Cement is created by a complex process involving multiple ingredients, testing is therefore essential to ensure compliance with specification and application requirements. Alfonso Rivera, Technical Department and Field Service Manager for materials testing ELE International, UK, reports.
Compressive strength is one of the most important properties of concrete and mortar. The strength of the binder therefore has a significant effect on the performance characteristics of the mixture and ensures the overall quality of the finished product.
The test is generally carried out by crushing cubes of hardened cement-sand mortar in a compression machine. Strength is determined by applying the highest stress 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.
Tensile strength tests can be carried out to gain an understanding of the cohesion between the cement particles. 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. 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 test is conducted by the application of uniaxial tension. This is done by placing cement-sand mortar briquettes in a machine that can apply a tension load. The 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, the 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, and test sand.
The final stage of cement production involves grinding to form a fine powder containing particles of significantly different sizes. The particle size distribution has a major influence on the rate at which 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 volume – 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.
Fineness is tested by measuring the air permeability specific surface area of the cement powder. This measurement is taken with the Blaine Air-Permeability Apparatus – a fineness determining test procedure developed by R L Blaine of the American National Bureau of Standards. Results are expressed as the total surface area in square centimeters per gram, or square meters per kilogram. A compacted sample is tested at room temperature, between 18–22˚C, by measuring the time taken for a fixed quantity of air to flow through the sample. 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 and liquid, reference cement, and a timer.
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.
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.
When water is added to cement, the resulting paste will begin to harden and gain compressive strength, and the Vicat needle test – needle units used to determine the initial and final setting times of cement and mortar paste – measures the time taken for the cement-water mixture to set. 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 specified weight, dimensions and drop height. The resistance to penetration is determined by the viscosity of the cement paste in a mould. The initial setting time is defined as the time taken for the needle to be able to penetrate the paste in the mould to a depth of 5mm. The final setting time is the time taken for the cement paste to harden sufficiently such that the needle cannot penetrate it in the mould and leaves no mark on the surface of the specimen. The required equipment includes a Vicat frame, needles and mould, glass graduates, and mixing tools.
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 that 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 to finish.
A cement paste or mortar mix is placed in a mould, which 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, and the sample raised and dropped from a height of 12.5mm, for 15 times in around 15 seconds. After the dropping sequence, the diameter 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 callipers.
Soundness and expansion of cement
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 will not develop cracks.
It can be determined by a variety of methods. In the Le Chatelier water bath method – used to determine the soundness of cement paste using boiling water – a specimen of hardened cement paste is placed in a mould and boiled for a fixed amount of time – approximately three hours – so that any tendency to expand is sped up and can be detected. After the boiling process is completed, the distance between two control indicator points is measured to the nearest 0.5mm and compared to the original length. This test requires a Le Chatelier water bath and mould, calliper, measuring cylinder, balance and length comparator.
Alternatively, soundness of cement can be determined by the autoclave method. This test covers the autoclave expansion of Portland cement on a neat cement specimen. Specimens are formed in an oil-covered mould with reference points attached at a length of 250mm. The specimens are extruded from the moulds and measured after 24 hours and placed in the autoclave at room temperature. After it is sealed, the specimens are exposed to high-pressure saturated steam for a defined length of time. Once 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.
The expansion of cement can also be determined by the length comparator method. This is a device used for measuring length changes of cement paste, mortar and concrete in accordance with The American Society for Testing and Materials – technical standards for materials, products, systems, and services – and The American Association of State Highway and Transportation Officials 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.
It is vitally important to follow the correct method and to use appropriate, properly serviced equipment. Errors or failures in the construction sector can be extremely dangerous and expensive, and the costs involved in testing are negligible in comparison with overall project costs, so there can be no excuse for inadequate or incorrect testing.
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