The crushing of concrete cubes or cylinder samples is required in a construction especially concrete structures. The reason for this is to verify the strength given in the design mix. Lets look at an example of a concrete column. Commonly, it has 60 megapascals of specified strength. When you crush it, the result should be equal to or more than 60 Mpa.
The specified strength should be achieved at a test age of 28 days but sometimes we experience failure results at 28 days. Thats the reason we have spare cubes or cylinder samples. The spares are the alternatives that you can check at 42 days or 56 days when a sample fails at 28 days test.
Now, there are many questions why we crush concrete cubes at 28 days. And you might be asking why. I was once the person asking about this until I came to know the reason. Read further and, youll find the answer later in this blog.
You also might be asking, why we crush cubes or cylinder at 7 days. In most common concrete elements, such as columns, beams, slab-on-grades, and conventional suspended slabs, we start crushing cubes at 7 days. The reason is that it is the time the cast concrete can be loaded, and we can proceed with the following construction activities, but only if the compressive test report at 7 days has passed the condition of 80 percent of the specified strength.
If a 7-day compressive test result is failed by having less strength than condition stated above, then you can use the spare cubes or cylinder samples to test at 14 days and then most probably youll get the desired result. But sample test failures rarely happen at 7 days, so 14 days test does not matter.
Concrete elements are designed on the basis of a 28-day cubes crushing strength. A 28-day cubes strength actually represents the characteristic strength of the concrete. It is mandatory to test the concrete cubes or cylinders at the age of 28 days as per almost all building code requirements.
The studies show that the characteristic strength of concrete reaches 99% at 28 days ofage. And it is said that the final strength will be achieved after 1 to 2 years. This means that a concrete slab or beam which has a specified strength of 40 megapascals or Mpa (N/mm2) shall be almost attained at 28 days of age in cubes or cylinders.
Although the ultimate strength varies with the kind of cement and curing duration, the concrete compressive strength in 28 days, is very near to its final strength that could be reached in one or two years. The usage of 28 days compressive strength for design strength is dependent on the Law of Averages, propounded by JACOB BERNOULLI.
It is believed that the 28 day consideration for the specified strength is based on the studies and test trials of the cube or cylinder sample. It is found that at 7 days you will get the 80 percent of the specified strength and at 28 days you will get 99 percent of the specified strength.
It is normal especially if you use concrete admixture and cement replacements GGBS and microsilica. If these three will work together your concrete will gather high strength at a very early age. How much more after 28days.
Im also working as QA/QC engineer in UAE. I Would like to buy some books from you but unfortunately, some errors found dear. Also, I would like to meet you so please allow me to get an appointment with you please
Sir, I have question regarding crushing of cylinders #7 or 28 days.. Why we crush cylinders @7 and 28 days??? Why not at 10 or 30 days??? As you said above that we curing the concrete at 7 days, so we tested the cylinders at 7 days if it shows the strength 80% @7 days. But my question is still the same why 7 days, why not 6 or 8 days??? What are the specific reasons??? Is there any cement, aggregates reactions or what??? Or strength gaining procedure? ?? Time frame??? Or is there heat of hydration effects up to 7 days??? Im sorry, simple saying that standard codes mentioned like that cant be satisfied? ? Bcz the codes also written by someone and it can be elaborate or explained by the research.
Please kindly improve your grammar to better understand your explanations. There are certain grammatical errors which cannot be understood by common persons. This is not to offend you but to help other readers understand simple grammatically correct English.
You can still send your spare cubes for a 56-day test and for over 65 days, you must discuss with your concrete supplier what the best thing to do is, so you come out with the best decision. You may also want to conduct coring to verify the actual concrete strength. Because sometimes cubes are not properly made, that is one of the reasons for a failed test report. I had an experience where I tested the cubes for 90 days and the results were too high, meaning the concrete would still gain strength after 3 months and beyond.
Im sorry, according to my opinion the crushing of cylinder cant give us 100% surety that our concrete is good. The crushing of cylinders only can give us some idea or some satisfaction that our concrete quality is good.. My opinion is; we should concentrate more on fresh concrete quality tests If our fresh concrete quality tests results are within limits then the crushing of cylinders if shows variation then its not a big problem. .. If cylinders fail then qe cant say I your concrete quality is not good or if cylinders are passed also we cant say our concrete quality is very good
Hi! Welcome to my blog. My name is Noel Mades and Im the author of qualityengineersguide.com. I am a Civil Engineer by profession but Ive specialized and taken the journey in the field of Quality Engineering. Read More Here.
This paper compares the use of jaw and impact secondary crushing for producing coarse recycled aggregates from concrete wastes, obtained from road pavement and demolished building materials. The crushing mechanism interferes directly with recycled aggregate properties at different levels: particle size distribution, aggregate shape, generation of micro-fractures, as well as regarding the detachment of porous hardened cement paste from particle surface in order to recover pure, non-porous natural aggregates. However, crusher selection in the recycling industry is mostly carried out by acquisition and maintenance costs, industry and manufacturer traditional habits, low cost associated with second hand equipment. It also does not consider essential parameters such as the final properties of the desired end-product. Representative samples from two recycling plants were collected after primary impact crusher and secondary crushing were performed in a controlled laboratory condition through jaw and impact crushers. The aggregates attained were characterized, demonstrating similar density, porosity, particle size distribution and content of attached cement paste. Minor observed differences do not justify the common belief in the industry that impact crushers provide an improvement in the quality of recycled aggregates due to the higher detachment of cement paste from aggregates.
Particle-size reduction, or comminution, is often required to prepare bulk material samples for testing. Crushing is often the initial step with materials such as mineral ores or aggregates using varying combinations of force, impact, and compression to initiate fracturing in the materials.