Sampling of Freshly Mixed Concrete at the Field (BS EN 12350-1:2009)

5
Sampling of fresh concrete at site.

This British Standard specifies two procedures for sampling fresh concrete which are implemented by composite sampling and by spot sampling.
Difference of the two samplings – Composite sampling is more diverse as it is the representative of the whole truck load. Scoopfuls are taken from four different parts of the load to form the standard sample. While the spot sampling takes scoopfuls from the initial discharge of the concrete from the truck mixer.
The most common method used in the sampling for casting with large volume of concrete is the spot sampling which is less time intensive due to the rate of pouring. In some big projects, imagine the time consumed to perform the composite sampling when there is a lot of concrete truck mixers in queue at the checkpoint area. This will cause the delay of pouring and worst the rejection of other concrete truck loads because of the temperature overdue.

SAMPLING FROM TRUCK MIXER
Steps Taken During The Composite Sampling:
Clean all the apparatus prior to use.
Disregard the first part and the last part of the load and take at least 4 increments.
Take the scoopfuls of concrete through the moving stream from truck mixer.
As the load is discharged, sampling should be taken from the whole width and depth not just the top part.
• Let the first concrete go.
• Take a scoopful from part 1
• Take a scoopful from part 2
• Take a scoopful from part 3
• Take a scoopful from part 4
• Let the last concrete go.
The elapsed time shall not exceed 15 min. between obtaining the first and final portions of the composite sample.
Record the date and time of sampling.
Quantity at least 1.5 times the quantity estimated as being required for the tests to be performed.
Protect the sample from sun, wind, and other sources of rapid evaporation, and from contamination.
Steps Taken During The Spot Sampling:
Clean all the apparatus prior to use.
Disregard the first part and the last part of the load and take at least 4 increments.
Take the scoopfuls of concrete through the moving stream from the truck mixer.
• Let the first concrete go.
• Take the scoopfuls from the initial discharge of the concrete.
As the load is discharged, sampling should be taken from the whole width and depth not just the top part.
The elapsed time shall not exceed 15 min. between obtaining the first and final portions of the spot sample.
Record the date and time of sampling.
Quantity at least 1.5 times the quantity estimated as being required for the tests to be performed.
Protect the sample from the sun, wind, and other sources of rapid evaporation, and from contamination.
Report from the sample includes:
Each sample shall be accompanied by a report from the person responsible for taking the sample. The report shall include:
a.identification of the sample;
b.type of sample (composite or spot);
c.description of where the sample was taken;
d.date and time of sampling;
e.any deviations from the standard method of sampling;
f. a declaration by the person technically responsible, that the sample was obtained in accordance with this standard, except as noted in item
The report can include:
g. ambient weather and weather conditions;
h.temperature of the concrete sample.
MEASURING TEMPERATURE OF FRESHLY MIXED CONCRETE (ASTM C1064:2012)

