what if concrete cube fails in test

Compressive Strength

Compressive strength is defined as the resistance to failure under the action of compressive forces.  It is the basic and old method to determine the performance of the material during service conditions.

In this Concrete specimens are a casted in cubes generally, we used molds of size 150 mm x 150 mm x 150 mm and tested under the action of compressive loads to determine the strength of concrete .

compressive strength is calculated by dividing the failure load with the area of application of load, usually after 28 days of curing.

But in this article i am going to tell you what is the next procedure if your Cube test is fail

Testing of Cube Samples:

Concrete cube samples area tested after 7 days curing or 28 days .The  application load is gradualy at the rate of 140 kg/cm2 Per minute till the Specimens fails.Because its strength increases with its age :-

Age	Compressive strength in %)
3 days	30 %
7 days	67.5 %
14 days	90 %
28 days	99.3 %

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

Failure of a concrete cube is a serious issue.First you have to analyze why the cubes have failed. By assessing a proper reason for failure, we may proceed for next process

Mainly , strength of concrete for general construction varies according to grade of concrete from 15 MPa to 30 MPa

When we concreting at site , there are four possible combinations are as following :-
1. Concrete is strong; cube is also strong.

2. Concrete is weak; cube is also weak.

3. Concrete is strong; but cube is weak.

4. Concrete is weak; but cube is strong.
If 7th day cube may fail you can slow down or stop that work and wait for 28th day cube tests , if they fail .Then must be some serious problem thats why cube test Fail

we know that must attain it 99 .3 % strength on 28th day cube test result and tests are considered as the deciding criterion for acceptance of the concrete.

According to IS 456 2000  For concrete Above M20 grade

Individual cube strength should not be less than fck – 4 N/mm2

Average strength of a group of 4 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

Note :-

Generally we do Rebound hammer test when the concrete cube fails.

But rebound hammer test is only for checking the hardness of the concrete . No absolute assessment can be done with a rebound hammer.

Because , The Schmidt Hammer test is only really usefull to gather an initial indication of concrete strength, usually the average of 12 blows is taken. Carbonation of the concrete causes the surface to harden and would therefore provide misleading readings

In my opinion , If hammer test is also fail then go with then go for 
Ultrasonic test and if this also fails then go for core cutter and test it to get actual strength of laid concrete.

If you dont want to dismantle the fail test concrete structure follow the following procedure :-

It may increase the cost of construction but strengthening the weak structure to take the original designed load

Fiber Reinforced Polymer (FRP) sheet bonding:

Reinforced Polymer (FRP) sheet  are wrapped around the surface of the concrete member and bonded with epoxy adhesive. This help to strengthen the structure
The strength enhancement of concentrically loaded columns bonded with external fiber-reinforced polymer (FRP) systems arises from two sources.

First, the confinement effect of transverse fiber sheets leads to an increase in the uniaxial compressive strength of the confined concrete, resulting subsequently in an increase in the contribution of concrete to the load-carrying capacity of the column.

Second, the longitudinal fiber sheets contribute directly to the load-carrying capacity of the column.

steel casing:

Providing steel casing around the structure, especially columns, may increase the strength of structure and decreases the chance of fail .But it double the cost of building

Pre-tensioned cables to the concrete structure :

This is the mostly not used method in construction because it is so uneconomical too

In this method we attached pre-tensioned cables to the concrete member method .This is the same method like Post tensioning of structure

Sprayed concrete / shotcreting:

In this Method ,wev can be done by drilling and grout the reinforcement around the periphery of the member

And shotcrete the surface of the member to modify the structural Strength