Taking control11 May 2000
Luis Ruiz Gaekel and George Casagran discuss the procedures and importance of a practical quality control and testing programme for RCC dams, and emphasise the need for more detailed technical specifications and innovative testing concepts to improve RCC quality control
The fast pace of RCC dam con-struction means that the quality and proportions of the mix components must be contin-uously monitored and tested in order to obtain a product of the required quality. RCC technical specifications are frequently revised and improved but the guidelines provided on quality control for key parameters of the RCC mix are still not sufficiently detailed. In addition investigative and preventive testing is usually not specified for checking potential variations in the properties, quality and quantity of the mix components. This paper outlines several tests and procedures which have proven to be a good complement to the standard quality control of lean RCC production. Many of these can also be applied to high paste mixes.
Variations in the quantity and quality of RCC mix components (particularly in lean mixes) can have a negative impact on the properties of the mix, particularly its strength. This impact can be compounded when the variations occur simultaneously and in more than one of the main components. Also, if the mix components are inadequately tested at the design and pre-production phase, different test results could ensue during the production phase. The experiences of the authors on several RCC dam projects have revealed that the degree and amount of testing specified is, in many cases, too general and limited in scope. This resulted in the need to set up additional quality control tests and procedures during the production phase.
The common practice for developing and implementing an RCC project is to begin a detailed trial mix programme, and then prepare the technical specifications. This is usually followed by the construction phase and the production quality monitoring programme. An intermediate phase between the trial and production phases or ‘verification mixes’ is also recommended.
Some of the tests presented may have been experimented or used by others on different RCC dam projects. The tech-nical aspects of these procedures have not been detailed or specified. Three elements proposed for assuring the quality of RCC for dams will be reviewed:
•Tests during the trial mix programme.
•Introducing verification mixes before the construction phase.
•Quality control procedures during the construction phase.
Trial mixes are usually produced during the feasibility or engineering phases. A series of laboratory mixes using the materials available for the RCC (ie cement, aggregates etc) are prepared and the strength parameters and physical properties of these mixes, as well as their respective components, are evaluated. A mix design can then be selected.
Trial mix programmes are extensive and detailed. However, the experiences of the authors have indicated that this testing phase does not adequately address the variable quality of the key components, particularly the cement and aggregates. On a few occasions the cement and aggregates initially used in the construction, although still conforming to the established standards and specifications, produced inadequate results because they were of a poorer quality than those used in the trial mixes. At the time no methods were in place to address the potential variability or its impact. Trial mix programmes should incorporate specific tests to examine this variability. The proposed tests are: cement and pozzolan testing; and the strength range/variability of the mix components.
In cement and pozzolan testing, cement and cement-pozzolan are systematically tested in parallel with all the RCC trial mixes. This concept arises from the fact that the quality of these components will often vary during the execution of the project, and that the normal use of mill certificates to monitor their quality was found to be insufficient, owing to:
•Inadequate frequency of mill certificates.
•Potential variation in the quality of materials between mill certificates.
•Potential impact on the quality of the cementitious materials due to transportation and storage.
Based on the correlation between the RCC and the cement and/or cement-pozzolan strength results, minimum strength values for the cement and cement-pozzolan can be determined to ensure that the required RCC strength is achieved. The engineer would then have the option to:
•Specify minimum strength values for the cement or cement-pozzolan which the contractor will need to supply, regardless of the permitted ASTM tolerances.
•Select a RCC mix design which takes into account the expected quality variations.
Regardless of the option selected, the results of the cement and cement-pozzolan testing programme can be used as benchmark references or requirements for monitoring the quality of construction.
Strength range and variability
In this test, the strength range or variability which can be expected with the selected cementitious materials and aggregates is evaluated. Mixes of aggregates and cement-pozzolan with variations in quality that are representative of the selected sources would be prepared and tested.
Cement and pozzolan
RCC mixes with cement and pozzolan from different productions are prepared. The aggregate quality is maintained, so that the effect of variations in cement and pozzolan quality is the only factor being assessed. In the event that this is not possible, the strength properties of the cement and pozzolan used in the trial mixes are compared with the historical mill certificates strengths. The historical mill certificates are still consulted to evaluate historical variations in quality.
