This article discusses everything about concrete. Basic definitions of concrete and its parameters are discussed in detail to provide better understanding.
In this article you will discover everything you need to know about concrete.
This article explains the following terms.
- Characteristic resistance of concrete
- Lightweight concrete
- Normal concrete
- Heavy concrete
- Free water/cement ratio
- Cement classification
- Additions
- Concrete durability
- Chlorides in concrete
- Maximum water/cement ratio and minimum cement content
- Restrictions on the workability of concrete
- Sampling rate for resistance compliance testing
Characteristic resistance of concrete
The strength value below which 5% of all possible strength measurements of the specified concrete are expected to fall. –BS 5328
Lightweight concrete
Hardened concrete with oven-drying density no more than 2000 kg/m 3
Normal concrete
Hardened concrete with a kiln-dried density exceeding 2,000 kg/m 3 but not more than 2,600 kg/m 3 .
Heavy concrete
Hard concrete with oven-dried density greater than 2,600 kg/m 3 .
Free water/cement ratio
The relationship between the mass of free water (minus the water absorbed by the aggregate to reach a saturated, dry surface state) and the mass of cement in a concrete mix.
Cement classification
One of the most important things to know when discussing concrete is the classification of cement.
Cement can be classified according to strength class, which is based on compressive strength.
The measure of cement strength is the compressive strength after 28 days, measured on a mortar prism in accordance with BS EN 196-1.
There are five standard strength-based classes. These are 22.5, 32.5, 42.5, 52.5 and 62.5.
In addition, there are two other intermediate strength classes, 37.5 and 47.5, for mixer combinations of Portland cement (BS 12 equivalent) with ground blast furnace slag (GGBS) (BS 6699 equivalent).
Additions
Additives must comply with British standards as shown in the table below.
Calcium chloride and chloride-based additives should never be added to reinforced, pressed or metal-encrusted concrete, or cement that complies with BS 915 or BS 4248.
If two or more admixtures are added to the same concrete mix, their compatibility must be checked.
Concrete durability
If we want to know everything about concrete, we need to delve into the durability of concrete.
Factors affecting durability can be identified as follows.
- The shape and mass of concrete
- The cover for built-in steel
- The environment
- The type of cement
- The type of aggregate
- Additive type and dosage
- The cement content and the free water/cement ratio of concrete
- Processing to achieve complete compaction, correct finishing and effective curing.
Chlorides in concrete
If there are chlorides in concrete, there is always a risk of corrosion.
The higher the chloride content and curing temperature or subsequent exposure to hot, humid conditions, the greater the risk of corrosion.
The maximum chloride content in prestressed concrete must be limited to 0.1% of the cement mass.
If chloride-containing marine or inland aggregates are used to produce concrete, thorough washing must be carried out to meet specified limits.
The chloride content specified in BS 5328 is as follows.
Maximum water/cement ratio and minimum cement content
The following table from BS 5328 gives the limits.
These values can be used as a guide for mixture composition.
Restrictions on the workability of concrete
According to British standards, the following restrictions can be achieved.
However, the limits shown in the table may vary depending on the project specification.
For example, the slump value of pumped concrete is given as 75 mm. However, in civil construction this value can vary between 150 and 200 mm.
Sampling rate for resistance compliance testing
When the concrete is poured, we check its strength to ensure it has the strength specified in the project.
The sampling rate generally depends on the amount of concrete to be filled.
Source BS 5328 Part 1
Working in hot weather
Unless otherwise specified by the buyer, the temperature of the concrete upon delivery to the site must not exceed 30°C.
This is done as a measure to reduce the heat of hydration or the increase in temperature during the reaction.
These guidelines must be observed, especially for thicker concrete.
