Concrete Curing and Finishing

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Questions and Answers

What is the primary purpose of curing concrete?

To increase the concrete's strength and durability (correct)

To reduce the mixing water from the surface of the concrete

To improve the aesthetic appearance of the concrete

To speed up the hydration process of the cement

When should brooming or tining be performed on concrete?

During the pouring process of the concrete

After the concrete has gained its full strength

After the concrete has thoroughly hardened

Before the concrete has hardened, but it should be sufficiently hard to retain the scoring impression (correct)

What is the purpose of membrane-forming curing compounds?

To improve the concrete's permeability resistance

To accelerate the curing process

To seal in the mix water from the surface of the concrete (correct)

To supply additional moisture to the concrete

What is the advantage of accelerated curing?

It accelerates the strength gain of the concrete Signup and view all the answers

What is ponding or immersion used for?

To cure horizontal surfaces, such as pavements and floors Signup and view all the answers

What is the result of not curing concrete properly?

Decreased volume stability and permeability resistance Signup and view all the answers

What is the primary reason for ensuring that fresh concrete is uniform throughout a given batch and between batches?

To achieve consistent properties in the hardened concrete Signup and view all the answers

What is the primary purpose of compacting fresh concrete?

To remove air pockets and increase density Signup and view all the answers

Which of the following factors affects the workability of concrete by reducing internal friction?

Water content Signup and view all the answers

What is the primary challenge in measuring the workability of concrete?

The absence of a universally accepted test method Signup and view all the answers

What is the primary purpose of using admixtures in concrete?

To improve the workability of the concrete Signup and view all the answers

What is the primary requirement for finishing fresh concrete?

It must be capable of being finished properly Signup and view all the answers

What is the primary purpose of using a hemisphere hammer in concrete testing?

To determine the workability of the concrete Signup and view all the answers

What is the result of segregation in concrete?

Weaker, less durable, and a poor surface finish Signup and view all the answers

Why is it important to mix concrete at the correct speed in a transit mixer?

To prevent segregation of the concrete ingredients Signup and view all the answers

What should be done when placing concrete straight from a truck?

Pour the concrete vertically Signup and view all the answers

What is the recommended time for mixing concrete in a transit mixer before discharge?

At least two minutes Signup and view all the answers

Why is it important to check the concrete's consistency before mixing?

To ensure the concrete is not too wet or too dry Signup and view all the answers

Which method is commonly used to consolidate concrete in walls, columns, beams, and slabs?

Internal vibrator (Needle vibrator) Signup and view all the answers

What is the primary purpose of screeding or strikeoff?

To bring the top surface of a slab to proper grade Signup and view all the answers

What is the purpose of bullfloating or darbying?

To eliminate high and low spots Signup and view all the answers

What is the purpose of adding 10% more cement in hand mixing of concrete?

To compensate for the inferior concrete produced by this method Signup and view all the answers

What should be done immediately after strikeoff?

Bullfloating or darbying Signup and view all the answers

What is the main advantage of machine mixing over hand mixing?

It produces uniform and homogeneous concrete Signup and view all the answers

What should be done before troweling?

Floating Signup and view all the answers

What is the key requirement for batching concrete?

Batching by mass Signup and view all the answers

What is the primary difference between central-mixed concrete and shrink-mixed concrete?

The method of mixing Signup and view all the answers

Why is floating necessary?

To embed aggregate particles just beneath the surface Signup and view all the answers

What is the purpose of thorough mixing in concrete production?

To produce uniform and homogeneous concrete Signup and view all the answers

What is ready mixed concrete?

Concrete mixed at a concrete plant and delivered to the project in a freshly mixed and unhardened state Signup and view all the answers

Study Notes

Brooming and Curing Concrete

Brooming or tining should be performed before the concrete has thoroughly hardened, but it should be sufficiently hard to retain the scoring impression to produce a slip-resistant surface.
Curing is the process of maintaining a satisfactory moisture content and temperature in concrete for a sufficient period to develop the desired properties.
Curing improves strength, volume stability, permeability resistance, and durability (including abrasion resistance and resistance to freezing and thawing and deicer scaling).

Curing Methods and Materials

Three curing methods: supplying additional moisture, sealing in the mix water, and accelerated curing.
Supplying additional moisture methods include ponding or immersion, spraying or fogging, and saturated wet coverings.
Sealing in the mix water methods include covering the concrete with impervious paper or plastic sheets, or applying membrane-forming curing compounds.
Accelerated curing involves supplying heat and additional moisture to the concrete to accelerate strength gain.

Properties of Fresh Concrete

Fresh concrete should be easily mixed and transported, uniform throughout a given batch and between batches.
Fresh concrete should keep its fluidity during the transportation period and have flow properties to fill forms completely.
Fresh concrete should have the ability to be fully compacted without segregation, set in a reasonable period, and be capable of being finished properly.

Factors Affecting Workability

Workable concrete is the one that exhibits very little internal friction between particles or overcomes the frictional resistance offered by the formwork surface or reinforcement.
Factors affecting workability include water content, mix proportions, size of aggregates, shape of aggregates, surface texture of aggregate, grading of aggregate, and use of admixtures.

Measurement of Workability

There is no universally accepted test method that can directly measure the workability.
Some tests measure parameters close to workability and provide useful information, such as the hammer penetration test.

Segregation

Segregation is the separation of the constituent materials of concrete.
A good concrete is one in which all the ingredients are properly distributed to make a homogeneous mixture.
Segregation makes the concrete weaker, less durable, and leaves a poor surface finish.

Avoiding Segregation

Check the concrete is not 'too wet' or 'too dry'.
Ensure the concrete is properly mixed and placed as soon as possible.
Load carefully when transporting the mix, and pour new concrete into the face of concrete already in place.

Mixing and Compaction

Thorough mixing of the materials is essential for producing uniform concrete.
There are two methods of mixing: hand mixing and machine mixing.
Hand mixing is used for small-scale, unimportant concrete works.
Ready mixed concrete is batched and mixed at a concrete plant and delivered to the project in a freshly mixed and unhardened state.
Compaction methods include hand compaction, vibration, internal vibration, and external vibration.

Finishing Concrete

Concrete can be finished in many ways, depending on the intended service use.
Finishing methods include screeding, bullfloating, floating, and troweling.
Screeding or strikeoff is the process of cutting off excess concrete to bring the top surface of a slab to proper grade.
Bullfloating or darbying eliminates high and low spots and embeds large aggregate particles.
Floating embeds aggregate particles just beneath the surface, removes slight imperfections, humps, and voids, and compacts the mortar at the surface.
Troweling produces a smooth, hard, dense surface.

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Description

Learn about the process of curing concrete and the importance of brooming or tining for a slip-resistant surface. Understand the necessary conditions for hydration of cement to occur.