Producing concrete kerbstone/curbstone
Today, concrete curbstones are considered as one of the most used concrete units in cities, and they are responsible for restricting spaces, beautifying cities, and guiding surface waterways of the highways.
Obviously, the exhaustion and damage of these kerbstones/curbstones have a reverse effect on the appearance of the city, and thereinafter repair, mend or replacement will cost a lot.
Hence, notice and accuracy and observance of the technical principles in the construction of these curbstones can multiply their useful life and reduce the many other problems.
In this research, we have tried to study the various factors affecting the quality of concrete kerbstones, including the choice of production method, the use of rock materials with high fracture percentage, the selection of maximum size of aggregate is limited (use of sieve), water-cement ratios are low and the use of bubble-based materials in concrete mix design.
The use of precast concrete products is one of the methods of using concrete, and concrete curbstones are one of the most used products. To achieve high quality in the production of concrete kerbstones, all the factors affecting it should be considered, such as the properties of concrete materials including aggregates, cement, water-to-cement ratio, construction methods, technical principles during construction, processing, and environmental conditions. The average lifespan of concrete kerbstones is about 15 years in most countries around the world, but this number is about 3 years in Iran. Therefore, it is possible to increase the life of concrete production by spending less money on production and observing the technical principles of construction and preventing the loss of national interests and public expenditure.
Today, concrete curbstones are produced differently: general method (vibration), dry press method and wet-press method. Due to the lack of a large number of kerbstone molds and a high rate of production, today's press method has been welcomed by more manufacturers. The ratio of water to cement in the dry-press method will improve the mechanical properties, and in particular the durability of concrete in these types of kerbstones.
The initial temperature of the concrete when pouring is one of the factors affecting the quality of the concrete. As the concrete temperature increases, when it is poured, the fluidity decreases. According to research, the highest compressive strength in concrete with an initial temperature of 25 ° C and the lowest compressive strength is observed in concrete at an initial temperature of 39 ° C. The highest flexural strength of the concrete is obtained at an initial temperature of 29 ° C and a minimum flexural strength and maximum penetration depth of water with a pressure of 39 ° C.
The aggregates contain about 60 to 80 percent of the weight of the concrete, it plays a significant role in the concrete properties, and the maximum size of aggregate directly affects the mechanical properties of the concrete. It can be seen that by decreasing the maximum aggregate size and increasing the softening ratio of grain, due to the increase in the lateral surface of aggregates in the unit volume of concrete, which increases the adhesion level of the cement dough and aggregate, as well as the reduction of stress concentration and cracking in the transfer region, properties Mechanical dry concrete is improved. The use of high-quality aggregates such as granite aggregates can also have a very positive effect on improving the mechanical properties of concrete curbstones. Although granite aggregate has a higher price than ordinary aggregates, the concrete made from it has high durability and a high lifespan.
Recycled aggregates of destroyed concrete structures can be used in the production of concrete curbstones. Depending on the mixing design, various percentages of these materials can be used in the manufacture of concrete. For example, the ratio of water to cement can be 0.45 and the ratio of aggregate to cement is 4 to 1. But before using this kind of material, its cost and efficiency should be thoroughly investigated. Concerning aggregates, other factors also affect the quality and mechanical properties of concrete. The aggregates must be clean and free of any chemicals that affect the hydration of the cement. Because these materials are very effective on the abrasion resistance of concrete, this should be carefully considered, especially for river aggregates.
Glass can also be used as a pozzolanic material (cement substitute) or as a substitute for aggregates in pre-fabricated concrete units. Before using the glass in the concrete, there are important things to consider including the amount of silica, the amount of chloride ion, the amount of sulfur trioxide, the specific surface of the glass powder, the amount of lead material, and other iron oxide-containing materials, sodium oxide, aluminum oxide, oxicalcium, potassium oxide, and magnesium oxide is determined by a lab equipped to control the range of this material.
Cement as the main adhering material of concrete component plays a major role in the mechanical properties of concrete. In the abrasion resistance of concrete curbstones, the amount of cement is directly effective. With a constant ratio of water to cement, increasing the amount of cement increases compressive strength and abrasion resistance. There is a correlation between the compressive strength and abrasive resistance, which means that as the compressive strength increases, the abrasion loss is reduced.
