester. This apparatus applies a force at a specified rate and records the force required to break the cylinder. Because the concrete cylinder is not constrained on the sides, this test is more accurately called "unconfined compressive strength". The point at which the concrete fails can range from almost unnoticeable for weaker concrete to spectacular for high performance concrete.
This test is used to assess the durability of concrete, specifically, the ingress of chloride into concrete. Chloride is detrimental because it can accelerate the corrosion of reinforcing steel within concrete. One way to prevent chloride ingress is to make the concrete less permeable, which is one of the benefits of using fly ash. The rapid chloride permeability (RCP) does not measure the migration of chlorides per se, but rather measures the conduction of an electrical charge through concrete. The underlying principle is that concrete which resists the passage of an electrical charge is less permeable and thus would also inhibit chloride ingress. This inference is somewhat controversial, although a general relationship between the RCP test results and actual chloride permeability data has been demonstrated.
The test is conducted by first cutting the top 2 inches from a 4 inch diameter by 8 inch long cylinder. The 2 inch section is then sealed between two plexiglass cells. The cylinder is in contact with brass screens, which are attached to the inner surface of the cells and wired to electrical contacts on the exterior of the cells. One cell is filled with an NaCl solution, whilst the other is filled with an NaOH solution. The electrical leads are connected to the RCP analyzer, and an electrical current is passed from one cell, through the concrete section, and out the other cell. The total charge passed in 6 hours is recorded as the test result. ASTM has devised a very general permeability scale based on the charge passed.