摘要
水泥(砂)浆在混凝土中起胶凝和润滑的双重作用,其用量随着混凝土流动性要求的提高而增大,已有研究发现浆体过多会导致混凝土的体积稳定性问题,环保绿色程度低,但骨料级配等因素对混凝土中浆体合理用量的影响研究尚不多见。在粗骨料低空隙率前提下固定粗骨料用量不变,调整砂浆用量来控制混凝土扩展度,研究骨料级配和砂浆流变特性对粗骨料包裹砂浆层厚度及用量的影响。试验结果表明:粗骨料的几何平均粒径增大,包裹砂浆层厚度单调增加,包裹砂浆用量会呈现先减小后增大的趋势,存在最优几何平均粒径;扩展度要求越高,包裹砂浆层厚度及包裹砂浆用量越大,最优几何粒径越小;当粗骨料级配不变时,包裹砂浆层厚度和包裹砂浆用量均随砂浆屈服应力和塑性黏度的增大而增大。
随着城镇化的推进以及基础设施的快速完善,混凝土也进入以大流态预拌混凝土为主的阶段。粗骨料作为混凝土组成中不可或缺的一环,占据了混凝土总体积的50%~70
混凝土中的粗骨料并不是简单的堆砌,通过硬化混凝土的切片和研究发现,粗骨料在浆体中处于悬浮状
笔者在粗骨料低空隙率的前提下定量研究粗骨料级配和砂浆流变特性对包裹砂浆层厚度和包裹砂浆用量的影响,以期得到粗骨料级配的优化和评价。
水泥:P·O 42.5水泥,其主要性能如
标稠用水量/% | 凝结时间/min | 细度(80 μm筛余)/% | 抗折强度/MPa | 抗压强度/MPa | |||
---|---|---|---|---|---|---|---|
初凝 | 终凝 | 3 d | 28 d | 3 d | 28 d | ||
26.9 | 160 | 240 | 0.4 | 4.6 | 7.5 | 22.8 | 47.5 |
粉煤灰:试验采用Ⅱ级粉煤灰,其主要性能指标如
45µm筛余/% | 密度/(g/c | 需水量比/% | 比表面积/( | 活性指数/% |
---|---|---|---|---|
10 | 2.24 | 93 | 352 | 80.0 |
矿渣:试验采用S105矿渣,其主要性能指标如
45 µm筛余/% | 密度/(g/c | 比表面积/( | 流动性比/% |
---|---|---|---|
4.01 | 2.81 | 438 | 91 |
减水剂:试验外加剂采用聚羧酸系高效减水剂,含固量为40%。
细骨料:细骨料为安山岩机制砂,产地为湖州,其性能指标如
细度模数 | 压碎指标/% | 石粉含量/% | 亚甲蓝值/(g/kg) |
---|---|---|---|
3.4 | 21 | 2.2 | 0.4 |
粗骨料:主要的性能指标如
表观密度/(kg/ | 紧密堆积密度/(kg/ | 吸水率/% | 压碎指标/% | 空隙率/% |
---|---|---|---|---|
2 575 | 1 751 | 0.6 | 6.8 | 41 |
C30和C50混凝土中粗骨料级配分别如
式中:为粗骨料的几何平均粒径,mm。
组别 | 分计筛余百分比/% | 几何平均粒径/mm | |||
---|---|---|---|---|---|
4.75~9.5 mm | 9.5~16 mm | 16~19 mm | 19~26.5 mm | ||
级配1 | 100.0 | 0.0 | 0.0 | 0.0 | 6.7 |
级配2 | 55.0 | 45.0 | 0.0 | 0.0 | 9.2 |
级配3 | 40.0 | 39.0 | 21.0 | 0.0 | 11.2 |
级配4 | 34.0 | 29.7 | 20.1 | 16.2 | 13.0 |
级配5 | 29.4 | 15.1 | 29.3 | 26.2 | 14.7 |
组别 | 分计筛余百分比/% | 几何平均粒径/mm | |||
---|---|---|---|---|---|
4.75~9.5 mm | 9.5~16 mm | 16~19 mm | 19~26.5 mm | ||
级配5 | 29.4 | 15.1 | 29.3 | 26.2 | 14.7 |
级配6 | 33.1 | 21.9 | 27.8 | 17.2 | 13.5 |
级配7 | 44.1 | 18.9 | 19.6 | 17.4 | 12.5 |
级配8 | 61.3 | 17.2 | 21.5 | 0.0 | 10.0 |
级配9 | 73.5 | 26.5 | 0.0 | 0.0 | 8.2 |
混凝土配合比的设计原则为:在粗骨料质量不变的前提下,调整C30和C50混凝土中粗骨料与总砂浆浆体的比例,使新拌混凝土扩展度达到500、600、700 mm,C30和C50混凝土的配合比分别如
砂浆用量倍数 | 砂率/% | 水泥用量/kg | 粉煤灰用量/kg | 矿粉用量/kg | 水用量/kg | 砂用量/kg | 石用量/kg | 减水剂用量/% |
---|---|---|---|---|---|---|---|---|
0.8 | 39.6 | 18.05 | 7.38 | 4.92 | 13.74 | 65.44 | 100 | 0.68 |
