The purpose of this lab is to prepare sample of structural concrete and to test and analyze its thermal behavior and strength characteristics.
SCHEDULE OF EXPERIMENTS
There are four sets of procedures in this lab (each at a different time):
Procedure 1: Test thermal properties (w/c=0.60 team)
Procedure 2: Prepare cylinders for strength tests (all teams)
Procedure 3: Save data from thermal tests (1 day after Procedure 1)
Procedure 4: Perform strength testing (at least one week after Procedure 2)
Concrete is a stone like material obtained by allowing a carefully proportioned mixture of cement, sand, gravel (or other aggregate), and water to harden into the shape and dimensions of the desired structure. The bulk of the material consists of fine aggregate (sand) and coarse aggregate (stone or gravel). Cement and water interact chemically to bind the aggregate particles into a solid mass. This lab seeks to understand and observe the properties associated with the strength of the hardened concrete and certain properties associated with chemical reaction between the cement and water of the concrete.
Heat of Hydration The chemical process involved in the setting and hardening of concrete liberates heat, known as the heat of hydration. In large concrete masses, such as dams, this heat is dissipated very slowly and results in a temperature rise. This rise in temperature causes volume expansion of the concrete during hydration. Eventually, the temperature rise reaches a maximum and then begins to slowly cool. With subsequent cooling the material begins to contract. As the material heats up and cools down, it expands and contracts and cracking can result. To avoid serious cracking and weakening, which may result from this process, special measures (like artificial cooling) are usually implemented.
The first major use of artificial cooling (post-cooling) of mass concrete was in the construction of Hoover Dam in the early 1930's (see Figure 1). In this case the primary objective of post-cooling was to shrink the concrete columns of the dam, so that contraction joints (voids to prevent cracking when expansion occurs) could be used. The cooling was achieved by circulating cold water through pipes embedded in the concrete. This cold water was circulated through the structure to prevent the effects of the heat of hydration. Circulation of water through the pipes was usually started several weeks or more after the concrete had been placed. After the Hoover Dam, post-cooling was used extensively by the U.S. Bureau of Reclamation in the construction of several large dams. Generally the practices followed were essentially identical to those followed at the Hoover Dam, except that the circulation of cooling water was initiated simultaneously with the placement of concrete, instead of several weeks after.
Figure 1—Pictures of Hoover Dam from Side and Above CHEMICAL PROCESS To make cement (a key component of concrete), manufacturers combine two abundant compounds to produce a more complex compound. This process, requiring high heat, produces solid pieces called clinker which are ground into fine particles of cement.
We can characterize the two natural compounds by the formulas CaCO3 for limestone and SiO2 for clay (although clay contains other compounds). These combine at about 1500ºC (or about 2700ºF) to give
This artificially made compound, called alite, is an unstable one which stores a chemical potential energy of high heat caused by its manufacturing. Combining alite with water forms a gel, some lime, and at the same time releases heat:
The gel (Ca3Si2O10H6) serves to bind sand and stones together to form concrete, the lime (Ca(OH)2) keeps the water basic (not acidic) and thereby prevents corrosion of steel reinforcing bars which is often used as a reinforcement in concrete, and the released heat (H) is what can cause the concrete to crack.
This cracking is especially dangerous in thick concrete structures such as heavy slabs and gravity dams. The chart in Figure 2 shows the temperature rise within the concrete at various depths versus time (after the mixing of the concrete). This indicates that the released heat builds up rapidly after 2 hours and after 100 hours (4 days) reaches a peak of about 75ºC (167ºF) for a slab 3 meters thick (about 10 feet). However, if the thickness is much greater, as it can be in a dam, none of the heat escapes to the surface and the maximum temperature can rise as high as 95ºC (203ºF).
Figure 2—Temperature Rise in Concrete versus Time For these large dams, the hot interior of the concrete tries to expand and the cooler exterior is forced to follow that movement such that the stress on the surface of a dam may reach
where E is the modulus of elasticity of concrete and a is the coefficient of thermal expansion. Typical values for concrete include:
E = 20.6 x 103 MPa (or 3 x 106 psi)
a = 10-5 per degree C (or 0.56 x 10-5 per degree F)
For example, if the maximum interior temperature were 73.7 degree Celsius and the measured surface temperature were 20 degrees Celsius, then the stress on the surface of the dam may reach 1074 psi. Since the tolerable cracking strength of this concrete would be about 410 psi (2.82 MPa), it is clear that such a dam will have substantial cracks unless something is done. As previously discussed, a solution used in the big dams of the 1930s was to build in a cooling system with inlaid pipes filled with circulating water to carry off the excess heat.