6.jpgTaking the temperature of fresh concrete at site

Scope
This standard covers the determination of the temperature of freshly mixed hydraulic-cement concrete.
It may be used to verify conformance to a specified requirement for the temperature of concrete.
Apparatus
Container, large enough to provide at least 75 mm of concrete in all directions around the sensor of the temperature measuring device, or at least three times the NMAS (Nominal Maximum Aggregate Size), whichever is greater.
Temperature Measuring Device, TMD, capable of measuring the temperature to ± 0.5 º C throughout a range of 0 º C to 50 º C.
Calibration of TMD
Reference Temperature Measuring Device
Accurate to ± 0.2 º C
Certificate or Report that verifies the accuracy shall be available in the Laboratory
Annually, or whenever there is a question of accuracy
As per project specifications requirement
Performed by comparing the readings at two temperature at least 15 º C apart.
Sampling Concrete
It is acceptable to measure the temperature in:
• Transporting Equipment
• Forms
If the transporting Equipment or placement forms are not used as the container, a sample shall be prepared in accordance with ASTM C172 / BS EN 12350-1:09.
Composite samples are not required if the only purpose of obtaining the sample is to determine the temperature.
Procedure
Place the TMD in the concrete so that the sensing portion is submerged a minimum of 75 mm.
Gently press the concrete around the TMD at the surface of concrete so that the ambient temperature does not influence the reading.
Leave the TMD in the concrete a minimum of 2 min.
Complete the temperature measurement within 5 min except for concrete containing the coarse aggregate of a nominal maximum size greater than 75 mm which may require up to 20 min for the transfer of heat from the aggregate to the mortar.
Do not remove the device from the concrete when reading the temperature.
Record the temperature to the nearest 0.5 º C.
SLUMP OF FRESH CONCRETE (BS EN 12350-2:2009)
Scope
This test method covers the determination of slump of hydraulic-cement concrete, both in the laboratory and in the field.
Significance and Use
The slump test was originally developed to provide a technique to monitor the consistency of unhardened concrete.
This test is considered applicable to:
Plastic concrete (slump greater than 10 mm and less than 210 mm).
Concrete having coarse aggregate up to 40 mm in size.
Summary of test:
A sample of concrete is placed in a mold, shaped as the frustum of a cone, and compacted.
The mold is raised and the concrete allowed to subside.
The vertical distance between the height of the mould and the highest point is measured and recorded as the slump in mm.
Apparatus
Mold

slump cone.jpg
Mold use for slump test of fresh concrete (Dimension in mm)

Metallic, Frustum of a cone
Alternative materials are allowed if requirements
are met (comparability tests)
Tamping Rod
Round, straight steel rod, length 600mm and diameter 16mm
Tamping end rounded to a hemispherical tip
Measuring Device (Ruler)
Marked in increments of 5 mm or smaller
Length of at least 300 mm
In addition, a scoop is needed.
Procedure for Slump Test

4.jpg
Apparatus being readied for slump test activity

Fill the mold in three layers of approximately equal height.
Move the scoop around the perimeter of the mold to ensure even distribution of material and minimize segregation.
Rod each layer 25 times using the rounded end of the tamping rod.
Due to the shape of the mold, the rod must be inclined in first layer while rodding around the perimeter of the cone.
Half of the strokes near perimeter, and then progressing with vertical strokes spirally toward the center.
Rod the bottom layer throughout its depth.
Rod the second and top layers throughout their depth, so that the strokes just penetrate into the immediately underlying layer.
If the tamping operation of the top layer results in subsidence of the concrete below the top edge of the mould, add more concrete to keep an excess above the top of the mould at all times.
Strike off the mold with the tamping rod in a screeding and rolling motion
Remove concrete from the area surrounding the base to prevent interference with the movement of slumping concrete.
Raise the mold carefully in a vertical direction with no lateral or torsional motion
Raise the mold in 2 to 5 sec.
Complete the entire test without interruption, from the start of filling to removal of the mold, without interruption within 2 ½ minutes.
Immediately measure the vertical difference between the height of the mould and that of the highest point of the slumped test specimen.
If a falling away or shearing occurs, disregard test and make a new test using another portion of the sample.

MAKING AND CURING CONCRETE TEST SPECIMENS IN THE FIELD (BS EN 12390-2:2009)

