Exposed Aggregate Concrete & concrete complete

Frequently Asked Question's

The first few items are question that we find are most frequently asked

My question is about pouring new concrete over top of old concrete. in order to restore what is otherwise old rough cement. What I have is an old front doorstep that I would like to have re-finished in exposed aggregate concrete. Can this be done without removing the step, as it is a cold cellar in my basement. and what should I look for in hiring a contractor.

Yours truly
J. Cosgrove

There are many factors when pouring over top of old concrete, the old concrete should be in relatively good condition, A minimum of 2in. or 5cm. are required in order to achieve a good bond between surfaces. Also, the use of Latex Adhesives are a must. all loose pieces should be removed and a acid etch should be done before the application of the latex adhesive. Be sure that the new concrete has air entrainment of 6%. When using exposed aggregate concrete use a small aggregate (between 3/8" - 1/2"). In respect to your cold room, if you are capping your step because your room is leaking, you should first locate the source of the leak, repair it then seal the old concrete with a water proof memo-brain. And allow for at least 3 inches or 7cm of coverage.

On Sun, 14 October 2001, "Brandon Riggs" wrote: 

> Name: Brandon Riggs 

>  

> eMail: riggsconst@qwest.net 

>  

> Subject: exposed aggregate 

>  

> Comments: I have heard of people using "sugar water" as a retarder for 

> exposed aggregate concrete. I was wondering about pros,cons and possibly the 

> mix ratio if in fact it is an acceptable solution 

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	         www.ExposedAggregateConcrete.com 

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Brandon, 

	 The process of exposing aggregate by way of retarder, is the neutralizing 

of the chemical reaction in the surface of the concrete. And for many years finishers 

have used merely a saturated solution of brown sugar and water, and this will work 

when used properly. But keep in mind that surface retarder development, and the ability 

to achieve a flawless finish is what has made exposed aggregate so popular today. 

I know that me as a concrete contractor I would never use anything but a quality 

retarder in order to satisfy my customers. I think when you compare the cost of surface 

retarders, verses the cost of the materials. Its just not worth the risk, I've heard 

many horror stories, play safe! 

good luck with your project. 

Art Sampson

On Thu, 13 September 2001, "Jamison Brown" wrote:

Name: Jamison Brown
eMail: ****************
Subject: Salt
Comments:    Is there any problem with exposed agg. finishes in cold climates
       where salt may be used heavily for snow and ice removal.
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www.ExposedAggregateConcrete.com
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Mr. Brown,
                 It is my opinion that if the concrete in question has been Air intrained to 6%, and a high strength concrete (5000psi) was used. The concrete will be resilient to damage from salt. As always I try to recommend that as little salt as possible should be used in the first year of the concrete, but after that you can use it regularly. Unlike white concrete which is a fine particle finish, exposed aggregate concrete has an aggregate finish of large particles and is the most resilient to damage. You may also choose a good acrylic sealer if one hasn't already been applied.

hope I've been helpful

Art Sampson

www.ExposedAggregateConcrete.com

On Sat, 01 September 2001, "KPRudolph" wrote: 

 

  

 Name: Paul Rudolph

 eMail: KPRudolph@techhead2.com 

 Subject: Repairing side of slab

 Comments:

             I have a front porch slab that was capped with exposed aggregate concrete. 

The side of the slab where the sidewalk joins the slab has cracked and fallen away. 

It was fixed once with epoxy but has fallen again. I am now planning on removing 

the broken concrete and trying to patch the slab. Besides bonding agents are there 

other ways to make sure the patch doesn't fall again. Could I fashion some sort of 

rebar into the slab? Also what is the best way to match the aggregate that already 

exists? Thank you for any information you can give me.

 Paul Rudolph 

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 www.Exposed Aggregate Concrete.com 

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Dear Paul, 

	    Bonding agents are great to allow the two surfaces to union. creating 

a strong bond. The use of rebar, or steel  is important to keep the surfaces from 

becoming delaminated. Sleeve anchors can be used to attach the rebar to the old 

concrete. Also drilling and dowels will work also in small areas. If you have a 

cutting saw, the corners can also be beveled (like dove tailing) inward so that 

once poured the concrete cannot become separated. 

	       

	    As to the aggregate color. Be sure to take the time to look at the available 

aggregate from the ready mix plant as they tend to change slightly from time to time. 

And use the aggregate that best represents your colors. 

 

I hope this will help you with your project 

GOOD LUCK 

 

Art Sampson

Click here to learn concrete terminology

Scaling

This is also called flaking. Most scaling can be prevented by:

The use of air entrained concrete
Your ready-mix producer and concrete contractor control the amount of air entrained in the concrete and the mix quality.

