Date posted: 15th January 2018 08:57
Cement performs a vital role in the construction industry, but as a manufactured material created by a complex process involving multiple ingredients, testing is essential to ensure compliance with specification and application specific requirements. In the following article Alfonso Rivera, Technical Department and Field Service Manager for ELE International, reviews the most important tests and provides an update on relevant standards.*
Compressive strength is one of the most important properties of concrete and mortar. The strength of the binder (cement) therefore has a significant effect on the performance characteristics of the mixture and ensures the overall quality of the finished product. The test for compressive strength is generally carried out by crushing cubes of hardened cement-sand mortar in a compression machine. The compressive strength of the cubes is determined by the highest stress applied to a cube specimen that causes it to fracture. The test equipment required for this purpose includes a compression machine, a mortar mixer, suitable moulds, a humidity cabinet, the cement itself and test sand. The applicable standards for Compressive Strength are: EN 196-1, ISO 679, EN 459-2, EN 1015-11, EN 13454-2, ASTM C109/C109M, ISO 7500-1 and ASTM E4.
The tensile strength of cement is relatively low when compared to its compressive strength. Concrete structures are vulnerable to tensile cracking due to a variety of effects including dynamic loading and temperature variation. Tensile strength tests can be carried out to gain an understanding of the cohesion between the cement particles. The tensile strength is proportional to the compressive strength and the tests are simple and inexpensive to perform, so they are relatively popular. The most common tensile strength test on cement is conducted by the application of uniaxial tension. This test is carried out by moulding cement-sand mortar briquettes which are placed in a machine that can apply a tension load. The tensile strength is calculated by measuring the load required to split the sample in half over the section of the fracture.
When a uniaxial tension machine is not available, tensile tests can also be carried out by other indirect methods. For example, in the split cylinder test, a sample in the shape of a cylinder is loaded laterally until fracture occurs. An alternative method is the flexure test in which a mortar beam is loaded between two supports to apply a bending stress; which causes the fibres in the lower half section of the beam to develop tension stresses, until failure occurs. It should be noted that these tests give a higher tensile strength value than the uniaxial tension method.
Typical laboratory equipment for tensile strength testing of cement would include a 10kN flexural/tensile testing machine, briquette moulds, a mortar mixer, a humidity cabinet, the cement itself and test sand. Applicable standards include ASTM C307 and EN 196-1.
The final stage of cement production involves grinding to form a fine powder containing particles of significantly different sized particles. The particle size distribution has a major influence on the rate at which a cement sets and gains strength, and can affect other factors such as workability and drying shrinkage. The smaller the particle, the larger the surface area of the powder in relation to the volume, so as cement reacts with water, the smallest particles contain the largest number of contacts and have a high participation in the process of setting and hardening.
The fineness of cement is tested by measuring the air permeability specific surface area of the cement powder. The measurement of fineness of hydraulic cement is undertaken with the Blaine air-permeability apparatus to determine the specific surface expressed as the total surface area in square centimetres per gram (or square meters per kilogram). The test is carried out on a compacted sample of cement at room temperature between 18 and 22 ˚C by measuring the time taken for a fixed quantity of air to flow through the cement sample, and this recorded time is a measure for the specific surface area. The method is a comparative test between a known and unknown material, therefore a reference sample with a known surface area is used for calibration. By measuring the time taken for air to flow through the reference material, the user can establish a correlation of the surface area based on the time taken on the test cement. The Blaine Apparatus is employed for this test and consists of a permeability test cell, a perforated disk, a plunger, filter paper, a manometer U-tube, manometer liquid, reference cement and a timer. The applicable standards are EN 196-6, ASTM C204 and AASHTO T153.
Consistency, Setting Time and Flow
The consistency of a mixture plays an important role in the performance of the mix when poured through reinforcing bars within a form, and in the time taken for the mortar or concrete to set. The consistency test is performed to estimate the amount of water needed to form a paste of normal consistency, defined as the percentage water requirement of the cement paste.
When water is added to cement, the resulting paste will begin to harden and gain compressive strength, and the Vicat needle test measures the time taken for the cement-water mixture to harden or ‘set’. Consistency is measured by the Vicat test which provides both initial and final setting times – measurements that can be regarded as the two stiffening states of the cement. The beginning of solidification, or the initial set, marks the point in time when the paste has become unworkable, and the time taken to completely solidify marks the final set. This should not be too long so that construction activity can be resumed within a reasonable time after the placement of concrete.