1
Mould for concrete cubes

Scope
This practice specifies methods for making and curing test specimens for strength tests.
Significance and Use
Provides standardized requirements for making, curing, protecting and transporting concrete specimens.
Apparatus
Cube molds for compressive strength and/or Beam molds for flexural strength testing
Compacting bar (Square 25x25x380mm)
Compacting Rod (Circular 16x600mm)
Vibrator, if necessary
Mallet
Small Tools (shovel, scoop…)
Sampling & Testing
Obtain a sample of concrete in accordance with BS EN 12350-1:2009 and shall be remixed in the container.
Minimum size of sample x 1.5 times the needed quantity for making specimens
Record the identification of the concrete with respect to:
The Location of concrete represented
The Time of casting
Compacting Methods
Internal Vibrators that will be used should have the following specs:
Frequency shall be at least 9000 vibrations per minute
The diameter of the internal vibrator not exceeding one-quarter of the smallest dimension of the test specimen
Vibrating table with a minimum frequency of 40 Hz (2 400 cycles per minute)
Compacting rod or bar
Preparation and filling of the moulds
Before filling the mould, cover the inner surface of the mould with a thin film non-reactive release material to prevent the concrete from adhering to the mould.
Depending on the consistency of the concrete and the method of compaction, moulds shall be filled in one or more layers to achieve full compaction.
In the case of self-compacting concrete, the mould shall be filled in one operation and no mechanical compaction shall be applied during filling or after the mould is filled.
Compaction of the concrete
The concrete shall be compacted immediately after placing in the moulds in such a way as to produce full compaction of the concrete with neither excessive segregation nor laitance.
Full compaction is achieved using mechanical vibration, when there no further appearance of large air bubbles on the surface of the concrete and the surface becomes relatively smooth with a glazed appearance, without excessive segregation.
Ways of how the sample shall be vibrated:
Mechanical vibration (using Compacting with an internal vibrator)
Apply the vibration for the minimum duration necessary to achieve full compaction of the concrete.
Avoid over-vibration, which may cause loss of entrained air.
Mechanical vibration (with vibrating table)
Apply the vibration for the minimum duration necessary to achieve full compaction of the concrete.
Avoid over-vibration, which may cause loss of entrained air.
Compacting by hand with compacting rod or bar
Distribute the strokes in a uniform manner over the cross-section of the mould.
Ensure that the compacting rod, or bar, does not forcibly strike the bottom of the mould when compacting the first layer, nor penetrate significantly any previous layer. Subject the concrete to a sufficient number of strokes per layer, (typically 25).
After compaction of each layer, tap the sides of the container smartly with the mallet until large bubbles of air cease to appear on the surface and depressions left by the compacting rod or bar, are removed.
Surface leveling
Remove the excess concrete above the upper edge of the mould using the steel trowel or float and carefully level the surface.

3
Concrete cubes at the site

Marking
The test specimens shall be marked clearly and indelibly, without damaging the specimen using a cub tag that contains the information related to the specimen (Casting date, ticket number, specimen ID, location of sampling…).
Records shall be kept to ensure the traceability of the specimen from sampling to testing.
Curing of test specimens
Leave the test specimens in the mould for at least 16 hours, but not longer than 3 days, protected against shock, vibration and dehydration at a temperature of (20 ± 5) °C (or (25 ± 5) °C in hot climates).
After removal from the mould, cure the test specimens till immediately before testing, in water at a temperature of (20 ± 2) °C, or in chamber at (20 ± 2) °C and relative humidity ≥ 95 %.
Transport of test specimens
Avoid loss of moisture and deviations from the required temperature at all stages of transport.
Pack the hardened test specimens in wet sand or wet sawdust or wet cloths (wet burlap), or sealed in plastic bags containing water.

What is the frequency of Sampling of Concrete?
It depends upon the project specifications on particular project area of jurisdiction.
In Qatar where Qatar Construction Specifications 2010 (QCS 2010) governs, it states in Section 5 – Concrete Part 6.4 Property Requirements – Quality and Testing Clause 6.4.2.4 – Slump tests (and VSI testing when applicable) shall be carried out at the rate of one test per load of concrete delivered to the Site, or one test per 10 m³ whichever is the lesser for the first 50 m³ of concrete then at a rate of 1 slump test for every 50 m³ if concrete was consistent during production. In the event of inconsistent slump values, the Engineer may instruct the Contractor to check the slump test on each truck of concrete at the plant. The Contractor shall carry out an investigation to establish the cause of the high variation in slump and shall take any necessary corrective measures. The slump requirements for the fresh concrete are to be approved by the Engineer depending on location and exposure and it shall be recorded on the batch ticket for each delivery.

Insight for QA/QC Engineer

 (Excerpt from 200 Questions and Answers on Practical Civil Engineering Works – Vincent T. H. Chu)

If on-site slump test fails for freshly mixed concrete, should engineers allow the contractor to continue the concreting works? 

