Planning in-situ testing of concrete for strength, durability and damages includes considering the most suitable tests to obtain the established goals of investigation, number of tests needed to show the real state of the concrete, and the location of these tests.
Planning a program for in-situ testing of concrete will be described in this article.
Planning In-Situ Testing of Concrete for Strength, Durability and Damages
- General sequential approach
- Visual inspection
- Test selection
- Number and location of tests
General Sequential Approach for In-Situ Testing of Concrete
It is significant to arrange an excellent program with analyzing and interpretation as ongoing activity regardless the motivation or the cause of the investigation.
Figure-1 illustrates a typical in-situ concrete test program and the investigation is continuing until strong relevant result is reached.
Fig.1: Typical Stages of In-Situ Testing of Concrete
Visual Inspection for In-Situ Testing of Concrete
It is possible to obtain useful information especially when well trained eye does visual inspection. Features of visual inspection might pertain to workmanship, material deterioration, and structural serviceability.
It is considerably significant for engineers to recognize various types of cracks which are likely to be encountered; numbers of typical cracks are shown in Figure-2.
Fig.2: Number of Typical Types of Cracks in Concrete
Honeycomb could be due to low standard workmanship however, excessive bleeding or segregation at joints of shutters might be resulted from issues in concrete mixtures and probably lead to plastic shrinkage cracking.
Large deflections and flexural cracking could reflect structural inadequacy and this often the cause for carrying in-situ assessment of structures.
Structural movements, thermal movements, and long term deflections are probably led to problems in door frames, cracking of windows, cracking of structure or its finishes. In these situations, visual inspection comparison of similar member may give important information.
Material degradation is frequently shown by concrete spalling and surface cracking and crack pattern assessment could give and initial indication of the cause.
The most usual causes are steel bar corrosion resulted from insufficient concrete cover thickness or large chloride concentration, sulfate attack, frost action, and alkali aggregate reactions.
As shown in Figure-2, the signs of sulfate attacks are developing arbitrary crack pattern with white layer leached on the surface but, concrete splitting and spalling in the direction of steel bars are assumed to be indications of reinforcement corrosion.
Occasionally, an alkali aggregate reaction is recognized by star shaped crack pattern whereas irregular surface spalling might show frost attacks.
It is reported that, regular crack mapping is an important technique to identify deterioration reasons and progressions and it gives detailed information about determination of crack types.
Table-1, which reported by Higgins, provides symptoms pertaining to the most usual cause of deterioration.
With regard to damages resulted from fire, modification of concrete surface color and texture could be beneficial guide and color change is broadly accepted as sign of extent fire damage.
Not only does visual inspection carried out on concrete surface but also it could include analyzing of drainage channels, expansion joints, bearings, post tensioning ducts and other similar feature of the structure. Ultra violet inspection systems can be beneficial to recognize alkali aggregate reactions.
Table-1: Diagnosis of Deteriorations and Defects of In-Situ Concrete
Concrete In-Situ Test Selection
After conducting visual inspection, numbers of factors for example damage, cost, access, reliability, and speed are considered for the selection of test for specific situation.
Durability Tests including Reasons and Amount of Deterioration
Preliminary tests used to examine the threat of reinforcement corrosion, which resulted from losing passivity due to chloride or carbonation, are commonly reinforcement cover measurements, chloride concentration, and carbonation depth in addition to half-cell potential and resistivity testing to achieve more comprehensive assessment of the large area.
If too much carbonation is discovered to be the reason behind deterioration, absorption tests and petrographic analysis might be carried out especially when knowing the cause of excessive carbonation is required.
More details on various tests conducted to examine concrete durability are provided in Table-2.
Table-2: Concrete In-Situ Durability Tests
In-Situ Testing for Concrete Strength
Core tests that are slow and expensive are the most effective method to assess concrete strength. However, Pulse velocity and hardness tests lead to minor damages and at the same time are economical and quick.
Even though, these tests are perfect to comparative and uniformity evaluation but their correlation to anticipate absolute strength create many issues.
The result of core tests might be employed as base for calibration of partially destructive and non-destructive test values which can be broadly used later.
Most of normal weight concrete test methods can be used for assessing light weight concrete strength but the correlation of the results is different.
When the only requirement is comparison with similar concrete quality, then test selection will depend on practical restrictions of different tests and occasionally back up tests might be carried out in some regions.
Table-3 provides different tests used to estimate concrete strength.
Table-3: Concrete In-Situ Strength Tests
Testing for Comparative Concrete Quality and Localized Integrity
Comparative testing is the most dependable application of several tests. Not only do these tests are led to small or no surface damage but also most of them are quick to use by which large area can be surveyed regularly. However, numbers of test methods require complex and high cost equipment.
In-Situ Testing of Concrete for Structural Performance
Large scale dynamic tests can be used to observe the performance of the structure. Nonetheless, large scale static load test in conjunction with monitoring of cracks by acoustic emission may be more suitable despite the cost and disruption.
The static load tests commonly include measurements of deflection and cracking but problem with isolated individual elements could be large.
Numbers and Locations for In-Situ Testing of Concrete
Setting suitable and adequate number of tests can be achieved through compromising between accuracy, cost, effort, and damage. Test results are related solely to the test locations from which test sample were taken.
That is why engineering judgment is needed to specify test number and locations and relevance of results to the entire member.
Therefore, connection between planning and interpretation of result is extremely significant. Moreover, it is considerably important to understand concrete variability adequately and have knowledge on reliability of utilized test methods.
It is considerably important to obtain adequate accuracy when core tests are used to determine concrete strength or employed as a base for calibration of other methods of testing.
For comparative purposes, non-destructive tests are the most efficient method because great number of location can test in short time due to speed test.
Minimum 40 locations are suggested for a member over whom tests stations are distributed uniformly, but smaller number of tests is required for comparative purposes.
When other test methods such as internal fracture or Windsor probe tests are used, practicalities may lead to decrease test numbers. Furthermore, tests for material specifications compliance should be made on typical concrete, so weaker top zones of the element must be avoided.
It is recommended that, for columns, beams, and wall should be taken around mid-height, and surface zone tests on slabs should be limited to soffits except if the top layer is removed.
It is advised to take at least four core tests from suspected concrete batch where specification compliance is investigated, and when small cores is employed minimum of 12 tests are needed.
Finally, the numbers of load tests which are conducted on a structure are restricted and should be taken at critical locations. The critical or suspect areas can be determined with the help of visual inspection and non-destructive tests.
Where destructive tests are carried out for members to give calibration for non-destructive methods, they should be selected to cover as wide a range of concrete quality as possible.
Table-4 Provides number of tests assumed to be equivalent to one single result. The strength prediction accuracy based on reliability of the correlation employed.
Table-4: Relative Number of Readings Recommended for Various Test Methods
Recommended Number of Readings at a Location
Ultrasonic pulse velocity