Concrete Pixels

Most of the construction projects big or small are estimating the in-place compressive strength of the newly cast structures using the cubes or cylinder specimens taken at the time of casting. It is “widely believed” that the specimens taken on the same casting have the same strength as the structure cast. This is extremely important for a contractor to know when he can proceed with other construction activities such as stripping the formwork or removal of scaffolds, end of curing, partial or full stressing of tendons, lifting of precast elements, etc. Some are storing the specimens in the curing tanks and or/fog rooms which is not correct as it does not represent the actual site condition and there is a danger that the strength is overestimated. While others are doing it right by storing the specimens at site close to where the structure in question is cast, this system has become more cumbersome especially if they are concreting many parts in a say high-rise buildings or other large projects. It becomes impractical bringing in cubes molds at the pouring area, storing and then taking the cubes thereafter to the Laboratory for crushing. There is always the possibility of mishandling the cubes especially at early ages that will impair the strength. With the advancement of technology, we can now get rid of these cubes or cylinders as specimens at site. What we need are just thermocouples embeded at critical points of the structure that monitors the temperature development. This temperature combined with age (hours or days) can then be used to calculate the strength of the concrete in the structure using the concept of “concrete maturity” which I will discuss separately. It is also possible that the temperature readings can even be transferred remotely to your personal computer or laptop in the office so that you can see the real time concrete strength of the structure. Interesting, isn’t it? I’m sure this is a big relief to everyone in the construction be it owner, contractor or independent consultant as it gives a very reliable esimate of compressive strength.

I was in Sweden last weekend and my day there was spoiled by the winter’s wrath. On the day I traveled through an overnight cruise accross the Baltic sea there was a forecast of snowstorm with up to 30+ m/s wind. Due to the urgency of my mission there I was forced to go. While in the middle of the cruise, I was awakened by the tilting here and there of the ship. The 2 cans of beer did not help me have a good rest as I intended to. We arrived at Stockholm safely though at 6:30 AM and I had to wait at the lounge for two hours hoping that the heavy snowfall will stop before proceeding to the Philippine Embassy at the city center but it didn’t. Though I manage to complete my mission, I was not able to roam around as I intended i.e. to visit the places that I have not seen during our last visit with my family in May this year. It’s almost impossible to move around the city by foot that day as the snow is so thick and much worst if you go with the public transport as you will see nothing from the inside. So what I did was just stayed at the small park accross the palace and took some photos.

The long, cold, dark & wet winter has once again come to our place, Finland. It’s long as this will last until March next year. Cold as expected but we still have not experienced though the real winter in this place at minus 30+. Last year was only about minus 15. Dark, as we go to office and the kids to school when the sun has not risen and be back when the sun is fully set. I remember my youngest son telling his Mom that he doesn’t want to go to school as it is still night time :-).At it’s peak we have barely 5 hours daylight. It’s wet too. If it is not rain, sleet or snow is falling. On the other side the kids are also happy that once again they will be skating. The skating rink in our hometown opened last weekend and the kids were so glad about it. They did not stay long though last Sunday, as they have not yet adjusted to the freezing temperature in the rink. The last time they skate here was on February this year. Even my eldest son who is supposed to be playing again with his hockey stick could not make it. My feet are getting itchy watching them glide on the snow. I think I can not wait for another month for the complete healing of my appendectomy before I joined them for skating :-).

Concrete when produced requires a certain measure of what we call workability. Until today I doubt if there is really a general agreement on the definition of workability. This is the reason why there are so many methods in measuring it like the slump, flow, vebee, compaction index, etc. Despite it’s many limitation, the slump is the most popular method for centuries and accepted globally in almost every local standard due to its simplicity and ease of use at job sites. A It does not directly measure the work needed to compact the concrete but it gives a reasonable indication of how easily a mix can be placed. The advancement of technology has also advanced the method in measuring workability. The latest of which is by employing the concept of rheology. Rheology refers to the study of flow of matter not only liquids but also soft solids where the flow can not be characterized by a single value of viscosity like concrete. Concrete rheology is the scientific approach to measuring workability and can be used to optimize mixture proportions and to ensure consistent fresh concrete quality.The characterization and control of rheology are especially critical for specialized concretes such as self-consolidating concrete (SCC).
An instrument was developed by the International Center for Aggregates Research (ICAR) - University of Texas at Austin to efficiently measure the workability of concrete. It is a pretty handy instrument with a four blade vane that is inserted into a sample of fresh concrete and rotated at a series of various speeds. The resulting torque acting on the vane at each speed is recorded and used to compute the Bingham model parameters of yield stress and plastic viscosity in fundamental units. The operation of the test is fully automated and can be completed in less than a minute. In addition, this instrument is capable of performing a stress growth test, monitoring workability retention, and characterizing thixotropy. It is also capable of testing concretes with slumps greater than 50 mm and is especially applicable to self-consolidating concrete (SCC). It is ideal for use on almost any jobsites and can even be brought as checked-in luggage on an airplane. Before using this new high-tech method check that it is acceptable by your codes.

