Concrete - Testing and Mixes

Cube test outlined

In a cube test, special moulds which are laboratory tested are used to create cubes from samples of fresh concrete. These cubes are then tested to determine the compressive strength. The cube test is the most important quality control available for checking on hardened concrete.

The main reason for performing a cube test is to verify that each batch of concrete is above the minimum “characteristic” strength specified.

The test can also be used to check the quality of production of the concrete throughout. It is important to ensure care is taken on site to ensure cubes are made cured and properly.

Badly made or improperly cured cubes will affect results. Care should also be taken to ensure that tests are carried out correctly and consistently.

Test Procedure and Apparatus

If the sample has to be taken from a dumped heap, scoop concrete from different areas of the heap. Do not take sample from the surface of heap, ensure scoops are taken from well below the surface.

As aggregate tends to gather at the edge of a heap, avoid taking samples from the edge. When collecting samples, collect scoopfuls in a wheelbarrow. Before testing, mix the sample thoroughly with a shovel.

Apparatus required (i.e. to create cubes in accordance with the British Standard)

  • Cube moulds and base plates. Wooden moulds are not sufficient, they must be steel or cast iron. 100mm cubes may be used if the largest aggregate is less than 20mm however, 150mm cubes are normally used.

  • A sampling tray or waterproof base plate for remixing. (Only if required)

  • A scraper, spanners and mould oil.

  • A sampling scoop capable of holding approximately 5kg of concrete and a smaller scoop for filling the moulds.

  • A steel float for levelling the top of the mould.

  • A standard compacting bar must be 380mm long, be 1.8kg in weight and have a square ramming face of 25mm sides.

  • Polythene sheeting, Damp sacks and a Waterproof crayon/marker are also required.

It is important that all equipment is well maintained and not left onsite open to rust.

Test Process

Part 1

  • Ensure that the clean mould is lightly oiled with all bolts tightened to prevent leakage.

  • Check that 2 correct halves of the mould are being used and that the pins lining the corners are correct.

  • The mix should be thoroughly remixed in a barrow using a shovel as outlined earlier.

Part 2

  • Fill the mould in layers of approximately 50mm.

  • Using the standard compacting bar compact for:

  • 25 tamps for both layers in a 100mm mould

  • 35 for all 3 layers in a 150mm mould

If the workability of the concrete is very high, it may not be necessary to carry out the minimum number of tamps.

Part 3

  • Scrape away excess concrete and smooth with float.

  • Wipe the mould edges clean.

  • Using the waterproof crayon/marker, number the moulds for identification and recording purposes.

Part 4

  • Using a damp cloth and a plastic sheet cover each mould.

  • Store cubes at normal room temperature (15°C < temperature < 25°C)

  • It is important cubes aren’t subjected to high or low temperatures (frost) and drying winds.

Curing And Storing Test Cubes

Step 1:

Record max and min overnight temperatures on certificate.

Step 2:

Store indoors

Cover with damp sacking

Cover over with polythene

Step 3*:

If testing cubes within 7 days, cubes should be stored for approximately 24 hours between 18°C and 22°C.

If testing cubes at 7 days or later, cubes should be stored in damp conditions for 16-18 hours between 15°C and 25°C

Step 4:

For cubes being tested within 24 hours, cubes should be sent in their moulds.

In all other cases, cubes should be removed from moulds and sent to the lab when they are 3-7 days old.

Stripping Cubes

Step 1:

Loosen nuts

Step 2:

Part the sides, tap the sides gently with a hammer.

Lift the mould off gently.

Exercise care when removing mould as new cubes can be easily damaged.

Step 3:

Number each cube on 2 of its cast sides.

After they are stored an appropriate length of time, the cubes then need to be water cured in a thermostatically controlled tank. (Between 18°C and 20°C)

Step 4:

When sending cubes to laboratory, wrap in damp cloths and plastic bags then pack in trays.

Attach all relevant certificates and information. Attach the sampling and cube making certificates as well as the order of testing.

