Taking core samples from concrete structures is required for comprehensive assessment of durability and structural performance of the structure. The compression test on core samples is known as the most reliable method for assessing the compressive strength and a means of quality control. The procedure for taking core samples and testing them has been standardized (ASTM C 42, and ACI 318). However, with all the benefits coring brings to the table, coring has certain disadvantages. In addition, the selection of core locations in structure can be a challenging task. In this article, we will review the benefits and disadvantages of taking cores. We will also discuss how non-destructive testing has changed the paradigm in recent years.
Concrete Coring | Challenges
Coring in concrete structures come with certain challenges. To make coring more efficient, one needs to address the following challenges:
1- How many cores are needed? ,and where?
The quantity and the location of cores should be selected such that they represent the strength distribution within the member. The cores should also be a realistic representation of concrete quality. It is safe to say that increasing the number of cores can deliver a more realistic distribution of strength, but at the same time, leave more weak points in concrete.
The type of condition assessment also plays a key role in selecting the location of cores.
1- Likely strength/stress: when the objective of an assessment is to evaluate the structure under service loads, the cores should be taken from areas where the minimum strength and maximum stress coincide (i.e. mid-span for the beams, or any face near the top of lifts for vertical elements).
2- Highly reinforced or slender members: when the element is highly reinforced, or coring can significantly affect the integrity or structural performance, the cores should be taken from nearest non-critical section
3- Aesthetic consideration: concrete coring can leave unpleasant scenes on the structure. The location of cores can be adjusted to reduce the impact on aesthetic features.
4- Durability concerns: when durability of concrete (i.e. permeability, protection against radiation), the number of cores should be very limited.
Concrete Coring | Benefits
Taking core samples provides invaluable information about an existing structure. Coring can also be used as a means of quality control when things go wrong with concrete material quality or the pouring and curing process. The following information can be obtained with further assessment of cores:
1- Compressive strength: Perhaps, measurement of compressive strength is the primary objective in most cases. Concrete cores can provide precise information about the strength of concrete.
2- Density: Cores can be used to determine the density of concrete.
3- Water absorption: concrete cores can be tested to evaluate the water absorption of concrete. Water absorption can provide basic information about the porosity, and inter-connectivity of pores.
4- Bulk Electrical Resistivity: Bulk electrical resistivity of concrete can be determined by testing concrete core samples. ER provides valuable information about the resistance of concrete against penetration of chloride ion into concrete.
5- Visual assessment of cores: Cores, and the drilled hole can be used to evaluate the visual condition of concrete defects, such as cracks, voids, and honeycombing.
Concrete Coring | Disadvantages
Taking cores from concrete elements have certain disadvantages, including:
1- Weak points: taking cores leave weak points within the element. The location of cores is normally repaired by standard or high strength patch repair materials. While repair materials often have adequate strength, the weak points can affect the integrity of structure, and durability performance.
2- Intervention: taking cores can damage structural performance of the element, especially if the coring damages the existing rebar. Cutting rebar in an already distressed structure is the last thing an engineer wants in the field.
3- Subjective test: The selection of core locations can be very subjective. After all, the very nature of tests means that the engineer or the technician may need to compromise between the number of cores, safety of the structure, and intervention to the structure.
Concrete Cores | Dimensions
When the purpose of coring is to evaluate the compressive strength of concrete, the minimum diameter of concrete core should be at least three (3) times the nominal maximum aggregate size. Different codes and standards set different values for the minimum diameter of concrete cores, ranging from 75 mm (in Australia) to ideal 150 mm (Britain). However, 100 mm core is a practical size in most structure inspections. The accuracy of cores for strength reduces when the ratio of diameter to nominal maximum aggregate size decreases.
Height to Diameter ratio (H/D)
Most standards refer to H/D of 2.0 as the standard ratio for structural cores. However, obtaining cores that meet this specification is not always practical. For example, in thin slabs (6” = 150 mm), it is very hard to obtain a core with this ideal aspect ratio. Correction factors have been used to correct the strength values for cores that do not have the preferred H/D of 2.0. The accuracy of the correction factors decreases as the H/D decreases.
Non-destructive testing | A new paradigm
Recent developments in the NDT methods can be used to improve the process of coring in concrete, such as location of cores, number of cores, and interpretation of results.
Ground Penetrating Radar (GPR): This method can be used to determine the as-built layout of steel rebar within concrete. The as-built layout of steel rebar can be used during the course for drilling cores in order to avoid from cutting steel rebar.
Combined Methods (UPV+ RN): NDT methods can be utilized to optimize the number of cores and core positions. To do so, it is recommended to combined NDT methods such as Ultrasonic Pulse Velocity (UPV) and Rebound Number (Schmidt hammer). The results obtained from these NDT methods can be used to evaluate the compressive strength of concrete. Based on these methods, maps of compressive strength can be developed to study the strength variation over concrete members. This map can be used to determine the number and location of cores. The combination of these two methods can cover the deficiency of each individual test, and improve the accuracy of strength prediction.