Abstract
Concrete acts as one of the most essential and cost-effective construction material made from a mixture of sand, cement, aggregates, and water. Cement acts as a binder for holding all the components together. Engineers working in the construction site must understand the purpose of the construction because different structures require different concrete strengths. The strength of cement paste used controls the overall strength of concrete that depends on the water-cement ratio. The following experiment was conducted to investigate different mechanical and physical properties of concrete.
Objective: To determine mechanical properties of concrete.
Materials used: Aggregate (coarse and fine), sand, cement, water, compressive machine, cylindrical and rectangular molds, timer, and computer.
Tests conducted: The main tests done to determine mechanical properties of concrete were slump test, compressive strength test, modulus of rupture, modulus of elasticity, and split tensile stress.
Results: after the lab experiment, it was realized that the material used to make concrete, coarse of fine aggregates, determine the strength of that concrete mixture. Additionally, water-cement ratio determines the strength of concrete. Samples with higher water-cement ratio recorded higher compressive strength, high modulus of rapture, higher split cylinder strength, and higher elastic modulus strength compared to samples from low water-cement ratio. On the other hand, air-entrained concrete had lower strength.
Conclusion: The experiment concluded that the strength of concrete mix depends on water-cement ratio, size of aggregates, and compaction rate.
Problem statement
Concrete mixtures are designed to meet specific physical properties depending on the purpose of the structure. Experts have been conducting many experiments to investigate the effects of different strengths of concrete. The following concrete lab aims at determining the strength of cement concrete. The main factors that influence the strength of concrete that will be tested are the type of mixing design used, mixing procedure followed, and curing period. The test determined the strength of concrete in 7 days strength and 28 days strength in order to determine compressive strength, modulus of rupture, and tensile strength for using different water-cement ratios. Additionally, the lab aimed at testing the differences in strength between normal concrete and air-entrained concrete. Air entrained concrete forms the best discoveries in the field of construction. The concrete contains small and stable air bubbles uniformly distributed over the cement paste. Air entrained concrete is freeze resistant, improves workability and reduces bleeding in areas exposed to extremely low temperatures (Kerkhoff 3). On the other hand, normal concrete occurs in densities of between 1900 and 2600 kilograms per cubic meter, and the majority of the aggregate occurs naturally.
Experimental method
Mix design method
The mixing procedure was conducted using three different batches of concrete; these were air-entrained, normal and super-high strength concrete. Each mix design used different water-cement ratios that define the strength of concrete. The main ratios used were 0.45, 0.5, 0.55, and 0.6. The three mixtures, air-entrained, normal, and super-high concrete were tested for each of the four water-cement ratio mentioned, the three. The class was divided into four groups with each group conducting the experiment using specific water cement ratio. Each concrete sample was supposed to be 10 cm for each mixture. Type I-II cement with a specific gravity of 3.15 was used in all tests. Additionally, the course aggregate had an average diameter of 18mm and with a bulk density of 2.53, and dry rotted unit weight of 1480 kg/m3. The moisture content of aggregate was 0.7% with an absorption rate of 1.9%. Moreover, the fine aggregates recorded a fineness modulus of 2.79, bulk specific gravity of 2.56, moisture content of 0.6% and absorption of 1.1%. According to Prowell, Jingna, and Brown, aggregate accounts for 60 to 70 percent of the total volume of concrete, and 80 percent of the total weight (12).
The amount of water content available in the aggregate determines the water cement ratio to be used. The ratio is calculated from PCA manual. With the known water-cement ratio, the minimum requirements for the cement mixture were identified. The fineness modulus helped in determining the exact volume of coarse aggregate content, also derived from PCA manual. In order to get the total volume of materials used to make the concrete mixture, the bulk specific gravity of each substance was divided by unit weight of water. The air content in the concrete is derived from the product of water content of aggregate and with 27 ft3. Testing for air content is done on concrete to control the damage caused from freeze. Air test helps in establishing the total content of the air entrained in concrete (National Precast Concrete Association 2). On the other hand, the volume of fine aggregate content was calculated by adding up all the constituents and subtracting 27 from the total. Moisture adjustments were done for both course and fine aggregates because they contain a certain level of moisture. Water was added to fine, and coarse aggregate that had low absorption rates and SSD in order to arrive at the required moisture level.