Admixtures can also be used to improve the mechanical properties of precast concrete components, such as concrete curbstones. These materials can improve the durability and ultimate strength of the concrete.
Some countries use their special standards for concrete curbs. For example, in the New Zealand standard, for the curbs of main roads, side passages and the connection roads produced by the curb machine and on-site, the minimum amount of cement in per cubic meter of concrete respectively is 240-280 and 260 kg, instead of the compressive strength of concrete, the concrete density, is usually used, which is specified by percent and should be at least 92.5%. This number is 95% in the Australian standard. In the Australian standard, in the case of on-site production curbs, the minimum cement content is 280 kg / m3, the maximum size of aggregate is 12.5 mm, and the minimum compressive strength is 20 MPa. n the American standard, according to different conditions, the minimum amount of cement is from 340 to 355 kg / m 3, the percentage of bubble materials from 5 to 8, slumped 30 to 80 mm, the maximum ratio of water to cement is 0.4 to 0.45 and the minimum and the compressive strength of 28 days is considered 32 MPa. However, these values can be changed depending on the employer or relevant organization.
In some countries, ready-mixes are also available by manufacturers that can be used in small amounts of work. The concrete slump in general method of vibration curbstone production can be up to 80 mm for concrete placement state by hand and 40 mm for concrete placement by machine and 0 to 10 mm for producing dry-press curbstones. The minimum compressive strength can be 20 to 32 MPa.
One of the disadvantages of the press production method of concrete curbstone is the weakness of these curbstones in the first hours of production against possible damages, as well as a weakness against inappropriate weather after production in the absence of necessary protection. Therefore, after production, it is necessary to take steps to protect them against such cases (proper processing). however, today there are various materials used to make curbs, each of them has advantages and disadvantages. For example, curbstones have compressive and abrasive strength, durable and high price. Plastic curbstones have easy production systems, but they have a durable, stable color and a low impact strength. So any of these methods can be checked in their place.
2. Factors affecting the production of dry press concrete curbstones
firstly, this research mentioned the factors affecting production and manufacturing, which must be followed. In the end, it has been investigated the mixing design of dry press concrete curbstones by ABA method. In all of these cases, it will be considered the determination of the ratio of cement content, fine aggregates, coarse aggregate, water to cement ratio, amount of bubble materials and the percentage of water absorption of concrete, which indicates the durability of concrete. To obtain an optimal layout for concrete kerbstones, first, it is necessary to study the consumption of materials and environmental conditions.
A: One of the issues that have been neglected in the production of concrete is the concrete temperature when pouring. Generally, the regulations take into account the temperature and conditions of the concrete placement environment and the reason for that may be the temperature measurement of the environment is usually easily possible. However, calculating the concrete temperature during pouring is usually difficult and far from the workshop conditions. According to the experiments, the temperature of the concrete during pouring completely affects the mechanical properties of the concrete, in such a way that the reduction of the concrete temperature slows down the speed of hydration of the cement and lower the concrete initial strength. But finally, due to the more uniform distribution of cement paste, the ultimate strength will increase. As the concrete primary temperature rises, the initial strength of the concrete increases due to the rapid formation of hydrated crystals. However, in the long run, the final strength of concrete will be reduced due to the uneven distribution of cement paste and the lack of filling of empty spaces and porosity. The effect of the initial temperature of the concrete on the concrete's flexural strength and compression strength is the same. Also, the absorption of concrete water at a higher initial temperature will be higher than the concrete at lower initial temperatures. Obviously, due to the creation of empty spaces and porosity in concrete with a higher initial temperature than concrete with lower temperature, the durability of concrete will decrease with initial temperature rise. Mohammad Reza Khosh Sima et al. Have achieved the above results in numerous experiments for concrete with initial temperatures of 17, 29, 39, 25.
