Testing of Hardened Concrete

Once concrete has hardened it can be subjected to a wide range of tests to prove its ability to perform as planned or to discover its characteristics if its history is unknown.

For new concrete this usually involves casting specimens from fresh concrete and testing them for various properties as the concrete matures. The ‘concrete cube test' is the most familiar test and is used as the standard method of measuring compressive strength for quality control purposes. Concrete beam specimens are cast to test for flexural strength and cast cylinders can be used for tensile strength. Specimens for many other tests can be made at the same time to assess other properties, eg. Drying shrinkage, thermal coefficient, modulus of elasticity.

For existing concrete samples will need to be taken from the structure (smaller precast units can be tested as found). Non destructive testing (NDT) methods are useful in some instances and can help identify areas from which samples should be taken. The normal method of concrete sampling is by coring although some chemical analysis techniques can be carried out on drilling dust samples. Once back in the laboratory many techniques can be used to examine and test hardened concrete to assess a wide variety of properties.

Visual examination of Concrete

Simple visual examination by an experienced technician can provide a valuable but quick assessment of the quality of a concrete sample looking at compaction, excess voidage, aggregate type, size and distribution, presence of cracking.

Microscopical examination of Concrete

Use of low and high powered microscopes to examine thin section and polished surface specimens can provide a detailed picture of the concrete including cement and aggregate type, additives (pulverised fuel ash or ground granulated blast furnace slag), carbonation depth, compaction, cracking and microcracking. Evidence of deterioration such as sulphate attack (thaumasite, ettringite), AAR or chemical attack can be identified and air void content of air entrained concrete can be measured by point counting. Fire or bomb damage can be evaluated and crack aging carried out.

Strength, Density and Related Properties of Concrete

• Density - As-received, saturated or oven dried.
• Compressive Strength - One of the fundamental properties used for quality control by testing cubes or cylinders. Core samples are used to assess insitu strength of existing structures.
• Tensile Strength - Direct tensile strength or indirect tensile (splitting) strength of cores or cast cylinders.
• Flexural Strength - 100mm or 150mm beams can be tested (often used for QC purposes for Pavement quality concrete). A modification of the test can be used to assess the flexural properties of fibre reinforced concrete.
• Modulus of Elasticity - Static Modulus of elasticity in compression of core or cylinder specimens or Dynamic Modulus using cores, cylinders or prisms.

Sandberg also provide a consultancy service giving advice and guidance for example where clarification is needed between ‘actual', ‘estimated insitu', ‘potential' and ‘characteristic' strength in dispute situations

Permeability

Permeability to liquids, ions and gases is one of the major factors when considering the durability of concrete. The resistance of concrete to chemical and environmental attack and thus the protection of reinforcing steel can be directly related to permeability. This property can be measured using a variety of techniques:

• Water absorption - Simple water absorption by total immersion on cores or cylinders (with indicative tests on irregular shapes).
• Initial surface absorption (ISAT) - Assess the uniaxial water penetration characteristics of the surface concrete.
• Capillary Rise or Sorptivity - Similar to ISAT , used to assess surface finish/weathering properties of cast stone and used for assessing water resisting admixtures in BS EN 934-2.
• Figg Air Permeability - Investigates the quality of concrete in the surface zone by measuring air flow.
• Chloride ion penetration (electrical method) - Provides a rapid indication of resistance to penetration by monitoring electric current across slices of concrete core , one face of which is in a chloride solution.
• Bulk Chloride Diffusion - Derives a coefficient of chloride diffusion by analysis of a chloride profile established after exposure of one face of a specimen to chloride solution. The diffusion coefficient can also be obtained from a chloride profile from an existing structure.

Movement

• Moisture movement - Standard and non-standard methods can be used to make accurate measurements of concrete drying shrinkage and wetting expansion.
• Thermal movement - Measurement of movement due to temperature change and calculation of coefficient of thermal expansion.

Other Tests

A wide range of other standard and non-standard laboratory tests can be carried out including ad-hoc tests tailored for specific project or client requirements.

• Non-destructive tests (NDT) - eg. Ultrasonic pulse velocity (UPV).
• Freeze-thaw - cyclic freezing and thawing of concrete specimens monitoring condition using NDT techniques such as resonant frequency or UPV.
• Pull-off or bond tests - assessing fixings or bond strength of coatings or screed.
• Slip Resistance - Often tested on precast products in the laboratory or on site.
• Abrasion resistance - A particular requirement for industrial flooring.

References: BS EN 12390 series, Testing hardened concrete
BS 1881 series, Testing concrete
BS 8204 series, Screeds, bases and insitu floorings
ASTM C457 and C856, Microscopical examination