Mixing procedures
The mixing stage is where all constituents were mixed carefully to come up with a concrete mixture. Coarse and fine aggregates were stored outside the lab while the cement was placed inside. All materials were put in different buckets for easier transport and volume determination. The weight of materials in the bucket was taken and recorded until the desired weight was reached. After getting the correct weight of each material, they were poured into the mixer starting with coarse aggregate, fine aggregates, sand, and then cement. The mixer was turned on and allowed to run for 10 minutes in order to mix all the contents. After 10 minutes, it was assumed an uniform mixture of constituents was attained, and the measured amount of water was slowly poured to the mixer in order to allow uniform bonding of materials. Additionally, the mixture was tilted at an angle of 90o, 45o from each side in order allow all material to mix uniformly from the bottom to the top. After the material was mixed for 30 minutes, the mixer was turned off, and the concrete mixture that was stuck to the sides was scrapped off to attain an uniform mixture. The mixer was turned on for the final turn in order to ensure all constituents were well mixed. The teaching assistant checked whether the content had fully mixed and ordered the machine to be turned off completely. The concrete mix was then poured into a wheel barrel and transported inside the lab.
Testing procedures
Slump test
Slump test on concrete is done to determine the consistency of fresh concrete. The test assisted in checking whether the correct amount of water was added to the mix. The test must be done immediately after mixing the constituents and when the concrete is wet (Lamond and Pielert 66).
Procedure
- A sample of concrete was taken from the wheel barrel. The sample taken was within a quarter of an inch.
- A steel slump cone with top diameter of 4 inches and bottom diameter of 8 inches and 12 inches high was used to carry out the test.
- The cone was placed on an impermeable solid ground with a level base and filled with fresh concrete in three equal layers
- Each layer was rod 25 times in order to ensure total compaction
- The third layer was leveled off with the top of the cone.
- After adding all layers, the cone was carefully lifted up in order to leave a heap of concrete that settled slightly.
- The steel slump was then placed on the base of the test to act as a reference. As the concrete settled, the differences in levels between the top of the cone and the top of the concrete was measured and recorded. The distance from the middle of a slump to the top of the cone was measured that represented the slump of the concrete. See figure 1.
Figure 1: Slump test
Compressive Strength Test
The compressive strength test for concrete helps in determining the strength of concrete. Concrete has many durability and mechanical properties that meet different purposes that the material performs in construction. Engineers use compressive strength of cement to design various structures. A compression testing machine is used to determine the compressive strength of concrete (Lamond and Pielert 136).
Procedure
- The compression testing machine was checked to ensure it was clean of any debris. A clean apparatus allows equal distribution of compressive load acting on the concrete cylinder and prevents any errors that might interfere with the actual results.
- The concrete was poured into cylindrical molds to form concrete cylinders
- The concrete cylinder was placed in a curing room for 24 hours after it was poured in molds. The mold was then observed for one week in order to ensure they had totally cured
- Moisture testing was done to concrete cylinders in the curing room in order to determine the level of dryness.
- The cylinders were then placed in a metal cap with rubber lining for effective distribution of load throughout the cylinder. The metal cap was placed on both sides of the cylinder in order to make contact with compressive machine.
- The metal caps were then placed in the middle of the machine, and the machine turned on. See figure 2 below.
- A constant load was maintained throughout until cracks were observed on concrete that eventually broke the cylinder a clear indication that the concrete had failed.
- The maximum compressive load of each cylinder was recorded. Compressive strength was acquired from dividing maximum compressive load before the concrete failed with the cross-section area of the cylindrical concrete specimen.
Figure 2: Compressive Testing machine
Modulus of rupture
The modulus of rupture of concrete us used to test the tensile strength of concrete beams. The strength depends on the length of the concrete beam. The test aims at calculating test distributions that cause cracking of the concrete beam prior to application of maximum load. It is calculated from the formula:
k = PLbd2 .eq. 1
Where:
P – Load applied
L- Length
b – Breadth of the beam
d – Width of the beam
Procedure
- The beam measuring 6 by 6 by 25 inches was used to calculate the modulus of rupture of concrete.
- The concrete was poured into a rectangular mold and left to cure for one week.
- Two lines were drawn three inches from each edge of the beam. A new line was marked after every 6 inches making a total of 36 lines drawn on each side of the rectangular mold. Additionally, there were two three by 6 inches rectangles drawn on each side of the specimen.
- The rectangular concrete specimen was placed into a compression machine, a different from the machine used during compression strength test as shown in figure 3.
- The specimen was centered in the machine in order for the lines drawn on the specimen to line with the semi-circles located on the device. The concrete specimen was carefully placed in order to allow for proper distribution of loads in all locations of the block.
- The metal caps of the machine were lowered to allow the rectangular specimen fit correctly on each side of the machine. The dial display was zeroed, and load applied at a rate of 2000 pounds per minute.
- The load was applied until the specimen started cracking. The maximum capture load was observed and recorded.