B: Another item that is usually not considered in the production of concrete curbstones is the maximum aggregate size. Because of the direct relationship between aggregate size and compressive strength, the role of aggregates in the quality of concrete cannot be ignored. By doing different experiments, it can be seen that increasing the size of aggregates will create more empty spaces and thus less strength to concrete. Contrarily, a more uniform mixture is obtained by reducing the maximum aggregate size and increasing the fineness modulus and will be increased the adhesion surface of the cement paste and aggregates. So, the mechanical properties of the concrete will improve. The results of the experiments confirmed this which conducted by Seyed Amin Yousefi et al. for concrete with a maximum aggregate size of 6.35 and 12.5 mm.
C: Another factor to be considered is the choice of the water-cement ratio is low which so far has not been considered by previous researchers. In this study, dry press method is chosen due to the use of concrete with a low water-cement ratio and also the possibility of using it in open areas such as workshops. The kerbstones produced by this method will have higher durability due to the low water-cement ratio. Indeed, the unwanted entrapped air is always tried to get out of the concrete, but the use of bubble materials in concrete will increase the durability of concrete. It is possible to decide on the use of bubble materials and it is depending on where concrete curbstones are produced and used. Of course, it should be noted that the use of bubble materials is allowed up to a maximum of 7%, and for every 1% of the air bubble, approximately 4 MPa of the final strength of the concrete will be reduced. Therefore, the required calculations in the mixing design should be considered, to achieve a specific resistance, when using bubble materials.
It seems that because there is not much compressive or flexural strength in the concrete curbstones (regardless of the special cases), the main determinant factor in the design of these products is their lifespan and their durability against ice and melting cycles, this is important especially in areas where the difference between the temperature of the night and the day is high, some of these curbstones have a lack of uniformity in the mixture and appearance is inappropriate. This may be because the female tableware was out of order and did not have the same press. Before production, the production band should be cleaned to ensure that the surface of the curbs is smooth and free of debris.
As can be seen in the figure, the hand-watering method is used in the workshop for supplying water to the product’s manufacturer. Therefore, it should be noted that watering should be carried out continuously and uniformly by the trained worker and cover the surfaces of the curbstones by the appropriate cover. After the production of some curbs by curbstones making machine, the surfaces of the table molding machine should be cleaned and, if necessary, lubricated to increase the functionality and longevity of the curbstone making machine and the production curbs have a good appearance. Usually, such machines have high depreciation. Therefore, the necessity of handling and technical control of the curb making machine is inevitable. Today, there are equipped factories that usually use fixed concrete curb machines on the pallet in their concrete curb production line. Obviously, due to more facilities and better control and supervision in the production and processing, products are far more qualitative.
Figure 1: dry press curb making machine and tables sample
3. Mix design of dry press concrete kerb by ABA method
Considering the minimum compressive strength of 30 MPa and 7% entrained air and the largest dimension of angular aggregate 9.5 mm:
1-3 water-cement ratio
According to Fig. 2, the water-cement ratio is 0.64 for cement 525 and 0.59 for cement 425, which is obtained by taking 7% entrained air, respectively ratios, by 0.29 and 0.24 We use cement 525.
In this study because the maximum size of aggregates used in dry press kerbs is 9.5 mm, the curve of Fig. 3 is identical to the grading curves of the ABA manual which are used for this size and since the appearance of kerbs should be uniform, and it is attempted to choose the finer aggregates of gradation. So gradation of materials is selected between range B and C. The fineness modulus is obtained at 4.57, which is equal to the sum of remaining percentages on all the sieves.
3.3 Determination of free water of concrete
Because in this research, aggregates with fractures of more than 90% are used to manufacture dry press curbs, and ballast need for more water and the concrete slump in these curbs should be less than 1 cm, as shown in Figure 4. So the amount of free water is 145 kg / m3.
3.5 to 4 percent of the water should be reduced for each percentage of entrained air. So here's 25 percent of the water loss, and finally, the amount of water is 120 kilograms. For this amount of water and the water-cement ratio of 0.29, the amount of cement is 415kg. While the curves prepared to estimate free water content based on 350, and by these regulations, for each 10 kg of change in the amount of cement, 1.5 to 2 kg of mixed water should be changed in the same direction.
As mentioned above, 12 kg of water should be added to the obtained amount. So the amount of water finally reaches 132 kg.