The Vicat plunger has a specified weight, dimensions and drop height, and the resistance to penetration by the plunger is determined by the viscosity of the cement paste in a mould. The initial setting time is defined as the time taken for the Vicat needle to be able to penetrate the paste in the mould to a depth of 5 mm. The final setting time is the time taken for the cement paste to harden sufficiently such that the Vicat needle cannot penetrate the cement paste in the mould and leaves no mark on the surface of the specimen. The required equipment includes a Vicat frame, Vicat needles and mould (EN or ASTM), glass graduates and mixing tools. Alternatively, ELE manufactures an automatic Vicat apparatus. Applicable standards include: EN 196-3, EN 480-2, EN 13454-2, ASTM C187, ASTM C191, AASHTO T129 and AASHTO T131.
Determination of consistency can also be carried out by using a Flow Table Test Apparatus. While flow is not usually included in hydraulic cement specifications, it is commonly used in standard tests which require the mortar to have a water content that provides a specified flow level. Cement paste acts as a separator for aggregates in mortar, and a lack of sufficient mortar results in a mixture of limited flow, which is prone to segregation and difficulty of finishing.
A cement paste or mortar mix is placed in a mould of defined volume and dimensions, and the mould is placed on top and in the centre of a flow table apparatus, where the sample is formed and compacted. The mould is then removed, leaving behind the formed sample of mortar, which is then raised and dropped from a height of 12.5 mm, 15 times in approximately 15 seconds. After the dropping sequence, the diameter of the spread of the mix is measured. The flow of the mix is the percentage increase in diameter of spread mix over the base diameter of the moulded mix. The key items for this method are the flow table, mould and calipers. ELE also offers an alternative motorised unit. The applicable standards are ASTM C230/C230M.
Soundness and Expansion of Cement and Fly Ash
Soundness refers to the ability of a hardened cement paste to retain its volume after setting without delayed destructive expansion. As such, soundness is an extremely important test. A sound cement paste will not undergo any appreciable change in volume after it has hardened, and shall not therefore be susceptible to the development of cracks. A sound cement paste is the base of a sound mortar or concrete mix.
Fly Ash is a by-product of coal burning power generation. During combustion, mineral impurities in the pulverised coal are expelled from the combustion chamber in the form of fused particles in suspension with the flue gases. As these particles cool, they solidify into spherical glassy particles. Fly Ash exhibits cementitious properties and is used in concrete as a replacement or partial replacement for Portland cement. This benefits the environment because it reduces material for landfill and lowers the requirement for quarrying virgin material. Un-soundness can arise from the presence of too much free magnesia or hard-burned free lime in the cement.
Soundness of cement can be determined by a variety of methods. In the Le Chatelier water bath method, a specimen of hardened cement/fly ash paste is placed in a mould and boiled for a fixed amount of time (approximately 3 hours) so that any tendency to expand is speeded up and can be detected. After the boiling process is completed, the distance between two control indicator points is measured to the nearest 0.5 mm and compared to the original length measured before the test. This test requires a Le Chatelier water bath, a Le Chatelier mould, caliper, measuring cylinder, balance and length comparator. Applicable standards include EN 196-3, EN 459-2 and EN-450.
Alternatively, soundness of cement can be determined by the autoclave method. This test covers the autoclave expansion of Portland cement by means of a test on a neat cement specimen. Specimens are formed in an oil covered mould with reference points attached at a reference length of 250 mm. The specimens are extruded from the moulds and measured after 24 hours and placed in the autoclave at room temperature. After the autoclave is sealed, the specimens are exposed to high pressure saturated steam for a defined length of time. After the heating is complete, the specimen is measured and compared to the original length measurements. The equipment required for this method includes an autoclave, test bar moulds, mixing equipment, a balance and a length comparator. Applicable standards include ASTM C151 and AASHTO T107.
The expansion of cement can also be determined by the length comparator method. The length comparator is a device used for measuring length changes of cement paste, mortar and concrete in accordance with ASTM and AASHTO specifications. The unit consists of a sensitive dial or digital indicator mounted on a sturdy, dual post construction frame. Movable and stationary anvils are shaped to receive the reference pins which are cast into the ends of the test specimen bars. ELE supplies a variety of devices for this method including the drying, shrinkage and moisture movement apparatus, as well as the ASTM length comparator. Applicable standards include ASTM C151, C157, C227, C490, C531, AASHTO T-107 and T-160.