Clean the moulds and then reassemble them.

Records

You will need to record the following:

-Date and time sample was taken

-The temperature and weather conditions when sample was taken

-The delivery docket number

-Where the concrete was used on site

-The manufacturer and aggregate

Slump Test (Concrete)

The slump test calculates the consistency (workability) of fresh concrete. It is important to measure the consistency of fresh concrete in order to assess the practicability of placing and compacting the mix and maintaining consistency throughout the job.

Workability or consistence tests can also be used as an indirect check on the water to cement ratio of the concrete.

Concrete slump test results

The table below shows the new slump classes S1, S2, S3 as well as their respective slump ranges and workability.

Table B2 - Slump classes and workability

Slump Test Process

  • Check to make sure inside of the mould is clean and damp before each test.

  • Place the mould on a level, smooth, rigid and impervious surface.

  • Using the foot rests, hold the mould down and fill with concrete. Fill the mould in 3 layers roughly of similar depth. After filling each layer, tamp with 25 uniformly distributed strokes of the tamping rod. Ensure that the entire depth of each layer is tamped. The concrete should be filled above the level of the mould before 3rd layer is tamped.

  • Once the 3rd layer is tamped, remove the excess concrete at the top of the mould. Use a sawing and rolling motion of the tamping rod to remove the excess. You should clean away any spillage from around the base of the mould.

  • Lift mould vertically from the concrete slowly.

  • Place the mould upside down beside the slumped concrete. Ensure that it is sitting level then by laying the tamping rod across the upturned mould as the original level, measure the slump of the concrete using the measuring rule. Measure from the tamping rod to the highest point of the concrete being measured.

  • If the concrete collapses or shears a new concrete sample should be used and the test should be repeated.

  • Once the test is completed, if the side of the slumped concrete is tapped gently with a tamping rod, it can be noted that a harsh mix will most likely shear or collapse whereas a well-proportioned, cohesive mix will not; instead, it will gradually slump further.

Slump Test Apparatus

The slump test is the most commonly used workability test and is suitable for normal cohesive concrete mixes. The test can be useful in ensuring the quality of concrete being produced as changes in slump value can indicate changes in materials, in water content and in the proportions of the mix.

Apparatus:

  • A 300mm high, truncated conical mould that is 100mm in diameter at the top and 200mm in diameter at the bottom.

  • A 600mm steel tamping rod, 16mm in diameter, hemispherical at both ends.

  • A measuring rule

  • A surface that is smooth, rigid and impervious.

Unreinforced Concrete - Typical Mixes for use in Housing

The current European standard for Ready Mix Concrete is EN 206-1: 2013, Concrete - Part 1: Specification, performance, production and conformity. It is important to ensure that all builders are familiar with the above document.

Compressive Strength Class

The most recent version of this document provides a dual description of compressive strength class i.e. C 16/20. The first number in this dual classification relates to the minimum compressive strength of a cylinder test however, this test is not typically used in Ireland and therefore the second number, which relates to the minimum compressive strength of a cube test, is relevant when building dwellings in Ireland.

It should be noted that it is important to detail both numbers when placing any orders to ensure that the manufacturer does not provide the incorrect concrete.

Consistence Classification

A slump test undertaken on site will detail the workability or consistence of the concrete. Table B2 provided above shows the different classifications. The builder will specify the workability required based on the proposed method of placing, finished required etc.

Exposure Classification

The exposure classification for unreinforced concrete for conventional domestic dwellings in non-aggressive soils will in general by XO. Information on all other exposure classes is available in EN 206-1.

Table B3 - Recommended mixes for unreinforced concrete in houses

Conformity

A declaration of conformity to EN 206-1 is required to be provided with all concrete orders on delivery. The declaration of conformity can only be provided through the obligatory production and conformity control scheme operated in accordance with this standard.

Please Note: The contractor must ensure good practice is maintained when using the concrete provided by the manufacturer to ensure the appropriate strength and durability is achieved.