Test on Fresh Concrete | Parameters to be Tested on Fresh Concrete

Concrete testing forms an important part of understanding the behaviour of fresh concrete and the strength of concrete in the case of hardened concrete. This blog covers important tests that are performed on fresh concrete. But first, it is important to know what are the parameters that are checked for fresh concrete and the same is explained further.

Parameters to be Tested on Fresh Concrete

Fresh concrete as the name implies is the freshly prepared concrete that is yet to harden and take a permanent shape and size. Some parameters that are needed to be tested for fresh concrete are mentioned below.

• Workability

• Segregation

• Bleeding

• Setting time of concrete

An explanation for these terms and tests to evaluate these parameters are discussed below.

Workability of Concrete

A concrete is said to be workable if it has enough lubrication for handling the concrete without segregation, for placing without loss of homogeneity, for compacting with the amount of effort forth-coming and to finish it sufficiently easily. Workability is different from consistency as consistency only indicates fluidity (mobility) and concrete with high consistency need not be of right workability for a particular job.

Every job requires concrete of particular workability. For instance, concrete that is workable for thin sections is not workable for thick sections concreting. Therefore, workability is a broader term comprising of the type of work, the thickness of the section, the extent of reinforcement and mode of compaction.

Tests performed

• Slump test

• K-slump tester

• Compacting factor test

• Flow test

• Flow table test

• Kelly ball test

• Vee Bee consistometer test

Segregation of Concrete

Segregation is the separation of constituent materials from concrete. For instance, if coarse aggregate separates out from the rest of the ingredients, then the concrete is said o exhibit segregation. Therefore, good concrete is one in which all the ingredients are properly distributed to make a homogeneous mixture.

Tests performed

• There are no specific tests to measure segregation. But it can be easily observed visually during concreting operation.

• The pattern of subsidence of concrete in the slump test or the pattern of spread in the flow test indicates the quality of concrete with respect to segregation.

Bleeding of Concrete

It is a form of segregation, in which some of the water from the concrete comes out to the surface of the concrete. It is predominantly observed in a highly wet mix. Bleeding can be reduced by proper proportioning and uniform and complete mixing. Though early bleeding i.e., when the concrete is fully plastic, is not harmful, delayed bleeding i.e, when the concrete has lost its plasticity, poses severe harm to the concrete.

Tests performed

• Test to estimate the bleeding water percentage

Setting Time of Concrete

The setting time of concrete varies widely from the setting time of cement. The setting time of concrete depends upon the w/c ratio, use of mineral admixture, and use of plasticizers. But, similar to setting time of cement, setting time of concrete also has two components i.e., initial setting time (time elapsed between the moment water is added till the time when the mix starts losing its plasticity) and final setting time (time elapsed between the moment water is added till the moment when the mix loses all its plasticity and begins hardening).

Tests performed

• Penetrometer test

Now that we have covered parameters to be tested, let's now see the tests to be performed in detail.

Test on Fresh Concrete

Slump Test

It is the most common test performed on fresh concrete used to measure consistency. Though it does not measure all the factors contributing to workability, it is used as a control test as it gives an indication of the uniformity of the concrete from batch to batch. Information on the workability and quality of concrete can be obtained by observing the manner in which concrete slumps.

• It can be performed on both field and lab

• It is not suitable for very wet or dry mixes (for very dry mixes compacting factor test is recommended and for very wet mixes flow test is recommended)

Apparatus

The apparatus required for the slump test is shown below. Other than this, a steel tamping with bullet end is used for tamping.

Slump Cone Apparatus

Procedure

1. The surface of the mould is cleaned and freed from moisture and adherence of any old concrete

2. Mould is then placed on a smooth, horizontal, non-absorbent surface

3. The mould is then filled with concrete in 4 equal layers with each layer being tamped 25 times

4. After tamping the top layer, the concrete is struck off level with a trowel

5. The mould is raised slowly and carefully in a vertical direction immediately after filling with concrete

6. Concrete subsides and this subsidence is called the slump of the concrete

7. The difference in height of the mould and that of the highest point of the subsided concrete is measured in mm and is called the slump value of concrete

8. The pattern of slump indicates the quality of concrete. There are three slump patterns.

   1. True slump - slumps evenly

   2. Shear slump - one half of the cone slides down - indicates non-cohesiveness

   3. Collapse

Types of Slump

Compacting Factor Test

Compacting test is more precise and sensitive than a slump cone test. This test works on the principle of determining the degree of compaction achieved by a standard amount of work done by allowing the concrete to fall through a standard height. Then the degree of compaction called the compacting factor is measured by the density ratio i.e., the ratio of the density actually achieved in the test to the density of the same concrete when fully compacted.

• This test was designed as a laboratory test but can be used in the field as well

• This test is designed for concrete of very low workability i.e., dry mix

Apparatus

The apparatus is shown in the diagram below.

