civil engineering concerete help
Abstract
The concept of this lab is to experiment compressive and flexural strength of concrete. Testing for the concrete cylinders and, were determined by a compressor machine. Concrete mixed with a specific water to cement ratio and testing them against each other over a certain period of time. The stress strain, load versus deflection were plotted for the samples to identify their stress strain relationship. These tests provided information about the mechanical properties, strength, and ductility of concrete. This lab helped to better visualize and analyze failures that are implemented in real life.
Introduction
Compressive strength is the ability of a material to withstand axial force. A recognized and widely spread material that can be modified to handle compressive strength is concrete. ACI318-11 provides regulations and specifications for concrete design that have been obtained from many tests. The mix itself can be designed to provide a wide range of mechanical and durability properties to meet the required ability. Following American Society for Testing and Materials (ASTM) procedure for does this making and curing concrete. This is important in order to pass the quality control and achieve the required strength. Most cases concrete is tested 28 days after mixing to achieve ultimate strength. In this lab this two objectives were test. The objective of the concrete lab was to evaluate the mechanical properties of Portland Cement Concrete. Five samples were tested within 28 days. The first sample was tested in the first 7 days, the second on the 14th day, the 3rd on day 21, and lastly on day 28. With the aid of a compressive machine the specimens were tested since it’s the most common performance measured used by engineers.
Determining the type of concrete mix was majorly dependent to the water cement ratio and aggregates used. These factors contributed on the overall strength of the concrete. In order to determine if the mix had high or low water to cement ratio, a slump test was performed. If the slump was high this means it is very workable, stating that it will end up as a weak sample compared to a low slump mix.
Methods
Concrete samples were designed regarding the ingredients constructed from water cement and aggregates. After collecting the ingredients it was then poured in a concrete mixer. They were then poured in a cylinder shaped mold and one beam shaped mold and a slump test was done. For every one-third filled 25 strokes were performed by a rod for each layer.
The samples were test by the compression machine after they were done curing. The dimension of the specimen was measured before the testing for later preference for calculating the cross section. The sample is attached to a compressometer to give the gage length of 6 inches with two caps for capping the concrete from the top and bottom. Before starting the test eyeglasses were required to be worn for safety reasons. The sample was then placed in the machine careful aligned with the center of the thrust. Load is then applied by using the double acting pump with the aid of human power. The fracture load is then recorded while observing carefully. Type of failure was determined at that stage. This was done to all concrete cylinder samples.
Results
Table 1: Data Summary
|
Cylinder based on curing days |
f’c (ksi) |
Load Capacity (kips) |
|
7 days |
3.75 |
106 |
|
14 days 14 days |
3.04 3.43 |
87 97 |
|
21 days |
3.75 |
106 |
|
28 days (Test Data) |
2.56 |
72 |
|
Theoretical f’c = 4 ksi |
|
|
|
Average f’c = 2.62 ksi |
|
|
|
Percent Difference = 34.5 % |
|
|
The data (Table 1) given showed the performance of the concrete for each time interval of four days. The results show a decrease in compressive strength in 28 days compared to the initial stage. Moreover, the theoretical compressive strength was 4 ksi and the tests gave an average of 2.62 ksi. Thus the percent difference is not acceptable resulting to 34.5 percent.
Discussion
This lab accomplished the objective by analyzing and exploring the properties of concrete. Beginning from the simple slump test, it was noticed that the 4.25-inch slump was very high and gave high workability. However, this will reflect on the strength of the mix, making it weaker.
Table 1 shows the difference from a 14-day cylinder and 28-day cylinder test. Twenty-eight days is an appropriate gauge of the strength of the concrete because at that stage 90% of the concrete strength is supposedly is achieved. Moreover, the industry had set it to be as reference point for all tests. Allowing the concrete to cure for longer period of time provide a much better compressive strength.
The graph on (Figure 2) shows that the ultimate compressive strength of the concrete cylinder cured for 28 days was 2.56 ksi. The max load capacity shown on Figure 1 was 72.36 kips. The results shown on Table 1 shows that cylinder cured in 28 days yielded a much lower compressive strength compared to the cylinders cured in other days. The compressive strength significantly drops overtime. One of the factors that could have caused the decrease in strength was the water cement ratio. Undergoing testing it was noted that the concrete cylinders had the same failure. They both cracked initial to the failure, which created a cracking sound. However, the time was very short in between cracking and failure. This means concrete gives minimal warning to failure. The concrete samples had diagonal fracture with most aggregate not shearing. This means hat the water cement ratio was weaker than the aggregates for both cylinder samples. The failure was also crumbly stating that the concrete was not mixed thoroughly containing some air voids too. This suggests that with the days left for the concrete to cure, water gaps evaporate and left the concrete weaker with air voids.
Conclusion
This lab confirmed the importance of the concrete mix and concrete’s characteristics. It showed why concrete is used widely for its compressive strength and how the water cement ratio affects the strength desired. The lab done proved that the test done while waiting 28 days is necessary for every concrete used in construction. With the amount of aggregates, cement, and water should be checked to avoid decrease in strength. Nevertheless, concrete have proven its ultimate characteristic of compressive strength when it comes to heavy loads.
Appendix A
Figure 1: Load versus Displacement
Figure 2: Stress versus Strain
Figure 3: Compressive Strength versus Curing Days