Proper finishing
The concrete contractor must be aware of the proper timing of the finishing operations, which can vary greatly depending on the weather. Finishing too early or over finishing can result in a weak concrete surface, susceptible to scaling.

Judicious use of deicing salts
Calcium or sodium chloride salts on their own will not chemically damage or etch your concrete, but the fact that they do allow the surface to stay saturated with water can damage your concrete. Deicing products and fertilizers made with ammonium nitrate or ammonium sulfate can chemically attack the concrete, causing severe damage.

Deicing salts are not recommended in the first year and over the long haul should always be used judiciously. Use plain sand instead.

Insufficient or no curing
Failure to properly cure your concrete often results in a weak surface skin which will be prone to scaling. Although proper curing should be done by your concrete contractor, it is absolutely necessary that you and your contractor work together on this because the curing method is determined by your plans for ongoing maintenance.

Curing Concrete - How is it done?

There are numerous methods for curing concrete from covering with plastic sheeting or wet burlap to the use of straw or ponded water.

Moist Curing

Probably the best method for curing concrete is to flood the surface continuously with water for the first seven (7) days after placement. However, it is important that the concrete not be allowed to dry out. Often, contractors will recommend to the home owner to wet the new driveway for the first week after its completion. But, if the concrete is allowed to dry between soakings, this alternate wetting and drying may actually damage the concrete. So if you are going to water cure, plan on keeping the sprinkler going for at least a week.

Membrane Curing

The most common method of curing new concrete driveways is the use of a liquid membrane-forming compound normally called a curing compound or a 'cure and seal'. These materials are usually sprayed or rolled on the concrete surface. Once dry, they form a thin film like varnish on wood which restricts the evaporation of moisture from the concrete.

The most important thing to remember regarding the use of a curing compound is timing. The application of these products should be done as soon as the final finishing operations are complete or as soon as their application won't mar the concrete's surface. So if someone says, "Let's wait until tomorrow," you will know it's not a good idea.

The next most important thing is application rate. A light sprinkling or dusting on the surface will not do the trick. A sufficient coat according to the manufacturer's recommended coverage rate is critical.

Curing with sealing in mind

When choosing the curing method and materials that will be used on your driveway, one important aspect to consider is how you intend to seal and maintain your concrete in the future. Sealing your concrete is addressed in the next section, but for now let's look at how the curing method can affect your sealing decision.

The most common type of membrane cure used is referred to as a 'cure and seal'. But let's make one thing clear, this is not a one step process for permanent concrete sealing. It does, however, dictate the use of a film-forming sealer unless the 'cure and seal' is chemically removed or sand blasted away.

If you plan to use a penetrating sealer for ongoing maintenance, then your concrete should either be moist cured or cured with an easily removed concrete curing compound.

By making the sealing choice before the driveway is installed, you can then inform your contractor on the curing method that you would prefer.

Choosing a Sealer

The 'Wet Look' vs the 'Dry Look'

Wet Look - Film Formers (most widely used) Dry Look - Penetrating Sealers

Advantages

tend to be less costly
better stain protection (i.e., oil, grease, etc.)
usually compatible with curing method used
glossy to medium gloss look.

ideal for exposed aggregate and stamped

should not change the concrete's appearance
less frequent application needed

great water proofing

ideal for trowelled or broomed finish

Disadvantages

can darken the concrete
may appear blotchy if not evenly applied
will wear away, requiring more frequent applications
may create a slippery surface

must be allowed to wear off before reapplying

usually more costly over time
not as good of a stain protector
cannot be applied over a film former

Film Formers - 'Wet Look'

The film formers are usually made from acrylic or rubber based compounds. They form a thin coating on the surface of your concrete, leaving a wet look, much like varnish does on wood. These products generally tend to be less expensive on a per gallon basis than their penetrating counterparts, but you'll probably find that they will need more frequent application since they will weather and wear away more quickly.

One significant advantage of the film formers is that there is usually not a compatibility concern with the method of curing used or whatever previous sealer might have been applied.

The biggest problem that can develop with the film formers is that they tend to darken the color of your concrete. Just like varnish will darken or enrich the color of wood, these will do the same to concrete. And just like it may take several coats of varnish to provide an even, rich color, don't expect the film forming concrete sealer to perform differently. If after one coat you get some dark areas and some light, you may want to apply another coat to make it evenly dark. These initial variations in color may be caused by natural variances in the porosity of the concrete and/or uneven application, but they are quite normal.

One other potential problem is too much of a film build up on the surface may reduce the friction that keeps feet or tires from slipping.

