Sir what should we do if our expected results of cude was not satisfied with compression test results after 28 days
Getting a low strength in concrete cube test can have many root causes for it, some of them are listed below
1. Cement composition - if there is change in C2S and C3S - it may have its effect on strength.
2. GGBS and Fly Ash -sometimes this may cause delay strength gain.
3. Water cement ratio - if moisture correction not done correctly - it may permanently reduce the strength of the concrete.
4. Admixture dosage and performance - admixture in some cases can also directly affect the strength of the concrete. mainly when admixture performs badly in giving workability, adding water to gain the workability can permanently reduce the strength.
5. Water - Hardness of water can also cause reduction in slump - but it may not harm up to that extent.
6. Sampling of concrete - if sampling done wrongly for concrete cube casting - it may give wrong results.
7. Cube casting activity - if cubes are not cast correctly - it going to show wrong results.
8. Method of concrete sampling and cube casting - it should be followed correctly - if there is deviation in cube test results of 3 specimen casted from single sample tested at any age is more than 15% of average - the results of tests are not acceptable - deviation above 15 % directly indicate the cube casting not done correctly.
9. Do also check the weight of the cube and compare its density with concrete density as per your mix design, if there is more deviation it also indicates the sample of the concrete is manipulated in fear or knowingly.
10. Temperature - it also plays roles in strength gain process, sometimes the temperature also leads to delay in strength gain process.
If the mix you used is having cementitious content like fly ash or ggbs, there are more likely chances of getting higher strength at lateral age, so if strength is not achieved in 28 days, you can test further at 35, 56 days.
The ultimate decision making factor is only concrete cube strength.
Rebound hammer wont predict accurate strength of concrete as its only measure of surface hardness.
UPV also wont make any prediction about concrete strength it only predict how dense is concrete as per the velocity of ultrasonic wave recorded during test.
Core test may give doubtful results and requires more coring which is also not a good option.
The prediction of the strength starts from early age of concrete testing.
We do testing of cubes at 3 or 7 days for de-shuttering of slab and beam formwork
at 14 or 21 days for long span beams.
at 28 days for acceptance criteria for concrete strength.
Low results are recorded in early age tests too, if there is low strength at 28 days.
if in early age strength is shown and in later age it is not achieving the desired strength - it directly indicate the problem with cement used for making concrete.
When there is low strength observed in concrete cubes at early age, immediately following things should be checked and verified;
1. Calibration of batching plant in case batching plant is used for production
2. Calibration of weight batcher in case weight batcher is used for production.
3. Water absorption and specific gravity of aggregates - compare with one which are used for designing of concrete.
4. Admixture performance test - compare the results of trial with original results of design mix for slump and retention time.
5. Do check for fineness of sand used for production of concrete and compare it with original design parameters.
6. Also do send all materials for third party testing - (do not mention the grade and type of cement when you send it to third party testing, you can always send sample without mentioning its details - this ensure testing is done by third party laboratory. you may send OPC 53 grade sample by mentioning OPC43 on sample to cross check weather the third party lab really do testing or not)
7. Do check pH value of Admixture used for concrete - pH value may decrease if admixtures are stocked in direct sunlight for long time and can become acidic.
No come to analysis
DO check weights of cubes and strength
is there difference observed in cube weight - indicates problem with either cube casting or variation in material properties.
is there difference observed in cube strength by more that 15% of its average - do ensure all three specimen tested are of same sample i.e. - from one sample of concrete 3 cubes should be casted and you can test any sample of that day concrete at any age. mixing of cubes from different samples for testing does not help in predicting what was wrong.
if the one sample is tested at one age - if there is difference of more than 15% - clearly indicates cube casting is not done correctly.
Once you found that there is no variation in cube weights and variation in strength of sample is within 15% of its average strength - there is no error in casting of cubes and sampling of concrete.
Now you can proceed further to analyze the issue as problem in concrete
Look for the previous records of concrete and observe the strength gain curve at 3 7 28 days or at 7 14 21 28 days.
As i said earlier, the ultimate reliable thing is concrete cube, you should test the concrete cube samples at further ages of concrete to check its gaining strength or not.
After doing all analysis and collecting all data, you should discuss this issue with your Structural consultant.
Concrete may be acceptable at lower strength up to some extent depending on various parameters for which structural consultant does calculations.
The final call on acceptance or rejection of structural member is taken by Structural consultant, when the desired concrete strength is not achieve.
If it fails on all front, there are some techniques which can be implemented for strengthening of weak concrete structure like injection grouting, carbon wrapping, sandwiching by steel plates, jacketing of columns with micro concrete and there are lot of techniques in market but all are costlier.
When you observed the strength of concrete achieved is less than required at early ages, you can keep curing concrete in structure for longer period like till 28 days or more.
Bad concrete can become good if cured well and good concrete can become bad if not cured well.
Low results are something which are indicating from early age testing and many precautions can be taken on it.
Low results in 28 days when cubes passed early age testing indicates problem with cement used for concrete production.
The thing we can do is wait for further age testing of concrete cubes to achieve strength.
do expensive treatment for strengthening of weak concrete structure.
Demolish the concrete in early age when it is indicating bad strength and the problem is identified as there is issue in quality of material used for production of concrete or issue in production process such as calibration of weighing equipment.
There is lot to write on this topic as it comes with endless possibilities in every aspect of it from material quality to placing and curing. Unless the data is available with us we can not make any statement on what should be done after getting low results in 28 days testing.
