The proportions must be so selected as to produce a concrete mixture of desired workability, strength, durability, and economy. The most common aggregates are gravel and crushed stone, although cinders, blast-furnace slat, burned shale, crushed brick, or other materials may be used because of availability, or to alter such characteristics of the concrete such as workability, density, appearance, or conductivity of heat or sound. Usually aggregate which passes a sieve with O. 187-inch openings (No. 4 sieve) is called fine aggregate, but that retained by a No. Sieve is oars aggregate, although the division is purely arbitrary. If all the particles of aggregate are of the same size, or if too many fine particles are present, an excessive amount of cement paste will be required to produce a workable mixture; a range of sizes aids in the production of an economical mixture. The best concrete for a given use is usually the one which will provide the necessary strength and the desired workability at the lowest cost. Unless otherwise indicated, strength, as applied to concrete, refers to the ultimate compressive strength of the moist-cured concrete at the age of 28 days.
Most concretes are batched to provide an ultimate compressive strength of 2500 to 4000 SSI after 28 days. The figure below shows a typical strength curve of concrete with the passage of time. The modulus of elasticity of concrete is about 1000 times the ultimate compressive strength. The strength of concrete depends chiefly on the water-cement ratio, with a low ratio producing a strong concrete. While only a small amount of water is required to complete the chemical reactions of setting concrete, more than this is used to make the concrete more workable.
The restorability of concrete is usually measured by its slump. The standard method of measuring slump consists of placing the freshly-mixed concrete in a mold in the form of a truncated cone, 12 inches high, 8 inches in diameter at the bottom, and 4 inches in diameter at the top. The concrete is placed in the slump cone in three layers, each layer eroded thoroughly to compact it. When filled, the mold is immediately withdrawn by lifting it gently, and the slump of the concrete is measured at the vertical distance from the top of the mass to its original 12 inch height.
An increase in the amount of mixing water will increase the slump, but it will also decrease the strength and increase the tendency of the ingredients of the concrete to segregate unless more cement is added. Increasing the amount of cement paste increases the cost, so all three factors- strength, workability, and cost-are interrelated in a complex way. Procedure: 1 . Concrete mixtures are commonly given as volume ratios as cement: sand: gravel. You will make two concrete mixtures at ratios given to you by the instructor.
Calculate the volume of the mold and determine the volume of moment, fine aggregate (sand) and coarse aggregate (gravel) for your mix ratio. Make one mixture and fill the mold, then mix the second mixture. 2. From the density of the materials, determine the weight of the required materials. Verify your calculations with the instructor. Show all calculations in your report. 3. Weigh out and mix the materials. (Refer to ASTM C 192). The mass of water used should be 50 – 60% of the cement mass. Record the mass of the cement, sand, gravel, and water (the mass of water used is approximately equal to the volume). . Record the type of cement of used: 5. Mix the dry ingredients and gradually add the water. You may not need to add the full amount of water calculated. Be sure to use the gloves provided as you mix your concrete. When adding the water note the concrete’s cohesiveness whether the concrete tends to hang together well or whether it tends to crumble readily – and the towering workability – if the concrete works smoothly and with little effort when using a trowel. Continue to add water until you have a desirable consistency. 6. Perform the slump test.
Fill the slump mold 1/3 full and tamp with the tamping rod 25 times; add more concrete until 2/3 full and tamp an additional 25 times; fill completely, tamp 25 times and then top off. Do not tamp more than 25 times. Measure the height of the concrete after removing the slump mold. The slump is 12″ (the height of the mold) minus the height of the concrete after removal of the mold. The greater the slump usually means the greater the workability. Your slump should be about 2 inches. 7. Be sure to measure and record the temperature of the concrete and the outside temperature. . Fill the molds, tamp 25 times, top off, and cover with a plastic bag to setup. 9. After a day or two, cut off the mold. Note any cracking or pores in the test samples. Put the concrete samples in a bucket, spray the concrete with water, and cover the bucket tightly with plastic. 10. On the designated “concrete crush lab day,” perform the compression test according to ASTM CO. Questions to be addressed in the lab report: 1. Fill in the group data sheet posted in the lab and include this table in your report. 2.
Your report should include all data related to the preparation of your encounter, the group data sheet, and any calculations pertaining to these items. 3. Plot compressive strength (y) versus cure time (x) for the two mixtures (on one plot). 4. From the class results, comment on how the mix ratio, weight % water, curing time, water/cement ratio, temperature, type of cement used and slump affect concrete’s compressive strength. In addition, you need to note the fineness modulus of the concrete sand from the sand sieve analysis in rotation A. 5. Be sure to include your observations of the physical properties of the concrete mixtures.