CIVE 302 Spring 2013 T. Johnson Dr. R.K. Dowell Lab 6. Compression Testing of Concrete Cylinders In this lab, the cylinders poured previously will be tested to failure in compression. While ASTM standard compression testing specifies that three cylinders be crushed at 28 days for the design strength f'c, this lab will deviate slightly and test one cylinder each week for 7, 14, 21, and 28 day strengths. As concrete is a material which changes over time, this process will help visualize the growth in strength and change in failure mode as concrete cures. To account for cylinder variability in strength, the force-displacement data gathered each week will be shared between lab sections and an average strength for that day computed. Any asymmetries between pours – such as slump differences, mixing quality, or variable mix quantities - will be provided along with this data for discussion. The mix used in this poor is manufacturer-specified to have a an ultimate design compressive strength f'c = 2500 psi. This value represents the minimum expected break strength of the concrete at 28 days and is the quantity used by engineers in the design process. To contextualize this design value, however, the stress-strain behavior of concrete should be understood. To measure deformation in the cylinder during the test, a compressometer – shown below in Figure 6-1 – will be attached to the cylinder over a 6 inch gage length. As the cylinder is compressed, both rings will pivot about the rod on the left which will in turn compress the dial indicator on the right. Figure 6-2 shows this geometry in detail. Figure 6-1: Concrete Cylinder with Compressometer Pivot Rod ¢ 118 ¢ Figure 6-2: Geometry of Compressometer Reading 128 128 6"-8 6" gage length From this, we can see that the dial indicator reads twice the displacement that the cylinder experiences. With displacement thus known over the gage length, strain can be determined for a given load increment. To determine the stress corresponding to this strain, the axial load should be recorded and divided by the average diameter of the cylinder. This average should be computed based upon three readings at the top, center, and bottom of the cylinder using a micrometer, totaling 9 diameter readings for each specimen. Concrete's compressive strength is strain-rate dependent: that is, the time over which concrete is strained will change its compressive behavior. Should the test be run in force control, the system equilibrates the applied force up through the ultimate point, after which the loss in strength results in an applied force equal to the concrete mass multiplied by the loading head's acceleration. Thus, to provide stability for the experiment the test will be run in displacement control. To prevent damage to the dial indicator from excessive strains or slamming effects, it should be removed prior to reaching the ultimate stress. With that in mind, concrete maintains some strength after reaching its ultimate load. Post-peak displacement results in reduced forces and stresses associated with increased displacements and strains - a concept known as negative stiffness. This is illustrated below in Figure 6-3 for the force-displacement profile and in Figure 6-4 for the stress-strain profile. Force (kips) Negative stiffness Fu 0.5F I I Compressive strain capacity in ACI Design Code D₁ 0.0360 p Stress (ksi) f'c 0.5f' Eci 0.0600 Displacement (in.) Figure 6-3: Force-Displacement Response of Concrete Negative stiffness 0.003 Compressive strain capacity in ACI Design Code 0.005 Strain Figure 6-4: Stress-Strain Response of Concrete Unlike the metallic specimens tested in previous labs, it is clear from these plots that concrete does not have a well-defined linear-elastic region. Standard design equations typically give the elastic modulus of concrete as E = 57000√ √f (6-1) C Where both E and f' are given in psi. This principle is based off taking a determining the stress equal to f'c, locating the corresponding strain, and drawing a secant line to this point. The axial stiffness of concrete is also often of interest, and for concrete is determined in the same fashion as the elastic modulus but using the force-displacement curve instead of the stress- strain curve. Theoretically, stiffness is defined as the amount of force required to compress or elongate an object axially and is given as (6-2) K= AE Where A is the cross-sectional area of the member, E is the elastic modulus, and L is the total length (not the gage length). The Experiment One of the cylinders poured in Lab 5 will be tested to failure in a concrete compression testing machine. Prior to loading, three diameter readings toward the top, three in the middle, and three toward the bottom will be taken using a micrometer. Space these readings roughly 120 degrees apart. Use the average of these measurements to determine the cross-sectional area of the cylinder. Once the diameter of the cylinder has been measured, attach the compressometer such that the center of the gage length is roughly at the center of the cylinder. Make sure that the clear space for both the top and the bottom rings is approximately equal on all sides. Place two loading caps with rubber pads inside to ensure the loading head applies the load evenly to the top and bottom faces of the cylinder. Take measurements approximately every 5000 lbs that include both a load and a dial gage reading. Use the following values of load to approximate the ultimate force: 7 Day: 60,000 lbf 14 Day: 70,000 lbf 21 Day: 75,000 lbf 28 Day: 80,000 lbf Note that the cylinder may reach higher strengths than these values, but they are conservative estimates for removing the compressometer early enough so that it will not sustain damage. Thus, a strain at the true ultimate stress may not be reached. Required Calculations (for 7, 14, 21, and 28 day tests) Axial stress (at each increment) Axial strain over gage length (at each increment) Experimental E from stress-strain curve as described above Design E as specified by ACI Experimental K from force-displacement curve as described above Theoretical K (use experimental E) Required Plots f'c for each day vs. time in days (include f'c from each lab section) Stress vs. Strain for 7, 14, 21, and 28 plotted together on one graph Required Discussion Discuss the mode of failure of each cylinder for 7, 14, 21, and 28 day tests. Did it fail as expected, or did you see any deviations? Explain what you observe using the following as guidelines: Mix procedure. Did your mix go well, or were there inconsistencies? If the latter, explain what went wrong and how this ties into the observed failure mode. Adherence to ASTM procedure during the pour. Were the cylinders properly prepared, or did you notice anything that may have contributed to an uneven cylinder (think in terms of the layering, tamping, and tapping process) Compare your results with the 7 and 28 day strengths from concrete pours used in previous experiments in the SDSU shaking table lab. Are your results close, or do you have a weaker or stronger mix? Explain.