Q.
b) Explain how the method of batching, placing, compacting and curing influenced the strength of concrete. (16 marks)
(16 marks, 2013 Q6b)
A.
b) See earlier posts:
Past year 2014 Q5 - Stages of concrete operation.
Factors affecting the strength of concrete.
The most important variables affecting the strength of concrete at a given age are the water/cement ratio and the degree of compaction. When concrete is fully compacted its compressive strength is inversely proportional to the water/cement ratio.
Concrete can be produced by employing either mechanical or manual mixing methods. The compressive strength of concrete depends so much on the consistency achieved through mixing. The question is how many times the mixture of the concrete ingredients can be turned over from one side to another. Adequate strength requirement is one of the major properties of concrete and this strength depends on the consistence of the mixing.
Ready mixed concrete:
If instead of being batched and mixed on site, concrete is delivered for placing from a central plant. It is referred to as ready-mixed or pre-mixed concrete. This type of concrete is used extensively abroad as it offers numerous advantages in comparison with other methods of manufacture:
- Close quality control of batching which reduces the variability of the desired properties of hardened concrete.
- Use on congested sites or in highway construction where there is little space for a mixing plant and aggregate stockpiles;
- Use of agitator trucks to ensure care in transportation of concrete, thus prevention segregation and maintaining workability
- Convenience when small quantities of concrete or intermittent placing is required.
There are two categories of ready-mixed concrete: central-mixed and transit mixed or truck mixed. In the first category, mixing is done in a central plant and then concrete is transported in an agitator truck. In the second category, the materials are batched at a central plant but are mixed in a truck.
Concrete Placing and Compaction of Concrete
The operation of placing and compaction are interdependent and are carried out simultaneously. They are most important for the purpose of ensuring the requirements of strength, impermeability and durability of hardened concrete in the actual structure. As for as placing is concerned, the main objective is to deposit the concrete as close as possible to its final position so that segregation is avoided and the concrete can be fully compacted. The aim of good concrete placing can be stated quite simply.
- It is to get the concrete into position at a speed, and in a condition, that allow it to be compacted properly. To achieve proper placing following rules should be kept in mind:
- The concrete should be placed in uniform layers, not in large heaps or sloping layers.
- The thickness of the layer should be compatible with the method of vibration so that entrapped air can be removed from the bottom of each layer.
- The rate of placing and of compaction should be equal. If you proceed too slowly, the mix could stiffen so that it is no longer sufficiently workable. On no account should water ever be added to concrete that is setting. On the other hand, if you go too quickly, you might race ahead of the compacting gang, making it impossible for them to do their job properly.
- Each layer should be fully compacted before placing the next one, and each subsequent layer should be placed whilst the underlying layer is still plastic so that monolithic construction is achieved
- Collision between concrete and formwork or reinforcement should be avoided.
- For deep sections, a long down pipe ensures accuracy of location of concrete and minimum segregation.
- You must be able to see that the placing is proceeding correctly, so lighting should be available for large, deep sections, and thin walls and columns.
Once the concrete has been placed, it is ready to be compacted. The purpose of compaction is to get rid of the air voids that are trapped in loose concrete. See here for more details about compacting.
Curing is the maintenance of a satisfactory moisture content and temperature in concrete for a period of time immediately following placing and finishing so that the desired properties may develop (Fig. 12-1). The need for adequate curing of concrete cannot be overemphasized. Curing has a strong influence on the properties of hardened concrete; proper curing will increase durability, strength, water-tightness, abrasion resistance, volume stability, and resistance to freezing and thawing and deicers. Exposed slab surfaces are especially sensitive to curing as strength development and freeze-thaw resistance of the top surface of a slab can be reduced significantly when curing is defective.
When portland cement is mixed with water, a chemical reaction called hydration takes place. The extent to which this reaction is completed influences the strength and durability of the concrete. Freshly mixed concrete normally contains more water than is required for hydration of the cement; however, excessive loss of water by evaporation can delay or prevent adequate hydration. The surface is particularly susceptible to insufficient hydration because it dries first. If temperatures are favorable, hydration is relatively rapid the first few days after concrete is placed; however, it is important for water to be retained in the concrete during this period, that is, for evaporation to be prevented or substantially reduced.
With proper curing, concrete becomes stronger, more impermeable, and more resistant to stress, abrasion, and freezing and thawing. The improvement is rapid at early ages but continues more slowly thereafter for an indefinite period. Fig. 12-2 shows the strength gain of concrete with age for different moist curing periods and Fig. 12-3 shows the relative strength gain of concrete cured at different temperatures.
The most effective method for curing concrete depends on the materials used, method of construction, and the intended use of the hardened concrete. For most jobs, curing generally involves applying curing compounds, or covering the freshly placed and finished concrete with impermeable sheets or wet burlap. In some cases, such as in hot and cold weather, special care using other precautions is needed.
Concrete mixtures with high cement contents and low water-cement ratios (less than 0.40) may require special curing needs. As cement hydrates (chemically combining with water) the internal relative humidity decreases causing the paste to self-desiccate (dry out) if no external water is provided. The paste can self-desiccate to a level where hydration stops. This may influence desired concrete properties, especially if the internal relative humidity drops below 80% within the first seven days. In view of this, membrane-forming curing compounds may not retain enough water in the concrete. Therefore, fogging and wet curing become necessary to maximize hydration (Copeland and Bragg 1955). Fogging during and after placing and finishing also helps minimize plastic cracking in concretes with very low water-cement ratios (especially around 0.30 or less).
When moist curing is interrupted, the development of strength continues for a short period and then stops after the concrete’s internal relative humidity drops to about 80%. However, if moist curing is resumed, strength development will be reactivated, but the original potential strength may not be achieved.
CURING METHODS AND MATERIALS
Concrete can be kept moist (and in some cases at a favorable temperature) by three curing methods:
1. Methods that maintain the presence of mixing water in the concrete during the early hardening period. These include ponding or immersion, spraying or fogging, and saturated wet coverings. These methods afford some cooling through evaporation, which is beneficial in hot weather.
2. Methods that reduce the loss of mixing water from the surface of the concrete. This can be done by covering the concrete with impervious paper or plastic sheets, or by applying membrane-forming curing compounds.
3. Methods that accelerate strength gain by supplying heat and additional moisture to the concrete. This is usually accomplished with live steam, heating coils, or electrically heated forms or pads. The method or combination of methods chosen depends on factors such as availability of curing materials, size, shape, and age of concrete, production facilities (in place or in a plant), aesthetic appearance, and economics.
As a result, curing often involves a series of procedures used at a particular time as the concrete ages. For example, fog spraying or plastic covered wet burlap can precede application of a curing compound. The timing of each procedure depends on the degree of hardening of the concrete needed to prevent the particular procedure from damaging the concrete surface (ACI 308 1997).
http://www.aboutcivil.org/batching-mixing-placing-compaction-of-concrete.html
PCA manual. Curing of concrete. Chapter 12, available at
http://www.ce.memphis.edu/1101/notes/concrete/PCA_manual/Chap12.pdf