Concrete is one of the most important construction materials. However, structural elements that are made from concrete are usually weak in tension but strong in compression. Nevertheless, concrete in its wet state is moldable and very versatile. It is this property that has enabled its application to various structures such as bridges, road, tunnels, and buildings, just to mention a few. However, there is nothing as the perfect concrete. Concrete design varies with regard to the use and this is known as well-designed concrete. As such, it has been in use for various centuries which date back to the Romans. In essence, the mix design depends on the plasticity required as well as the structural requirements.
Therefore, there are various mix requirements that have led to the design of various types of concrete. In this, there is high strength concrete, pervious concrete, and normal concrete, among others. Each concrete use is justified by its properties.
As stated initially, concrete has been in use due to its various advantages. The amount of concrete production all over the world is about 10 times that of the production of steel (Mehta, 2009). To begin with, the stability of concrete is unchallenged. It is able to withstand harsh environmental weathers and other deleterious effects that enables it to stand for a long time, making it ideal for most structural uses. Secondly, the elements that are designed from concrete can easily be shaped into various forms, shapes, and sizes. This makes it ideal for use in the current generation where buildings are of different shapes and sizes. Thirdly, the price of concrete is relatively lower than that of other building materials. As such, it is readily available for structural applications. Moreover, the resistance of concrete to very high temperatures is commendable. Concrete can withstand very high temperatures making it ideal even in the harshest of environments. Last, but not least, there is usually no need for protection when it comes to concrete. This is a sharp contrast to other materials such as timber.
THE TYPES OF CONCRETE
The various types of concrete depend on the use. This is usually determined by the composition of concrete as well as its applicability. In other instances, concrete may be modified using other additives.
The regular concrete.
As the name suggests, it the most commonly used form of concrete .Nevertheless, its compression quality is very high and can be used in a number of structural applications (American Concrete Institute (ACI 318-08), 2008) The initial mixture is composed of cement and aggregates but later on, water is added. This ensures that it attains its plasticity as well as its structural strength.
High strength concrete
Usually used in structural application s requiring strengths higher than that of regular concrete. The design mix ensures that it can sustain higher loads and pressures (American concrete institute, 2008)In essence, the water to concrete ratio is usually lower while the aggregates used are of higher strengths. This requires the careful selection and testing of the materials that are used. Aggregates that are weaker tend to compromise the nature of this concrete.
Moreover, there is the presence of silica fumes (Awanti & Harwalkar, 2016) which are very important in preventing calcium hydroxide formation. However, its use is limited by the fact that it is not workable (Mehta & Monteiro , 2014). Engineers who tend to use this concrete have various mechanisms in place to ensure that the concrete is placed in the shortest span of time. The problem of workability is aggravated when there is need to use reinforced concrete. This is mainly achieved through the use of additives which improve the workability.
The project of developing this type of concrete was initiated by the American society of civil engineers in 2013 (American Society for Testing and Materials, 2009) due to the increase in strength requirements of concrete. Ain essence, this concrete has very high compressive strengths. The compressive strength of concrete is usually tested after 28 days (American Concrete Institute (ACI 318-08), 2008).
The use of this concrete has highly been influenced by the increase in labor costs of construction projects (Wanderwerf, n.d.) The use of the previously mentioned concrete types requires the use of equipment that remove the air entrained in the mixture and this has the effect of increasing the cost. However, it is important to ensure that there are no air bubbles formed in the concrete, regardless of its use.
Furthermore, the use of self-compacting concrete may reduce the use of higher strength materials for the framework. As with the other types of concrete, there is usually an increase in the stress on the framework due to the vibration required in removing air bubbles. Reduction in the vibration requirements of concrete means that it can be easily reused and can be applied in various layouts.
In essence, the design of this mixture limits the use of fine aggregates (Wanderwerf, n.d.). As such, it allows only small amounts of air and water to go through in order to attain the optimum strength. A reduction in the number of fine aggregates used means that there is an increase in intergranular spaces which requires the use of more concrete. Concrete is used in ensuring that these spaces are at a minimal.
The increase in porosity has ensured that this concrete is highly recommended by the environmental protection agency (Crouch, et al., 2007). This porosity enables the concrete water to go back into the groundwater and recharge the water table. On the other hand, the porosity of the regular and high strength concrete is limited which limits the ability of the groundwater to recharge.
Concrete is strong in compression but it is weaker in tension. On the other hand, steel is strong in tension but weaker in compression. Therefore, designing a composite material that is made up of concrete as well as steel has the effect of ensuring that the building is strong both in tension and compression. Its first development was witnessed in the year 1844 (American concrete institute, 2008) by Joseph Monier and since then, its application is tenfold. In essence, the elements that are designed from reinforced concrete tend to withstand some unprecedented loads, as compared to the elements designed from plain concrete.
