1. Cement

水泥质量不好是造成混凝土强度不足的关键因素。

The poor quality of cement is the key factor causing the insufficient strength of concrete.

2.石子石子质量不好包括石子强度低、体积稳定性差、石子形状与表面状态。在混凝土试块试压时，有时会发现石子被压碎了，这说明石子强度低于混凝土的强度，导致混凝土实际强度下降。有些由于多孔燧石、页岩、带有膨胀粘土的石灰岩等制成的碎石子，在干湿交替或冻融循环作用下，常表现为体积稳定性差，而导致混凝土强度降低。针片状石子含量高影响混凝土强度。石子表面光滑则无法与水泥砂浆结合好，也会影响混凝土强度。

2. Poor stone quality, including low stone strength, poor volume stability, stone shape and surface state. During the pressure test of concrete test block, it is sometimes found that the stone is crushed, which shows that the strength of stone is lower than that of concrete, resulting in the decrease of actual strength of concrete. Some crushed stones made of porous chert, shale and limestone with expansive clay often show poor volume stability under the action of dry wet alternation or freeze-thaw cycle, resulting in the reduction of concrete strength. The high content of needle flake stone affects the strength of concrete. If the stone surface is smooth, it cannot be combined with cement mortar, and it will also affect the strength of concrete.

3.砂子配制混凝土用砂子的粗细程度按细度模数分为粗、中、细3个等级，配制混凝土宜优先选用中砂。砂子质量的好坏可用细度模数、含泥量、泥块含量等几项指标来评定。其中含泥量、泥块含量对混凝土强度影响。这是因为泥块遇水会呈现浆状，胶结在一颗或数颗砂子表面，在混凝土中与水泥起着隔离作用，从而影响了水泥与石子的粘结力。

3. The thickness of sand for preparing concrete is divided into three grades: coarse, medium and fine according to the fineness modulus. Medium sand should be preferred for preparing concrete. The quality of sand can be evaluated by fineness modulus, mud content, mud block content and other indicators. Among them, mud content and mud block content have an impact on concrete strength. This is because the mud block will appear slurry when encountering water, which is cemented on the surface of one or several sands and plays an isolation role with cement in the concrete, thus affecting the adhesion between cement and stone.

二.未严格按照配合比施工及施工工艺不正确。

II The construction is not in strict accordance with the mix proportion and the construction process is incorrect.

1.用水量比较常见的有搅拌机上加水装置不准确；不扣除砂、石子的含水量，甚为了方便施工在浇筑地点上任意加水。这一现象除了对水严格计量外，还应根据现场当时砂、石子的含水率将试验室配合比换算为施工配合比，而按试验室配合比施工是不正确的。

1. The water consumption is common, and the water adding device on the mixer is inaccurate; The water content of sand and stone is not deducted, and even water is added at the pouring site for the convenience of construction. In addition to the strict measurement of water, the laboratory mix proportion should also be converted into the construction mix proportion according to the water content of sand and stone at that time, and the construction according to the laboratory mix proportion is incorrect.

2.外加剂掺量不准确普通型减水剂，如木质素磺钙、糖钙等掺量过大是常发生的掺量不准确现象。超掺严重时，会造成对混凝土强度的不足，值得注意的是，掺量少也会使28d强度达不到要求，但长龄期强度仍可达到空白混凝土强度。

2. The dosage of admixture is inaccurate. It is a common phenomenon that the dosage of ordinary water reducing agents, such as calcium lignosulfonate and calcium sugar, is too large. When the over mixing is serious, it will cause the deficiency of concrete strength. It is worth noting that a small amount of mixing will also make the 28d strength fail to meet the requirements, but the long-term strength can still reach the strength of blank concrete.

3.其他原因如外加剂使用不当；砂石计量不准确和水泥用量不足；搅拌不佳、时间过短造成搅拌不均匀；因模板问题造成的水泥浆漏失；养护不当，如早期缺水干燥、冬期受冻等原因都会造成混凝土强度不足。

3. Other reasons, such as improper use of admixture; Inaccurate measurement of sand and gravel and insufficient cement consumption; Poor mixing and short time cause uneven mixing; Cement slurry leakage caused by formwork problems; Improper curing, such as water shortage and drying in the early stage and freezing in winter, will cause insufficient concrete strength.