A variety of physical tests can be carried out on samples taken from a structure.
Compressive Strength Determination
Cores may be drilled from a structure to assess one or more of the following:
The quality of the concrete provided to the construction (Potential Strength).
The quality of the concrete in the structure (In-situ or Actual Strength).
The load factor of a structure to carry:
- the actual loading system
- the designed loading system
- a projected system for a new use
Deterioration in the structure due to:
- chemical reaction
- fire or explosion
The engineer may wish to conduct core tests for a variety of reasons e.g.
To assess the actual or in-situ strength at a specific location to use in structural calculations.
To assess actual strength in locations affected and unaffected by deterioration in order to establish the extent of deterioration.
To assess the potential strength of concrete to compare with the values for the concrete supplied, where there is a dispute, or where it is suspected that cube test results may be inaccurate.
It must be appreciated that the in-situ core strength, or actual strength, cannot be compared directly to the cube strength that would have been obtained at the time of placing. The in-situ concrete is likely to have a higher void content and is unlikely to have the same curing that a cube should depending on where in the structure the core sample is taken. Higher strengths can be found near to the base of a column, for example, whereas, lower strengths are likely to be found near to the top, owing to settlement effects. The planning and interpretation of core testing is covered in BS 6089 and Concrete Society Technical Report No. 11.
If cores, are well cut, trimmed and capped, a coefficient of variation of some 6%± 12%/(n)0.5 only of the true strength of the concrete contained in the core samples Where potential strength is being determined, the average estimate of potential strength is likely to be at best within ± 15% only of the true value for the batch.
The determination of the expansive characteristics of concrete affected by alkali aggregate reaction can give useful information on the state of the reaction, whether the reaction is exhausted or whether there is potential for more reaction and further expansion. The test involves the monitoring of length changes in concrete cores over a period of time during which the cores are kept in a constant environment at a controlled (usually 38°C or 20°C) and approximately 100% relative humidity.
Expansion Testing at 38° and 100% Relative Humidity
Preparation of the Cores
The test is carried out on 100mm diameter cores. Demec gauge points are fixed at even 120E intervals around the core and 50mm intervals along the core commencing 20mm in form the outer surface of the core. Each Demec gauge length is marked for later identification.
The cores are stored in a sealed unit which is kept at 38°C. The unit contains water to allow saturation of the surrounding atmosphere to approximately 100% relative humidity. The cores are not allowed to come into direct contact with the water. The temperature and humidity of the unit are checked at regular intervals.
The cores are removed from the test environment for measurement for the minimum period of time. Measurement of the Invar bar is taken prior to readings on the cores so that appropriate corrections may be made for any variations in temperature outside the sealed unit.
Demec readings are taken at 0 days, 1 day, 3 days, and then 7 day intervals up to 60 days.
Reporting the Results
Individual readings are tabulated. Information recorded includes core number, position of reading on core, day, Demec gauge readings, Invar bar correction and gauge factor. An average result for each Demec gauge length is calculated from the three sets of readings taken around the core and converted to a strain measurement in millistrain. Strain measurements recorded along the length of the core are then plotted against time.
BS 6089:1981. "Guide to the Assessment of Concrete Strength in Existing Structures".
Concrete Society Technical Report No. 11, "Concrete Core Testing for Strength" Report of a Concrete Society Working Party, May 1976.