THE PRODUCTION OF PERVIOUS CONCRETE
This type of concrete uses the same materials as any other form of concrete. However, there is two important aspect of this concrete that makes it exceptional for specific uses. For one, the concrete has no fine aggregates in the sense that the mix design does not consider the properties of this type of aggregates. Secondly, the grading that is used for the coarse aggregates is kept narrowly. The narrow grading of coarse aggregates ensures that there is a bigger spacing of the grains (Montes & Haselbach, 2006). As such, there are exceptional requirements for the placing, curing, and compaction among other concrete properties. In essence, this concrete is designed so that there is an increase in the void ratio to allow the free movement of water.
However, the pervious nature of the concrete, as well as the void ratio, will be influenced by the proportioning, the character of the concrete and the consolidation. Nevertheless, the void ratio of the concrete should be between 15% and 30 % (Tawatchai, et al., 2012). However, it has been noted that an increase in the void ratio results to a decrease in the strength of concrete and as such, it is important to determine the characteristics of the trial batches prior to placement and consolidation.
A concrete that has been tested with a void ratio of about 20% will have a significant different void ratio in the actual field due to the method that has been used for compaction (Montes, et al., 2005). There is software that has been used to estimate the void ratio of the trial batch in order to ensure the desired properties are achieved in the field. However, the workability will also play a significant role in this achievement.
As with the cementitious materials used, Portland cement, as well as blended cement, may be used. However, additional materials such as fly ash, furnace slag and pozzolans may also be used. Nevertheless, the testing of these individual materials should be conducted in order to ensure that the desired strength parameters can be achieved. Some of the tests required ion these materials include the setting time, the porosity, the permeability the rate at which strength can be developed, just to mention a few (Yang & Jiang, 2003).
On the other hand, the aggregates used is limited in these types of concrete. Larger aggregates have the effect of increasing the rougher surface. Nevertheless, standards such as ASTM C33 number 67 and 69 are also important in determining the grading (American Society for Testing and Materials, 2009). Other shapes of aggregates that may be used in the production of this concrete include gravel and angular aggregates.
The water-cement ratio should be between 0.27 and 0.36 (Montes & Haselbach, 2006). However, this requires the proper application of admixtures to improve the properties. Unlike the conventional concrete, the void ratio has not been understood in relation to the concrete strength since there is a difference in the paste content. Nevertheless, the void content within the aggregates in these types of concretes is less than the paste within the concrete (Crouch, et al., 2007).
Finally, the admixtures are used in order to ensure that desirable properties are achieved. However, the most common types of admixtures used are the retarders (Tennis, et al., 2004). These admixtures reduce the setting time of the concrete. On the other hand, the use of air-entraining admixtures is meant to limit the effect of freezing and thawing.
INGREDIENTS OF CONCRETE
It is a heterogeneous material that is composed of aggregate, cement, sand and water. To begin with, the aggregate makes about 75% of the total mixture and can either be sand, crushed rocks, recycled materials or any other appropriate Materia (American concrete institute, 2008)l. It forms the main component of the materials and the strength of the aggregates to determine the strength of the concrete. On the other hand, the cement makes up about 7 to 14 % (Awanti & Harwalkar, 2016) of the concrete and is used as the binding material. Nevertheless, the American society of civil engineers has come to place cement in five classes. Class 1 is the most commonly used class of cement.
Moreover, there is also the use of additives that may act as accelerators or retarders (Awanti & Harwalkar, 2016). The accelerator additives are used to reduce the time that the concrete requires to set and on the same token, improve the early strength of the concrete. These types of additives are usually common in weathers that are usually cold in order to ensure that the concrete sets within the shortest time possible. On this note, the most commonly used type of accelerator additive is calcium chloride. However, these additives may result in a corrosion effect on the reinforcing steel or any other reinforcement.
On the other hand, retarding additives are used in order to ensure that there is a delay in the time required for the concrete setting. As such, common applications are witnessed in hot weather environments as well as where there is concrete transportation. The most common retarding additive is sugar and it has the effect of improving the workability.
The properties of concrete
Considering that concrete is a homogenous material, its nature is dependent on the properties of the individual ingredients. Furthermore, these properties are dependent on the mix ratios Nevertheless, some of the properties of concrete include workability, setting time, the shrinkage, creep, compressive strength, just to mention few. These properties are discussed in the following sections.
To begin with, the setting refers to the process through which concrete hardens before it actually hydrates (Montes & Haselbach, 2006). It is usually the phase between the plastic state of the concrete and the hardened state. In essence, this is dependent on the properties of the cement paste that binds the aggregate materials. Nevertheless, the setting properties of concrete are dependent on the ratio of the water to cement, the temperature, the type of cement, the use of additives, the admixtures used, the aggregate amount and the relative humidity among others.
