What to do if concrete cubes failed after 28 days , Such
Concrete Cube Compressive strength :
The Concrete cube test provides details about all the characteristics strength of concrete. From result of this cube test, we can judge whether Concreting has been done properly or not. The strength of concrete for general construction varies according to grade of concrete from 15 MPa (2200 psi) to 30 MPa (4400 psi) and higher in commercial and industrial structures.
Compressive Strength Definition:
The Compressive strength is the ability of material (concrete) or structure to carry the loads on its surface without any crack or deflection.
Compressive Strength Test of Concrete Cubes:
For Checking compressive strength of concrete two types of specimen samples are used. Cube sample size 15cm X 15cm X 15cm or 10cm X 10cm x 10cm depending upon the size of aggregate are used. Generally, we used molds of size 15cm x 15cm x 15cm.
During Concreting work random concrete sample is poured in the mold and tempered properly so as not to have any voids. On next day, concrete from these moulds are released and test specimens are put in water for curing. For Cube test it is essential to make the top surface of this specimen should even and smooth. It should be done by putting cement paste and spreading smoothly on the whole area of the specimen.
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Testing of Cube Samples:
Concrete cube samples area tested on compression testing machine after 7 days curing or 28 days curing. The application load is gradua at the rate of 140 kg/cm2 per minute till the Specimens fails.
The compressive strength of the concrete cube is the load applied at the point of failure to the cross-section area of the face on which load was applied
Compressive strength of concrete can be calculated from following Equation for (15 cm x 15 cm x 15 cm):
Compressive Strenght = Load at failure in Machine(N) / C/S Area of Cube (mm²)
Strength of Concrete at Various Ages:
|Age of concrete||Compressive strength Gain (%)|
|3 days||30 %|
|7 days||67.5 % (70% for safety)|
|14 days||90 %|
|28 days||99 % (70% for safety)|
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Concrete Strength of Cube at 7 and 28 Days Test:
|Grade of Concrete||Minimum compressive strength N/mm2 at 3 days (30%)||Minimum compressive strength (N/mm2) at 7 days (70%)||Specified characteristic compressive strength (N/mm2) at 28 days(100%)|
|M – 15||4.5||10||15|
|M – 20||3||13.5||20|
|M – 25||7.5||17.5||25|
|M – 30||9||21||30|
|M – 35||10.5||21||35|
What to do if Concrete Cubes Failed After 28 days:
The first reaction is ” Prevention is better than cure”- The failure of the Cube test depends up on the various aspect even the way it has been tested is also important. The sampling for the cube is also very important.
The Failure result of a concrete cube is a serious issue. You need to analyze why the cubes have failed. By finding out a proper reason for failure, we may avoid further reoccurrences of the same. concrete cubes failed after 28 days
When we are doing concreting on site and collecting Sample for the test there are four possible combinations behind Concrete Cube fails in Test as listed below.
1. It may be, Concrete is strong; cube is also strong.
2. It may be, Concrete is weak; cube is also weak.
3. It may be, Concrete is strong; but
4. It may be, Concrete is
Generally, we focus on the first two possibilities while analyzing cube test results, the third and fourth possibilities are generally being neglected. The 3rd option does not harm the structure. But the option four….well…it is better to be optimistic in such situations.
While you get reports showing that the 28th day cube tests are failed, remember that it is something serious. This test result of 28th days cannot be treated just like in the case of 7th day test failure. As we know that must attain it 99 % strength on 28th day cube test result and tests are considered as the deciding criterion for acceptance of the concrete.
Before considering that our 28th day cube test is fail or pass and going deep into our scenario of cube failure, let us quickly see how the cube results are generally interpreted as per IS code.
There are two conditions are to be met for accepting a concrete after cube test as per IS 456 2000 Cl 16.1,
For concrete of grade M – 20 and above,
- The average strength of a group of four consecutive test results should not be less than the greater of fck(20)+ 4 N/mm2 or fck+0.825σ where σ is the standard deviation established
- Individual cube strength should not be less than fck – 4 N/mm2
Let us analyze this with an example.
If 28th days concrete cube test results fails check if result of all 7 days cubes is the same. If even one passes the acceptable criteria you are sure that the concrete mix is OK.
- Now the question of workmanship.
- Even if one of the cubes is
OK waitfor 28 Day result.
- If 28 days result fails then again there is a possibility that cubes are not filled properly or they were not cured properly in first 7 days. But it is possible that slab concrete is OK. Go for Hammer test.
hammertest is also as per failed cube result then go for core cutter and test it to get actual strength of laid concrete
- Take this core cutter result to consultant and he will take a decision whether he can retain the concrete with rectification like grouting and all. If not then
weatherhe can find ways to strengthen the slab. If not then he has to get the slab recast.
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We must consider following before any decision is taken by site engineer on discarding concrete:
- Whether cubes filled by trained supervisor?
- Whether the strength is unacceptable as per the acceptable formula?
- if so whether they satisfy the designer with corrective methods as suggested and NDT test results received from labs?
- Still doubtful go for
In many cases, I have experienced horrible results by few labs but the site engineer should have his own judgment about place of concrete where failed and taking risks. Contractor shall always happy if reduced rates are paid for such works but always note any failures needs your pre action reports as suggested above and that only a site person can decide to discard concrete
What to do if we do not want to dismantle the concrete member:
Providing steel casing:
Providing steel casing around the structure, especially columns, to make it a composite member to improve its structural
Providing steel stiffeners:
Providing Steel stiffeners on the sides of the member to increase its load carrying capacity. The steel stiffeners are may bolted or epoxy bonded to the concrete
Providing Fiber Reinforced Polymer (FRP) sheet bonding:
Reinforced Polymer (FRP) sheet basically with glass fiber – GFRP or carbon fiber and lesser thickness are wrapped around the surface of the concrete member and bonded with epoxy adhesive. So, Concretes structural behavior is improved
Sprayed concrete / shotcreting:
Sprayed concrete / shotcreting can be done by drilling reinforcement around the periphery of the member and shotcreting is done on the surface of the member to increase its cross-section to modify the structural performance.
Attaching pre-tensioned cables to the concrete members :
Attaching pre-tensioned cables to the concrete member method is not so commonly used. In this method, we induce the effect of the pre-stressed member into the existing member.
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The concrete Jacketing involves covering the structural member at any or all sides with skillfully placed reinforced concrete. In this method, reinforcement is welded to the existing reinforcement to impart proper bonding of jacket to the existing member. Concrete Jacketing increases the cross-section area of the member and thereby improving its structural performance.
From the above Solution option, 6 would seem to be the easiest one to