Rules for selecting concrete strength grades

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This post was last edited by “Hasty Passerby” on September 16, 2018, at 14:48. Rules for selecting concrete strength grades; Range of selection for concrete strength grades. I. Concrete strength grades: According to the standard **GB 50010–2002 “Code for Design of Concrete Structures”, concrete strength grades shall be determined based on the characteristic value of the cube compressive strength. The standard value of compressive strength for cubes refers to the compressive strength, with a 95% confidence level, measured using standard testing methods on cube specimens with an edge length of 150 mm that were prepared and cured according to standard procedures; this value is denoted as fcu,k. Ordinary concrete is classified into fourteen strength grades: C15, C20, C25, C30, C35, C40, C45, C50, C55, C60, C65, C70, C75, and C80. The concrete strength grade is an important basis for the design of concrete structures, quality control during construction, and project acceptance. Different construction projects and building components require concrete of different strength grades, with generally defined ranges for selection. II. Range of selected concrete strength grades: Different construction projects and different parts of a structure often require concrete of varying strength grades. Given the current level of concrete engineering in China, the general range of selected grades is as follows: ① C10–C15 – used for cushion layers, foundations, floors, and structures that are not subject to high stresses. ②C20～C25 —— Used for ordinary reinforced concrete structures such as beams, slabs, columns, stairs, and roof trusses ； ③C25～C30 —— Used for large-span structures, structures requiring high durability, precast components, etc ； ④C40–C45 — Used for prestressed reinforced concrete components, crane beams, and special structures; suitable for buildings with 25–30 floors ； ⑤C50～C60 —— Used for high-rise buildings of 30 floors to over 60 floors ； ⑥C60～C80 —— Used in high-rise buildings, with high-performance concrete ； ⑦C80～C120 —— Ultra-high-strength concrete is used in high-rise buildings. In the future, concrete of grades C130 and above may be widely used. Concrete is classified into various strength grades based on its strength, with these grades being determined by the standard value of compressive strength of cubes, fcu,k. The standard value of cube compressive strength is a value within the overall distribution of cube compressive strengths; the percentage of specimens with strengths below this value is no more than 5%, providing a 95% confidence level. III. Concrete mix ratio The concrete mix ratio refers to the proportional relationship among the various components of concrete (cement, water, sand, aggregates). There are two ways of expressing this: one is by indicating the amount of each material per cubic meter of concrete, such as 300 kilograms of cement, 180 kilograms of water, 690 kilograms of sand, and 1260 kilograms of gravel ； Another way is to express it using the ratio of the amount of various materials per unit mass of cement, along with the water-cement ratio of the concrete; for example, the previous example can be written as: C:S:G=1:2.3:4.2, W/C=0.6. ①. Common grade C20: Water – 175 kg; Cement – 343 kg; Sand – 621 kg; Gravel – 1261 kg. Mix ratio: 0.51:1:1.81:3.68. C25: Water – 175 kg; Cement – 398 kg; Sand – 566 kg; Gravel – 1261 kg. Mix ratio: 0.44:1:1.42:3.17. C30: Water – 175 kg; Cement – 461 kg; Sand – 512 kg; Gravel – 1252 kg. Mix ratio: 0.38:1:1.11:2.72. ②. Reference mix ratios for ordinary concrete: C20 concrete mix ratio – Slump range: 35–50 mm ； Sand type: Coarse sand; Design strength: 28.2 MPa ； Stone: river stone ； Maximum particle size: 31.5 mm; cement strength grade 32.5, with an actual strength of 35.0 MPa. The mix ratio for C20 concrete (by weight) is: cement:sand:aggregates:water = 1:1.83:4.09:0.50. In each cubic meter of concrete, the amount of cement is 326 Kg, sand amounts to 598 Kg, and aggregates amount to 1332 Kg. The mix ratio for C25 concrete: slump range of 35–50 mm ； Sand type: Coarse sand; Design strength: 28.2 MPa ； Pebbles: River stones (cobblestones) ； Maximum particle size: 31.5 mm; cement strength grade: 32.5, with an actual strength of 35.0 MPa. The mix proportion (by weight) for C25 concrete is: cement : sand : crushed stone : water = 1:1.48:3.63:0.44. In each cubic meter of concrete, the amounts are as follows: 370 kg of cement, 549 kg of sand, and 1,344 kg of crushed stone. During actual construction, the mix proportions of mortar and concrete may vary depending on construction and material conditions; therefore, adjustments must be made on-site according to the actual circumstances. Therefore, the aforementioned mix ratio is only a reference value. But the magnitude of the change won’t be very large. IV. Mix ratio of cement mortar http://img.civilcn.com/d/file/zhishi/jggc/2018-07-24/b39f2c0fc1609fd28a38a072ffe1e064.jpgMix