Find a Concrete Contractor and get questions answered. Our Concrete Forum provides a place to ask expert advice and post your own valuable information. That’s been one of our key missions for decades.

In addition, our Concrete Calculators help you get quick numbers to make your project a success.

Lauren Concrete was selected to provide ready mix for ABC’s award winning show, Extreme Makeover: Home Edition when the show traveled to Bastrop, Texas to build a home for a family that lost all in the Labor Day fire of 2011 in Texas.

Mizzy and Chris Zdroj and their three children were the recipients of the new home as EFC Custom Homes of Bastrop and “Extreme Makeover: Home Edition” harnassed the power of more than 3,000 volunteers. Mizzy is an unpaid volunteer firefighter who battled the Bastrop wildfire while her own house burned. Her fellow firefighters nominated her for the home makeover.

The builder, EFC Custom Homes of Bastrop, selected Lauren Concrete for this precision timetabled build. “We are working under an amazing schedule and Lauren Concrete has the state of the art dispatching and technical systems to coordinate the pour with precise deliveries” said one worker on location at the site. “Lauren Concrete has built a reputation as the go to company when you need the finest materials and a disciplined team. That’s why they were selected from all the ready mix companies in Central Texas”.

There was an added surprise for the Bastrop community, Extreme Makeover: Home Edition” remodeled the Heart of the Pines Volunteer Fire Department building located next to the Zdroj home and other companies also donated firefighter uniforms and additional equipment.

If you have any questions you can always visit the Concrete Forum and post your questions. Here’s an example of the great information shared for one person who had a question about why their concrete was cracking after only four months.

I am in the late stages of an addition to my home which includes a master bath and covered patio. The slab was poured in late November 2010 all at the same time (bath slab and patio). We had a major storm roll thru the afternoon/evening the concrete was poured but the concrete did sit for 4-6 hours before it started raining. As the addition progressed, I started noticing cracks in the concrete. The tile guy found nine hairline cracks across the bath area alone. Last month I noticed a crack all the way across the patio, which is about 16′ x 30′. I refuse to believe that this is “normal” like the contractor says. I had a detached garage and driveway extension built summer of 2009 and that concrete doesn’t have any cracks at all…..even with the recent trucks driving over it for the current addition. I believe the concrete supplier didn’t mix the concrete properly. Is there any way to test a sample of the concrete or have it inspected by an expert? What are my options if I find out the concrete is substandard?

Some of the helpful replies:

It’s extremely common for concrete to crack. it’s far more uncommon for it NOT to crack. There are several reasons for cracks. The two most likely are:
1) The concrete was poured much too wet, leading to excessive shrinkage as it cured. Many times, contractors add a lot of water to the mix to make it easier to pour it out of the truck. When the concrete cures, it loses water and shrinks a bit. Too much added water leads to a high water to cement ratio and therefore a lot of cracking, and typically within a couple of days of the pour!
2) There were not enough crack control joints, or they were improperly installed. These are the joints that are either tooled in while the concrete is wet, or sawed in as soon as possible after the concrete sets up. They are designed to control random cracking. There are specific ways they need to be installed. Probably the least likely scenario is that the concrete was defective. It’s more typically contractor error. Did the same contractor pour both of the slabs you mentioned? I wrote an article about concrete cracks. It can be found at www.4greatconcrete.com under the artcles and publications link. It goes into much more detail as to other causes of cracking as well. Good luck.

Although you were not there when they actually filled the truck with the appropriate materials, once the truck comes to the job site, the contractor or the man in charge of the concrete usually takes a look at the concrete by pouring a little into the shute. So the driver of the concrete truck gives this blank look and looks at the contractor or the man in charge and sometimes the man in charge will have the driver add water so he can work the concrete easier. At that point the question would be, who is responcable for the mixture of the concrete? As it is not feasable to hire a state licensed testing company to be there during the pour on a small job, that would have been possibly the only way to protect yourself. So the options are tear it out or live with it, sorry to be blunt.

If concrete is of perfect mixture and of perfect placement and perfect subgrade(of course there is nothing perfect in concrete)over a period of time will crack every 15 feet. The control joints just try to get the crack to happen at the control joint. So the object is to make the control joints asthetically pleasing to the eye with the surrounding architecture. The more control joints the better chances you have of hiding the cracks. Climate – soil conditions – placement voids – where the rebar is inside the concrete and mix all will have a contributing factor in a crack.