Figure 3—Table of Useful Formula Weights
For example, a normal cement, assumed here to be made entirely of alite, releases heat, H = 216 BTU per pound of cement. The formula weight of the alite is 456.6 pounds (see Figure 3 above), so that the released heat would be 456.6 x 216 = 98,500 BTU. The total weight of the products (gel plus lime) is 564.6 pounds and the concrete usually weighs about four times the weight of the products or 2,258 pounds. Furthermore, the heat capacity of concrete is about one joule per degree Celsius per gram or 0.43 BTU per degree Celsius per pound. This means that the temperature of one pound of concrete will rise by 1 degree Celsius for every 0.43 BTU of heat that it absorbs. Thus the temperature of 2,258 pounds of concrete will rise by (1/2258) degrees for every 0.43 BTU or
for 98,500 BTU. This large value, boiling the water, will cause cracking.
A first solution would be to use a different type of cement, one that liberates less heat upon hydration. Belite is such a low-heat cement that has the form Ca2SiO4 and it hydrates with H = 112 BTU per pound of cement. This is useful later in your calculation section.
STRENGTH OF MATERIALS
To take into account the forces acting on an object and the objects size, the notion of stress is used. Stress is defined as force exerted per unit area. Strength is the ultimate stress at which a specimen fails (for tension or compression respectively). The shaded region in Figure 4 is subject to a compressive stress, fc‘,such that
Figure 4—Compressive Forces in a Column The shaded segment of the cable in Figure 5 is subject to forces tending to pull it apart. For this reason, it is said to have a tensile strength, ft’, such that
where A is the cross-sectional area of the cable, but Fu is now the upward force exerted on the shaded element by the part of the cable above it, and Fd is the downward force exerted by the part of the cable below it. Figure 5 also shows the free body diagram.
Figure 5—Tensile Forces in a Column The compressive strength of a material is defined as the compressive stress needed to break a piece of the material (also known as the ultimate compressive stress). Similarly, the tensile strength of a material is defined as the tensile stress needed to break a piece of the material (also known as the ultimate tensile stress).
For a concrete cylinder the compressive strength (fc’) can be calculated by:
where P is the ultimate load (in lbs) at which the concrete specimen fails and r is the radius of the specimen (in inches).
To measure the tensile strength of a specimen, a special test called the split-cylinder test is used. This test places the cylinder on its side and applies a load to the side of the specimen (as shown in Figure 6). The specimen fails by splitting in half, thus a split cylinder (also Figure 6).
Where P is the load at which the specimen fails (lbs), d is the diameter of the specimen and L is the length of the specimen.
Theoretical values for the tensile strength of concrete are typically defined as
One of the most important factors affecting the strength of concrete is the proportioning of the different materials. The water-cement ratio is the chief factor that controls the strength of concrete. Figure 7 shows the various tradeoff relationships for higher and lower w/c ratios. For larger w/c ratios, the strength of concrete decreases. However, a higher w/c ratio means more water and therefore the concrete is easier to mix and to work with. For lower values of the w/c ratio, the strength is greater, but the cost increases. Strength, workability and cost are factors that engineers take into account when selecting the best mix for their particular design.
Figure 7—Effect of w/c ratio on strength, workability and cost Figure 8 shows the compressive strength of concrete as a function of the w/c ratio. You will use this Figure to later compare with actual values measured in lab.
Figure 8—Compressive Strength as a function of w/c ratio
Procedure 1:Test thermal properties
Setup Group (w/c =0.60 ONLY)
(Mixing Group for w/c = 0.60: see Procedure 2)
(All other w/c ratios: see Procedure 2)
Obtain a cylinder cap and regular drinking straw.
Cut a strip of clear tape as long as the straw. Place the clear tape on ½ of the straw and fold over covering one end of the straw opening. Flatten the folds of this tape together so that this end of the straw is sealed. This is later used to protect the temperature sensor from the concrete.
Take your cylinder cap and fold it in half. Using scissors, make a ½” cut into the center of the folded cap. Then fold you cap in the other direction and make another ½” cut. Now you should have a cap with a cross-shaped opening.
Insert the straw through the cap cross opening until the cap is in the center of the straw.
From the other half of you group, obtain the black cylinder filled with the proper cement and water mixture.
Using a paper towel to protect the scale, weigh the filled cylinder (remember to tare the scale with an empty black cylinder mold). Record this weight.
Add the cap with the taped-straw end first into the cylinder and mix. The open half of the straw should stick out above the top of the cylinder.