I’ve been grounded for almost a month now at home due to an appendix operation. Three days before the operation I feel an abdominal discomfort that leads me to take at first medicine for diarrhea. The next day, it does not get any better and I thought perhaps my gastric has come again so I took some medicines for it. Day after, I went to the office feeling uncomfortable and losing my appetite for anything. I manage to hold on until the COB on that day but I almost can not walk due to severe pain and so I called my sweetie and I went straight to a nearby private hospital. It was almost closing time for the doctors and upon my request the hospital agreed to take me on. Being the last patient for the specialist that day, he checked and immediately referred me to the main hospital in Pori (Satakunta Keskussairala). Without delay, I called for an ambulance that brought me to an emergency ward. In severe pain, they took 6 vials of blood sample, gave IV and a very strong pain killer that lets the pain out but made me drowsy. Due to some scheduled operation that night, I waited for almost 5 hours before finally having my turn to be operated. The nurses, doctors and anaesthologist introduced themselves and explain the process to be done afterwhich antibiotic and the general anaesthesia was given thru my IV. That was all I remember and when I woke up after 6 hours I was already at the recovery room feeling the cut on my tummy.

The doctors has given me the green card to check-out after nearly 2 days. I was so happy to check out as everything seems to be well. But then after a week, I got a high fever and chilled just when I was going to bed for rest. The following day, I went to the hospital and they found some infection. This is the most unforgetable and painful experience I ever had. They took the stitches and open the wound inorder to remove the puss and …. without anaesthesia… wahhhhh. The doctor explained that local anaesthesia will not be effective in this case. Then they put inside the wound an anti-bacteria cloth. But this is just the beginning of the painful process. Everyday after this for one week they have to replace the cloth. Taking out is not a problem but putting in is a BIG problem for me. But thanks to God and to my caring Nurse sweetie, I am now 90% fully recovered and hoping to be back to normal routine in my work by next week.

As I am involved in the splitting tensile strength of concrete in our project, the construction of OL3 Nuclear Power Plant, I have made some research on this subject. There have been so many studies/models made to date inorder to estimate the splitting tensile strength of normal weight concrete from higly respected institutes like ACI,CEB-FIP and from other individual so called concrete experts. But there is one study made by Nihal et al as published in the ACI Materials Journals in early February 2006 that summarizes and evaluates around 12 splitting tensile equations and proposed a very convincing equation. The splitting tensile strength of concrete is normally expressed as A*fc^B where A & B are some constants and fc is the cylinder compressive strength of concrete. Of the 12 models, it’s very interesting to note the constants; A varies from 0.294 to 0.59 and the power B from 0.50 to 0.71. The evaluation of the different equations was made using the integral absolute error (IAE). The lower the IAE the better is the result. What surprised me most is the equation in ACI 363R & 318 are giving poor estimates. To some extent it is fairly acceptable for concretes between 20-40 MPa compressive strength. For strengths > 40MPa it is underestimating the splitting tensile strength. The equation by CIB-FIB where A=0.3 and B=0.6667 is far better in most range of compressive strengths from 0-120 MPa. A slightly more accurate though is the one proposed by Nihal, et al in which A=0.387 and B=0.63. So for those that requires a higher degree of accuracy for the splitting tensile strength, try the equations of Nihal, et al or the CEB-FIP or better still do some extensive experiments to validate this finding if not convinced. Given the splitting tensile strength of concrete, the direct tensile strength can be easily estimated.

I’m so glad to be a part of this international concrete expert team for the pioneering generation III+ nuclear power plant project in Olkiluoto, Finland known as the OL3 Project. It is called OL3 being the 3rd Nuclear Power Plant in this island (Olkiluoto). Our team is composed of Dr. Bernd Schnuetgen from Germany. He is a retired professor of Concrete Technology and a real “walking concrete encyclopedia” who is so kind and unselfishly sharing his immense wealth of knowledge to us. Along with him is Jouko Lehtimaeki of Finland who compliments Bernd’s technical knowledge with his more than 30 years of practical experience in concrete. He knows how to perfectly make whatever type of concrete and knows exactly what is wrong when the desired property of the mix is not achieved. Combining the experties of these two fine gentlemen would really make you a top concrete specialist (if you can). Anders Thorin from France just joined us recently trying still to gain the practical side of concrete after learning it in school. He will be with us for about a year then he will go back to complete his engineering degree in France. And yours truly who has just over 15 years of experience in construction in various countries around the world in different capacities as Office Engineer, Site Engineer, TEchnical Chief Engineer, Design Coordinator, Project Manager and Consultant. I have worked with Jouko in the precast yard for the worlds’s longest and widest segmental viaduct in the world, the 55 km Bangna EXpressway in Thailand where we produced around 1800 cubic meters of concrete daily in at least 18 months.