Depending on the observed quality variation of the rock in the proposed quarry, materials from different areas are sourced and crushed for use in the trial mixes. The properties of these rock/aggregates are compared with those of the cores extracted from the proposed quarry to assess representativeness. If sourcing of the rock for the trial mixes is limited the properties of the cored material can still be compared with the sourced rock. This will help to indicate any potential variability in the quality of the rock. The crushing method used to prepare the aggregates should be well documented, as should the particle shapes produced.
The information obtained from these tests will enable the engineer to select a mix design with more confidence. The test results also highlight the need to identify specific quality ranges for some of the mix components.
The amount and extent of testing needed to evaluate the impact of quality variation in the mix components should be a function of the desired level of confidence in the selected mix, and the strength variations which can be tolerated at the production phase.
The conditions in which the RCC is produced during the construction phase are usually quite different from the controlled conditions of the trial mixes (varying cement quality associated with transportation and storage, different aggregate production system, gradation and moisture variations, etc). Most technical specifications specify some tests which the contractor is to execute before the construction phase. However, they usually do not outline a specific testing programme, its purpose, or when these tests are to be carried out. Also, most specifications are normally prepared with the intent that once the ‘in-spec’ gradation is obtained, the RCC production can begin without having to verify the mix design using the production components. Most specifications require conformity tests or a test section before construction commences. However, these tests are more related to confirming the adequacy of the contractor’s entire RCC placement operations, and the consistency of the RCC produced by the plant.
The experiences from several projects have confirmed the need for very specific laboratory mixes or verification mixes. The main objective of these mixes are:
•To fine tune the selected mix design.
•To establish the potential variability of the mix to changes in the quality and proportions of the mix components at the site production facilities.
•To identify site-specific preventive tests to consider during the construction phase.
These verification mixes should be produced as soon as the crushing plant is operational, and when the in-spec material is representative of the rock quality in the quarry. Most technical specifications require that a certain percentage of the aggregates for the dam are produced and stockpiled before beginning construction, and these mixes would be executed over this period.
Some basic tests are recommended for the verification mixes which will also help the laboratory staff to familiarise themselves with the specific parameters which may need closer or additional monitoring during the construction phase. These basic tests include:
•Strength range and variability of components: laboratory cylinders are prepared with aggregate and cement of varied quality and moisture content. The aggregates are selected from representative in-spec productions obtained from rocks of varying quality and locations in the quarry. Three different production batches can be used for the cement. An appropriate number of cylinders with varying moisture contents are prepared for these batches.
•Moisture content: moisture variation tests indicate the sensitivity of the mix to variations in water content, and confirm the optimal moisture content selected during the trial mix programme. These tests are also useful for the contractor’s and the engineer’s field staff to familiarise themselves with what an optimum mix feels like.
A correlation between the moisture content of 2-5kg samples of varying gradations with that of the 30kg samples can be evaluated to confirm the adequacy of using smaller samples for moisture assessment and control during the production phase.
•Cement-pozzolan quality: the same testing concept used in the trial mix stage for preparing cement and cement-pozzolan mortar cubes at the same time as the RCC cylinders is performed. With these results, and the corresponding RCC results, a correlation can be investigated to assist in monitoring the cement and cement-pozzolan during the production phase. The observed properties (strength, heat of hydration etc) of the different batches used can be compared to previous and historical cement and pozzolan productions.
Some of the cement and cement-pozzolan tests should include the same testing ages as the complete set of RCC cylinders. This will allow comparisons to be made between the strength development of the cubes and the RCC cylinders, and allow us to understand how the mix components are influencing the RCC results.
Production quality control
The procedure normally required for quality control of the cementitious material is the presentation of mill certificates. Some specifications require the quality testing of the cementitious components to be specified, but these are presented in general terms and specific testing concepts or procedures are usually not detailed. Consequently, the presentation of mill certificates is often the only way the quality of the cementitious material is monitored.
