The unit weight of concrete is specified in EN1991-1-1 Annex A. For plain unreinforced concrete = 24 kN/m3. For concrete with normal percentage of reinforcement or prestressing steel = 25 kN/m3.
The characteristic compressive strength fck is the first value in the concrete class designation, e.g. 30 MPa for C30/37 concrete. The value corresponds to the characteristic (5% fractile) cylinder strength according to EN 206-1. The strength classes of EN1992-1-1 are based on the characteristic strength classes determined at 28 days. The variation of characteristic compressive strength fck(t) with time t is specified in EN1992-1-1 3.1.2(5).
The characteristic compressive cube strength fck,cube is the second value in the concrete class designation, e.g. 37 MPa for C30/37 concrete. The value corresponds to the characteristic (5% fractile) cube strength according to EN 206-1.
The coefficient cc takes into account the long term effects on the compressive strength and of unfavorable effects resulting from the way the load is applied. It is specified in EN1992-1-1 3.1.6(1)P and the National Annex (for bridges see also EN1992-2 3.1.6(101)P and the National Annex).
The coefficient ct takes into account long term effects on the tensile strength and of unfavorable effects, resulting from the way the load is applied. It is specified in EN1992-1-1 3.1.6(2)P and the National Annex (for bridges see also EN1992-2 3.1.6(102)P and the National Annex).
The elastic deformation properties of reinforced concrete depend on its composition and especially on the aggregates. Approximate values for the modulus of elasticity Ecm (secant value between c = 0 and 0.4fcm) for concretes with quartzite aggregates, are given in EN1992-1-1 Table 3.1 according to the following formula:
According to EN1992-1-1 3.1.3(2) for limestone and sandstone aggregates the value of Ecm should be reduced by 10% and 30% respectively. For basalt aggregates the value of Ecm should be increased by 20%. The values of Ecm given in EN1992-1-1 should be regarded as indicative for general applications, and they should be specifically assessed if the structure is likely to be sensitive to deviations from these general values.
where bt is the mean width of the tension zone and d is the effective depth of the cross-section, fctm is the mean tensile strength of concrete, and fyk is the characteristic yield strength of steel.
The minimum reinforcement is required to avoid brittle failure. Typically a larger quantity of minimum longitudinal reinforcement for crack control is required in accordance with EN1992-1-1 7.3.2. Sections containing less reinforcement should be considered as unreinforced.
According to EN1992-1-1 184.108.40.206(1) Note 2 for the case of beams where a risk of brittle failure can be accepted, As,min may be taken as 1.2 times the area required in ULS verification.
where where bw is the width of the web and s is the spacing of the shear reinforcement along the length of the member. The angle corresponds to the angle between shear reinforcement and the longitudinal axis. For typical shear reinforcement with perpendicular legs = 90 and sin() = 1.
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The UK energy industry relies on unseen assets to keep it ticking over. Buried deep below ground, often carrying hazardous substances, and frequently managed by different operators, pipelines quietly provide our countrys energy needs and more. But how do we ensure that theyre safe, compliant, and conform to a standard that makes the whole thing work for the greater good?
Failure to do this can result in the need to spend even more money undertaking complicated calculations to prove compliance with British Standards or even drilling and crushing cores to prove design and reliability! Why waste so much time and money when its so easy!? What we have here is a sure fast way of doing it properly and getting it right first time. Enjoy!
Concrete is used mostly for structural purposes such as foundations, columns, beams and floors and therefore must be capable in taking the loads that will be applied (unless youre just after a large paperweight!). One of the methods of checking its fit for purpose is to carry out a concrete cube test which measures the compressible cube strength of the concrete and relates directly to the required design strength specified by the designer.
Make sure all people involved are trained and competent to carry out the task at hand and make sure you read and understand the risk assessment and method statement, because if you don't, itll only come back to bite you later. If youre not sure how to do it, ASK!
Never take a sample from the first or last section of the pour, it wont be a true representation of the batch. The concrete is usually sampled after the 1st metre of concrete has been poured to ensure a good sample is taken. As said in BS EN 12350-1, take a few samples throughout the pour for the best representation of the batch and make sure you take 150% of what you think youll need. The sample is taken and used to make the cubes. The sample must be a good cohesive mix, it may require some mixing once taken from the concrete batch to be suitable for a slump test and cubes.
Slump Test always do your slump test before making your cubes to ensure the concrete is usable. If the slump test fails to meet the range limit as dictated by the British Standard (shown in the table below) then the load should be rejected.
Place the damp slump cone on a flat, hard surface. Fill the cone with the concrete sample in three stages. Once each stage is filled, tamp the mix with the tamping rod 25 times. After the third tamping the excess concrete shall be struck off flush to the top of the cone. Lift the mould carefully upwards, to minimise disturbance of the concrete inside. The concrete will slump. Place the cone next to the concrete slump and measure the difference in height in mm between the top of the cone and the top of the highest point of the concrete.