0.85 | 41.0 | 19.18 | 7.90 | 5.23 | 14.56 | 69.54 | 100 | 0.68 |
0.9 | 42.4 | 20.31 | 8.31 | 5.54 | 15.38 | 73.64 | 100 | 0.68 |
0.95 | 43.7 | 21.44 | 8.82 | 5.85 | 16.31 | 77.74 | 100 | 0.68 |
1.0 | 45.0 | 22.56 | 9.23 | 6.15 | 17.13 | 81.85 | 100 | 0.68 |
1.05 | 46.2 | 23.69 | 9.74 | 6.46 | 17.95 | 85.95 | 100 | 0.68 |
1.1 | 47.4 | 24.82 | 10.15 | 6.77 | 18.87 | 90.05 | 100 | 0.68 |
1.15 | 48.5 | 25.95 | 10.67 | 7.08 | 19.69 | 94.15 | 100 | 0.68 |
1.2 | 49.5 | 27.08 | 11.08 | 7.38 | 20.51 | 98.26 | 100 | 0.68 |
1.25 | 50.6 | 28.21 | 11.59 | 7.69 | 21.44 | 102.36 | 100 | 0.68 |
1.3 | 51.6 | 29.33 | 12.00 | 8.00 | 22.26 | 106.36 | 100 | 0.68 |
1.35 | 52.5 | 30.46 | 12.51 | 8.31 | 23.08 | 110.46 | 100 | 0.68 |
1.4 | 53.4 | 31.59 | 12.92 | 8.62 | 24.00 | 114.56 | 100 | 0.68 |
1.45 | 54.3 | 32.72 | 13.44 | 8.92 | 24.82 | 118.67 | 100 | 0.68 |
1.5 | 55.1 | 33.85 | 13.85 | 9.23 | 25.74 | 122.77 | 100 | 0.68 |
砂浆用量倍数 | 砂率/% | 水泥用量/kg | 粉煤灰用量/kg | 矿粉用量/kg | 水用量/kg | 砂用量/kg | 石用量/kg | 减水剂用量/% |
---|---|---|---|---|---|---|---|---|
0.8 | 35.7 | 27.18 | 3.88 | 7.77 | 12.84 | 55.66 | 100.00 | 0.39 |
0.85 | 37.1 | 28.91 | 4.10 | 8.31 | 13.59 | 59.12 | 100.00 | 0.39 |
0.9 | 38.5 | 30.64 | 4.42 | 8.74 | 14.46 | 62.57 | 100.00 | 0.39 |
0.95 | 39.8 | 32.25 | 4.64 | 9.28 | 15.21 | 66.02 | 100.00 | 0.39 |
1.0 | 41.0 | 33.98 | 4.85 | 9.71 | 16.07 | 69.47 | 100.00 | 0.39 |
1.05 | 42.2 | 35.71 | 5.07 | 10.25 | 16.83 | 73.03 | 100.00 | 0.39 |
1.1 | 43.3 | 37.43 | 5.39 | 10.68 | 17.58 | 76.48 | 100.00 | 0.39 |
1.15 | 44.4 | 39.05 | 5.61 | 11.22 | 18.45 | 79.94 | 100.00 | 0.39 |
1.2 | 45.5 | 40.78 | 5.83 | 11.65 | 19.20 | 83.39 | 100.00 | 0.39 |
1.25 | 46.5 | 42.50 | 6.04 | 12.19 | 20.06 | 86.95 | 100.00 | 0.39 |
1.3 | 47.5 | 44.23 | 6.36 | 12.62 | 20.82 | 90.40 | 100.00 | 0.39 |
1.35 | 48.4 | 45.85 | 6.58 | 13.16 | 21.57 | 93.85 | 100.00 | 0.39 |
1.4 | 49.3 | 47.57 | 6.80 | 13.59 | 22.44 | 97.30 | 100.00 | 0.39 |
1.45 | 50.2 | 49.30 | 7.01 | 14.13 | 23.19 | 100.76 | 100.00 | 0.39 |
1.5 | 51.0 | 51.02 | 7.34 | 14.56 | 24.06 | 104.31 | 100.00 | 0.39 |
根据粗骨料堆积试验得到粗骨料的紧密堆积空隙率,用总砂浆用量减去粗骨料的紧密堆积空隙后再除以粗骨料几何平均粒径下的比表面积,即为骨料表面的包裹砂浆层厚