Penetrating Sealers - 'Dry Look'

Most penetrating sealers are made from derivatives of silicone called silanes or siloxanes. These materials are designed to penetrate into the pores of the concrete, and once there, react with the alkaline materials and moisture present to form silicone. The silicone filled pores then make your concrete water repellent.

While the silanes and siloxanes penetrating sealers are usually more expensive than the film formers, they should last longer too. Another reason that the penetrating sealers are gaining in popularity in spite of their price, is that, when properly applied, they don't change the appearance of the concrete. Their biggest disadvantage, or at least the major concern in their application, is that there can be no other membrane cure or sealer on the concrete when applying and the concrete must be at least 28 days old.

A note on linseed oil sealers

Other commonly sold sealers for concrete are based on linseed oil in solvent. These are very effective concrete sealers. However, linseed oil is a very dark material and many people find the dark, blotchy result objectionable

Why is my concrete cracking after only a few weeks. What's happening, and can it be fixed?

First off let me begin by saying "All concrete cracks." Due to a measure of physical characteristics. It has to crack because it contracts during the active curing or drying process, and when you consider that even microns of shrinkage, multiplied over a large area. Will eventually add up to a fracture. The bond between the cement paste and the aggregates is not strong enough to withstand that stress. The best way to prevent unsightly cracking is to put control joints in your concrete. The basic rule of thumb is 1 cut every 2 to 3 times in feet the thickness in inches (ie. 4" slab should have joints every 8 to 12 feet). Frost is a major contributor to cracking, and where applicable, extra precaution should be taken. The proper curing of concrete is essential in the reduction of crack, in freshly poured concrete. Before repairing any concrete cracking, determine the source of the cracking and remedy that first. Epoxy injections and epoxy grout is an excellent crack repair agent.

What is the relationship of SLUMP to concrete?

Slump is a test preformed on concrete, that indicates the flow ability or workability of the concrete. This is important to help testers determine how much water is in the concrete. To ensure that the concrete or ready mix has been manufactured within the guidelines of the engineered mix. The lower the slump, the less water used, and the stronger the concrete. The slump is measured by working concrete in a 12" tall cone in intervals (must be performed by professional). Then pulling the cone up and allowing the concrete to slump down. The distance the concrete falls (in inches or cm) or slumps from the top of the cone to the top of the slumped concrete is the slump. With today's concrete, you can achieve a high slump without using much water with the application of superplasticizers or mid-range water reducers. In these instances thou, concrete is usually tested before the addition of the plasticizers.

How do I remove rust stains from concrete?

If the stain is fresh, use a heavy duty cleaning compound containing a large proportion of tri-sodium phosphate or TSP, available at any hardware store. Apply with a damp cloth or pad. Rub the discolored surface until the stain is removed and rinse thoroughly.

If that doesn't work, make a paste of 1 part sodium citrate crystals to 6 parts water and enough whiting or talcum powder to make a paste. Spread the paste on the rust stains. Scrape the dried paste. The rust should be removed. Repeat if necessary.

Another rust removing product is naval jelly, which is available from paint stores. Apply a small amount of the jelly with a soft paintbrush or cloth. Allow the jelly to cover the stain for 30 minutes to an hour or until the stain disappears. Then wash with clear water.

What is the best Formula for concrete?

Measure: 4 Units clear stone, 3 units washed sand, 1 Units cement ( type 10 portland ) and 1 Units water and mix until well blended, adding water to increase workability. This will ultimately sacrifice compressive strength.

units equal by weight or volume

What is Cement? is it the same as Concrete?

Cement is a gray powder that chemically reacts( caustic calcium reaction), When water is added. This intern acts as a binder for the aggregates and sand to make concrete. Therefore Cement is a powder, and Concrete is a mixture of cement, stone, and sand.

Why is my floor dusty? And what can I do about it?

Any time concrete begins to segregate and is then altered or changed (ie. freezing, adding water, over finishing...) this in turn doesn't allow the molecules to properly bond and will result in a slow but continual break down of the concretes surface. The blame usually goes to old concrete, or over worked concrete. Now there are products available that when applied will re-create those reactions found in concrete, and actually re-bond the fine loose molecules together again. This is a very effective way to water-proof concrete as well.

Was-sup with Fly-ash?

Fly-ash is a fine granular carbon used as an additive to help with the compressive strength of concrete. Although I believe fly-ash has its place in concrete. I must admit I'm not a big fan of it. Produced by power generating plants, and knowingly containing heavy metals and other known carcinogenic I don't feel it belongs in our homes and or our work places. As a person who works in concrete on a daily basic I am concerned. But, where else can the government get rid of it. Out of sight.....Out of mind............!!