Question sent from a reader: I have a large concrete driveway that has obviously been enlarged over the years and pieced up with mortar at various points. Consequently it’s a mismatch of colours. I was thinking about painting or staining the driveway with exterior paint or concrete stain, assuming I can find any here in the UK, and sealing it the same day using a concrete seal?

Would such a process be effective or would I be wasting time and money?

Here’s our expert’s answer:

It won’t be miraculous buy it will be effective.
We have a waterbased stain that will color everything including your sneakers. Very potent.

Acid stain may not be as effective with different surfaces and patched areas.

How long should I cure (keep the plastic, straw, blankets, water, etc) on my concrete (driveway, sidewalk, slab, etc.)?

This question can only be answered by the professionals associated with the project. Concrete cures at different rates depending on the constituent ingredients and the ambient conditions it is exposed to. Your contractor (concrete supplier) should know what sort of curing is required for the particular mix being used. Also, by following their recommendation, you maintain whatever warranty that might be associated with the work performed. If there is a problem that occurs later on, they can’t blame you for not curing it properly if you did it according to their instructions.

How do I level (screed) the concrete over an area that is wider than my screed board.

You can tackle this several ways. The professionals will drive grade stakes inside the forms at convenient intervals (less than the width of the screed board). The grade stakes will have the proper level marked on the stake, usually a nail in a wooden stake. You can place some concrete in the forms and using the grade stakes as a guide, begin screeding the concrete. As you get the proper grade on the concrete, you can pull the grade stakes. As you can tell, this requires a little experience to get it just right, but beginners can do it with less accuracy.
Another method is to get a probe, such as a stick or steel stake, mark it with the depth you want the concrete to be, then place the concrete in the forms. Screed the concrete and probe to ensure the proper depth. This is not as efficient as the first method, but it will get the job done with a little more effort. You can set construction joints (1×4’s, metal keyway forms, etc) inside the forms at intervals of 25-30 times the depth of the slab. You can use these as screed guides.

Finally, you can set screed guides inside the forms using 2×4’s and stakes so that the bottom of the 2×4’s are at the top of where you want the concrete to be. Nail ears on the end and top of your screed board so that when you set the screed board on the screed guides, the bottom of your screed board is even with the bottom of the screed guide, thus level with the top of where your concrete will be. Once you get your concrete down and screeded, you can pull these screed guides and fill in the stake holes and re-level by eye.

What standard specifications should I know or understand regarding information received from a contractor in anticipation of signing a contract to have my concrete driveway replaced? What is better rebar, reinforcement mesh (both?) , fiberglass reinforced cement? Depth of the driveway poured, 2inches 4 or more? How far should expansion joints be put in to handle heat and cold.

Reinforcement steel in flatwork is strictly for crack control. The best crack control is affected by putting the steel in the upper half of the slab. If your contractor can guarantee he will keep the wire mesh in the upper half of the concrete, that will do as well as rebar. Make sure he gives at least 3/4″ cover to whatever reinforcement he uses. The standard driveway depth is 4″, but that doesn’t mean that depth is right for you. It really depends on the soil conditions and the load you will be putting on the concrete. Thickened edges (beams) are highly recommended for the outsides of the driveway. This will enable the edges to take that occasional load on the edge or coming onto or off of the edge. Normal
Portland cement concrete likes to crack every 10-12′, so jointing in all directions no more than 10′, with the joints being a minimum of 1/4 the slab depth, should help the expansion/contraction problem. There are two types of joints: contraction and construction. Contraction joints are usually installed with a concrete saw after the concrete gets hard or a jointing tool while the concrete is still fresh. Construction joints are preinstalled dividers such as redwood strips, aluminum keyways, or other physical barriers. Either type of joint will allow for expansion/contraction, so if you don’t want redwood every 10′, you don’t have to have it. Make your joints as square as you can. Try to avoid rectangular jointing sections. Make sure the contractor orders concrete that has 4-6% entrained air, since you live in conditions that probably require salting of roads. The air will enhance the concrete’s freeze/ thaw durability and make it less susceptible to salt damage. I could spend at least 8 hours going over things to look out for and things to avoid. The main thing is sitting down with the contractor before you sign a contract, and tell him how you want the thing to look when he gets finished. Discuss whether visible cracks are acceptable or not. Discuss a warranty. Tell them what you want in the end, put it in your contract, then get out of his way and let him give you what you want. If he doesn’t give you what he says he would in the contract, you have legal standing for a remedy. Also, let him buy the concrete so
he can’t blame you for buying an inferior product that he couldn’t work with.

Periodically, as a service for our readers, we’ll feature a select job posting with a top concrete industry company.  This month we are proud to present a posting for a Ready Mix Dispatcher.

