Ensuring the quality of concrete in structures is crucial, as hardened concrete becomes stronger over time. To achieve this, various tests are available that can assess different properties of the concrete. These tests serve as a means of quality check and can help to identify any issues or areas for improvement. Depending on the specific property being evaluated, different types of tests may be utilized. Overall, performing regular tests on hardened concrete is an essential aspect of maintaining the integrity and durability of structures.
Properties of Hardened Concrete
Early Volume Change in concrete can lead to cracking and is caused by plastic and drying shrinkage. Plastic shrinkage happens when fresh paste loses water through evaporation or suction, leading to a volume reduction of up to 1%. Drying shrinkage occurs due to water loss and cooling, and accounts for 15-30% of the total shrinkage in the first 2 weeks and 65-85% within the first year. Drying shrinkage can be influenced by several factors, including curing conditions, water-cement ratio, cement content, coarse aggregate content, and the presence of steel reinforcement. The extent of shrinkage may vary based on the size and shape of the concrete structure, and it can be non-uniform due to non-uniform water loss. However, continuous curing of concrete in water after settling may lead to swelling, which does not cause significant problems.
The creep properties of concrete refer to the deformation that occurs over time under a constant load. The amount of creep depends on several factors, including the magnitude of the load, the age of the concrete, and the environmental conditions. Creep can lead to long-term deformation and may affect the serviceability and durability of concrete structures.
Permeability is a critical property of concrete that influences its durability and resistance to water and chemical penetration. Concrete with high permeability is more susceptible to damage from freeze-thaw cycles, chemical attack, and corrosion of reinforcing steel. Factors that affect permeability include water-cement ratio, cement content, aggregate characteristics, curing conditions, and the presence of admixtures.
The stress-strain relation of hardened concrete refers to the response of concrete to external loading. Concrete is typically assumed to be linearly elastic up to a certain point, beyond which it exhibits nonlinear behavior. The stress-strain relation is influenced by several factors, including the age of the concrete, the strength of the concrete, and the environmental conditions. Understanding the stress-strain behavior of concrete is essential for designing safe and reliable concrete structures.
In the realm of structural design, it is crucial to understand the mechanical behavior of concrete, which is a composite material made up of aggregate and cement paste. When subjected to stress, aggregate and cement paste each exhibit a linear stress-strain relationship. However, the behavior of concrete as a whole is non-linear, which can be attributed to micro-cracking that occurs at the interface between the aggregate and cement paste.
To effectively design structures that utilize concrete, it is necessary to determine the compressive strength (fc) and modulus of elasticity (E) of the material. These properties provide important information about how concrete will perform under various types and magnitudes of loads. Compressive strength refers to the maximum amount of compressive stress that concrete can withstand before failure, while modulus of elasticity indicates how much the material will deform under a given amount of stress. Understanding these properties is essential for ensuring that structures made of concrete are safe and reliable.
Tests on Hardened Concrete
When testing hardened concrete, there are several tests that can be performed. The most common test is the compressive strength test, which is performed using destructive testing (DT) methods. Other DT methods include the split-tension test and the flexural strength test.
In addition to DT methods, there are also non-destructive testing (NDT) methods that can be used to assess the properties of hardened concrete. These include the modulus of elasticity test, the rebound hammer test, the penetration resistance test, the ultrasonic pulse velocity test, and the maturity test.
Overall, the choice of testing method will depend on the specific properties of the concrete being tested and the information that is required. DT methods are generally more accurate but also more destructive, while NDT methods are less destructive but may be less precise.
Compressive Strength Test (fc’)
The ASTM C39 test requires the use of a cylindrical specimen with dimensions of 6 inches by 12 inches to determine the compressive strength of normal-weight concrete. This strength ranges from 21 megapascals (MPa) to 34 MPa (3000 pounds per square inch (psi) to 5000 psi). The compressive strength of concrete is affected by the size of the specimen, with larger specimens having a greater probability of containing weaker elements. This reduces the overall strength of the concrete and increases the variability of the results obtained from the test. Therefore, larger specimens are preferred as they provide a better representation of the actual concrete strength.
In terms of how failure occurs in concrete, it typically happens at the weakest location of the hardened material, which is the interface between the cement paste and aggregate. This is where the bond between the cement paste and aggregate is weakest, and under the application of a compressive load, the material will fail at this interface. The failure of the concrete will occur gradually, and as the load increases, the cracks will propagate until the material eventually fails. The compressive strength of the concrete is determined by measuring the maximum load the specimen can bear before it fails.
Modulus of Elasticity – Ec Test on Hardened Concrete
ASTM C469 is a testing method that is utilized to determine the chord modulus. This technique involves the use of a single sample and requires 3-4 loading steps. The results obtained from this method are significant as they provide a useful correlation with strength. The range of test results obtained from ASTM C469 falls between 14 GPa – 41 GPa (2000 ksi to 6000 ksi).
In addition to determining the chord modulus, ASTM C469 is also used to calculate Poisson’s Ratio. This value typically ranges between 0.11 to 0.21, and can be affected by factors such as aggregate, moisture, age, and compressive strength.
Overall, ASTM C469 is a valuable testing method that can provide critical information about the strength and properties of various materials. With its ability to accurately determine both the chord modulus and Poisson’s Ratio, this method has become an essential tool for engineers and researchers working in the field of material science.
Split Tension Test on Hardened Concrete
The purpose of the ASTM C496 test is to measure the tensile strength of concrete. This is achieved by subjecting a cylinder of concrete to a compressive load along its vertical diameter at a constant rate until failure occurs due to tension developed in the transverse direction. The failure occurs along the vertical diameter, which is caused by tension in the transverse direction.