Construction nowadays uses different types of “cosmetics” in producing concrete inorder to achieve concrete properties that the situation demands. These cosmetics that I am refering to are commonly called admixtures. Admixtures are actually materials other than cement or aggregate or water that are added during the production of concrete i.e. before or after mixing. Though the practice of adding admixtures to concrete is now a common practice and accepted by standards such as ASTM C 494 or ASTM C 1017 , the chemistry of admixture remains a complex process that requires indepth knowledge & experience depending on the application. the most common types of admixtures are:
Superplasticizers or commonly called as plasticizers. These type of admixtures are also high range water reducers that allow large water reduction or greater flowability without substantially slowing set time or increasing air entrainment. Their water reducing effect increase the strength due to the lower water cement ratio.
Retarders as the name implies are admixtures that slow down the hydration of cement thereby allowing for a more longer setting time. They are beneficial for hot weather concreting to overcome the effect of temperature to quickly set the concrete. Like plasticizers, retarders are also water reducers thus increasing the strength of concrete.
Accelerators are admixtures that has the opposite effect of retarders i.e they shorten the setting time of concrete, thereby beneficial for cold weather concreting, early removal of forms, early surface finishing, and in some cases, early load application. Proper care must be taken while choosing the type and proportion of accelerators as they may cause an increase in the drying shrinkage of concrete.
Air-entraining admixtures entrain small air bubbles in the concrete. This is commonly used in cold areas. The major benefit of this is enhanced durability in freeze-thaw cycles. Designing such air-entrained concrete is quite a challenge but the real challenge based on experience is on the stability of the air during production. Although some strength loss maybe there due to the presence of air in concrete, it generally can be overcome by reducing the water-cement ratio or through the use of other appropriate admixtures.
Bonding admixtures are those that improves bonding of new / fresh concrete with old / set concrete. They contain materials such as polyvinyl chlorides and acetates, acrylics and butadiene-styrene co-polymers.
Coloring admixtures are commonly used in patios and walkways. In most cases it is applied at the surface and has an additional effect of surface hardening. Such surface applied coloring admixtures generally should not be used on air-entrained concrete.
Waterproofing and damp proofing admixtures are commonly use to decrease the amount of water penetration into the larger pores of concrete.
Choosing an appropriate admixture for a specific application should be done with extreme care. An experienced expert in this field to evaluate the situation and the recommendations by the supplier is a must. Alternatives to the use of admixtures should always be explored whenever possible. In addition, the environmental impact of certain admixtures must always be taken into account. Moreover, admixtures should only be combined in a concrete mix by a competent professional because some of them can interact in undesirable ways. Finally, always remember that admixtures cannot compensate for bad practice and low quality materials.

I served as a Consultant with one of the development projects of UNOPS-United Nations Office for Project Services in Indonesia early last year (2007). The project is the reconstruction of hundreds of schools devastated by the 2004 notorious tsunami in Banda Aceh and neighboring areas and the high magnitude earthquake that struck the island of Nias in March 2005. I was assigned in this island, the biggest of the group of islets in the West of Sumatra. I came here 2 years after the 8.++ magnitude earthquake and I was struck by the devastation and stories of the people. I have the strong desire to serve with this kind of work especially after looking at how the children in this island are coping with their temporary schools. This island is unique in many ways. It is the only island where majority are Christians in “world’s biggest Muslim nation.” The Sunday worship is vibrant in almost every church. Majority are protestants. It is the only island I saw where you can find corals even at the mountain tops, a manifestation that this island rose up to where it is now from the bottom of the ocean many million years ago. This island sits on the Pacific ring of fire. Building structures in this island requires very good foundation design as the lower areas are swampy and the higher grounds have mostly soft rocks/(coral-like). Construction projects in this island face a real challenge due to the lack of construction materials,equipment, laboratories and skilled manpowers. It’s very difficult to attract reliable firms in the island due to the remoteness of the place, isolated for many years. It is unknown to the world until the deadly earthquake struck the island in 2004 except perhaps from few surfers as deep south in the island is the famous surfing paradise. Nias is considered as one of the surfing capital in the world. Each party will not be complete without the traditional Nias dance. But this is not the reason why I call this place “the dancing island”. It’s physically “dancing” every week even without music… it’s an unbelievable earthquake that frequently occurs sometimes twice a day. There’s never been a time when I was there when the island never rocks at least once a week. It’s amazing to note however that while the modern concrete houses and bridges collapses, a traditional house like this survived the killer earthquake:-)