This basic requirement may be sufficient when the consistency and quality of the material is well established, and where the storage and handling of this material is adequate. Whether the material is supplied in bulk or in big bags may be a determining factor in the final quality of the material prior to use, and may consequently affect the test results.
Experiences on several projects have indicated that relying on the mill certificates as the sole means of monitoring the quality of the cementitious material is not sufficient. The geographic location of the project, cement source, frequency of submission of mill certificates, transportation and storage etc are all factors which should be closely evaluated when establishing the level of confidence in the quality and consistency of the material, and when determining the amount of testing to be done at the site.
The proposed testing programme is basically a follow-up of the cement and cement-pozzolan testing performed during the trial and verification phases. The programme has two components:
•Random sampling of all incoming cement or cement-pozzolan shipments.
•Sampling of cement or cement-pozzolan at the RCC plant during production.
The first component involves sampling and testing cement from each shipment as it arrives at the site, or at selected intervals. If the test results obtained are less than the benchmark results (associated with the acceptable RCC results from the trial and verification mixes), the shipment can be used in a less critical section of the dam, or rejected. In the case where a smaller tolerance range than the standard one for the type of cement selected was agreed with the supplier, the material could be rejected. The second part, unlike the random sampling which was more of a preventive measure, is used in the analysis of the RCC results. It involves sampling the cement, or cement-pozzolan directly at the RCC plant through a bypass at the feeder. The sampling is done in conjunction with the sampling of the field mix from the placement area, or with a plant sample. This allows a comparison to be made between the strength development trend of the RCC cylinders with that of the cement or cement-pozzolan cubes. This will show how the various mix components are influencing the RCC results.
On one particular project the results obtained from the cement-pozzolan testing was a key element in under-standing many of the RCC strength variations which could not otherwise be adequately explained. By performing these tests, and comparing them to the benchmark results from the trial and verification mixes, a projection of the expected RCC strengths can also be made.
The above sampling and testing procedure may seem excessive for some projects. However, past experiences have indicated that specifying some form of parallel testing programme for the cementitious components, right from the trial mix stage, is highly recommended.
The methods used to monitor the cement content of RCC are expensive, and there is no universal consensus on their advantage. Some suggest that these methods, if not carefully and properly conducted, may do more harm than good. In most projects where these methods are optional, they are rarely carried out. Efforts should be directed to determine the suitability of simpler and less expensive methods such as heat neutralisation, developed in Australia.
A common and more practical way to monitor the cementitious content is to monitor the cement and pozzolan fed into the mixer and compare it with the compacted volume of RCC placed in the dam. This method is not as accurate as the laboratory test but gives an acceptable indication of the cementitious material content. Also, past experiences have indicated that it may be possible to correlate the cementitious content with the differences in passing #200 sieve samples taken before and after the mixer. If the test conditions are controlled and the materials are consistent over time (ie passing #200 of sand and pozzolan), this may be a practical means of monitoring the cementitious content. In cases where the mix contains a high content of cement and clean sand, this may be more evident. Again, this method is not as accurate as the standard test, but it may indicate a relative increase or decrease in the content instead of the exact amount.
The aggregate gradation can have a considerable impact on the mix’s density and strength, depending on the deviation from the optimal curve of the specified envelope, the specific gravity of the various aggregate sizes, and the relative content of each group size. Technical specifications require gradation tests to be performed at certain frequencies, based on either the production rate or time. The end analysis usually involves confirming whether the resulting gradation is within the specified envelope. In terms of evaluating the impact of the gradation, particularly on the density and strength results, it is recommended to track and monitor certain gradation groups daily. The most influential group sizes of the combined gradation are usually passing sizes of 3/4”, #4 and #200. Important changes to these group sizes, even though the overall gradation may be acceptable, may indicate a need to monitor the crushing and blending operations more closely.
