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I thoroughly researched how to waterproof concrete throughout 2012. Since then I have been very public criticising admixture manufacturers for not being as honest as they make out. I could be out of date now, but no one has contacted me in any way to say that they have improved anything - so I don't expect that I am out of date. Text book explanation of how concrete can waterproof itself. Written by Phil Sacre 2010 to 2015, then a small amount of editing in 2026 when this was added to www.basementexpert.co.uk. Quick Conclusion: Waterproof concrete, that is: concrete that is waterproof from water, sand and cement alone, is too dense for water to get through under pressure, by capillary action or as vapour. Waterproof concrete is totally waterproof. Deleterious chemicals dissolved in water (such as salt and oxygen) cannot penetrate waterproof concrete to attack steel reinforcement. Drying cracking cannot occur because waterproof concrete has insufficient excess water to dry off and create tension within. (Note. Flyash or microsilica would need to be added to concrete to give concrete sulphate resistance better than class two but great care is needed choosing the proportions to avoid leaving capillary routes between unreacted grains of flyash). Important note added 2026. There is no test an ACAS registered laboratory could carry out to prove concrete is vapour proof. If you suspect your basement slowly increases in humidity for reasons other than your breathing and perspiring, blackjack the entire inside during a dry spell in the weather. References: Within the concrete industry: quarrying, cement manufacture, readymix and so on, there is a post-graduate diploma one can study for and a series of four text books contain all the necessary knowledge to obtain it. These are "Advanced Concrete Technology" edited by John Newman and Ben Seng Choo. I have read large parts of these books and this information paper is written as a direct result of the understanding I have gained, particularly from the chapters "Durability concept: pore structure and transport processes" by Lars-Olaf Nilsson in the Volume 'Concrete Properties'; "Admixtures for concrete, mortar and grout" by John Dransfield in the Volume 'Constituent Materials'; and "Concrete construction for liquid retaining structures" by Tony Threlfall in the volume 'Processes'. Two other important sources are the e-book "Understanding Cement" by Nick Winter (from www.understanding-cement.com); and "Advanced Concrete Technology", 2011, by Professor Zongjin Li of Hong Kong University published by John Wiley and Sons. Other sources have been used as well to increase understanding but not quoted unless acknowledged. Concrete is. Concrete is aggregate of various sizes, cement and water. It might also have cement substitute and chemicals such as water reducing agent. All these materials vary enormously and they can be in any number of proportions. Some concrete would be very porous and some very much denser. C35A (A is for Aqueous and this is a water treatment industry classification) is a standard mix specification, containing extra sand and cement but less water by proportion compared to C35, that the water treatment industry uses for sewage treatment tanks because it is dense enough to prevent water visibly leaking. But it has capillary pores because the excess water providing the workability pushes all the solids slightly apart and vapour would always be lost from these capillary pores, so it is unsuitable (on its own) for habitable accommodation. Properly waterproof concrete is not only a very dense mix of aggregate, sand and cement. It includes the most powerful third generation super plasticiser so that it can be packed tight without excess water providing the workability. Pores close off.
The extra cement partially reacts with much of the limited water until the concrete simply runs out of space inside (at which time remaining water cannot form gel with remaining cement). This does not seem to happen between grains of cement substitutes that have very limited reaction with water, such as flyash. Possibly because flyash is a spherical klinker, largely hollow and with a lot of space between packed grains. THE CONCRETE MIXES WE WILL COMPARE. Create 3 pictures in your mind's eye: fully waterproof concrete, C35 and C35 with extra water added on site against the rules.
What goes on in these concretes?
So, to begin with, all the cement grains are wet all over and just touching each other rather like mixed-sized marbels in a jar. Just the spaces in between are filled with water. The grains of cement are not pushed apart. In due course (this will be after the concrete is placed and compacted) the surface of the cement grains mix with the water coating their surfaces, forming gel. The gel takes up 2.2 times (1) the space originally occupied by the cement. So after the cement and water combine, gel from one grain is forced to mix with gel from its neighbour. Gel is forced against sand and aggregate as well. The reasons for this dramatic increase in volume of the cement are that to form gel the surface of a grain of cement takes up a great deal of water. Heat is given off as well and the rise in temperature causes thermal expansion. When crystals form from the gel not all the water within it forms crystals, so water is now released. But this is after the gel in such a tightly packed environment has been forced into any available space. Hours later the gel turns to crystals. The released water mixes with the new surface of the cement revealed after the first surface gelled and turned to crystals, the pressure pushes it through the first crystals thickening the continuous skeleton of crystals throughout the concrete. The secret behind fully waterproof concrete is the exceptional super plasticiser that gives the workability to completely pack concrete with vibration, there being no space taken up by water added only to provide workability. Cement and water turning to gel can only continue while there is space. Because initially the cement needs to expand 2.2 times to create gel. After only a few days the rate of change from cement and water to crystals of concrete is far slower because remaining water cannot get through the tightly-packed crystals already formed (2). The concrete is already completely waterproof. Within a few more weeks nothing has any space to move. No molecule of water can get through to any unreacted cement grain. The last gel to form crystals will always release water so there will always be free water left inside concrete. In our concrete it is trapped. However, if the concrete were to crack then the space created would allow remaining water to reach remaining cement, gel to form and turn into crystals and the concrete would self-heal (autogenous healing). Cubes made with waterproof concrete crush at well over 50N/mm2 after less than 28 days. In fact it almost stops gaining any more strength in less than 28 days because it runs out of the necessary space inside sooner than that.
