20 17 key points of civil engineering for cost engineer: physical properties of sand

Are you ready for the exam? Is the column of cost engineer sincere in sorting it out? 20 17 key points of civil engineering for cost engineers: physical properties of sand? Candidates are welcome to study.

Knowledge points in civil engineering metrology textbooks: physical properties of sand

The apparent density is generally 2.55 ~ 2.75 g/cm3, the bulk density of dry sand is generally 1450 ~ 1700 kg/m3, and the bulk density also changes with the change of sand moisture content. If the dry sand is taken as the standard, when the water content is 5% ~ 7%, the volume will increase by 25% ~ 30%, which is caused by the water film attached to the sand surface. However, when the water volume continues to increase, the rupture volume of water film will decrease, so it is more reliable to control the sand content of mixed concrete.

The water content of sand can be divided into four States: completely dry, air-dried, surface dry and wet, with different water contents. In order to eliminate its influence on concrete quality, the standard stipulates that the mixture ratio design of aggregate should be carried out in dry state, and the water content in other States should be converted.

Knowledge points in civil engineering metrology textbooks: maximum particle size and particle gradation

(1) Maximum particle size. The maximum particle size of stone shall not exceed 1/4 of the minimum size of structural cross section, and shall not exceed 3/4 of the minimum clear distance between steel bars. For concrete solid slabs, the maximum particle size of coarse aggregate should not exceed 1/3 of the slab thickness, and should not exceed 40 mm If pumping concrete is used, it should also be selected according to the pump pipe diameter.

(2) Particle grading. The principle of gravel grading is basically the same as that of sand, but its grading is divided into continuous grading and intermittent grading.

Continuous gradation refers to the continuous gradation of particle size from large to small, in which stones of various grades account for an appropriate proportion. The dosage of intermittent graded cement with continuous ratio is a little more, but the mixed concrete has good fluidity and cohesiveness, which is the most commonly used grading form in cast-in-place concrete.

Intermittent gradation is an aggregate gradation that omits one or several intermediate particle gradations, and the gap between large particles is filled with small particles several times smaller than it, which reduces the void ratio and saves cement. However, due to the large particle difference, concrete mixture is easy to segregate. Therefore, discontinuous gradation is more suitable for mechanical vibration hard mixture with low fluidity.

Knowledge points in civil engineering metrology textbooks: classification of additives

① Admixtures to improve the mixing rheological properties of concrete, including various water reducing agents, air entraining agents and pumping agents.

(2) Admixtures for adjusting the setting time and hardening performance of concrete include retarders, early strength agents and accelerators.

③ Admixtures to improve the durability of concrete, including air entraining agent, waterproof agent, antifreeze agent and rust inhibitor.

(4) Admixtures to improve other properties of concrete, including air entraining agent, expansion agent, colorant, etc.

Knowledge points in civil engineering metrology textbooks: The commonly used water reducing agents mainly include

① Ordinary water reducer lignosulfonate, such as calcium lignosulfonate (hereinafter referred to as calcium lignosulfonate powder, M-type), the appropriate dosage is 0.2% ~ 0.3% of the cement mass, and the water reduction rate is 10% ~ 15% when the slump remains unchanged. Under the same requirements of strength and fluidity, cement can be saved by about 10%.

② Superplasticizer, such as NN0 water reducer, concrete mixed with NN0 is superior to ordinary concrete in durability, sulfate resistance, impermeability and corrosion resistance of steel bars. The appropriate dosage is about 1% of the cement quality, and the water reduction rate is 14% ~ 18% while keeping the slump constant. In general, the strength of concrete can be increased by 60% in 3d and about 30% in 28d. Under the requirement of maintaining the same concrete strength and fluidity, cement can be saved by about 15%.