Curing Room for Masonry Blocks Pavers

[bearmatt]

Hello,

Sorry this is long..I was wondering if anyone knows of a book/publication/other that may help us decide the best type of curing room to install for concrete pavers.

The main reason we are considering curing (CO2 maybe) is to try and curb a mild secondary efflorescence problem, occuring when the products are in the yard. We dont use anything at the moment, just ambient cure.

The confusing facts as I have recently learnt (right or wrong ?) are:-

- Steam curing at 140F+/- was the old way
- New method adopts Max humidity at Max 80F-100F
- Also new method introduces 3-5% CO2 in Max Humidity at Max 80F-100F
- Does SCM (Fly Ash, GGBFS) react significantly in 18hrs at only 80F (I didnt think so)
- CO2 introduction will interfere with CSH reaction & stop SCM reaction

Our factory area reaches 110-115F in summer, hence 80F seems pretty cold. I will need an iceberg to bring the temperature down to 80F in a curing room full of concrete.

Is it fair to say that a perfect curing scenario might be

Concrete with SCM = Preset – Steam Cure 140F – Moist Cure (+CO2) 80F
Concrete without SCM = Preset – Moist Cure 80F - Moist CO2 Cure 80F

I appreciate there are too many questions for an easy answer, so a 'point' in the right direction would be great.

Thanks for any assistance.

Matt

 

[Ron]

CO2 will likely increase your efflorescence, as the primary reaction that causes efflorescence is Calcium combining with water to hydrate, then exposed to CO2 which creates calcium carbonate, a primary constituent of efflorescence.

Steam curing is a good method. Just because it is "old school" doesn't mean it needs to be thrown out.

Not sure what your coarse aggregate is, but if it is a carbonate aggregate, you might consider a change or a mixture to reduce the carbonates.

Adding GGBFS or fly ash will retard the strength gain of your mix.

 

[concretemasonry]

Those kind of steps are rare in the manufacture of pavers if you are referring to the interlocking concrete pavers made using a zero slump mix and technology. After all the ASTM strength requirements are only 8000 psi and many producers have set their own specifications at 10,000 psi.

Do you have automated equipment that allows pavers to be taken of of an unheated curing room, without cooling it off? Many of these use no extra heat, but just a light water mist and use the heat of hydration to maintain a satisfactory unless the aggregates are cold or the kiln is drafty.

I would take a hard look at your aggregate gradations and mix design. You may have aggregate over 3/8", which is a problem. All aggregate must be clean. Obviously, more water in the mix is the cheapest and fastest way to get strength, but the problem comes with maintaining the necessary tolerances. You may also be using an excess of cement to compensate for a poor gradation. High cement contents and "dirty" aggregate are a major reason for efflorescence. Fly ash can also contribute to efflorescence.

What type of manufacturing equipment is used. If it is a dedicated paving machine, the European equipment manufacturers have a wealth of information and usually know far more about mix design and the curing and they are always willing to help. If the pavers are made on a block making machine, it is a little more different because of the feeding/filling a mold and different type of vibration. But I have had 12,000 psi pavers made as an exercise to determine the potential and have made 8,000 psi units routinely using Indian cement that frequently has some fly ash in it.

CO2 really does not penetrate quality concrete to a significant depth, the the long term carbonation the requires long times (months or years) plus humidity and can be beneficial since it ties up the free lime that is always present in a concrete mix.

If you are concerned with the larger slab-type pavers, the technology for this much smaller market is somewhat different.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[bearmatt]

Dick,
Thank you very much for your comments. You have given me a bit to think about, and as always your comments are remarkably insightful.
Matt

 

[ione]

A curing atmosphere enriched with CO2 is an acceptable (but expensive) method to prevent efflorescence. The goal is to prevent calcium salts migration to the surface, promoting the reaction described at page 6 of the attached paper within the pores and blocking calcium hydroxide there. Keep the ambient moisture as high as you can (within saturation limit to avoid condensation and dripping) since high evaporation rates are likely to force calcium hydroxide to precipitate on the paver surface.

 

[concretemasonry]

The most important thing about the design of a curing room/chamber is the method of putting concrete products into the area and removing them. Controlling the temperature and humidity may be necessary. Some admixtures can help, but that requires fine tuning of the mix and compatibility with the equipment.

If the access is open to fill and then to remove the products, it is thermally horribly inefficient and you will have little control because of the mass on the walls and concrete products.

Modern manufacturing uses automated equipment and transfer equipment that trigger the doors to open and close as needed quickly and no people are in the curing area. This preserves the thermal inertia and the heat of hydration also generates heat. This results in a very uniform temperature and humidity with little regard to the exterior conditions. Normally, mixing water is heated to about 140F and in some cases with very cold winters, but the amount of water used in low, but it does help to "kick off" the curing and generation of the hydration heat.

I have seen many plants that successful use heat of hydration plus a water mist hydration system than can maintain 140F. Some also supplement with heated CO2 (often direct fired), the the amount of CO2 can be variable especially in warm climates.

The cement content of a paver can be quite high, which creates more heat and in warm climates, so a concrete block room with a tarp at one end will give adequate strength for handing (70% 0f ultimate). All concrete products tend to absorb moisture, during the process and some monitoring (weight gain over time during and actual curing cycle - not laboratory work) is used to fine tune a plant production.

Of course, the ultimate curing method are insulated autoclaves, but it can be costly unless the plant operates 24 hours per day. One you get beyond the magic 300F, the chemical reaction is different and the amount of free lime is almost eliminate because of the cement replacement of high silica. The benefits are the much low cement cost, uniform curing and colors and elimination of efflorescence. The down side is the investment and the requirement to go to few days per week at 24 hours/day rather than shortening number of hours per week. With this system, 90-95% ultimate strength can be obtained in a 4 1/2 hour actual curing cycle and the concrete products have almost no moisture when removed. This system can provide 12,00 psi pavers or 9000 psi hollow block with the short curing cycle.

There are hundreds or thousands of successful paver plants in the world that learned and still make high quality pavers using "egg-layer" molding machines that just mold and place the products on the ground in the open with mist or under a roof.

The curing "room" is not just a building, but is an integral part of the production and operation and must fit the operating criteria, since that is where the real investment ($2,00,00 to $5,00,000)is. Marketing will determine what is needed for the market. Usable curing information really wont be found in a book the cause of the variables (climate, raw materials, products produced and finished product criteria) and demands of the production equipment. - It sort of like an art.

Dick


Engineer and international traveler interested in construction techniques, problems and proper design.

 

[chicopee]

I worked for a cast concrete company that used a direct fired steam generator for their curing chamber and curing cast products covered with canvas in which steam was release. A direct fired steam generator generates steam relatively high in carbon dioxide that is part of the products of combustion.

 

[concretemasonry]

The use of a direct fired steam generator is mainly to provide heated moisture to improving/shorten the curing cycle to permit stripping form dailyeven in a temporary shelter. One of the plants was in almost daily used a Johnson steam generator in some kilns enclosed with draped canvas. They also cast some slabs that were meant to be exposed architecturally. The higher quality slabs are cast but the excess moisture is sucked out by a vacuum through a applied filter paper.

Cast pipe is a totally different product than a typical paver that is made using "zero slump" concrete instead of wet cast. It is not meant visual exposure.

Much depends on the quality and appearance. Machine molded and vibrated pavers are in the mold for 10 to 25 seconds before stripping and still get 8000 psi due to the molding/vibration process and the aggregate requirements (ASTM specs are too loose and meaningless and the exception is noted in ASTM C33).

Zero slump pavers are a huge international market and have a totally different technology. That is why there are repeated 15-40 acre applications.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.