Building Materials and Construction (建筑材料与构造)
建筑材料与构造 tests material selection and the measured properties behind it: cement type, concrete grade, mortar, steel grade, waterproofing and insulation. Questions often turn on one number or one incompatibility.
Cement and concrete
- 硅酸盐 and 普通硅酸盐水泥: high early strength, high 水化热 - suited to precast and winter work, not to mass concrete.
- 矿渣硅酸盐水泥: low 水化热, good sulphate resistance, poor early strength; the usual choice for 大体积混凝土.
- 火山灰质硅酸盐水泥: good impermeability, needs curing, unsuitable for dry or frost-exposed work.
- Concrete grade C30 means the 立方体抗压强度标准值 fcu,k = 30 MPa on 150 mm cubes at 28 days, with a 95% guarantee rate - not the strength every batch reaches.
Mortar, steel, waterproofing, insulation
- 砂浆 grades M5, M7.5, M10 are measured on 70.7 mm cubes at 28 days.
- HRB400 rebar has design tensile strength 360 N/mm²; the number is the yield strength, not the design value.
- 防水: SBS/APP modified bitumen for roofs, 聚氨酯 coatings for wet areas, 水泥基渗透结晶 for basements; thickness and number of layers are specified, not optional.
- 保温: EPS, XPS, 聚氨酯 and 岩棉 differ in λ and in 燃烧性能 (A, B1, B2); a fire-rated detail may force mineral wool where a plastic foam would insulate better.
Worked example: for C30 concrete the target mean strength is fcu,0 = fcu,k + 1.645σ. With σ = 5.0 MPa for C25–C45, fcu,0 = 30 + 1.645 × 5.0 = 38.2 MPa. The mix is therefore designed for about 38 MPa so that 95% of cube results clear 30 MPa - a frequent exam calculation.
Traps
- Treating the grade as an average or a guaranteed minimum for every cube.
- Choosing 普通硅酸盐水泥 for mass concrete, then wondering about thermal cracking.
- Using 火山灰质水泥 in dry interiors.
- Giving the 屈服强度 when the question asks for the 抗拉强度设计值.
- Specifying a B1 foam where the detail demands an A-grade 岩棉.