D6913和D7928取代D422粒度分析
这篇客座文章是由 杰克杰曼. 他是北京大学土木与环境工程学院的研究教授 塔夫斯大学.
Determination of the particle size distribution (PSD) is one of the most requested soil characterization tests. 结果可用于工程参数的分类和估计. 几乎每一个地质测试实验室都进行了这些测量. 不幸的是，PSD结果中的散射是远远不能接受的. 的 AASHTO资源能力测试程序数据 提供一个关于这种分散的优秀例子. 为了改善现状, ASTM Committee D18 on Soil and Rock has discontinued the use of D422/D421 for measuring the particle size distribution of soils. D422已被两个新标准所取代. D6913 is the standard for the sieve analysis of the coarse-grained portion and D7928 is the standard for the sedimentation analysis of the fine-grained portion.
杰曼教授 of the Civil and Environmental Engineering Department at Tufts University and past Chair of Committee D18 was involved in the original decision to replace D422 with the new standards. He was also a member of the task group charged with writing the new standards and is now the technical contact for revisions.
放弃D422的决定是在近20年前做出的. 然后用了大约15年的时间来制定和投票新标准. D6913 was first passed in 2004 and D7928 in 2017 so both have been around a significant amount of time. D422作为有效标准一直保留到2016年，以方便过渡. 不幸的是，由于种种原因，新标准的采用非常缓慢. D422在这个行业中根深蒂固. 它在建筑规范中有广泛的规定, 建设规范, 以及各种政府机构的文件. 除了, most labs routinely perform the test procedure and inspection agencies provide certification. 的 combination of these factors with the length of the new documents has proven a formidable obstacle to acceptance.
多年来，D422一直因各种缺点而受到批评. 最主要的问题是，它是一个一刀切的标准.
该程序需要以下步骤:
- 风干
- 在10号筛上分离
- 筛掉残留的部分
- 通过部分的比重计
- 在200号洗衣机上清洗
- 筛掉残留的部分.
然后将结果组合以获得完整的PSD. 这一过程适用于某些土壤，但并非适用于所有土壤. 风干需要时间，会产生结块，还会改变一些粘土矿物. Handling large quantities of dried soil creates a dust hazard and promotes segregation of the fines from the coarse particles. 并不总是需要同时进行筛析和沉降分析. It is a waste of time and money to run a hydrometer on a clean sand or perform a sieve analysis on a fine-grained sample.
D422还有几个严重的技术问题. 该标准缺乏对材料取样和次取样所用程序的控制. 这可能是导致结果分散的主要原因. 的 standard has an inconsistent criterion for the amount of material required for the test specimen. 的 criterion is based on the maximum particle size in the sample but requires too much material for the 3/8 inch size and far too little material for the 1.5英寸及以上尺寸. It does not specify how to reduce the mass of material from the whole sample to the test specimen nor does it address subsampling the plus #10 fraction.
的 new standards address the limitations of D422 and are based on sound physical principles with consideration of material attributes. 这些标准提供了不同的工作流程，以适应最大粒径, 可用材料的数量, 需要其他类型的测试. 的se workflows provide the quality control measures necessary for precision in the test results and allow variations in the procedure to improve testing efficiency.
该标准要求将给定尺寸的质量通过值报告为1%. 设定这样一个目标，就有可能建立防御控制. This criterion is the basis for calculation of the minimum dry mass requirement based on the maximum particle size. It limits the impact of having one of the largest particles being included or excluded from the analysis. 的 criterion is applied to collection of the test specimen as well as any subsamples obtained for various parts of the analysis. 它为过程中的任何步骤提供了一个简单的统一标准. It also allows the technician to use different sieves to separate samples depending to other tests being performed or to adjust for different grain size distributions. 然而，它确实如此, dramatically increased the required specimen mass when dealing with materials with large particles.
确保筛分操作是有效的，是一个严重的问题. D6913为此添加了两个控件. 第一个被称为重载标准. This sets the maximum mass that can be retained on a given sieve at the end of the sieving operation. 的 calculation is based on the mass of a layer of the particles that will completely cover the surface of the sieve. 随着颗粒变小，允许的层数从1到6不等. 的 limit is intended to make sure that each particle has an opportunity to pass through an opening. 第二个控制是评价筛振器有效性的要求. 的 standard provides a detailed process to evaluate the equipment and sets a numerical criterion for passing.
D6913 is a long standard primarily because it provides flexibility to accommodate different materials and situations. 给定由最小试样质量和筛超载所建立的约束条件, 有必要分段进行筛分分析. 该标准根据最大粒径分为3种不同的工作流程. Single set sieving uses one rack of sieves and is applicable when the maximum particle size is about 3/8 inch. Single separation composite sieving uses two racks of sieves and is applicable when the maximum particle is about 1 inch. Double separation composite sieving uses three racks of sieves and used went the maximum particle is above 1 inch.
D6913提供了使该过程更容易和更有效的选项. 例如，可以用两种不同的方法获得样品或子样品的干质量. 的 dry mass can be obtained using the sample prior to testing or based on the moist sample mass and a companion sample water content. 对于细小的样品，干燥然后处理是快速和容易的. 然而, 高细粒含量的材料在潮湿的条件下更容易加工, 所以同伴样本方法效果更好.
沉淀标准D7928与D422的比重计部分非常相似. 的 procedure covers processing from the minus 3/8 inch size with the test performed on <#10 or finer material. 结果以超过#10(或更小)尺寸的百分比表示. 材料必须在潮湿的条件下制备. 分散剂以粉末的形式加入，浆料隔夜回火. 比重计的尺寸改变，以满足目前可用的设备. A hydrometer calibration equation is provided to eliminate the need for running a companion cylinder. 这些变化很小，但使测试更有效.
新标准比D422更长、更复杂. I believe it will take considerable education and training to gain acceptance of the standards by the profession. 然而, 一旦实验室制定了内部程序, 测试应该与使用D422一样简单和快速. ASTM和D18积极参与提供教育工具. For example, elearning modules are in development which are expected to be available this summer. 的se training videos will provide visual support for the procedures as well as illustrate example workflows to show how the standard is applied to various materials. 除了, video content will be added to the standards to illustrate the procedure in practice. 几个月前, 我在ASTM网络研讨会上介绍了这两个标准的发展背景. 点击此处可免费获得本次网络研讨会 在这里.
参与! ASTM标准通过协商一致的投票过程不断修订. D7928 is being balloted at the subcommittee level and D6913 is being revised for balloting this summer. 如果你有兴趣发表意见的话, 加入ASTM并参与标准制定过程是很容易的. 成为会员没有资格要求，每年的费用仅为75美元. 有关更多信息，请使用此 link.