In conjunction with the aggregate gradation monitoring, the quality of the aggregates should also be closely monitored. Other than specific chemical and mechanical tests which are usually carried out during the trial mix phase, the tests which are generally specified and performed during the production phase are the mortar unit weight and the specific gravity of the various gradation sizes. But these tests do not provide specific testing frequencies and guidelines for monitoring the specific gravity and gradation sizes. Experiences on some projects have indicated the need to specify minimum specific gravity values
to ensure that the quality of aggregates produced will produce RCC with the required properties. Adequately establishing these values during the trial and verification phase is essential for proper quality control during the production phase.
The density of the RCC is the basic parameter used to monitor the mix’s quality during production. Nuclear density is the most practical method to determine in situ density, so most technical specifications rely on this test to evaluate the quality, and the subsequent acceptance or rejection of the final product. Also, densities of ‘fresh’ field samples recovered from the placement area are determined in the laboratory. Both of these methods are essential, and serve a specific purpose.
The in situ tests allow the engineer to evaluate the quality of the finished product, monitor the compaction process and implement the necessary adjustments to the process, and understand the mix’s behaviour. The field sample densities provide a better insight into the mix’s behaviour because of the controlled compaction conditions. This better qualifies the entire RCC operation, and provides benchmark values for in situ tests.
Both these tests also have disad-vantages. The in situ density tests are not always suitable for analysis purposes because of the varying compaction process (number of passes, operator efficiency, possible delayed compaction, etc). The field sample densities, although more appropriate for analysis purposes, may also be influenced by ‘field’ related causes. Also, because these tests are usually only done on fresh mix samples recovered for the preparation of strength cylinders, the testing frequency is low and consequently not adequate for production density monitoring.
The experiences of the authors on lean mix projects resulted in the density testing of fresh mix samples taken directly off the production belt at the plant (ie plant sample). The main reason for doing this was to improve mix production monitoring, particularly when variations in the quality of some of the key components were experienced. Other advantages associated with this testing procedure are:
•Quicker plant modifications, minimising the potential impact of placing non-conforming material.
•Differentiation between field and plant causes.
•Provision of more appropriate results for analysis and troubleshooting purposes.
•Temporary adjustment of the theoretical air free density (TAFD) or maximum density.
The purpose of the plant sample testing is not to replace the density tests usually specified (in situ and field samples), but to complement them, providing a better overall quality monitoring process during production.
Maximum density or TAFD
Most production procedures involve a method to determine the maximum density the RCC mix can theoretically, or physically, achieve. Methods range from calculating the TAFD, to compacting the sample until the maximum achievable density is obtained. The TAFD is the theoretical density of the RCC mix without the air content. This value is usually calculated in conjunction with the mix design calculations. The placed RCC is acceptable if the in situ densities are within certain percentages of the TAFD. These values (% of TAFD) are the cutoff points where lower density results will lead to low or unacceptable cylinder strengths.
Ideally, the quality of the quarried rock will not vary considerably, in which case the TAFD should not change significantly. But the TAFDs and laboratory densities calculated during the verification mixes should give an indication of the expected variability in rock quality. Should the quality of the quarried rock change the TAFD must be recalculated accordingly. The main indicators of a change in the quality of the aggregates are usually simultaneously varying RCC plant densities and specific gravities.
The exact mechanism for adjusting the TAFD is usually not detailed. The logical procedure would be to repeat the specific gravity tests when a change in the aggregate quality is observed and recalculate the TAFD. However, because of the time needed to recalculate the TAFD, the RCC placed and compacted during this time may be inadequately monitored.
The plant sample density can be used as a quick and effective method to temporarily adjust the TAFD. An average of the plant densities can be obtained and the air content deducted to give what is referred to as the production air free density (PAFD). The use of the PAFD as a temporary substitute for the TAFD, would only be acceptable if the other key parameters affecting the density are consistent, within specifications, and not a factor in the differing densities obtained (ie moisture content, gradation, etc).
The use of a representative TAFD is critical as over or under-compaction of the RCC can significantly, and adversely alter the properties of the in situ material.
The moisture content can influence the RCC cylinder strength and density of the mix. Most technical specifications for lean RCC mixes specify the use of a 30kg sample to determine the moisture content of the mix. This is the most effective method of determining the moisture content for analytical purposes. However, in terms of production quality control, this method is not effective because of the time delay in obtaining the test results.
