San Francisco State University School of Engineering ENGR 200, Materials of Engineering Laboratory 3 Concrete Compression Test I. OBJECTIVES The purpose of this experiment is to investigate the strength of a general purpose concrete at a specific proportion of its constituents, and to study the effect of curing time on the strength of concrete. Note: Please refer to course handout for further reading on concrete. Materials Portland Cement, Sand, Gravels, Water. Apparatus II. EXPERIMENTAL PROCEDURE Table 1: Proportion of constituent materials Constituent Ratio Total Cement 1 20 Sand (Fine Aggregate) 1.8 36 Gravels (Coarse Aggregate) 2.7 54 Water 0.65 13 Bucket, metal rod, measuring tape, brick trowel, shovel, scale Wheelbarrow Slump Cone (12 height, Figure 1) 4 x Test Cylinders (6 diameter x 12 height) Compression Tester Concrete Preparation Procedure 1) Table 1 indicates that sufficient amount of concrete mix will result from using approximately 20 lbs of cement Figure 1. A slump cone. 2) Slowly add aggregates and cement into a wheel barrow, mix thoroughly and then pour water. Mix concrete until it reaches a reasonable consistency. 3) Next, perform Slump Test as described below and in Figure 2: i) Wet the cone with water and put it in an upright position with the bigger base flat down against a pan. Fill a third at a time and poke a stick into the concrete 10-20 times all around it each time to compact the mixture, i.e. there should be no air pockets inside, especially near the walls.
ii) After the cone is completely filled, lift it slowly counting 5 to 10 seconds for removal. iii) Let the concrete settle for 60 seconds after the cone is completely removed. iii) Measure how much the pile has slumped from the 12 original position If slump is more than 3.5 (i.e. pile height is below 8.5 mark), add more fine and coarse aggregate in equal amounts and repeat the slump test. If slump is less than 2.5 (i.e. pile height is above 9.5 mark), add water and cement at the ratio of 0.60:1 and repeat slump test, until slump falls in specified range (between 8.5 and 9.5 mark) 4) After passing the Slump Test, fill four Test Cylinders with the concrete mix, make sure to compact the content to remove air bubbles by repeating the action in 3 (i). The top surface of the concrete should be even and flush with the edge. Seal the cylinders properly. (a) (b) (c) Figure 2. Slump Test. (a) Pour concrete mix into slump cone. (b) Slowly remove slump cone. (c) Let concrete pile sits for 60 seconds, and measure the slump (i.e. distance from original height). (Source: http://upload.wikimedia.org/wikipedia/commons/archive/1/12/20070616102812!slump_test.png) III. COMPRESSION TEST & DATA COLLECTION Compression Test: To measure the strength of hardened concrete. This test will be carried out over a period of 4 weeks, starting one week after the concrete has been mixed. 1) Four compression tests will be carried out to determine the compressive strength of the concrete as a function of curing time. The first test will be performed one week from today, followed by one test per week in the subsequent three weeks. 2) Place a specimen in the concrete tester and adjust the compression head clearance to bring it in contact with the specimen 3) Place the red follow marker on the load indication face to zero 4) Activate the load cell by selecting the appropriate valve position and pump on the hydraulic cylinder using the manual lever fitted to the side. Continue until failure of the specimen occurs. 5) Record the fracture strength values (Note: you will record 4 values, corresponding to the fracture strengths at the end of Week 1, 2, 3, and 4)
IV. DELIVERABLES FROM YOU 1. Lab Assignment: Answer questions in (2) below and turn in a Tech Memo on the designated date, after you have completed 4 weeks of compression tests. 2. Please answer the following questions: i) Why is it important that during the first few days of the curing of concrete the evaporation of water from its surface be prevented or reduced? ii) What is the disadvantage of too high a water-cement ratio? What is the disadvantage of too low a water-cement ratio? iii) What happens to the compressive strength of the concrete between 7 to 28 days? Plot a graph of Compressive Strength (Psi) vs. Time (week), label it as Figure 1 and discuss the trend observed. iv) How does the compression test result compare to expected value at the end of 28 days, as illustrated in Figure 3 (below), based on the water-to-cement ratio of your sample? v) How do the aggregates affect the mixture? Discuss the effects of having too much and too little aggregates. vi) Suggest possible chemical bondings involved in concrete forming. Briefly explain your answer. Figure 3. Effect of water-to-cement ratio by weight on the compressive strengths of normal and airentrained concrete at the end of 28 days.
San Francisco State University School of Engineering ENGR 200, Materials of Engineering Data Sheet for Experiment on Lab 03: Concrete Mixing and Testing. Group Members: Date: Constituents: Constituents Cement Sand Gravel Water Adjustment Total Slump Test on Wet Concrete: (between 2.5-3.5 in) Final Slump (in) Compression Tests on Hardened Concrete: Curing Time (Week) One Week Two Weeks Three Weeks Four Weeks Compressive Load (lbf)
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