So, when the cement begins to gel with water, more gels at once because there is more water available for each grain and more space for gel to form. With so much space available, the first gel from one grain barely mixes with the first gel from the neighbouring grain. Crystals form but they don't intertwine, they don't form a continuous skeleton, until much later. The skeleton formed out of crystals is not as neat or complete. Neither are the crystals from neighbouring grains so thoroughly interlocked. So, although after any short period more cement in C35 has crystalised - become concrete, it is not as strong a structure and the spaces between grains are connected allowing water to travel through. This sounds fairly awful. However, in practice, C35 is made with flyash as well which gives the skeleton a nice shape to form round. Whilst my picture above is true for waterproofing, in actual fact, with flyash, the crystal skeleton is quite neat and strong even though the flyash is barely involved. And that is why flyash concrete eventually becomes stronger. But flyash concrete will never be waterproof if cement gel cannot get in to the water-filled space between neighbouring grains of flyash. If flyash reacts at all with gel it is only the glassy surface, expert opinion differs on this.
We can also picture the worst mix, C35 that the workmen wetted up with more water. The water-filled spaces are bigger so lots of gel from one grain needs to form before any mixes with any gel from its neighbour, reducing strength considerably. Poor concrete with too much water is very weak and porous. Because the exposed surface of any concrete can expand freely, when water is added on site and formwork removed within hours, concrete that should protect reinforcing steel, such as our motorway bridges and sign supports, is free to expand as it cures and becomes extremely porous which is why the Hammersmith Bridge, the bridges along the M62 and so on, cost more to upkeep than they cost to build. We still use the ridiculous system so condemned by the Latham report years ago, that consultants draw up long-winded specifications that form the basis of adversarial contracts that cause everyone to cheat. And no-one polices any of it. The system just wastes money and results in poor concrete. Clients who want a quality job might consider bringing back the hated Clerk of Works. Porosity. Let us now consider porosity. How easily water might be pushed through under pressure, for instance water outside a basement wall; or pulled in by capillary action. The wetted up C35 is full of water anyway. So water will flow through easily. The C35 is denser but there is still inter-connected water, so, under pressure, water may be pushed through. But, for certain, if water dried from the concrete, let us say into a basement so the basement feels and smells damp, capillary action would replace the water in the concrete whenever any was available. I have explained that, in the waterproof concrete example, no molecule of water can travel the microns through crystals to get to more cement; and remaining pores containing water are not connected. In fact, nothing can move. To put 'nothing can move' into perspective, it may be a whole year until absolutely nothing, not a single ion, can move anywhere at all. But from site experience even before any structure is sealed and backfilled nothing could travel right through concrete with extra cement, limited water and PCE plasticiser. These pictures now in your mind's eye are evidenced by BS 8007:1987. 'Code of practice for design of concrete structures for retaining aqueous liquids'. This standard contains a mix design for concrete of structural strength that will not allow any visible sign of water through. This is called C35A, the A standing for Aqueous. The mix is for use with structures that will retain aqueous liquids, such as sewage treatment plants and water purification plants. Water will not visibly be seen to leak but water is allowed to dry off as vapour and be replaced by capillary action. This is known as watertight rather than waterproof. It relies on some extra sand, some extra cement and a little less water than a readymix supplier would put in a plain C35. Chemical resistance. Chemicals deleterious to reinforced concrete are either those that attack the concrete or those that attack the steel. Anything that can attack steel needs to be dissolved in water to do so (mostly road salt and oxygen). By keeping out the water on the surface very good C35 and waterproof concrete protect the steel. Very good (well made as well as well placed on site without adding water) C35 possibly for 60 years, a waterproof concrete indefinitely. The other chemicals that attack the concrete do so at the surface. These are mainly sulphate in aggresive ground and sea water and carbon dioxide. The solution here, no matter what the concrete, is to add a cement substitute that the salt and carbon dioxide cannot get through: Fly ash, GGBS, microsilica or volcanic pozzolan (which the Romans discovered proofed concrete against sea water). With a substitute in the concrete, when PC cement crystals are eroded the chemical will quickly expose a surface of alternative material it cannot attack, and the structure is protected. Note that concrete containing flyash or GGBS may need curing for up to 56 days to get all the benefit. Concrete with cement substitutes may not be able to become fully waterproof on its own. The user may need to fund research. Crack Proof. Equally important as the concrete is the reinforcing steel within it if the concrete is to remain waterproof. I have another page dealing with the minimum steel requirement in waterproof concrete here. In a nutshell, a waterproof concrete structure will cool after setting and the stresses will cause cracks within it. The job of the steel is to take up the strain across a crack limiting its width to a space that the concrete can self heal. The correct steel makes concrete crack and heal many times instead of opening up one wide fissure. I, Phil Sacre, am not a chemical company. I was a Land Surveyor and what is now called a Construction Materials Technician in the British Army, turned site engineer turned basement expert who got some sets of letters after his name in the 90s and since studied concrete technology a lot further in the library at Kingston University. It is my preference to increase the horizontal reinforcing steel to 0.35% to restrict crack widths in the hotter environment of fully waterproof concrete. However the alternative practice of pouring no more length than 3 times the height, leaving the ends of pours free to shrink and fill in short gaps later, is perfectly valid. It just means a lot more work creating stop ends and if you are buying tapes or strips you will be persuaded to buy a whole lot more. Further thought for all. I have thought about these issues more than most. No one could stop all the men on every site wetting up C35 with more water. Any important structure, such as a motorway bridge, should be made with a waterproofed concrete instead of C35 so that
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