Vật liệu bê tông trong xây dựng

  1. Construction concrete is defined as an artificial stone, created from a mixture of:

    • Loose objects such as sand, rocks or gravel with different particle sizes. The smallest type is sand with size from 1-5mm, the largest type is gravel or crushed stone with size from 5-40mm.

    concrete-material-1.jpg

    • The binder is usually cement mixed with water or other plasticizers. Note that the amount of water participating in the chemical reaction should only account for about 1/5 of the weight of cement.

    concrete-material-2.jpg

    • In addition to the main ingredients above, people often add additives to improve some properties of concrete during construction or use. Such as admixtures to improve the ductility of concrete mixes, admixtures to speed up or prolong the setting time of concrete, admixtures to enhance the strength of concrete, or admixtures to increase waterproofing. for concrete,…..

    concrete-material-4.jpg

    Types of concrete on the market

    Depending on the composition and structure of concrete, people will have different classifications such as:

    How to classify concrete based on its structure:

    • Solid concrete.
    • Porous concrete (this type usually uses less sand than solid concrete).
    • Honeycomb concrete.

    concrete-material-6.jpg

    How to classify by density of concrete:

    • Special heavy concrete: With density >2500 kG/m3.
    • Heavy concrete: With density = 2200-2500 kG/m3.
    • Heavy concrete with small aggregate: With density = 1800-2200 kg/m3.
    • Lightweight concrete: With density < 1800 kG/m3.

    How to classify by composition of concrete:

    • Regular concrete.
    • Small aggregate concrete.
    • Concrete filled with stone pits.

    How to classify by scope of use

    • Concrete as a bearing structure.
    • Heat resistant concrete.
    • Insulated concrete.
    • Anti-corrosion concrete.

    Concrete strength and how to determine

    Concrete strength is an important criterion to determine the bearing capacity of materials
    And concrete strength is determined through sample testing method with 1 of 2 following ways:

    • Method 1: Sample failure test, that is, people will cast test samples and then proceed to destroy the sample to determine the strength value of concrete.
    • Method 2: Non-destructive sample testing, that is, people will use ultrasonic waves or use concave pressing of balls on the concrete surface to determine the strength of concrete.

    When determining the strength of concrete, we need to determine the following two important criteria:

    • Compressive strength of concrete.
    • Tensile strength of concrete.

    How to determine the compressive strength of concrete

    Step 1: Prepare the test sample

    2 ways to take concrete samples:
    concrete-material-8.jpg

    • Method 1: Before pouring concrete, people will take the mixed concrete mixture to cast samples..
    • Method 2: Use specialized equipment to drill and cut concrete samples from existing structures.

    Sample shape
    concrete-material-10.jpg

    • Shape 1: Square block with side a (with a=100, 150mm or 200mm ).
    • Shape 2: Cylindrical block with square bottom side a.
    • Shape 3: Cylindrical block with round bottom with bottom area A=200cm2, axial block height h=2D=320 mm (this shape will often be used when sampling by drilling and cutting method).

    Step 2: Conduct sample experiment

    Using a compressor, increase the compression force slowly until the specimen fails as shown below.
    concrete-material-11.jpg
    After the test, we will determine the compressive strength of concrete according to the formula R=P/A. In there

    • P: Is the destructive force of the concrete sample.
    • A: Cross-sectional area of ​​the concrete sample.
    • And the unit of R is MPa with 1MPa=10^6 Pa= 10^6 N/m2= 1N/mm2= 9.91 kG/cm2.

    Step 3: Analyze the failure chart of the test specimen

    When the test piece is compressed, in addition to the shrinkage strain of the test piece in the direction of force application, the concrete also expands horizontally. The main cause of concrete cracking and breaking is due to this excessive horizontal expansion.
    In the experiment, when compressing, if we do not lubricate the contact surface between the test piece and the compressor, friction will appear at the contact surface between the concrete and the compressor. And it is this frictional force that prevents the horizontal expansion of concrete. Leads to the test sample being damaged in two opposite pyramids as shown below.
    concrete-material-13.jpg
    If in the experiment, we lubricate the contact surface between the test piece and the compressor so that the concrete is free to expand horizontally. Then the concrete sample will appear longitudinal cracks as below.
    concrete-material-14.jpg

    How to determine the tensile strength of concrete?