Tests done by the authors have indicated that correlating moisture content of smaller samples (2-5kg) of varying gradations with the standard moisture content, may be an effective and quicker means of assessing relative moisture content.
The feel test, although not generally specified because of its subjective nature, is a common field method used to evaluate the moisture content of the mix. However, from past experiences, it has been concluded that more field staff training is necessary in order to find a consensus on what the optimum moisture is.
Sampling for cylinders
The sampling practice generally used for preparing cylinders for lean mixes, is to take the fresh sample from the layer. This is good practice because it takes into account all the quality variations associated with conveyance, transportation, exposure, placement and spreading, which might effect the RCC from the pugmill to the placement area. It is also helpful to simulate delays in the compaction of the RCC.
However, sampling and handling may adversely affect the strength results obtained with these samples. Consequently, some of the results may not be indicative of the actual RCC quality, and will hinder proper monitoring and analysis of the results. To account for this, it is recommended that a set of cylinders using fresh mix samples from the production belt (plant sample) also be prepared. This will eliminate any adverse field factors, which may otherwise alter the strength test results, and will confirm the production mix potential.
When periodically evaluating the the mix design potential, it is recommended that laboratory cylinders are prepared using the in-spec aggregate production and the mix design proportions. Preparing this mix under controlled conditions, will confirm the results ultimately achievable with the components available.
Revised and improved
Although RCC technical specifications are being continually revised and improved, the experiences of the authors on several RCC projects have indicated that they are often too general to allow site engineers to exercise a more effective quality control of the key mix components.
Many specifications still do not adequately address the potential variability of RCC mixes, the site-specific conditions of many projects, and the quality control conditions associated with the fast pace of RCC construction.
Based on these experiences, three concepts, each covering the main stages in the development and implementation of an RCC dam project are proposed:
•Implementation of a parallel testing programme during the trial mix stage geared specifically to the testing of the cementitious components and the
strength range and variability of the proposed RCC mix.
•The execution of ‘verification mixes’ between the trial mix and production stages with the main objective of confirming the behaviour and adequacy of the selected RCC mix using the actual production materials.
•Implementation of additional testing procedures and concepts during the construction phase to facilitate the day to day quality control operations of the RCC components, and assist in the analysis of the test results.
|Quality assurance of materials for dams Major dam projects normally establish a quality control (QC) programme and a quality assurance (QA) programme to monitor design and construction, as David Kleiner from Harza Engineering explains Quality control Quality control involves the routine testing of concrete, soil and rock materials that are used in the dam. Concrete quality control involves testing the quality of the ingredients, such as cement, flyash, fine and coarse aggregate and any admixtures that might be used. Concrete mixes are designed and tested in an on-site laboratory. The concrete is then sampled during placement and tested for various properties. Soil and rock materials are tested in a similar way. The various sources of materials are tested and approved. The materials are also tested after placement in the embankment. Tests, such as moisture, density, gradation, plasticity and compaction characteristics are routinely performed. In some cases, the contractor is entrusted with the responsibility of managing the QC programme. The specifications spell out the material requirements and frequency of testing. In other instances, the engineer or the owner will be responsible for managing the QC programme. Quality assurance A QA programme is normally managed by the owner or the owner's agent (engineer). The QA programme is designed to evaluate whether the design intent is being fulfiled during construction. This programme, depending on the size of the project, includes a number of elements. On-site staff will monitor all aspects of the construction. This involves review of test data collected during construction; monitoring of placement of all materials; monitoring of surface and subsurface excavations and support; monitoring of foundation preparation and treatment; checking field construction activities against the approved construction drawings and specifications; and responding to changes in field conditions. When the contractor is entrusted to prepare the detailed design of the project, the QA programme will also include a detailed review of the design and the analyses supporting the design. The engineer and/or owner would undertake this review. Often, a separate panel or board of consultants is engaged to provide broad oversight and review of the design and construction of the project as it relates to the fundamental safety and stability of the various project features.|