ratio for M5 cement mortar: Conditions: construction level, general ； Sand, medium sand ； Sand moisture content: 2.5% ； Actual strength of cement: 32.5 MPa. M5 mix ratio (by weight): Cement:sand = 1:5.23. Per cubic meter of brick masonry, 0.238 m3 of M5 cement mortar is required, of which 67.59 Kg is cement ； Medium sand: 354 Kg (0.26 m3). Mix ratio for M7.5 cement mortar: Conditions: normal construction level ； Sand, medium sand ； Sand moisture content: 2.5% ； Actual strength of cement: 32.5 MPa. Mix ratio (by weight) for M7.5: Cement:sand = 1:4.82. Per cubic meter of brick masonry, 0.251 m3 of M7.5 cement mortar is required, of which 77.31 Kg is cement and 373 Kg (0.27 m3) is medium sand. Selection of concrete grade in construction projects: The impact of the concrete grade on columns and shear walls (regulation via axial compression ratio): Increasing the grade can significantly reduce the dimensions of columns and walls, thereby increasing the actual usable area of the building. Effect of concrete grade on beams: Under normal conditions, it has almost no impact on the load-bearing capacity of beams; therefore, its effect on the beam’s cross-section and reinforcement is minimal, and high-grade concrete should not be used. Effect of concrete grade on floor slabs: Under normal circumstances, it has almost no impact on the load-bearing capacity of the slabs; however, it may increase the reinforcement ratio required for the slabs, and it also raises the risk of cracking in those slabs. It is therefore advisable to use lower-grade concrete whenever possible. Recommendations for selecting concrete grades in practical engineering: The concrete grade for ordinary structural beams and slabs is generally C25 or C20. For beam and slab concrete subjected to high stresses, a grade of C30 can be used, such as in the floor slabs and roof slabs of basements, as well as in the slabs of rooftop gardens ； The concrete grade of shear walls and columns is controlled based on the axial compression ratio, so as to bring it as close as possible to the upper limit specified for this ratio; at the same time, the vast majority of vertical members are provided with structural reinforcement. Generally, the same grade is used for beams and columns; however, for columns and walls, it depends on the calculations. It is best not to use C40 or higher for multi-layer structures, as C40 cannot be mixed on site. High-rise buildings generally use C40-C50 grades. For residential buildings, C25 is sufficient for beams and slabs. If it is a large public building, then C30 is used throughout; the floor slabs and side walls of the basement are also made of C30. This is the standard practice in such cases. For ordinary multi-story structures, if the building design does not require extremely large cross-sectional dimensions for the vertical elements, there is no particular need to use high-grade concrete; generally, C25-C40 is used, with C30-C40 being the most common choice. Although an increase in concrete grade can reduce the amount of rebar required to a certain extent, the degree of this reduction seems to offer little economic advantage compared to the increases in concrete costs and the additional expenses arising from higher requirements for construction quality and standards. Therefore, unless there are special circumstances, neither the client, the designer, nor the contractor is willing to use high-grade concrete. From a mechanical standpoint, for multi-layer structures, the axial compression ratio of columns is not very high; thus, increasing fc has little effect on enhancing the load-bearing capacity of the members ； Since a beam is a bending member, provided its height is sufficient, the economic benefits resulting from increasing fc are not very significant ； Generally speaking, raising the strength grade is not worth the effort. However, as the number of structural floors increases, in high-rise and super-high-rise structures, the axial compression ratio of the vertical elements at the base level – namely columns and walls – is usually high. In such cases, increasing the strength of fc helps to reduce this axial compression ratio, thereby allowing for smaller cross-sectional dimensions of the elements and less reinforcement required. After a comprehensive comparison, it is common to use high-strength concrete for the vertical elements at the base level. As for horizontal members – beams, by the same logic, C40 is usually used at most. Although increasing fc in beams does contribute somewhat to their load-bearing capacity, when loads become higher, deformation and crack control become issues. Moreover, high-strength concrete members have poor ductility, so additional restraint bars are needed to improve this ductility; as a result, the benefits brought by high-strength concrete are significantly reduced or even outweighed by the drawbacks. http://img.civilcn.com/d/file/zhishi/jggc/2018-07-24/1d37fddb0c3b0539b174170effd28457.jpg