| Concrete - Fresh
(Click on the questions below as this will take you directly to the answer, or scroll down to see all questions and answers)
- In what forms can concrete be manufactured?
- What is Hydration?
- How do you control the strength of concrete?
- What are recommended mix proportions for good concrete?
- Can it be too hot or too cold to place new concrete?
- Can I use any water for mixing concrete?
- Why is it so important to "cure" concrete?
- What is air-entrained concrete?
- Why does concrete crack only a short time after placing?
- What are the most common tests for fresh concrete?
- How do I perform a slump test?
- Why do concrete surfaces flake and spall?
Concrete is produced in three basic forms:
Soon after the aggregates, water, and the cement are combined, the mixture starts to harden. All portland cements are hydraulic cements that set and harden through a chemical reaction with water. During this reaction -called hydration - a node forms on the surface of each cement particle. The node grows and expands until it links up with nodes from other cement particles or adheres to adjacent aggregates.
The building up process results in progressive stiffening, hardening, and strength development. Once the concrete is thoroughly mixed and workable it should be placed in forms before the mixture becomes too stiff. This hardening process continues for years meaning that concrete gets stronger as it gets older.
The key to achieving a strong, durable concrete rests in the careful proportioning and mixing of the ingredients. A concrete mixture that does not have enough paste to fill all the voids between the aggregates will be difficult to place and will produce rough, honeycombed surfaces and porous concrete. A mixture with an excess of cement paste will be easy to place and will produce a smooth surface; however, the resulting concrete is likely to shrink more and be uneconomical.
Cement and water form a paste that coats each particle of stone and sand. Through a chemical reaction called hydration, the cement paste hardens and gains strength. The character of the concrete is determined mainly by quality of the paste. The strength of the paste, in turn, depends on the ratio of water to cement. The water-cement ratio is the weight of the mixing water divided by the weight of the cement. High-quality concrete is produced by lowering the water-cement ratio as much as possible without sacrificing the workability of fresh concrete.
Generally, using less water produces a higher quality concrete provided the concrete is properly placed, consolidated, and cured. The following chart provides a range of trial mixes for a given strength of concrete at 28 days.
Cements with higher extender contents (e.g. CEMII/B or CEM III) may develop strength more slowly and will require particular care with curing.
Almost any natural water that is drinkable and has no pronounced taste or odour may be used as mixing water for concrete. However, some waters that are not fit for drinking may be suitable for concrete.
Excessive impurities in mixing water not only may affect setting time and concrete strength, but also may cause efflorescence, staining, corrosion of reinforcement, volume instability, and reduced durability. Specifications usually set limits on chlorides, sulphates, alkalis, and solids in mixing water unless tests can be performed to determine the effect the impurity has on various properties.
Curing is one of the most important steps in concrete construction, because proper curing greatly increases concrete strength and durability. Concrete hardens as a result of hydration: the chemical reaction between cement and water. However, hydration occurs only if water is available and if the concrete's temperature stays within a suitable range. During the curing period - from five to seven days after placement for conventional concrete - the concrete surface needs to be kept moist to permit the hydration process. New concrete can be wetted with soaking hoses, sprinklers or covered with wet burlap, or can be coated with commercially available curing compounds, which seal in moisture.
Air-entrained concrete contains billions of microscopic air cells per cubic metre. These air pockets relieve internal pressure on the concrete by providing tiny chambers for into which water can expand when it freezes. Air-entrained concrete is produced through the use of air-entraining portland cement, or by the introduction of air-entraining agents, under careful engineering supervision as the concrete is mixed on the job. The amount of entrained air is usually between 4 % and 7% of the volume of the concrete, but may be varied as required by special conditions.
Air-entraining agents are used to produce a number of effects in the concrete mix:
· To improve cohesion and reduce bleeding
· To improve compaction of low workability concrete
· To provide stability to extruded concrete
· To give improved handling properties, stability and cohesion to bedding mortar
· To improve freeze/thaw resistance of hardened concrete (not a major problem in South Africa)
When designing air-entrained concrete it should be remembered that the compressive strength is reduced, compared to non-air-entrained concrete.
Plastic settlement cracking occurs after the concrete has been compacted. After compaction there is a tendency for solid particles to settle and displace some mixing water which rises to the surface. This settlement will continue until the concrete stiffens. In a section where there is no restraint (e.g. top reinforcement, changes in section, etc.), such settlement rarely causes any problems.
Slump, air content, unit weight and compressive strength tests are the most common tests.
Slump is a measure of consistency, or relative ability of the concrete to flow. If the concrete cannot flow because the consistency or slump is too low, there are potential problems with proper consolidation. If the concrete won't stop flowing because the slump is too high, there are potential problems with mortar loss through the formwork, excessive formwork pressures, finishing delays and segregation.
Air content measures the total air content in a sample of fresh concrete, but does not indicate what the final in-place air content will be, because a certain amount of air is lost in transportation, consolidating, placement and finishing. Three field tests are widely specified: the pressure meter and volumetric method are ASTM standards and the Chace Indicator is an AASHTO procedure.
Unit weight measures the weight of a known volume of fresh concrete.
In essence, the compressive strength test consists of crushing three cubes from the same sample of concrete. The cubes are tested in a saturated condition. The strength of the concrete is defined as the average of the strengths of the three cubes. For the test to be valid, the ranges of strengths within the set of three cubes must not exceed 15% of the average.
The surface is struck off by rolling the tampering rod across the top edge of the mould. After careful removal of the mould, the slump of the concrete present is measured to the nearest 5 mm. The slump as measured is the distance between the top of the inverted mould and the highest point of the concrete.
- In areas of the country that are subjected to freezing and thawing the concrete should be air-entrained to resist flaking and scaling of the surface. If air-entrained
concrete is not used, there will be subsequent damage to the surface.
- The water/cement ratio should be as low as possible to improve durability of the surface. Too much water in the mix will produce a weaker, less durable concrete
that will contribute to early flaking and spalling of the surface.
- The finishing operations should not begin until the water sheen on the surface is gone and excess bleed water on the surface has had a chance to evaporate.
If this excess water is worked into the concrete because the finishing operations are begun too soon, the concrete on the surface will have too high a water content
and will be weaker and less durable.