Usually a minimum of 3 cubes are taken from each sample, so make sure you taken enough from the pour before it finishes. Do check the specification you are working to, as sometimes the quantity of cubes you have to take may vary. The frequency of sampling should be identified in client specifications or by the designer. This could be per batch / load or even per volume poured. Check before you start.
Cube moulds are usually 150mm x 150mm x 15 mm (or 100mm x100mm x100mm) and can be made from steel or polyurethane. The cube moulds must be manufactured to the specifications / standards of the relevant body, in the UK it is the British Standards Institute to this specification BS EN 12390-1:2000.
Before the concrete is scooped into the moulds, the moulds must be lightly coated in a mould release agent. This ensures that the concrete does not stick to the mould and makes it easier to remove the cube. When using a 150mm mould, the concrete sample is scooped into the mould in 3 equal layers (50mm) and compacted between each layer. There are various methods to compact the concrete into the moulds.
When using a 150mm mould, each layer compacted is tampered using a certified compacting rod /bar, 35 tamps per layer is required. Once the 3 layers have been tampered, tap the side of the mould with a hammer. Tampering and tapping removes trapped air in the concrete and allows compaction of the sample.
Each layer is filled and vibrated till no more bubbles are on the surface of the layer, this is repeated for the 3 layers. It is very important not to over vibrate the layers as it may lead to segregation / disruption of the concrete mix.
Its very important to uniquely identify each of the cubes (and moulds) and to record where they have come from. Usually companies will have a process of labelling or tracking the cubes so make sure you ask first and record it properly.
The cubes should be covered with a damp cloth and a plastic sheet and stored in dry environment at a temperature range of 20 5 degrees. The concrete cubes are removed from the moulds between 16 to 72 hours, usually this done after 24 hours. Make sure the cube ID is transferred to the cube from the mould before placing into a curing tank. The curing tank needs to operate at a temperature between 20 2 degrees and provides a moist environment that allows the cubes to hydrate properly. Ensure the cubes are fully submersed at all times and record the tank water temperature at least daily.
The cubes are generally tested at 7 & 28 days unless specific early tests are required, for example to remove a concrete shutter safely prior to 7 days. Usually 1 cube will be tested at 7 days and 2 cubes at 28 days, however this may vary depending of the requirements, check the design first. The cubes are removed from the curing tank, dried and grit removed. The cubes are tested using a calibrated compression machine. This can be carried out internally by competent personnel or by a certified test house.
The cubes are tested on the face perpendicular to the casting face. The compression machine exerts a constant progressing force on the cubes till they fail, the rate of loading is 0.6 0.2 M/Pas (N/mm/s). The reading at failure is the maximum compressive strength of the concrete. BS EN 12390-2: 2009 / BS EN 12390-3:2009.
The 40 is the compressive requirement of 40 N/mm of a crushed 100m concrete core and the 50 is a compressive requirement of 50 N/mm for a crushed concrete cube. Therefore using the method of testing using concrete cubes, the tested compressive strength should be compared to the second number.
Once the cubes have reached failure, the shape of the cube has been altered due to the compression. The failure shape can indicate whether its a satisfactory / unsatisfactory failure. The image below shows the various failures of a cube as show in BS EN 12390-3:2009.
Concrete cube testing as with all methods of testing, fresh / hard concrete are governed by standards set by the British Standards Institute and or the client in-house specifications. These standards specify all aspects involved in the process of carrying out tests, from the equipment to the method of testing.
We cant stress enough the value of carrying out cubes tests on concrete within the construction industry. Not only does it verify compliance with design soon after construction but also can save time and costly investigations later if things go wrong.
From the editor: We have tried to make sure the above article is as accurate and up-to-date as possible. If you think we have something wrong, or you feel we need to update it, please get in touch here.
Structural loads, structural analysis and structural design are simply explained with the worked example for easiness of understanding. Element designs with notes and discussions have added to get comprehensive knowledge. Also, construction materials, shoring system design, water retaining structures, crack width calculations, etc. have discussed in addition to other aspects.
The compressive strength of the concrete and concrete testing is must known information in structural design. Compressive strength is initially checked by doing mix design to make sure the grade of concrete considered in the structural design is achieved. Concrete cube testing or cylinder testing is done to check the development of the strength of the concrete.
Based on the test results, conformity can be checked as per the relevant standards. In this article, we discuss methods provided in BS 5328 and BS EN 206-1. The following areas are covered in this article.
More importantly, testing cubes are kept under the water until they are testing. Therefore, they get maximum curing. However, during the construction concrete not cured mostly the dates specified above. Therefore, the expected strength could not be developed in the actual condition within 28 days.
Further, there are certain risks when the test cubs achieve the required strength marginally as at the site it could not have developed due to the lack of curing. Therefore, due care shall be made when checking the test cube results.
As discussed above, the time frame for the testing will vary project to project though generally they are tested in 7 days and 28 days. Concrete testing is done to make sure that the assumed characteristic strength during the design.
Compressive strength is the parameter that represents the concrete in the structural design. Mainly, there are two materials such as concrete and steel in the mix. Therefore, knowing the compressive strength is uttermost important for the designer.