The split tension is computed using the formula T = 2p/BLD, where T is the tensile strength in MPa or psi, p is the load at failure in N or psi, L is the length of the specimen in mm or in, and D is the diameter of the specimen in mm or in. The tensile strength can vary from 2.5 MPa to 31 MPa (360 psi to 450 psi), which is approximately 10% of the compressive strength of the concrete.
Flexure Strength Test (ASTM C78) on Hardened Concrete
The Modulus of Rupture (MR) is a critical parameter for assessing the strength of concrete pavements used in road and airport construction. To measure MR, a specific test is employed, which involves subjecting a beam specimen with a square cross-section to a 3-point loading apparatus.
The test apparatus applies a load at two points on the beam’s surface, while a third point is located between these two points, providing support. The resulting stress causes a bending moment within the beam, which leads to failure when the MR is exceeded.
The 3-point loading apparatus is a widely used method for testing MR due to its simplicity and reliability. It enables accurate and consistent measurements to be obtained, which are crucial for ensuring the safety and durability of concrete pavements. Overall, the MR test is an essential tool for evaluating the strength of concrete pavements and ensuring that they meet the necessary requirements for use in road and airport construction.
Calculation of Modulus of Rupture
In the given context, the flexure strength (R) of a specimen that has experienced a fracture in the middle third of its span can be calculated using the formula R = PL/(bd^2), where P is the maximum load applied, L is the span length, b is the average width of the specimen, and d is the average depth of the specimen. The units for R are in MPa (psi), while the units for P, L, b, and d are N (lb), mm (in.), mm (in.), and mm (in.), respectively.
It is important to note that if the fracture occurs outside the middle third of the span, the results obtained from the above formula must be discarded.
Alternatively, the flexure strength (R) can be estimated using the following relationships, which give the values of R in either S.I. or English units, depending on the units of the input parameters:
- R = (0.62 to 0.83)fc’^0.5 (S.I. units)
- R = (7.5 to 10)fc’^0.5 (English units)
In these relationships, fc’ represents the compressive strength of the concrete specimen.
Rebound Hammer Test (Schmidt Hammer Test) on Hardened Concrete
The given passage describes a non-destructive testing method that is commonly used to evaluate the strength of hardened concrete. The test involves a spring-loaded mass striking the surface of the concrete, and a scale measures the distance that the mass rebounds. This rebound distance is an indicator of the hardness of the concrete surface, which is directly related to the concrete’s overall strength.
To determine the strength of the concrete, a calibration chart is used, which relates the rebound distance to the concrete’s strength. Typically, 10 to 12 readings are taken per specimen to ensure accurate and consistent results. This testing method is useful for assessing the uniformity of the concrete, as it can quickly identify areas of the concrete that may be weaker or more prone to damage than others. By using this non-destructive testing method, engineers and builders can ensure that the concrete used in construction projects is strong, durable, and able to withstand the demands of the intended application.
Penetration Resistance Test (Windsor Probe Test)
A non-destructive testing method is utilized to assess the strength of concrete structures. This technique involves the use of a gun-like device that shoots probes into the concrete to obtain readings of its structural integrity. The test is performed by inserting probes into each of three holes that are positioned within a special template. The depth of the probes is then averaged to determine an overall strength value.
The depth of the probes is inversely related to the strength of the concrete, meaning that the deeper the probes penetrate, the weaker the concrete’s structure is likely to be. This method is preferred over the rebound hammer test because it provides a more accurate estimate of the concrete’s strength. The rebound hammer test only measures the surface strength of the concrete, while the penetration resistance test makes measurements that reach into the depth of the sample. Overall, the penetration resistance test is a valuable tool for accurately evaluating the strength of concrete structures.
Ultrasonic Pulse Velocity Test (ASTM C597)
The purpose of the test is to determine the velocity of an ultrasonic wave as it passes through concrete. The method involves measuring the length between transducers and calculating the travel time of the wave. This calculation results in the average velocity of the wave propagation. The test is commonly used in the inspection of concrete structures to detect any discontinuities, cracks or internal deterioration that may be present. By identifying such issues, appropriate measures can be taken to maintain the integrity and safety of the structure.
Maturity Test (ASTM C1074)
The degree of cement hydration in concrete, known as maturity, is a variable that changes over time and is affected by temperature. Researchers have found that the strength of concrete is closely related to its maturity for a specific mixture. To monitor the maturity of concrete, devices are used to measure the temperature of the material as it sets and cures. This information can provide valuable insights into the strength and durability of the finished product. By understanding the relationship between maturity and strength, engineers and construction professionals can make informed decisions about the selection of materials and the timing of construction activities.
Permeability Test on Hardened Concrete
The given context describes a test that assesses the durability of hardened concrete by measuring its ability to resist the penetration of water and chemicals. This test can cause a reduction in the concrete’s resistance to frost, alkali-aggregate reactivity, corrosion of steel rebars, and other chemicals. The presence of air voids in the concrete can also affect its permeability. These voids can be caused by incomplete consolidation during the pouring process or by the evaporation of mixing water that is not used for hydration of the cement. Increasing the water/cement ratio can also have a significant impact on the permeability of the concrete. Other factors that can influence permeability include the age of the concrete, the fineness of the cement particles, and the use of air entraining agents.
Creep Properties of Hardened Concrete
Creep is a phenomenon that occurs when a sustained load is applied to a structure, resulting in a gradual increase in strain over time. This process can take several years to become noticeable and can vary depending on the type of structure in question. The effects of creep include increased deflection and stress in steel, which can compromise the integrity of the structure. Additionally, creep can lead to a gradual transfer of load from concrete to steel, which can further exacerbate the problem. In the case of prestressed concrete, creep can cause the structure to lose some of its prestress force, which can have serious consequences for the safety and stability of the building.