This is a very classical question raised by many graduate engineers. In fact, there are two schools of thought regarding this issue.

The first school of thought is rather straightforward: the contractor fails to comply with contractual requirements and therefore as per contract the engineer could order the suspension of the Works. Under the conditions of Contract, the contractor is not entitled to any claims of cost which is the main concern for most engineers.
This is the contractual power given to the Engineer in case of any failure in tests required by the contract, even though some engineers argue that slump tests are not as important as other tests like compression test.

The second school of thought is to let the contractor continue their concreting works and later on request the contractor to prove that the finished works comply with other contractual requirements e.g. compression test. This is based upon the belief that workability is mainly required to achieve design concrete compression strength. In case the compression test also fails, the contractor should demolish and reconstruct the works accordingly. In fact, this is a rather passive way of treating construction works and is not recommended because of the following reasons:
(i) Workability of freshly placed concrete is related not only to strength but also to the durability of concrete. Even if the future compression test passes, failing in slump test indicates that it may have an adverse impact on the durability of completed concrete structures.
(ii) In case the compression test fails, the contractor has to deploy extra time and resources to remove the work and reconstruct them once again and this slows down the progress of works significantly.

Hence, in view of such likely probability of occurrence, why shouldn’t the Engineer exercise his power to stop the contractor and save these extra time and cost?

What is the time limit before placing of fresh concrete?

There is often debate about the time limit between placing fresh concrete against fresh concrete already placed and compacted i.e. minimizing the risk of a cold joint being formed.
Whereas also the QA/QC Inspectors are confused about the prescribed time limit about the placement of concrete based from the allowed period stated from the specifications.

BS 8500-1 Concrete – Complementary British Standard to BS EN 206, clause A.9.1 takes a more flexible approach in stating:
“Concrete should be placed into its final position as soon as practicable. Extended delays in placing can lead to a significant loss of consistency with time making it impossible to place with satisfactory compaction. Where there is an extended delay between the placement of one layer of concrete and the following layer then there may be a cold joint between layers. For this to occur the delay should be sufficient for the lower layer to develop an initial set or harden. The time for this to occur depends on the concrete temperature, cement type, mix proportions, the use of admixtures with set retarding properties and consistency of the concrete.”
BS 8500-2 requires concrete to be delivered within 2 h after the time of loading where transported in truck mixers or agitators or within 1 h after the time of loading where non-agitating equipment is used unless a shorter time is specified or a longer time permitted by the specifier. These limits are sufficient for normal UK temperatures.

Where required, the specification of a maximum time between placing layers of concrete should take account of the likely temperature of the concrete, cement type, mix proportions, the use of admixtures with set retarding properties and consistency of the concrete.”

Ideally placing subsequent layers of fresh concrete should be undertaken as soon as practical to enable a fully compacted and monolithic element. The time a surface remains ‘open’ is not easily defined without taking account of:
• the time the concrete has taken to be discharged after batching
• the ambient temperature
• the cement type
• the admixture(s) used
• the time it takes to be compacted.

But still, all of these pose hanging question from the QA/QC Inspectors where during casting there is a chance that they experience concrete beyond the allowable limit of placement from specifications.

Actually beforehand, the solution should be discussed at pre-construction meetings so all parties agree under what conditions the waivers for the discharge time limit can be applied. Usually, especially for big projects where a large quantity of concrete is cast, both parties come up with the designed mix and validated through trial mixes to compensates for this particular problem. After this, the design mix specific to the particular type of structure recommends the discharge time limit waived against what is stated in the specifications. Both parties will agree to the specific time limit as substantiated from the concrete design mix.
Now, if the placement of concrete goes beyond these waived time limit, the concrete should be rejected.

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3 thoughts on “Sampling of Freshly Mixed Concrete at the Field (BS EN 12350-1:2009)

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  1. Thank you for another wonderful post. The place else may anybody get that type of info in such a perfect means of writing? I have a presentation subsequent week, and I am on the look for such info.

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