Wipe the outside of your cylinder with a dry paper towel to make sure it is clean. Place a label on the cylinder and label it with the date, your AI and one of your last names. Place this cylinder in the center of the inside of the Styrofoam box. Don’t put the Styrofoam lid on yet.
Get the lid of your square Styrofoam container. It should already have a hole in the center of it. Fit the lid over the box, so that the straw goes through the hole in the lid and sticks up on the outside of the top of the box.
Put the thermocouple (temperature sensor) through the hole of the lid and into the straw.
Use two pieces of gray duck tape and tape two opposite ends of the Styrofoam box shut.
Plug the thermocouple plug into the scientific-workshop center (the black box). Plug it into any one of the available sockets (A, B, or C).
On the computer, double click the icon “Process-thermal” from the desktop. The screen will list the temperature measured by the thermocouple.
Press ”START” to start recording your heat data. (Do not hit, “mon” because your data will not record).
NOTE: This procedure starts the recording of your data. It will record the temperature within your specimen every 15 seconds for the next day. You will be able to plot a curve like the one in Figure 2. To save your data, you need to follow Procedure 3 the following day. One member of your group should come back the next day before 1:30 and save your data. You may want to bring a 3 ½” floppy disk with you to save your data for later use.
Write down the first recording of temperature for your sensor.
On the top of your computer monitor, there should be a piece of paper taped and flapped over. Flap that paper down so that it covers the monitor and write the day and time on the paper. It says that an experiment is in progress and the computer shouldn’t be disturbed. This will protect your experiment until you save your data.
Procedure 2: Prepare cylinders for strength testing
Setup Group (all groups (including w/c=0.60))
Each station is assigned with a particular mix design (taped to the top of the table). Measure the required amount of gravel and sand in separate containers. (Remember when measuring to subtract the weight of the measuring cup by using the “tare” on the scale.) Give the gravel and the sand to the other part of your group.
When ready, get the finished mix from the other half of your group.
Get four black cylinder containers and spray each of them lightly with lubricant spray oil - be careful to hold the cylinder away from you when you spray (some of the spray will probably come back out of the cylinder).
The mix that you have should fill four black cylinders. Fill each cylinder ½-full with concrete. Using the metal tamping rod supplied, push the rod into the concrete about ten or so times. This eliminates voids and air bubbles that would later weaken the concrete.
Add more concrete to the cylinders until they are slightly over filled. Again, use the metal rod and push it into the concrete about ten times to eliminate voids.
Then pack more concrete on the top, so that it overfills the cylinder slightly. Using the rod, in a rolling motion, “roll off” the excess concrete, so that it is flush with the surface of the cylinder. Then you should be able to easily place the cap for your cylinder on. Add you cap to each cylinder.
Give your metal bowl and tools used to the other half of your group for cleaning.
Wipe your cylinders clean. Place a label on the cylinders and label it with the date, your AI and one of your last names. Also mark the water/cement ratio of your mix on this specimen (w/c = weight of water divided by weight of cement).
NOTE: This procedure produces the specimens that you will test for strength at least one week later when you do Procedure 4.
Fill in the appropriate data on page 17. Discuss what you did with the other half of your group and find out what they did.
Work only in side the mixing room. Use vinyl gloves, eye goggles and a particle mask to protect yourself. Please be careful when working with the cement, because it can irritate your skin and damage clothing. Put on latex lab gloves to protect your hands from the irritating effects that cement may sometimes cause to your skin and do not touch your eyes unless your hands have been washed. Make sure that you also take your watch off to prevent that from getting dirty. If you would like to protect your clothing then there are several labs coats that are available for use.
Each station is assigned with a particular mix design (taped to the top of your table). Measure the required amount of water and cement in separate containers. (Remember when measuring to subtract the weight of the measuring cup by using the “tare” on the scale.)
Add the water to the metal mixing bowl of the mixer. Then add the cement. You may need to remove the mixing blades and the bowl to add everything.
Make sure that the bowl is secure on the mixer. Using the metal handle (in the back of the mixer) raise the bowl. Turn the mixer on to a setting no higher than 3. Higher settings may cause your mixture to splatter and get on you and your clothing. Let it mix for about 3 to 5 minutes.
Stop the mixer. Then lower the bowl using the handle in the back.
Group with w/c ratio = 0.60 ONLY: Carefully remove the bowl and pour your cement and water mixture into one black cylinder (it does not need to be lubricated) – fill the cylinder to the top. This cylinder is for the thermal test. All other groups skip this step!
Get the gravel and the sand from the other part of your group. Add about ¼ of the sand and gravel to your metal bowl.
