Compressive strength of concrete in structures is a spatially varying quantity and is considered as the quality index of hardened concrete.

Core test is a partially destructive test commonly recommended for ascertaining the compressive strength of concrete in structures.

Compressive strength of core taken from structures is influenced by many factors such as location of core, size of core,  moisture condition of core, voids in the core, ratio of maximum aggregate size to core diameter, diameter of core, length/diameter ratio of core, method of drilling, direction of drilling, method of trimming, method of capping etc,

Excess voidage of concrete could be estimated by finding the potential density based on the available values for 28 day cubes of same mix.

Detailed guidelines for interpretation of results of core test  is available in " Testing of concrete in structures" , 4 th Edition  by John H. Bungey

If possible coarse aggregate sample may be tested for crushing value/impact value for confirmation with standards.

Concrete Segregation – Segregation of Concrete

Segregation generally indicates poor aggregate grading or mix design. Segregation may occur in mixes which are too wet or too dry, and most frequently in under-sanded mixes.

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https://www.theconstructioncivil.org/concrete-segregation-segregation-of-concrete/

Rebound number does not give reliable results to estimate compressive strength of concrete. The NDT method can only give an idea about concrete quailty although it shows a certain comp. strength according to its relation curve.

The beams are seen as road members. Thus, if they could be exposed to intensive CO2 and carbonate reaction might have been occured on the surface layer of the concrete members. The reaction can be increase surface hardness of concrete, and then rebound numbers of hammer and related compressive strength can be read higher than actual.

Moreover, "Size Effect" is an important point for compressive strength of cores. Last picture that is broken sample shows that Dmax of aggregate is high. So, core size of concrete is set as at least 3 times higher than the Dmax. If it is possible, cores having 150 mm diameter will give satisfactory results to reach actual values.

Relationship between hammer rebound number and compresssive stenght values is constituted for 150 or 200 mm cube strength. So, there is need a correction from cube to cylinder. For this correction related standards and reports can be used.

Additionally, if segregation is clearly observed from the hardened cross sections it can be quantified and proved by sevaral methods. 

According to segregation direction (generally concrete placement direction);

1.At least 2 layer's cross section image from top and bottom are quantifed by  an image analysis program indir terms of coarse aggregate distribution.

2. At least 2 layer's rebound and ultrasonic pulse velocity values from top and bottom are evaluated.

note:generally segregation reach highest values at the end points of concrete members such as top and bottom ends for normal consistency concrete or initial and final ends for self consolidating concete

NGLAIS 

In addition to all that has been mentioned by the  colleagues, one of the main reasons for segregation is vibration. Indeed, if the concrete is too vibrated during its pouring  or too liquid (too much water in the formulation), the aggregates with the largest diameters tend to flow and those with small diameters to go up if all the aggregates are the same density. In this case, the combined action of gravity and vibration generates the separation

Effects on Concrete

A segregated concrete is very weak in strength

Due to segregation after placing, the concrete in the lower part of a pour of any significant depth can be stronger than that in the upper part.

Segregated concrete does not give a homogeneous mass throughout the structure

Rock pockets, sand streaks and porous layers in hardened concrete are the result of segregation.

Due to segregation excess mortar comes to the top of the surface, which causes plastic shrinkage cracks.

A segregated concrete is difficult to compact properly.

Dear

1- Check the variation in specific gravity of core samples to check the homogeneity.

2- The diameter of the core should be three times the maximum size of aggregate.

3- I disagree with Sardar, the segregated concrete is not a very weak concrete.

4- NDT is suitable to check the homogeneity of concrete rather than the compressive strength.

Dada sarsam, the specific gravity of 3 core samples is almost the same (around 2.1), the astm recomendations states that the core should be at least 3 times the size of corsé aggregate, but for the beam that was samppled chosing a higer diameter was impossible due to the steel reinforcent.

i was shocked that the ndt

If the current problem is to rectify the existing concrete having suspected low strength, it is strongly suggested to undertake grouting of the seggregated concrete and obtain fresh strength after 28-days for comparative study of tests on specimen.

Segregation may occur due to  improper mix design, higher water cement ratio. Size and shapes of aggregate, cement content  also affect workability of concrete. The reasons for poor strength of concrete may be  lesser cement,  higher water cement ratio  and over compaction resulted  segregation, Nothing is mentioned about curing. Quality of cement also affect strength of concrete.  Quality control on core sample is essential. The factors affecting core  results are  well explained by Mr. Elango P. 

Segregation in concrete is definetly a problem that will affect the overall compressive strength of hardened paste. This may be as a result of poor grading of aggregates, over Vibration using Poker among other actions. Also it should be noted that results of NDT are always influenced by surface texture which may be high on meeting segregated aggregate surfaces compared to Partial destructive test using Coring Method. I presumed that the core test specimens must be from the same surfaces the rebound hammer was used? If not the Core test  result may a better reflection of the compressive Strength of the concrete as it closure to DT Method.

http://www.buk.edu.ng

Dear

Based on the photographs shown above, i suggest to take core specimen from a position perpendicular to the sides of the girder rather than the existing position

Regarding the crack, it appears that it is due to pre-stressing action and not to poor concrete

From my experiences, the most typical reasons of segregation are these:

1) the lack of filler,

2) high amout of mixing water (as high W/C ratio -> cement paste on the top),

3) high amout of superplasticizer (as Plast./C ratio -> very low density "cement water" on the top after consolidating).

4) human factor during consolidating (over-vibrating or re-vibrating the same section).

From the theory point of view, acceptable range of segregation (like bleeding) reduce W/C ratio in lower parts of fresh concrete (on the other hand, it leads to deterioration of surface layer by increasing W/C ratio in this part). Considering fracture area, from the photo of crushed sample, which pass through the aggregate grains while CS is approx. 20-30MPa, I also assume low aggregate strenght used in concrete mix.  

Yes it is always so