How To Measure Concrete Strength
Factors affecting the strength of concrete
The strength of concrete is one important indicator to measure the quality of concrete. The compressive strength of concrete is in proportion to the strength of concrete cement. If the water-cement ratio is the same, the compressive strength of concrete prepared with high-grade cement is higher than that of concrete prepared with low-grade cement, while the water-cement ratio is in inverse proportion to the compressive strength of concrete.
When the water-cement ratio remains fixed, it can only increase workability instead of compressive strength by increasing the amount of cement in use, resulting in the deformation of concrete.
Therefore, the cement strength and water-cement ratio are closely related to the compressive strength of concrete. So, we must control the cement dosage and water-cement ratio to improve the comprehensive quality level of the concrete.
In addition to the water-cement ratio and cement dosage, coarse aggregate will also affect the compressive strength of concrete. During the construction, we should guide and clarify the coarse aggregate, especially when the stone strength is the same, the gravel surface is slightly thicker than the pebble surface, which increases the adhesion with water mud mortar, which is higher than that of the pebble;
When the water-cement ratio is equal, gravel and pebble are used for concrete preparation. The strength of concrete prepared with gravel is higher than that of a pebble.
If you want to determine concrete structure strength on-site, there are a non-damage method and a local damage method.
Nondestructive method
1)Ultrasonic rebound method
In determining concrete strength, we can determine concrete’s strength and existing defects by using a frequency of 20-200khz. Why? Since ultrasonic detection can not only directly reach the cavity position in the concrete, determine the depth of concrete cracks and the position and range of non-dense areas, but also determine the thickness of the concrete damage layer, to provide a real basis for construction processing.
In practical application, it is necessary to evaluate concrete’s bonding quality and homogeneity according to different pouring times. However, there are many interference factors when using the ultrasonic method to determine the strength of concrete, decreasing the accuracy of testing results and increasing the testing cost of concrete to a certain extent.
We can generally combine the rebound and ultrasonic methods for engineering applications. First, determine the approximate range of concrete strength and eliminate the influence of other factors, and then use the ultrasonic method to determine the strength of concrete to improve the comprehensive quality level of concrete strength.
The rebound method estimates the compressive strength of concrete according to the correlation between rebound value, carbonation depth and compressive strength.
In determining concrete strength, the rebound method mainly uses the concrete rebound value and carbonation value to evaluate the concrete construction strength. It will not damage the concrete structure or internal components, as so is widely used in engineering.
How to do it?
We can use the rebound instrument to determine the concrete surface strength and carbonation value and to determine the concrete load-bearing capacity and bearing limit range.
The determination of rebound value must be based on the rebound instrument and use a particular analytical curve to formulate the rebound value data and make a clear judgment according to the specific test results.
This method will not impact the concrete structure or external surface. The measured results will reflect the actual use of concrete.
Pros:
simple and low cost
Cons:
It is only a rough estimation of the strength of concrete
Testing conditions of rebound method
1) It is suitable for testing the compressive strength of ordinary concrete in engineering structures.
2) There is no apparent difference between the surface layer and the interior. Since the rebound value only represents the quality of the concrete surface layer, it must be consistent with the internal quality when using the rebound method.
3) The time-limited by the rebound strength measurement curve (14 ~ 1000 days).
4) The rebound strength measurement curve (10 ~ 60MPa) limits the strength.
5) It is not suitable for determining concrete with loose and peeling surfaces caused by freezing injury, chemical erosion and fire.
Number of sampling and layout of a testing area for rebound method
When testing a single component, we should evenly arrange the measuring area on the component and the number of measuring areas on each component more than 10.
For components (short columns) with a dimension of less than 4.5m in one direction and less than 0.3m in the other direction, we can reduce the number of measuring areas appropriately, but not less than 5.
When the components are sampled and tested by batch, the number of components sampled should be more than 30%, not be less than 10 pieces.
When sampling and testing by batch, the components that meet these conditions can be regarded as the same batch: the same strength level of concrete, concrete raw materials, mix proportion, forming process, curing conditions and age are basically the same, and the same type of components and the same state in the construction stage.
Local damage method
Pull out method
The pull-out method is mainly used to install the pull-out instrument on the hardened concrete surface or drilling hole, carry out the pull-out test on the concrete strength, determine the ultimate pull-out force of the concrete strength, and determine the concrete strength in combination with the pre-established correlation between the pull-out force and the concrete strength.
In the practical application, it needs to ensure the concrete surface’s cleanness and dryness, remove the finishing layer’s floating paddle, and grind it if necessary.
Cons:
it is easy to cause inaccurate results due to it will produce eccentric tension when using the pull-out method
The pull-out force may affect the concrete structure or surface.
Shear compression method
The shear compression method is to apply pressure on the edge of the concrete component by a special extrusion Jack to make the edge of the component produce local shear compression failure.
We can estimate the concrete strength according to the local bearing pressure. The local failure surface after detection is small and does not affect the bearing capacity of the component itself.
Core drilling method
The core drilling method is to drill core samples from the structure by a special drill and evaluate the concrete’s strength according to the core samples’ compressive strength.
Range: used to determine the estimated value of concrete compressive strength or modify the indirect strength testing method. The core sample should be a core sample with a diameter of 100 mm, and more than 3 times the maximum particle size of the aggregate, or a small diameter core sample with a diameter of more than 70 mm and not less than 2 times the maximum particle size of the aggregate.
Pros:
direct and reliable
Cons：
It belongs to the damage method to determine the compressive strength of concrete. When drilling cores on concrete components, ensure the number no too large, and avoid concentrated stress parts.
It is easy to cut off the reinforcement in the concrete component, resulting in the decline of structural and mechanical properties. The height diameter ratio of the core sample should be within the range of 0.95 ~ 1.05, and the end of the core sample needs to be levelled. Otherwise, it will cause a test error.
It is mainly used to modify other indirect methods to determine the compressive strength of concrete.
Coring position and quantity
It is suitable for testing the compressive strength of new and old structures or concrete with a strength grade of C10 ~ C80.
Coring position:
1)the position where the structure or member is less stressed;
2)Representative parts of concrete strength;
3)Position where is convenient for placement and operation of core drilling machine;
4)Avoid the location of primary reinforcement, embedded parts and pipelines.
5) randomly selected and took each core sample from the local part of a component or structure.
When determined by batch, We must determine the number of core samples with the same specification according to the sample size in the same test batch.
When the nominal diameter is 100mm, the number of core samples should not be less than 15;
When the nominal diameter is 70mm or 55mm, the number of core samples should not be less than 20.
Core sample processing requirements
1) The core specimen’s actual height diameter ratio (H / D) should be between 0.95 and 1.05.
2) The difference between any diameter and the average diameter along the height of the core specimen should be less than 2mm.
3) The unevenness of the end face of the compression core specimen should be less than 0.1mm within 100mm.
4) The non-perpendicularity between the end face of the core specimen and the axis should be less than 1 °.
5) There are no cracks or other major defects in the core sample.
Summary:
The above determination method of concrete strength has advantages and disadvantages for various testing methods. We need to choose reasonable testing methods according to actual needs.
a. When it will take a large number of qualitative tests on concrete components, the rebound method is preferred.
b. When needed to test accurately, it had better choose the core drilling method. When core drilling conditions are unavailable, the post anchoring or shear compression method is commonly used instead.
c. When there are differences between the concrete surface and the interior, it had better choose the core drilling method.
d. Ultrasonic rebound method is mainly used in concrete with measurement conditions.
e. There are high requirements for the specification and shape of concrete when using the shear compression method.