图1 C30混凝土包裹砂浆层厚度与几何平均粒径关系
Fig. 1 Relationship between C30 concrete mortar coating thickness and geometric mean particle size

图2 C50混凝土包裹砂浆层厚度与几何平均粒径关系
Fig. 2 Relationship between C50 concrete cladding thickness and geometric mean particle size

图3 C30混凝土不同扩展度要求下粗骨料包裹砂浆用量与几何平均粒径的关系
Fig. 3 Relationship between coarse aggregate mortar coating content and geometric mean particle size under different slump-flow requirements of C30 concrete

图4 C50混凝土不同扩展度要求下粗骨料包裹砂浆用量与几何平均粒径的关系
Fig. 4 Relationship between coarse aggregate mortar coating content and geometric mean particle size under different slump-flow requirements of C50 concrete
选择两种不同强度(C30、C50)的混凝土中砂浆用量倍数为1时的配合比作为参照,配制砂浆时除去粗骨料,而水泥、粉煤灰、矿粉、机制砂、水的比例不变。两种砂浆的流变曲线如

图5 两种砂浆的原始流变曲线
Fig. 5 Original rheological curves of two mortars
组别 | 屈服应力/Pa | 塑性黏度/(Pa·s) | 包裹砂浆层厚度/μm | ||
---|---|---|---|---|---|
扩展度500 mm | 扩展度600 mm | 扩展度700 mm | |||
C30 | 191.7 | 3.2 | 1 598.7 | 1 987.8 | 2 588.7 |
C50 | 346.5 | 5.1 | 2 933.2 | 3 483.5 | 3 666.4 |
总砂浆用量中的填充砂浆可以看作粗骨料的紧密堆积空隙体积,而现有的骨料密实级配堆积理论有k法、N法和I法。I法和N法为无穷级数,无法规定粗骨料的最小粒径;k法则通过规定最小粒径,从而得到最大粒径和最小粒径区间的密实堆积级配,笔者通过k法得到粗骨料的堆积级配曲线。k法是通过颗粒分级质量递减系数k设计堆积级配曲
式中:为级数;为骨料颗粒最大粒径;为第x级筛孔粒径,mm。
总级数为
式中:为骨料第n级粒径;n为总级数。
某一级筛的通过率为
=
式中:k为k法计算系数。
在对级配进行优化时先将粗骨料的低空隙率作为前提,几何平均粒径随着k值的增大而减小,通过调整3种扩展度下k值的变化范围,从而限制几何平均粒径的范围。水胶比不同的两种混凝土填充砂浆用量始终小于包裹砂浆用量,曲线走向较为平稳,在k=0.75时取得最小用量25 L;包裹砂浆用量在总浆体量中占据主导地位,且包裹砂浆的最小用量均随着扩展度的增大而增大。在骨料级配、配合比以及工作性要求都相同的情况下,骨料粒形综合指数如果差异较大,那么浆体用量也会上下浮动5%~10

(a) 500 mm扩展度时系数k与各浆体用量的关系

(b) 600 mm扩展度时系数k与各浆体用量的关系

(c) 700 mm扩展度时系数k与各浆体用量的关系
图6 C30混凝土不同扩展度要求时系数k与各浆体用量的关系
Fig.6 The relationship between the coefficient k and each mortar content at different slump-flow requirements of C30 concrete

(a) 500 mm扩展度时系数k与各浆体用量的关系

(b) 600 mm扩展度时系数k与各浆体用量的关系

(c) 700 mm扩展度时系数k与各浆体用量的关系
图7 C50混凝土不同扩展度要求时系数k与各浆体用量的关系
Fig. 7 The relationship between the coefficient k and each mortar content at different slump-flow requirements of C50 concrete
在试验条件下,得到影响骨料包裹砂浆层厚度及用量的主要因素和规律如下:
1)包裹砂浆层厚度随着粗骨料几何平均粒径的增大而增大,而包裹砂浆用量则呈现先减小后增大的趋势。
2)随着砂浆流变特性的变化,达到相同扩展度时所需包裹砂浆用量随之增加。当砂浆的屈服应力、塑性黏度分别增大了80.8%和59.4%时,粗骨料包裹砂浆层厚度增加了41.6%~83.5%。
3)随着混凝土扩展度要求的提高,粗骨料的最优几何平均粒径取值区间减小,而包裹砂浆层厚度和包裹砂浆用量增大。当C30和C50混凝土的扩展度要求从500 mm增大到700 mm时,最小包裹砂浆用量分别增大了24.7%、37.9%,而对应的最优几何平均粒径分别减小了16.3%、8.7%,较小粒径的粗骨料更适合配制大流态混凝土。
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