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RapidAir457 air void analyzer– Distributor agreement with Drennan Company Limited in China

Concrete Experts International, and Drennan Company Limited of Hong Kong, has signed a distributor agreement about Drennan acting as a sole distributor of RapidAir systems in China, including Hong Kong and Macau. The first RapidAir system was shipped to Drennan in April 2003 and is at present used for quality control of the concrete at the construction of the Three Gorges Dam in China - the world’s largest dam.

The distributor agreement between CXI and Drennan is expected to introduce the ease of air void analysis of hardened concrete on the Asian marked.

RapidAir 457 – validation by Cementa Research

In collaboration with Cementa Research (CR) in Sweden (Slite, on the isle of Gotland), an evaluation of the reproducibility of the RapidAir system was performed on a suite of 10 different concrete samples. Manual linear traverse results were obtained by CR prior to sending the samples to CXI in Denmark for analysis on the RapidAir – the manual LT results were kept unknown to CXI. The results of the RapidAir analyses were very similar to CR’s own manual LT data, prompting CR to purchase a RapidAir system. The RapidAir system was delivered to Cementa Research in May 2003. The delivery was followed by a 3-days successful training course held by CXI’s Dr. Jakobsen. For more detailed information on the Cementa validation study please visit the CXI website.

Result of validation study. The graph shows the air content measured by manual LT and RapidAir (RA).

Fluorescence Microscopy – Training Courses

CXI visited in March 2003 Aalborg Portland in Northern Jutland in order to train a course in fluorescence microscopy involving all the steps from sample preparation to analysis of thin sections.

The next training course will be held in Copenhagen in mid-August with attendees from the United Kingdom. For more information on our training courses please contact Dr. Ulla Hjorth Jakobsen at CXI.

Micrograph taken in crossed polarized light with the gypsum plate inserted. This technique is used to identify the various mineral phases in the concrete.

W/C-CHECK – DEMO version now available!

W/C-CHECK is a brand new software package developed by Concrete Experts International for the determination of water to cement ratio in hardened concrete using fluorescent epoxy impregnated thin sections. The W/C-CHECK software is compatible with a wide selection of digital, and analog cameras, and offers image acquisition as well as image analysis capabilities. Through the camera and microscope, fluorescent light images are acquired directly into the software’s interface. As the next step, the images are color segmented to separate the paste from the aggregate and the air voids. Subsequently, the average light intensity of the paste is measured, and the corresponding water to cement ratio is determined by mathematical comparison to a calibration curve. The W/C-CHECK software is backed by more than 25 years of successful experience with the determination of the water to cement ratio from fluorescent thin section microscopy in Northern Europe. During recent years the technique has slowly spread into the Americas and Asia. Contact Mrs. Ulla Jakobsen for a free DEMO version of the software.

The user-interface of the W/C-CHECK software. Color segmentation is performed based on the intensity-histogram of the green color. The average green color intensity of paste correlates well with the w/c ratio of the concrete.

How to, Case Study

Concrete having very soft cement paste (high w/c). The quality of the polishing is very poor and the sample should not be analyzed for its air void content.

The surface is well polished having voids with sharp edges. The sample is suitable for air void analysis.

Polished surface of poor quality. The surface has relief resulting in a haze of zinc paste covering the surface. This surface should not be used for air void analysis

 
Polished surface of good quality. The surface has no relief and air voids are white, have sharp edges, and have maximum contrast to the cement paste.

Preparation of concrete samples for Air Void Analysis

In order to perform any type, manual or automatic, air void analysis of hardened concrete it is fundamental that the sample preparation is of highest quality. In our experience, too often air void analysis is performed on polished surfaces of improper quality, e.g. exhibiting poor void edges and relief. Whether the analysis is performed manually or automatically poor sample preparation will result in faulty data.

In the following a polishing procedure that has been found by CXI to result in very high quality surfaces is described. The specific procedure described involves a special polishing machine. However, the procedure has been successfully modified to accommodate different polishing equipment 

1. The concrete sample is cut to size in the concrete saw - as close to plane parallel as possible.

2. Draw a grid with a piece of chalk (“crayon”) on the surface to be analyzed.

3. Grind the surface using fixed diamonds 250 until the chalk grit is just removed.

4. Draw a new grit is on the surface.

5. Grind the surface again using fixed diamonds 125 until the chalk grit is just removed.

6. Polish the surface with grit 600 silicon carbide powder suspended in a solution of 1:2 liquid third generation superplasticizer:water. Use one large tee-spoon of powder to 100 ml solution. The solution is poured into the polishing device, and the sample is polished for 10 minutes.

7. Polished the surface with grit 800 silicon carbide powder suspended in a solution of 1:3 liquid third generation superplasticizer:water. Use one large tee-spoon of powder to 100 ml solution. The solution is poured into the polishing device, and the sample is polished for 10 minutes.

8. Polished the surface with grit 1200 silicon carbide powder suspended in a solution of 1:4 liquid third generation superplasticizer:water. Use one large tee-spoon of powder to 100 ml solution. The solution is poured into the polishing device, and the sample is polished for 3 minutes.

It is very important that the surface is cleaned with a soft brush under running water between each grinding step.

If the cement paste is very soft and friable a cellulose based varnish diluted 1:10 with acetone may be applied to the surface with brush prior to step 6. Low viscosity varnishes should be preferred. The varnish is removed by immersion in acetone for 5 minutes after final polishing step (step 8).

After having checked the polishing quality in the stereoscope the sample is now ready for manual air void analysis. If analyzing the sample automatically by using e.g. RapidAir 457 the sample will need to be contrast enhanced.

The best analysis result is obtained if the concrete surface is prepared to obtain maximum contrast between the air voids and the rest of the sample. The best contrast is best obtained by coloring the polished concrete surface black and subsequently filling the air voids with e.g. white zinc paste or white BaSO₄ powder.

Whatever method used to contrast enhance the surface it is important that the void edges appear sharp as in the example to the left of here.

For further information contact Mrs. Ulla Jakobsen