Physical testing complements the data provided from chemical and microscopic testing, providing important information regarding the physical properties of the member under examination.  Most commonly requested, of course, is strength, to enable the Engineer to assess the loading capabilities of a structural element.  This is usually supplemented with data on the reinforcement configuration, type and size, from a covermeter survey and localised breakouts.

Tests Available

Notes on Physical Testing

A Modern Concrete Crushing Machine (Photograph Courtesey of Qualitest)

Compressive Strength Determination

The compressive strength of a concrete has traditionally been used as a specification tool by engineers to specify the quality of concrete delivered to a job. It has been used for many years because it is a relatively simple property to measure, unlike chemical analysis which requires a well equipped chemistry laboratory to determine whether an adequate cement content has been used. On site, a simple slump test will normally be carried out to show whether the concrete complies with the specified workability.

The concrete will normally be specified as a "designed mix", meaning that the engineer will have asked for a concrete of a certain strength, either to ensure that he has adequate strength for structural purposes, or, more usually, to ensure the overriding requirement that the concrete has adequate durability. The first thing to realise when measuring concrete strength is that concrete is not a uniform material and, when placed, it is even less so. Even with good quality control, a spread of results of 16 N/mm² is likely with a Grade 40 concrete. This means that to be sure of getting most (95%) of his results above 40 N/mm², the concrete supplier will aim for a target mean strength of some 48 N/mm².

Designated mixes give the specifier a simpler means of specifying concrete for a particular job.  For example, the specifier wanting a concrete suitable for a domestic driveway would call for a PAV1 mix. This automatically requires a minimum strength of 35 N/mm², a minimum cement content of 300 kg/m³ and a maximum water/cement ratio of 0.6, with air entrainment.  For Heavier duty, a PAV2 mix may be specified, and so on.

It must be appreciated that the results gained from concrete core samples will not be directly comparable with those from the original cube tests. Cube samples are fully compacted and have been stored under ideal curing conditions for 28 days, prior to test. Core samples, on the other hand, have been taken from insitu concrete, cured in the structure, often inadequately, with perhaps less than perfect compaction. There are also settlement effects, with results on cores from the bottom of a wall or column differing by 15-30% from those of cores taken near the top. Not surprisingly, there are large differences in the strength measured on cores and on cubes made from the original concrete, as supplied. Concrete Society Technical report No. 11 gives excellent guidance to anyone wishing to find their way through the maze of measuring strength in concrete. It provides guidance on planning an investigation, interpreting the results and comparing the test values with those from the original concrete as delivered. A further Concrete Society technical report has studied the effect o a range of different placement situations and also the use of ggbs and pfa cement replacement materials.

Measuring Concrete Strength

Core samples are the most common form of sample for this purpose, removed from the structure by diamond drilling. Typically cores will be 100mm in diameter, and should ideally be at least three times the maximum aggregate size in diameter.

The cores are usually visually described and photographed, concentrating especially on compaction, distribution of aggregate, presence of steel etc. and then trimmed to a length to diameter ratio approaching 1:1. Various methods are available to ensure accuracy of the ends of the core; grinding to a perpendicular flat surface, capping with high alumina cement mortar and capping with hot sulphur are all methods which are used.

The capped core is then crushed (after appropriate curing for HAC capped specimens) in a calibrated compression testing machine. The resulting failure load is converted firstly to a cylinder strength and secondly to an equivalent insitu cube strength. This will generally be some 75% or so of the original cube result, depending on where the core was taken, before allowing for any additional correction due to compaction effects.