Make sure that the bowl is secure on the mixer. Using the metal handle (in the back of the mixer) raise the bowl. Turn the mixer on again to a setting no higher than 3. Higher settings may cause your mixture to splatter and get on you and your clothing. Slowly add the remainder of the sand and gravel and let it mix for about 5 minutes (if it’s difficult to add while still mixing you may stop the mixer and lower the bowl (steps 5,7,8)).
Give the mix with the metal bowl to the other half of your group.
Clean up!!! This involves:
Getting the bowl back from the other half of your group when they are finished with it.
Empty your mixing bowl into the left over concrete box.
Thoroughly rinsing the bucket and the tools and containers that you used in the trashcan filled with water (not in the sink). Dry these also. With a damp paper towel, wipe the scale and mixer. Then dry with another paper towel.
Clean the plastic on the desktop of any concrete particles.
Throw any trash way.
Have your AI inspect your cleanup.
Wash your hands thoroughly in the sink
Fill in the appropriate data on page 17. Discuss what you did with the other half of your group and find out what they did.
Save data from thermal tests (1 day after Procedure 1)
Go to the computer that you setup your thermal experiment on and move the mouse around and wait for a minute to de-activate the screen saver if it is active.
Using the mouse click on the “stop” key on the monitor in science workshop.
Click on the tabular form of your data (making that window active).
Go to to the File menu and click on “Save”.
Give the file a unique name and save it within the “C:\Mydocuments”. Save it in the folder with your lab AI’s name.
You should transfer this file to disk or email to yourself or ftp it to yourself (ask for help if you need this).
Now activate the graph window clicking on it.
Click on the little magnifying glass in the lower left-hand corner. This will make the graphed data full size. Print this graph for yourself. A sample of what the graph should look like is shown below.
Record your maximum temperature and your cooled temperature. The temperature rise is simply the difference between the two values.
Procedure 4: Perform strength testing
at least one week after Procedure 2
There should be 3 groups of students. Each group has prepared a different W/C ratio. Trade 1 specimen with each of the other 2 groups for one of theirs. After you trade, make sure that you have 3 different w/c ratio specimens (W/C ratios 0.55, 0.60 and 0.65) and one extra of yours for the split cylinder test.
Remove a concrete cylinder from its black mold. To remove the concrete, from the mold, you will need to use a hammer and a mold splitting tool (it looks like a big screw driver). Place the screw driver tip on the top edge of the black mold. Use the hammer to drive the screw driver into the mold, splitting it. To successfully remove your cylinder from you mold, you may need to split the cylinder in several places around the cylinder. Continue with the rest of the procedures for this first cylinder and let the other members of your group continue to free your other specimens from their molds.
For each specimen, make sure that you label which w/c ratio was used to prepare the cylinder. Use a marker and write the w/c ratio right onto the concrete.
Measure the dimensions of your concrete specimen. You will use these values to convert your load values (pounds) into stress values (pounds per square inch) later.
This procedure must be done with the assistance of a lab AI or assistant. Get one and go to the load-testing machine with your four specimens. Testing should alternate between groups one cylinder at a time.
First you should test you compressive strength specimens (the three different w/c ratios). Place your cylinder on the metal table of the load machine. On the bottom and top of the cylinders, you should place the load caps. They have rubber in them and they evenly distribute your load so that no bumps or irregularities on the surface of your cylinder affect your results.
Make sure that your cylinder is centered in your circular load caps.
Make sure that you cylinder and load cap combination are centered on the metal testing table and under the circular load head above the table.
These next few procedures are performed by your lab AI. Observe what is done.
Start the load machine, by pushing the start button on the stand-alone panel in between the load platform and the computer unit. You should be able to hear the machine start.
Next, you want to get the loading head above the cylinder lowered down close to the top of the cylinder. Try to get the load head as close as possible to the top of the cylinder without touching the cylinder or loading it. (The head should be NO MORE than a few cm from the top of the cylinder.) To move the top of the head, use the buttons on the side stand-alone panel labeled "Xhd" up and down.
Have the students place the plastic safety shield in front of the cylinder and load head on the load head table.
On the computer desktop, double-click the icon labeled " Concrete Cylinder Test". This starts the loading program.
On the dialog box on the screen enter values for cylinder diameter, specimen identification (enter the w/c value, ie. "w/c=0.55").
On the dialog box on the computer screen, click "Run test". The test will run and slowly load will be applied to the cylinder. The computer will record values of load that are applied. Watch what happens to your cylinder.