3.4 Determine the amount of cement
Due to the water-cement ratio of 0.29 and the free water content of 132kg, the amount of cement is 455kg/m3.
5-3 Determining the amount of aggregate
The aggregate weight is saturated with a dry surface of 1672 kg / m3 by using the absolute volume relation presented in the ABA Code. Each of the aggregates (60% sand and 40% gravel) consists 1003 kg of sand and 669 kg of gravel.
2.65 = Aggregate Weight [1000-142-132-95] = 1672
The obtained weighted values with the ABA mix design are presented in Table 1.
As some issues mentioned in this study, we can briefly point the following:
- Dry press method in the production of concrete curbs will be preferred, because of the no need for the high number of concrete curb mold and high speed in the production.
-In the dry press method, to achieve a more uniform mixture, the concrete aggregate must have a higher fracture percentage and at the same time a smaller size.
-Since the curbs usually do not tolerate compressive forces and so on, their durability and longevity will be crucial in their design.
- In the dry press method, due to the low water-cement ratio, the produced curbs will have longer durability than the curbs produced by other methods.
- Depending on where the curbs are used, if used in cold areas, it is recommended to use air-entraining materials in their manufacture.
- If the air-entraining materials is used, the maximum permissible limit is 7% and the final compressive strength approximately 4 MPa shall be deducted for each percentage of entrained air.
-Maintenance, care and post-production process are important and significant to ensuring the ultimate quality of dry press curbs.
- If you follow the technical principles and producing high-quality curbs, you can be assured that they will suffer the least damage during loading, carrying and unloading.
Therefore, it can be assured that, given the above considerations and the use of dry press concrete curbs in the collection of surface waterways, the future heavy costs of repairing and replacing the kerbs will be largely avoided.
Concrete kerb making machines
concrete curbstones are considered as one of the most used material in modern urban building, most commonly used for landscaping and separating one or more surfaces in aqueducts and runnels. As the concrete curbs are exposed to cold, rain and all weather conditions, they must be of high strength and quality, so the quality and strength of the concrete curbs depend more on the type of table machine and the materials used which the following is an attempt to introduce you to different production methods.
Concrete kerbs are generally produced in two traditional and press ways:
In this way, the concrete is poured into the moulds and after the concrete has taken the form of the concrete mould and is processed, they remove the curb which is low in the water to cement ratio to extend the longevity of the curb as well as admixtures are also helped. Air entraining materials are also used to increase the strength of these tables against frost and cold.
The curbs produced in this method have a perfectly smooth and clean appearance but are not of sufficient strength.
Press and vibration method:
This method uses press machines called curb making machine which can be mobile and stationary (production line). The stationary model has a lot of press and vibration strength. The general process in these machines is as follows: First, materials with the dry mixing design are inserted into the mixer, then they enter the kerb making machine. In the curb-making machine, vibrations are used to fill in the materials inside the mould, so materials agglomerated and damages disappeared. The kerbs are then pressurized completely by using the press system and gets stronger and then removed from the mould in the final step to be dried.
Types of concrete kerb making machine by press and vibration method:
Kerbstone egg layer making machine (Cadona model)
(Cadona curb making machine’s name is taken from an Italian company which is semi-hydraulic)
Kerbstone egg layer making machine (packaging model)
(Packaging kerbstone making Machine, which is wrongly known in the market as Automatic kerbstone making Machine, is a fully hydraulic machine)
Concrete kerb production line (stationary)
(stationary machines are known as paving or paving machine in the market)
Wet press method
In this method, the concrete is wet, which is poured into the mould and its water removed by using a vacuum system and then extracted from the wet press. The system does not use vibrate.
- The sand should not be crusher and should be as round as possible, crusher sand increases the cement consumption by 10% and has no technical-economic justification.
- processing and mixing appropriate time is 1.5 minutes unless assured that less than this time-homogeneous concrete is achieved.
- Bulk cement should be stored in a metal warehouse or silo cement not in building warehouses and maximum storage time should not exceed 4 months.
-When using pocket cement, the floor of the warehouse should be 80 cm higher than the ground floor and as far as possible be without windows.
- Crusher gravel can reduce cement consumption by up to 5%