Compacting Factor Test

Procedure

1. The concrete is placed in the upper hopper up to the brim

2. The trap door is opened so that the concrete falls into the lower hopper

3. Then the trap door of the lower hopper is opened and the concrete is allowed to fall into the cylinder. Slight poking shall be used to set the dry concrete in motion

4. Excess concrete above the top level of the cylinder is removed and the outer surface of the cylinder is wiped

5. Concrete along with the cylinder is weighed and this weight is called the weight of the partially compacted concrete

6. The concrete is removed from the cylinder and is refilled with concrete from the same sample in layers with each layer being heavily rammed or vibrated

7. Excess concrete above the top level of the cylinder is removed and the outer surface of the cylinder is wiped

8. Concrete along with the cylinder is then weighed and this weight is called the weight of fully compacted concrete

9. The compacting factor is then calculated as, compacting factor = weight of partially compacted concrete/weight of fully compacted concrete

The compacting factor is then related to the workability of the concrete i.e., a value of 0.78 to 0.8 represents low workability and a value of more than 0.95 represents high workability. Compacting factor test measures all the contributing factors to workability and therefore it is one of the good tests to depict workability.

Flow Table Test

Flow test gives an indication of the quality of concrete with respect to consistency, cohesiveness and the proneness to segregation. In this test, concrete is subjected to jolting and then the spread of concrete is measured and this flow is related to workability.

• This test is performed on laboratory

• This test is performed for very wet mixes

Apparatus

The apparatus for performing the flow test contains a table with a jolting arrangement, slump cone-like mould apparatus to fill the concrete. A wooden tamping rod is also required.

Procedure

1. The top of the table and mould is cleaned of all gritty materials and is wiped with a damp cloth

2. Mould is kept on the centre of the table and is filled in two layers with each layer being tamped 10 times

3. The excess concrete is removed from the top of the mould

4. The mould is lifted vertically upward after half a minute of removing excess concrete

5. Then the top of the table is raised by the handle and allowed to fall 15 times in 15 seconds

6. The diameter of the spread concrete is measured in two directions parallel to the table edge eg and the average spread is noted as flow

Test for Bleeding of Concrete

This test is done to determine the relative quantity of mixing water that will bleed from a sample of freshly mixed concrete.

Apparatus

1. A cylindrical container having an inside height of 280 mm and an inside diameter of 250mm is used

2. A tamping bar similar to one in slump test

3. A graduated jar

4. A pipette for drawing off free water from the surface

Procedure

1. A sample of freshly mixed concrete is filled in 5 layers with each layer being tamped and the top surface is made smooth by trowelling

2. The specimen is weighted and the quantity of water in the container is also noted

3. The cylinder is then kept on a level surface free from vibration and it is covered with a lid

4. Water coming to the surface is pipetted out at 10 mins intervals for the first 40 minutes and at 30-minute intervals subsequently, till the bleeding ceases

5. All the bleeding water is filled in the graduated jar

6. Bleeding water percentage = (Total quantity of bleeding water/Total quantity of water in the sample concrete) * 100

Setting Time of Concrete

The setting time of concrete is found by the penetrometer test and it is covered by IS 8142: 1976.

Apparatus

A container with a minimum lateral dimension of 150 and a minimum depth of 150 m, and six penetration needles with bearing areas of 645, 323, 161, 65, 32 and 16 mm^2. A device to test the force required to cause penetration of the needle is also required.

Procedure

1. A sufficient quantity of concrete is collected and sieved through a 4.75mm sieve

2. Fill the sieved concrete mortar in the container and compact it by rodding or tapping

3. Bring the bearing surface of the needle in contact with the mortar surface and gradually apply a vertical force downwards until the needle penetrates to a depth of 25mm

4. Record the force required to produce 25 mm penetration and time of inserting from the time water is added to the concrete

5. Find the penetration resistance value as force divided by the bearing area of the needle

6. Avoid the area where mortar is disturbed for subsequent penetration

7. Plot a graph between penetration resistance and elapsed time. It should be noted that at least six penetration determination is made and at least one penetration resistance value of 27.6 MPa is reached. Connect the points by a smooth curve

8. Draw a horizontal line to intersect the smooth curve from the penetration resistance value of 3.5 MPa and read this point on the x-axis. This gives the initial setting time of the concrete

9. Similarly, reading the x-axis value for penetration resistance of 27.6 MPa gives the final setting time of the concrete

We hope the information provided is more than enough to understand the tests on fresh concrete and what parameters are tested in fresh concrete. It requires some time to grasp all the details, so we stop this blog here.

Shortly, in our next blogs, we will be covering the "Test on Hardened Concrete" topic. Don't want to miss out on any blog updates? Provide your mail Id below and get subscribed.