Once the cylinder breaks, you will see on the graph on the computer screen that load is dropping off (the curve is decreasing instead of increasing). Once this cylinder breaks, stop the automated test, by pushing the little stop sign icon on the top toolbar in the computer screen window.
For this w/c ratio specimen, record from the screen the maximum load (compressive load with units lbf). You can also record the compressive strength (units in psi). You can calculate this value later and double check, by dividing the load by the applied area.
Print your results by selecting the "File" menu and selecting "Print". Print a copy for each member within your group.
Clean off the remains of your specimen from the load table. Dispose of the concrete in the cardboard box near the load machine.
Repeat tasks 2 thru 18 for your other 2 w/c ratio specimens. To have the computer initialize for the next test Press the “F9” key.
The previous three specimens were for compressive strength (crushing strength). This next part deals with the tensile strength specimen test (pulling apart strength). For this lab we use an industry-based standard called the “split-cylinder test”. Get you tensile specimen ready by performing procedures 2 thru 4.
Place your specimen on the load table. This time, do not use any load caps. The cylinder should be placed with the long end of the cylinder parallel to the table.
Position you cylinder under the load head. The load head it 6” in diameter, so the length of your cylinder should fit EXACTLY under it. Make sure there is no overlap and that the entire length of the cylinder will make contact with all of the load head.
Run the test using tasks 9 thru 18. For this part it will not be necessary to print you results. Record only the ultimate load (in lbf) of your tensile specimen. To calculate ultimate tensile stength (or stress in psi) you will need to divide that load (lbf) value by the area around the cylinder. The stress (psi) value calculated by the computer on the screen is not accurate for the tensile test because it uses the wrong area to calculate it. Remember to clean up also.
Problem 1: (should be completed for homework in week one of the lab)
The chemical equation for hydration of belite into gel and lime is
The formula weight of belite is 172.2 (pounds). In the following, consider 2(172.2) = 344.4 pounds of belite.
a) Calculate the released heat in BTUs from the reaction.
b) Assuming that the concrete weighs four times the weight of the products (gel and lime) calculate the weight of concrete. Refer to the table of formula weights.
c) Calculate the temperature rise in the concrete assuming the heat capacity of concrete is unchanged (0.43 BTU per degree Celsius per pound) and assuming no heat escapes.
d) Compare your answer with that found for alite in the lab write-up.
Problem 2: (should be completed for homework in week one of the lab)
To avoid cracking, the surface tensile stress must be kept below the tensile strength of concrete, which is usually about 410 psi (2.82 MPa). Water cooling must carry away the excess heat so that < 410 psi. If Tsurf = 20C (68F) again what is the maximum value of Tmax that will not exceed the tensile strength of the concrete?
After the construction of the Hoover Dam, the general process of post-cooling changed. What changed, why did it change? Do your experimental results from Procedure 3 support your conclusion? If so, how?
Given the curve produced in Procedure 3:
What is the maximum temperature rise within the specimen?
How do these results compared with the chart in Figure 2?
What is the stress on the surface of your specimen if you use the equation:
where E is the modulus of elasticity of concrete and a is the coefficient of thermal expansion. Tmax is the maximum temperature recorded from your thermal experiment. Tsurf if the recorded starting temperature of your thermal experiment. Assume:
E = 20.6 x 103 MPa (or 3 x 106 psi)
a = 10-5 per degree C (or 0.56 x 10-5 per degree F)
If the tolerable cracking strength of this concrete is 410 psi, will cracks be likely to form?
Using the example in the lab (for belite) calculate the temperature rise for this specimen’s weight.
Compare this temperature rise with the actual temperature rise measured in the experiment. What is the percent error?
Compare the theoretical and actual values for tensile strength of your split cylinder test. What is the % error?
Problem 6 Compare you graph for strength as a function of w/c ratio with the theoretical graph in Figure 8.
Heat of Hydration (Temperature vs Time inside concrete specimen) from Procedure 1 and 4
Load (lbf) vs time curve for compressive tests (one for each w/c ratio)
Ultimate strength (psi) vs W/C ratio
Useful values to record Procedure 1: Weight of cement cylinder (lbs) . .
Starting temperature (degrees C) . .
(not used in calculations…recorded for good measure)
Procedure 2: Amount of water used (lbs) . .
Amount of cement used (lbs) . .
Amount of gravel used (lbs) . .
Amount of sand used (lbs) . .
Water/cement ratio . . Procedure 3: Maximum Temperature (degrees C) . .
inside the specimen (also known as Tmax recorded 1 day after setup)
Cooled Temperature (degrees C) . .
(this is the temperature after the cylinder cools down)