- The modulus of rupture was calculated using the formula in equation 1 above.
Figure 3: Setup for the modulus of rupture test
Modulus of elasticity
The modulus of elasticity (E) was tested using a cylindrical concrete mold measuring 6 inches in diameter and height of 12 inches. It is calculated by dividing stress by strain.
E = τΔl . Eqn. 2
Where: τ – Stress
Δl-Change in length
Procedure
- A 6 b y 12 inch concrete cylinder was molded the used to test the modulus of elasticity for concrete.
- The testing device was screwed into the concrete cylinder.
- The device was first screwed on the top of the cylinder and later on the bottom in the central position.
- After the set up the test, equipment were placed in the compressive machine as shown in figure 4.
- The machine was first set to full load and later adjusted to moderate load of 2000 pounds.
- The compressive machine reached 10% and 40% strength levels as a close look was made to the device and the concrete specimen to observe any change.
- The change in length of the specimen was recorded at each percentage levels above, and the test continued.
- The strain was calculated by multiplying the change in length with 0.0001. With the amount of load applied known, the modulus of elasticity was calculated using the formula in equation 2.
Figure 4: Modulus of elasticity test setup
Split tensile Stress
Split tensile stress was determined by testing a concrete cylinder measuring 6 by 12 inches. It is calculated from the equation:
T=2Pπld 3
Where:
P - Load applied
l- Height of the cylinder
d – Diameter of the cylinder
Procedure
- A steel cylindrical mold was used to make the cylindrical concrete specimen measuring 6 inches in diameter and 12 inches high
- The cylindrical specimen was placed in the compressive testing machine
- A metal bar wad centered at the top of the cylinder, and the load lowered to fix the metal bar to the machine as shown in figure 5.
- The compression machine was automatically controlled using a computer where load values were observe on the screen.
- The computer level was zeroed, and load applied using the split cylinder test.
- The load was applied until the specimen split into two parts. The Split tensile stress was calculated from equation 3.
Figure 5: Split tensile stress test set up
Curing procedures
After the finishing the above tests, the concrete mix was poured into different molds for curing. Curing was done in order to preserve the concrete for future experiments. The molds were smeared with oil on the inside part in order to prevent concrete from sticking to the sides of the mold. The concrete mix was poured into the mold. The concrete mix was compacted into the mold to eliminate any air. The compacting rod was dropped 25 times for circular molds and 36 times for rectangular molds to ensure total compaction of concrete in the mold. A metal plate containing group number was placed on top of the mold. The compacted concrete mix was allowed to stay in molds for 24 hours. The molded specimens were then removed from the molds and taken into a curing room where they would stay for 28 days.
Conclusion
The lab results contributed more in understanding the mechanical properties of concrete. It was realized that the type of material and the mode of mixing used plays a critical role in determining the strength of a particular concrete mix. The type of cement water ratio used to make concrete mix determined the compressive strength and the curing time. Samples with high water-cement ratio took long to cure and were the strongest. Additionally, cross-sectional areas of molds used determined the curing time of the concrete mix, as well as the strength. 4 by 8 inches molds had stronger compressive strength with similar water-cement ratio compared to the 6 by 12 molds. Compressive strength of the material is inversely proportional to its cross-sectional area, the small the area the stronger the concrete.
On the other hand, the lab test revealed that the air-entrained concrete recorded lower tensile and compression stress. Additionally, air-entrained samples had lower modulus of elasticity compared to normal samples when made up from high cement-water ratio. The air-entrained concrete mix recorded lower strength because the voids left in the concrete reduce bonding causing weak points. Finally, the coarse and fine aggregate concrete mix recorded varying results in terms of tensile and compressive strength. Concrete mix made from fine aggregate was the strongest in compression, but poor in tension compared to sample of coarse concrete. In conclusion, the strength of concrete mix depends on water-cement ratio, size of aggregates, and compaction rate.
Works Cited
Lamond, Joseph F, and J H. Pielert. Significance of Tests and Properties of Concrete and
Concrete-Making Materials. Philadelphia, PA: ASTM, 2006. Print.
Kerkhoff, B. “ Benefits of air entrainment in HPC”. HPC Bridge views, 23, 2002.
National Precast Concrete Association. Concrete sampling and testing. Tech Notes. 2013. Web
http://precast.org/wp-content/uploads/2011/05/TechNote_Concrete_Sampling_and_Testing.pdf
Prowell, Brian D, Jingna Zhang, and E R. Brown. Aggregate Properties and the Performance of
Superpave-Designed Hot Mix Asphalt. Washington, D.C: Transportation Research Board, 2005. Print.
