Roller Compacted Concrere (RCC)
Roller compacted concrete is a new type of concrete capable of reaching high strengths and durability with minimum cement content. Read the article to get an idea about the significance of RCC. Due to its differences, a good understanding of quality control in roller-compacted concrete is important before construction.
Placing RCC usually follows the simple steps of transporting, spreading, compaction, and surface preparation. The performance of RCC is highly depending on the placement quality of the mix. Even though transporting and spreading works simply cover using earthmoving machinery, compaction, and surface preparation are tricky. They are highly sensitive to the performance and durability of the structure.
The key tests and areas to pay attention are as follows.
RCC contains the same ingredients that you use in conventional concrete despite different proportions. When placing a good volume of concrete the uniformity of the mix is important. Otherwise, the characteristics of the concrete can vary between batches. Maintaining a steady & uniform mix is totally up to the batching plant. However, dumping and spreading should do in a manner where segregation is least. Maintain a good quality control in roller-compacted concrete both batching and placing is essential to perform well.
Normally mixes give good compressive strengths but the parameter to focus is consistency. When water content varies, consistency changes making difficult to reach optimum packing density. Further, the main parameters affecting the mix proportioning of RCC are as follows.
Moisture content in the aggregate is important to monitor at the batching plant. However, changing weather conditions can affect this. ASTM C566 is the related standard test for the moisture content of aggregates. Moisture content is one of the important parameters to control the consistency in an acceptable range.
Aggregate content in RCC is optimized to achieve maximum packing and reduce the void content. For that reason, the gradation of fine and coarse aggregates are adjusted. The typical RCC mix contains aggregates about 80% by volume. One of the theoretical methods to predict the gradation curve is by Fuller’s equation. However actual test results are required to finalize the aggregate gradation for the mix.
Gradation of the aggregate is directly affecting the workability and compaction of the mix. You can tune the gradation of the aggregate to reach the best possible packing. This gives you the least void content. Then the mix will be economical because cement paste to fill up the voids will be minimum. Make a set of samples with different gradations and check the compacted density. You can see what mix gives the optimum packing.
Preferable coarse and fine aggregate gradations are as follows.
Coarse aggregate gradation according to Engineering Manual 1110-2-2006 US Army Corps of Engineers
|Sieve size||Cumulative passing %|
Fine aggregate gradation according to Engineering Manual 1110-2-2006 US Army Corps of Engineers
|Sieve size||Cumulative passing %|
|4.75 mm||95 – 100|
|2.36 mm||75 – 95|
|1.18 mm||55 – 80|
|600 µm||35 – 60|
|300 µm||24 – 40|
|150 µm||12 – 28|
|75 µm||6 – 18|
Normal Portland cement is suitable for RCC. However minimum cement content is using as a major characteristic of RCC. Supplementary cementitious materials like fly ash and granulated blast furnace slag are popular in the mixes. Generally, mixes use fly ash nearly 40 to 50 % of cement weight. ASTM C150 is available to check the characteristics of cement.
Air entraining and water-reducing admixtures are using commonly with RCC. These can vary the consistency, setting time, and air content in the mix. Particularly introducing admixtures affect positively on the workability which leads to reduce Vebe time, makes easy for compaction, and even lower the cement consumption.
Common tests on RCC
Vebe test: This is one of the most important tests that you do quality control in roller-compacted concrete. The standard vebe test is reformed for checking RCC’s consistency as a modified vebe test ASTM C1170. Firstly, fill the container with the RCC mix and place it on the machine. To add the effect of vibratory rollers, there are 2 surcharge loads (12.5kg, 25kg) you can select according to the workability of the mix. An eccentric motor vibrates the table. The test measures the time taken to separate mortar from the mix and reach to the top under a standard vibration.
The test is suitable for consistencies between 10 s – 60 s. Consistencies changing 15 s to 25 s is generally easy to place and vibrate. However, mixes with higher vebe times are more economical due to the high amount of filler materials and less cement consumption. Do some trial tests to find the most suitable consistency for the work.
Placing temperature of the mix largely affected by the temperature of aggregates using. When the pour is large, it is important to maintain placing temperature to mitigate the risk of thermal cracks. There are few Programs to analyze and predict the internal temperature rise during hardening. One of the free and easy software available is Concrete works.
Modified Proctor compaction test.
The same Proctor compaction test that use to find optimum moisture content for soil is using for RCC too. Here, you have to measure the mixture density after compaction with a variation of moisture content for a series of samples. Then select the moisture content giving the best possible compaction using all data drawn in a graph.
Moisture content testing
Even though this is one of the simplest tests, it reveals important information on the performance of mix during quality control in roller-compacted concrete. Moisture content is the most common variable during pouring. In wetter consistencies, vibration may separate the paste quickly even sinking the rollers in concrete. When it’s dry, compaction is hard.
Due to low water content in the mix, RCC is highly sensitive to environmental conditions. As an example, if some dry wind is present at the site, your top surface of RCC will tend to dry quickly making not suitable for next layer placement. So maintaining desired moisture content compatible with site conditions is a must with RCC.
