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The failure of the concrete cylinder test at 28 days occurs due to several factors like improper casing and curing of specimens and an incorrect mix of concrete used for construction. The cause of the failure should be identified by engineers, based on which proper actions need to be taken.
The first step should be checking whether the failure is due to inappropriate casting, curing, or testing procedure of the specimen. If the process for making samples was right, then the fault could be due to incorrect mix, which produced a low-strength concrete.
Engineers need to conduct tests on the structure like a core cutter test to ensure whether the concrete utilized in the construction of the building achieved designated strength or not. If the strength of the concrete was low, then use suitable strengthening and rehabilitation techniques to increase the load-carrying capacity of the structure.
The concrete cylinder strength at 28 days indicates whether the strength characteristics of the concrete used in the construction of a structure are good enough or not.
1. Failure Criteria of Cylinder Specimens
According to ACI 318-19, the strength of concrete is not acceptable if it does not meet the following two criteria:
- The average compressive strength of three consecutive cylinder specimens should be equal or greater than the designated compressive strength.
- For specified compressive strength of 35 MPa or less, single compressive strength should not be less than the designated compressive strength by more than 3.5 MPa. For specified compressive strength higher than 35 MPa, the individual compressive strength should not be less than the specified compressive strength by more than 10%.
2. Cylinder Test Failure at 28 days
Sampling procedure and testing steps may be the reason behind the specimen failure. Two scenarios should be analyzed properly:
1. When concrete strength is high, but cylinder strength is weak
2. Both concrete and cylinder strength are low
2.1 When Concrete Strength is High, but Cylinder Strength is Weak
The high concrete strength but weak cylinder specimen strength can be due to several factors which are listed below:
- Inadequate cylinder compaction
- Leaving cylinder specimens in adverse environmental conditions such as too hot or cold weather
- Improper alignment of molds lead to distorted sample production
- Insufficient filling of molds
- Testing wrong samples
- Errors in testing machine
- Errors during sample testing
- Concrete cylinders taken from the wrong mix
Therefore, engineers should closely monitor all procedures of cylinder preparation and curing to ensure that specimens properly represent the concrete of the structure.
2.2 Both Concrete and Cylinder Strength are Low
If the scenario discussed above is not the reason for cylinder failure, then further tests and calculations should be carried out to determine whether the strength of the structure has reduced significantly or not. This scenario may be due to:
- Incorrect mix design; the designated mixture cannot achieve the required strength.
- Error during mixture preparation which may be due to mechanical or software faults.
In this scenario, consider the following actions:
2.2.1 Hammer Test
Conduct a hammer test to evaluate concrete compressive strength. If the strength is still low in cylinder specimens, then you should go for a core cutter test.
2.2.2 Core Cutter Test
Carry out the core cutter test to properly evaluate the concrete strength of the structure. Based on specifications of ACI 318-19, “three cores must be taken for each strength test that falls below the specified compressive strength by more than the limit allowed for acceptance."
If the average strength of three cores is at least equal to 85% of designated compressive strength, and if no individual core strength is lower than 75% of specified compressive strength, then the concrete area represented by core samples is assumed to be structurally sufficient, as per ACI 318-19.
If the concrete fails to reach the acceptance limit of the core cutter test, then consider a proper strengthening technique.
3. Rehabilitation Methods
The rehabilitation methods can increase the load-carrying capacity of structural members to gain the designated strength. One should investigate the suitability and various aspects of each strengthening technique to select the most suitable, economic, and effective method for the structure under consideration.
The rehabilitation methods may include fiber-reinforced polymer sheet bonding method, concrete jacketing, external steel bonding technique, near-surface mounted FRP technique, steel casing methods, shotcreting (spray concrete), and attaching pre-tensioned cables to the concrete members.
Concrete gains strength with time after casting. It takes much time for concrete to gain 100% strength and the time for the same is still unknown. The rate of gain of concrete compressive strength is higher during the first 28 days of casting and then it slows down. Read more about it here.
Concrete gains strength after 28 days but at a slow rate.
The compressive strength of concrete cylinders is determined by applying continuous load over the cylinder until failure occurs. The test is conducted on a compression-testing machine. Read more about it here.
1. The average compressive strength of three consecutive cylinder specimens should be equal or greater than the designated compressive strength.
2. For specified compressive strength of 35MPa or less, single compressive strength should not be less than the designated compressive strength by more than 3.5MPa. For specified compressive strength of higher than 35MPa, the individual compressive strength should not less than the specified compressive strength by more than 10%.