On the other hand, the workability refers to the ease of concrete application. This usually refers to the ability to be transported and placed without having to bleed or segregate too much (Palmer, 2018). However, this definition can also be based on the amount of work done in order to overcome the internal frictional forces so that the concrete can be easily compacted.
The strength of the concrete is usually dependent on the workability and as such, it is important to ensure that there are no voids in the mix. Removal of voids within the mix ensures that the density of the mixture is very high while the presence of voids will have the effect of reducing the strength as well as the density. Void of about 5% has the effect of reducing the strength of the concrete by as much as 30% (American Concrete Institute (ACI 318-08), 2008).
The slump test is used to test the workability of any concrete. Nevertheless, factors that affect the workability of concrete include the ratio of water to cement, the aggregates that are used, and the weather, the use of admixtures and the ratio of sand to the aggregate. The slump test should be done immediately after the mix design.
It is used to determine the water gain of the concrete (Huang, et al., 2010). Water has the lowest specific gravity as compared to the other ingredients of the concrete and as such, it can come out to the surface. As such, it is a form of segregation. Nevertheless, bleeding is predominant in mixtures that are wet, mixtures that are badly proportioned and those that have been improperly mixed. In other instances, the water may move upwards with some amount of cement and this paste is known as laitance (American Society for Testing and Materials, 2009).
The effect of bleeding is that there is an increase in the transverse channels from the bottom of the concrete to the top. These channels will increase if the water-cement ratio is about 0.7 (Tennis, et al., 2004). In essence, there will be an increase in the permeability of concrete which may reduce the strength. Moreover, the water may accumulate at some certain regions within the concrete. Accumulation may lead to a reduction in the bond between the cement and the aggregate or the cement paste and the reinforcement. In turn, the concrete will be of lower strength.
One of the remedies to reduce bleeding is proper proportioning. This ensures that there is complete mixing of all the concrete ingredients. Secondly, it may be reduced by improving the use of fine cement. Fine cement will increase the bond and moreover, will ensure that the water travels a long distance to the surface. The use of agents that entrain the air is also an effective remedy. Finally, the use of vibration will ensure the even distribution of water and as such, will reduce bleeding.
This is highly influenced by the weather conditions as well as the concrete ingredients. To begin with, Weather temperatures, as well as humidity, will increase the shrinkage of concrete. Moreover using more water in the mix will also increase the total shrinkage. It has been recommended that the total shrinkages of concrete should be about 0.0003 (American Society for Testing and Materials, 2009). The effects of shrinkage include cracks as well as deflection.
The compressive strength
The strength of concrete in respect to compressive loads. From the lab results, this is the strength below which less than 5% of the test specimen may fail. Nevertheless, M 20 is the concrete class that is recommended for reinforced concrete work.
The Modulus of elasticity
The modulus of elasticity is of significant importance in calculating the deflections of structural elements. Nevertheless, the modulus of elasticity and the compressive strength are directly related. As such, an increase in strength will mean a reduction in the modulus of elasticity.
DISADVANTAGES OF PERVIOUS CONCRETE
To begin with, honeycombing, as well as rough textures, are common in pervious concrete (Tennis, et al., 2004). As such, there is a likelihood of surface raveling. This, in turn, means that such concrete cannot be used in roadway surfaces that may have high traffic. Tire sheer can result in dislodging of the surface aggregates.
The second disadvantage is the fact that these types of concrete tend to lose water very fast As such, it is important to ensure that there is a substrate beneath any element made from this type of concrete and indirect contact with the soil. Unlike in other types of concrete, this may require the services of a specialized engineering team specific to the use.
Thirdly, this type of concrete is not as strong as the conventional concrete. A consistency pressure test is likely to reveal that there is usually a collapse of the pore when subjected to heavy loads. Therefore, it cannot be used to design pavements such as aircraft runways or even the highways
Considering the void ratio, most conventional methods would not be appropriate for testing. Nevertheless, testing is important in determining consistency as well as acceptance. This will determine the methods for placing, curing, compaction etc.
This can be conducted using the round-robin test. Nevertheless, it estimates the flow that can be observed from a certain portion of the concrete from a specified head pressure. The use of this concrete is common in parking pavements and as such, the head is used to simulate a specific rain event.
This is used to determine the density of a fresh batch of pervious concrete. It is also used to establish the air void ratio and the pervious nature of the concrete. However, a comparison can be done by using conventional concrete.
The strength of the concrete in cylindrical forms
This test is used to indicate the likely strength of the element constructed from pervious concrete. As such, the consolidation is the most significant part of the test. Nevertheless, it is important to simulate the standard field condition before actually subjecting it to the compression test.
The air test
Method conducted through the use of the pressure method. The fresh pervious concrete is first collected from different sections of the batch. Later on, the two buckets are weighed after one has received 20 blows from the standard proctor hammer. The difference in weight will determine the weight of the air present.
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