    Step 1: Prepare the test sample

    The tensile specimen has a square cross section of side a (as shown in figure a below) or a flexural specimen of rectangular cross section bxh (as shown in figure b below).
    Or the upper cylindrical sample (as shown in figure c below) if the tensile strength of the sample is determined by the method of splitting the circular cylindrical specimen.
    concrete-material-16.jpg

    Step 2: Conduct sample experiment

    Using a tensile or bending or splitting machine, gradually increase the force until the specimen fails as shown below.
    concrete-material-17.jpg

    Step 3: Calculation of tensile strength

    After the test, we will determine the tensile strength of concrete according to the formula R=2P/(πlD). In there

    • P: Is the load acting to split the sample
    • l: Sample length
    • D: Diameter of test piece
    • =3.14

    Factors affecting the strength of concrete

    Factor 1: Quality and quantity of cement

    Normally 1m3 of concrete we will need 250-500 kg of cement.
    The amount of cement more or less depends on the quality of the cement used to make concrete. If the quality is high, the amount of cement used will be less.

    Factor 2: Hardness, cleanliness and composition ratio of the mix

    Note to choose the correct proportion of materials corresponding to the expected concrete strength, check the level of concrete to know this ratio.
    And the harder a material such as sand, stone or gravel, the more it is cleaned before application, the better the concrete strength will be.

    Factor 3: Ratio of water and cement

    If this ratio is not reasonable, especially too much, it leads to a decrease in the density of the concrete, and an increase in shrinkage strain, leading to poor strength of the concrete.
    Look up the reasonable ratio between water and cement is specified according to the norm of concrete mortar mix.

    Factor 4: Quality of concrete mortar mixing

    Concrete mortar should be thoroughly mixed before pouring concrete.

    Factor 5: Quality of concrete mortar compaction

    When pouring concrete into the mold, the concrete mortar should be thoroughly compacted in accordance with regulations.

    Factor 6: Quality of concrete curing. Here are some tips for good maintenance of concrete

    concrete-material-20.jpg
    Spraying water on wooden formwork is the most effective way to keep moisture. Note to spray evenly, do not leave any dry area that will cause cracks in the crow’s feet on the surface of the small jet water sprayed continuously in a constant cycle.
    The watering cycle should also be regular. It is important to remember that a small continuous spray of water that provides frequent moisture is better than a rush of water but with a dry interval between sprays.
    When pouring concrete on the floor, the roof has a favorable surface, it is possible to build a row of walls to soak the cement water. Note that after 1 hour, stir the cement in the water with a brush because the heavy cement particles tend to stay in one place.
    During the first week, it is necessary to continuously water the concrete surface. If within two days after pouring concrete, it is raining, it must be covered, not to let the rain fall directly to pit the concrete surface.
    During the first 3 days, do not walk or leave on the freshly poured concrete floor. Time can go on reinforced concrete floor after 1.5 days in summer and 3 days in winter.
    In the first 7 days, watering every 3 hours during the day. At least once a night. From 14 to 18 days must be watered at least 3 times per day and night.
    Maintenance work must be maintained regularly for one week after the date of concrete placement. If it is cooler or more and can be reduced, but in hot weather, must be more frequent and longer.
    After the concrete begins to set, the concrete surface is covered with a layer of sawdust sand, rice straw or water hyacinth… The coatings have the highest water retention efficiency. In the hot summer weather, it is recommended to use paper (preferably a cement bag) or use a polyethylene film to cover the freshly poured concrete surface to keep moisture, use adhesive tape to stick at the joints.. The sheet must be covered. watered regularly.
    concrete-material-21.jpg
    Only remove the formwork when the concrete structure has reached sufficient strength of the material to stabilize the structure. Normally, people often consider the wrong time from 3 to 4 weeks when pouring under normal conditions ( 20 degrees C – 30 degrees C ) to be enough to unload the formwork, but if possible, the longer the better.

