What To Do If Concrete Cube Fails After 28 days

What To Do If Concrete Cube Fails After 28 days

What To Do If Concrete Cube Fails After 28 days, Such a situation arises many times on the construction site. This article explains to you what to do and not do during such type of situation on the construction site.

So, What To Do If Concrete Cube Fails? There are many repair and rehabilitation options if the concrete cube fails in the test

Cube Test of Concrete

The Concrete cube test provides details about all the characteristics and strengths of concrete. From the 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 the grade of concrete from 15 MPa (2200 psi) to 30 MPa (4400 psi) and higher in commercial and industrial structures.

Compressive Strength Definition

“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 the 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.

Sample Preparation

During Concreting work random concrete sample is poured into the mold and tempered properly so as not to have any voids. On the next day, concrete from these molds is released and test specimens are put in water for curing.

For the 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.

CONCRETE CUBE FAILS IN TEST

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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 gradual at the rate of 140 kg/cm2 per minute till the Specimens fail.

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 the load was applied.

The compressive strength of concrete can be calculated from the following Equation for (15 cm x 15 cm x 15 cm):

Compressive Strength =   Load at failure in Machine(N) / C/S Area of Cube (mm²)                        

Strength of Concrete after 28 Days?

The concrete achieves a total strength of 99%age of the compressive strength after 28 days, also the rate of compressive strength of various concrete grades, the worth will vary from grade to grade.

Suppose the concrete cube of grade M25 of size 15cm x 15cm x 15 cm compressive strength of around 25 N/mm2.

Strength of Concrete at Various Ages

It is a known fact that concrete strength increases with age. Below shows the strength of concrete at different ages in comparison with the strength at 28 days after casting.

• concrete cube test results- 28 days
• concrete cube test results- 7 days as per is code
• 3 days concrete cube strength

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 Cube Test Fails In Test After 28 days

• The first reaction is ” Prevention is better than cure”- The failure of the Cube test depends upon 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 cube fails in test after 28 days
• When we are doing concreting on site and collecting Samples for the test there are four possible combinations behind Concrete cube fails in a test as listed below.

1. It may be, Concrete is strong; the cube is also strong.

2. It may be, Concrete is weak; the cube is also weak.

3. It may be, Concrete is strong; but the cube is weak.

4. It may be, Concrete is weak, but the cube is strong.

• Generally, we focus on the first two possibilities while analyzing cube test results, the third and fourth possibilities are generally neglected.
• The 3^rd option does not harm the structure. But 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 the 28th day cannot be treated just like in the case of the 7th-day test failure.
• As we know we must attain 99 % strength on the 28th-day cube test result and tests are considered the deciding criterion for acceptance of the concrete.
• Before considering that our 28th-day cube test is failed or passed 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,

1. 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 fail check if the 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.

1. Now the
2. question of workmanship.
3. Even if one of the cubes is OK wait for a 28 Day result.
4. If 28 days result fails then again there is a possibility that cubes are not filled properly or they were not cured properly in the first 7 days. But it is possible that slab concrete is OK. Go for the Hammer test.
5. If the hammer test is also as per failed cube result then go for the core cutter and test it to get the actual strength of laid concrete
6. Take this core cutter result to the consultant and he will make a decision whether he can retain the concrete with rectification like grouting and all. If not, then whether he can find ways to strengthen the slab. If not, then he has to get the slab recast.

We must consider the following before any decision is taken by the site engineer on discarding concrete

• cubes filled by a 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 loading test?
  In many cases, I have experienced horrible results from a few labs but the site engineer should have his own judgment about the place of concrete failure and taking risks.
• The contractor shall always be 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

Concrete Cube Test Failure Reasons

The following are the major concrete cube test failure reasons,

1) Wrong Mix Poured: The concrete mix prepared is not as per the approved mix design for that particular strength.

2) Wrong Mix Design: There is a mistake in the concrete mix design. It is not properly designed and its strength is not properly calculated.

3) Improper Sample Collection: Sometimes concrete sample for the cube test is not collected by a skilled worker or it may not be checked by a technical person on site.

So, there may be a possibility that the collected concrete sample is not truly representing the fresh concrete supplied at the site.

4) Improper Cube Preparation: The concrete cure must be prepared as per the guidelines given in IS 516.

The compressive strength of a concrete cube might be affected by several errors during cube casting such as,

Improper alignment of cube mold: Is resulting in distorted cube shape.

Improper/inadequate tamping: Improper tamping results in voids formation in cube sample

Improper filling of the moulds: This may result in a weak zone in concrete leaving only mortar without aggregate

5) Inadequate Curing of the Cube Samples:

The curing of the concrete cube must be as per the guidelines given in IS 516.

Once the concrete cube is prepared the cubes must be covered with wet gunny bags for the least period of 24hrs from the time when water is first added to the concrete.

Then after removed the concrete cube from the molds and kept it under clean water until the time of the test. Make sure the temperature of curing is between 24˚C to 26 ˚C.

6) Testing Wrong Samples: Properly identify the cube preparation marking on the cube. Sometimes wrong cube sample test.

7) Errors During  Testing: This includes improper placing of moulds over the CTM, cleaning the surface, seating the cube properly, rate of load application, etc.

8) Errors With the Testing Apparatus: This may include errors with reading display, loading or hydraulic systems, or a manual error in reading and interpreting the displayed value.

9) Unauthorized Water-Cement Ratio Correction of the Mix: Sometimes for making concrete easily peaceable labor adds extra water without consulting a technical person on site.

10) Change in Quality of Materials:  Sometimes the quality of concrete materials used for preparing the concrete mix on-site may be different from those used while preparing the trail mix.

11) Batching Plant Error: There may be the possibility of batching plant error wrong materials quantity added in the mix, due to any mechanical / software error, which went unnoticed.

What to do if we do not want to dismantle the concrete member

We can strengthen the weak member/members to take the originally designed load. This is more advisable than redesign since this method of modification is mostly limited to the members that are weak hence is easy to keep track of the process.

However, the suitability of this method depends on other factors also. There are many options to strengthen a weak concrete member if a concrete cube fails in the test

1. Providing Steel Casing

Providing steel casing around the structure, especially columns, to make it a composite member to improve its structural performance.

2. 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 member

3. Providing Fiber Reinforced Polymer (FRP) sheet bonding

Reinforced Polymer (FRP) sheets 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, Concrete’s structural behavior is improved

4. Sprayed Concrete / Shot crating:

Sprayed concrete/shotcrete can be done by drilling reinforcement around the periphery of the member and concreting is done on the surface of the member to increase its cross-section to modify the structural performance. 

5. 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.

Read More: Free Download Civil Codes

6. Concrete Jacketing

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 the jacket to the existing member.

Concrete Jacketing increases the cross-section area of the member and thereby improves its structural performance.

Conclusion

From the above Solution option, 6 would seem to be the easiest one to overcome the situation of Concrete Cube Fails in Tests, but there are several impediments with this method, like disposal of debris, and the practical difficulty in demolition without damaging other members.

There is also the risks to the safety of the workmen, practical difficulty in shuttering and casting new member in the original position, quality control, etc are a few hurdles to mention with this option.

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