A simple moisture content test using drying methods is applicable for RCC too. The standard test is ASTM C566. However, the easiest and quickest method to check the moisture content using a nuclear density gauge. Especially it gives the degree of compaction and moisture content in a few seconds at the location you use.
Get an idea about testing frequencies using in RCC dam construction.
|Grading of fine and coarse aggregates||ASTM C136||Daily or every 2300 m3|
|Aggregate Moisture content||ASTM C566||1 time per shift|
|RCC moisture content||ASTM C566||2 times per shift|
|Vebe test||ASTM C1170||2 times per shift|
|Nuclear gauge calibration||Daily|
|Wet compacted density||ASTM C1040||10 times per shift or 1 per 50 m3|
|Temperature||ASTM C1064||2 times per shift|
|Casting test cubes||ASTM C1176||1 time per shift|
As we do in conventional concrete, cube strengths and cylindrical strength are the key parameters to evaluate the strength of RCC. Usually, 150 mm cubes or cylindrical specimens are using at the site for compressive strength. Larger aggregates than 38 mm are removed using wet screening and prepare the cube. Vibrating hammer with plate foot can give similar compaction as the rollers do it at the site.
RCC shows good initial compressive strength due to aggregate interlocking. However, strength gain is much slower compared to conventional concrete because of its low cement content. Considering this slow strength gain, the compressive strength of RCC is considering at periods of 90, 180, 360 days.
Construction with RCC
After mixing, the first thing is to transport it to the site. The contractor can use various methods for this. But the most common and effective methods are conveyer belts and dump trucks. If easy access is available, dump trucks will be the easiest. Truck tires should wash before entering to the concrete because they may bring unwanted material.
Immediately after the delivery, the temperature is checked before spreading. 300 mm is the common layer thickness which easily compactable using 10 to 15-ton steel drum rollers. It is better to compact the RCC as fresh as possible. In general, 60 minutes is the limit for compaction after the addition of water in mixing.
Especially, the first roller pass and finishing pass should be non-vibratory roller passes for a quality finish on the surface. The amount of vibratory roller passes to acquire design compaction will vary according to the mix design characteristics. The best practice is to do a pilot test on a few mixes. This will surely help to identify the number of vibratory roller passes that needs to reach satisfactory compaction.
An important rule for compaction during quality control in roller-compacted concrete in dams is always driving the roller perpendicular to water flowing direction. This will prevent any poorly compact material between two adjacent roller paths falling along water flowing direction.
It is risky to operate roller with vibration near formworks. In the areas, jumping jack vibrators can use with multiple thin layers. A Further effective method is mixing cement grout separately at the site near formwork locations. This grout enriched vibratory RCC (GEVR) can vibrate with poker vibrators.
Compaction can be tested using nuclear density meters. This instrument frequently uses with quality control in roller-compacted concrete. It operates using a nuclear pulse which is harmful to be exposed. So after you install, keep your distance about 5 m for safety. Before every pour, the best practice is to calibrate the instrument. Calibration block with known density is the key here. Prepare an RCC block of 0.76 m3 (1yd3) for the work. Use the instrument on the block first and calibrate before bringing in to the site. A typical mix of RCC may have theoretical density of 2240 kg/m3 – 2560 kg/m3. Achieving 98% of it is generally acceptable.
The number of density testing points for a layer of concrete is depending on the scale of the work. In large concrete pours one point for each 150 – 200 m2 surface area of each 300 mm thick layer is the thumb rule using. Use the following simple steps in testing density using a nuclear density gauge.
- Prepare the location as much as horizontal and wipe any loose material
- Hammer the drill rod into the concrete 2” more than the testing depth
- Remove the drill rod and install the instrument on the same location
- Input parameters like time, depth and proctor
- Lower the source rod as required and make sure it contact with concrete
- Start the pulse and keep yourself 1 minute aside until a beep sound indicate the test is complete
- Record the data with the location and depth
Main parameters that can test using nuclear density gauge are,
- Wet and dry density
- Moisture percentage
- Degree of compaction
Compaction can improve even after the test with extra roller passes. However, moisture content cannot be adjusted much at the site. Best is to change the moisture content at batching and protect the concrete by polythene sheet to reduce evaporation. Another method to control evaporation is to saturate the air with moisture. So there won’t be any moisture loss from concrete. Use high-pressure water spraying machines to moist the air.
The bond between RCC layers is one of the essential areas to keep your attention on. Many dams have shown water seepages along the interlayer joint. Cement rich RCC mixes have successfully reduced this risk of failure. Best practice to mitigate this is good compaction and placing the next layer before the setting time of concrete. In case, if the surface got dried, placing a mortar layer of 2-3 cm can improve surface quality.
As similar to conventional concrete RCC shrinks with hardening. Too long RCC layers may end up with shrinkage cracks. This displacement is provided by cutting the layer into pieces. 25 m length RCC block can survive without shrinkage crack. It directly relates to the mix design proportions. The easiest method is to cut the layers separately after compaction. Perform another roller pass along the joint to compact any loose material.