    Should concrete strength be tested after 7 days or after 28 days?

    The relationship between R and t

    Suppose call the age of concrete as variable t (ie time in days) from the date of manufacture to the time the sample is tested to determine the strength of the concrete sample.
    Let R be the variable of the strength of the concrete sample.
    Then according to the experimental results, when t is changed, we will have a graph of the relationship between t and R as shown below.
    concrete-material-22.jpg
    t=0 corresponds to the time when we knead and pour the mold, at that time the concrete is still in a paste state, so there is no strength, so R=0
    During the curing process of concrete, the strength R gradually increases. At first, R increases very quickly, but then it increases slowly.
    If concrete using Portland cement is manufactured and cured according to regulations, the strength will increase rapidly in the first 28 days. And that’s why it is common to test concrete samples after 28 days to determine R
    With favorable environmental conditions (positive temperature, high humidity) R can last for many years. In dry weather or low temperature, the increase in R strength of concrete is not significant
    If the strength enhancer is used, the concrete strength increases very rapidly during the first few days, but it also has the consequences that the concrete becomes more brittle and has a lower final strength (after several years) than that of concrete. The cardboard is cured under natural conditions without the use of additives.

    Answer the question when pouring fresh concrete, commercial concrete, should we test concrete strength 7 days or 28 days?

    The result of compaction of concrete samples at the age of 28 days is the highest legal value for acceptance of concrete quality. However, to ensure the reliability of concrete quality, it is possible to compress concrete samples at the age of 7 days to monitor the development of concrete.
    The simultaneous regulation of the results of compressing concrete samples at the age of 7 days and 28 days shall comply with the provisions of the technical instructions or the requirements of the design documents approved by the investor.
    If in the technical instructions or requirements of the design documents approved by the investor, it is not required that the results of compression of concrete samples at the age of 7 days are required by the Supervision Consultant, the investor must agree. written.

    What is the standard value of concrete strength?

    During structural calculations, you will often come into contact with the concept of the standard value of concrete strength, or standard strength for short.

    Formula for calculating standard strength of concrete

    Suppose call Rch: is the characteristic strength of the concrete test piece.
    Call γkc: Is the structure coefficient (usually γkc= 0.7-0.8 depending on Rch ).
    Then we have the formula for the standard strength of concrete: Rtc=γkc*Rch.

    Why does the coefficient kc . appear?

    This coefficient is generated by taking into account the different behavior between the concrete in the actual structure and the concrete of the test specimen. From there, the standard strength value of concrete is generated.

    What is the difference between durability grade and concrete grade?

    Concrete grade

    Concrete grade, or symbol M, is the average strength of the standard test specimen with units of kG/cm2.
    And if you don’t know what the standard test piece is, it’s a cube test piece with side a=15cm and 28 days old.
    Concrete grade is an old concept called by the old standard on reinforced concrete (Standard TCVN 5574-1991).
    According to the old standard TCVN5574-1991, we have the following concrete grades: M50, M75, M100, M150, M200, M250, M300, M350, M400, M450, M500, M600.

    Durability Level

    The concrete strength class, or symbol B, is the characteristic strength value of the standard test piece with units of MPa.
    The concrete strength level is specified according to the latest reinforced concrete structure design standard TCVN 5574-2012 replacing the standard TCVN5574-1991.
    Types of durability levels in concrete as specified in the new standard TCVN 5574-2012 include: B3,5 . B5 . B7.5 . B10 . B12,5 . B15 . B20 . B25 . B30 . B40 . B45 . B50 . B55 . B60

    Conversion table from concrete grade to concrete durability level

    Conversion formula: B=αβM

    • α: Is the unit conversion factor from kG/cm2 to MPa, with α=0.1
    • β: Conversion factor from average intensity to characteristic intensity, with β=0.778

    Conversion table from concrete grade to durability level.
    concrete-material-23.jpg

    Table of tensile and compressive strength of concrete

    concrete-material-24.jpg
    concrete-material-25.jpg

    Table of elastic modulus of concrete

    concrete-material-26.jpg

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