Jeff what exactly is the difference between a HRWR and a superplastisizer??? I havent gotten to the bottom of this. But they seem to do similar things.
Water reducers are a class of admixture that permits less water to be used in concrete in order to maintain a given level of workability. High range water reducers (HRWR) are simply more potent water reducers that allow for a greater level of water reduction. Superplasticizers are, for all intents and purposes, HRWR's. Often the confusion between the different names stems from the two different ways they can be used. Water reducers, including HRWR, are often used to reduce amount of mix water needed to maintain a given slump. Superplasticizers are often used to increase workability while maintaining a constant mix water amount. It's a matter of symantics, really. Your experience shows that they are basically the same thing.
jeff, this is my first concrete polish project, i am doing a outdoor fire pit. we framed and poured it last week after we stripped it, the surface is very rough,( we did not juk it ). what can we do to get it smooth so we can polish it is there a mix we can spreed over and then polish.
It sounds like you don't want to be exposing aggregate in the final finish. If you do, then you don't really need to worry about the rough casting surface because you'll simply grind it off to achieve a smooth surface. Polishing concrete floors often start with a metal bond cup wheel (or similar coarse tooling) to level the floor and remove roughness and uneven surfaces). This almost always exposes some aggregate.
If you want to leave a surface that has little or no aggregate showing after polishing then the cast surface needs to be smooth enough so that you can skip the coarse grinding steps.
From your comment it seems like you don't want to expose aggregate, so you'll need to fill in the low spots with a concrete mortar. Often this is called sacking, and it essentially is a fine grained cement mortar that's rubbed into the holes and low spots to fill them. Common sack mix blends don't specify proportions, but a 1:1 fine sand to portland cement mix is a good start. Always use an acrylic polymer additive (sometimes called a fortifier) as a bonding agent. The acrylic polymer helps the sack mix stick. I prefer a commercial bonding agent called Acryl 60, but there are others that you can use. Follow the directions from the manufacturer. Often the acrylic is blended with the mix water at a 1:1 ratio.
Keep in mind the areas that are sacked will look different from the base concrete. You won't really notice this until you start to polish, so if you suspect the sack mix is too dark you can blend some white portland in with the gray portland cement to lighten the shade. If you don't have samples of the base concrete you won't be able to do a trial batch to see how well the sacking matches.
For future projects you might want to address why the casting came out rough in the first place. If you were using rough formwork then simply using smoother material (like melamine) would have created a smoother cast finish. If concrete stuck to the forms and left holes where the concrete pulled away from itself a commercial form release oil would have helped. I have always liked Cressett's 880 VOC release agent, since it's an architectural grade of oil. If the roughness was caused by honeycombing and rock pockets then better vibration and consolidation, or a different concrete mix would have helped create a tight, smooth cast surface.
4 Comments:
Jeff what exactly is the difference between a HRWR and a superplastisizer??? I havent gotten to the bottom of this. But they seem to do similar things.
By
Anonymous, at 10:51 PM
Water reducers are a class of admixture that permits less water to be used in concrete in order to maintain a given level of workability. High range water reducers (HRWR) are simply more potent water reducers that allow for a greater level of water reduction. Superplasticizers are, for all intents and purposes, HRWR's. Often the confusion between the different names stems from the two different ways they can be used. Water reducers, including HRWR, are often used to reduce amount of mix water needed to maintain a given slump. Superplasticizers are often used to increase workability while maintaining a constant mix water amount. It's a matter of symantics, really. Your experience shows that they are basically the same thing.
By
Jeff Girard, at 6:07 PM
jeff, this is my first concrete polish project, i am doing a outdoor fire pit. we framed and poured it last week after we stripped it, the surface is very rough,( we did not juk it ). what can we do to get it smooth so we can polish it is there a mix we can spreed over and then polish.
By
bruce, at 6:56 PM
It sounds like you don't want to be exposing aggregate in the final finish. If you do, then you don't really need to worry about the rough casting surface because you'll simply grind it off to achieve a smooth surface. Polishing concrete floors often start with a metal bond cup wheel (or similar coarse tooling) to level the floor and remove roughness and uneven surfaces). This almost always exposes some aggregate.
If you want to leave a surface that has little or no aggregate showing after polishing then the cast surface needs to be smooth enough so that you can skip the coarse grinding steps.
From your comment it seems like you don't want to expose aggregate, so you'll need to fill in the low spots with a concrete mortar. Often this is called sacking, and it essentially is a fine grained cement mortar that's rubbed into the holes and low spots to fill them. Common sack mix blends don't specify proportions, but a 1:1 fine sand to portland cement mix is a good start. Always use an acrylic polymer additive (sometimes called a fortifier) as a bonding agent. The acrylic polymer helps the sack mix stick. I prefer a commercial bonding agent called Acryl 60, but there are others that you can use. Follow the directions from the manufacturer. Often the acrylic is blended with the mix water at a 1:1 ratio.
Keep in mind the areas that are sacked will look different from the base concrete. You won't really notice this until you start to polish, so if you suspect the sack mix is too dark you can blend some white portland in with the gray portland cement to lighten the shade. If you don't have samples of the base concrete you won't be able to do a trial batch to see how well the sacking matches.
For future projects you might want to address why the casting came out rough in the first place. If you were using rough formwork then simply using smoother material (like melamine) would have created a smoother cast finish. If concrete stuck to the forms and left holes where the concrete pulled away from itself a commercial form release oil would have helped. I have always liked Cressett's 880 VOC release agent, since it's an architectural grade of oil. If the roughness was caused by honeycombing and rock pockets then better vibration and consolidation, or a different concrete mix would have helped create a tight, smooth cast surface.
By
Jeff Girard, at 1:57 PM
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