Candidates must have experience with Command Alcon software.

This is not an entry level position. Professional experience as a ready mix dispatcher is a prerequisite. The geographic location for this job is the Austin, Texas area.

How to apply:  please email resume and requests to: [email protected]

*No phone calls, please – email correspondence only – send resume in PDF or doc format

So, you want to put in a patio, driveway or sidewalk, and you are going to use concrete. A very wise choice, we can all agree. One thing to know before you put in your concrete all concrete cracks. You say, “Wait a minute, I’ve seen concrete that doesn’t have any cracks. How can you say all concrete cracks?” Concrete typically consists of cement, rock, sand and water. In the fresh, or plastic stage, concrete is fluid.

By Kenneth Wayne Meyer
Contributing Editor
www.concrete.com

As it hardens, the cement and water begin to shrink, and the stresses created by this shrinking cannot be overcome by the small amount of strength developed by the young concrete. If you place the concrete on a windy day, the top may start to harden before the bottom, which will cause the concrete to shrink unevenly (plastic shrinkage cracks.) Also, if the ground underneath the concrete is not level, there will be an unequal dragging force while the concrete shrinks, also causing stresses the new concrete cannot withstand. So, how do you get concrete with no VISIBLE cracks in it? By following a few simple steps before and after you place the concrete, you will have a very nice looking structure that will require very little maintenance, and give you years of enjoyment.

Before you place the concrete, make sure your subgrade (ground beneath the concrete) is thoroughly compacted and level. The absolute best thing to do is get a garden tiller, till the soil to a depth of 6 inches, then rent a hand operated compactor and compact the soil vigorously. This will help ensure there are no soft spots. You can apply a layer of cushion sand if you want. This will help achieve a totally level surface and allow a consistent friction to the shrinking concrete. Four inches of washed sand ought to be plenty for the cushion. If you use a wire mesh for reinforcement, use panels and not rolls. The rolled wire mesh is extremely difficult to keep in the top half of the concrete, where it HAS to be in order to do its job. You can also use reinforcement bars (rebar) tied together with steel wire, but spacing and size requirements vary based on load and soil conditions, so it is hard to recommend a standard set up for that. If you do use rebar, it is essential that you keep it in the top half of the concrete. You can use stones, broken brick or you can buy plastic chairs that the steel will sit on to keep it in the proper position when you place the concrete. You can also have the ready mix concrete company supply fibers to the mix. These fibers are usually nylon or polypropylene. They help keep the cracking of the concrete on a micro level instead of a macro level (where you can see the cracks with your naked eye.) Steel reinforcement also helps keep cracking in check, but if cracking does occur, the steel, when properly placed in the concrete, will hold the concrete together, whereas fibers will not do that.

Okay, you’ve got your subgrade ready, you have placed a plastic vapor barrier on the subgrade for slabs that will support dwellings, your steel is sitting nicely on your plastic chairs in the proper position, and you now have 14 of your closest friends on their way over to help you place the concrete you have coming. When the concrete arrives, if you don’t have a vapor barrier, wet the subgrade without puddling the water so that the water in the concrete will not be absorbed by the dry subgrade, thus causing uneven drying and the dreaded plastic shrinkage cracks. Once the concrete is placed, make sure to protect it from high winds and direct sunlight so the concrete will dry evenly from top to bottom. You are now ready to perform the most important step in preventing noticeable cracking. Contraction joints are the secret to no cracking! By placing contraction joints that are at least 1/4th the depth of the concrete and on intervals of 25 to 30 times the depth of the concrete (usually easiest with a jointing trowel or tool while the concrete is still fresh), you will almost ensure there will be no visible cracking in your concrete. If your slab is 4 inches thick, the joints must be at least 1 inch deep and placed every 100 to 120 inches. If you cannot use a jointing tool to put the joints in, you can hire a concrete sawing contractor to do this for you. Make sure he cuts the joints a minimum of 1/4th the slab depth. This jointing method helps the concrete crack at the weakest point. This is why it is so important for the joints to be deep enough. Variations in subgrade levels could cause greater stress in the concrete in an area where the joint isn’t deep enough, and the concrete will crack outside the joint. Once your joints are in place, and the concrete has cured for about two weeks, you are ready to seal the joints. This will prevent water from migrating into the subgrade and expanding and contracting, or getting into the joints and freezing, causing the water to expand and breaking out the concrete around the joints. You now have a concrete structure that will serve you well.

We are regularly asked to determine the amount of cement in hardened concrete and mortar. The request is normally made for one of two reasons; the most common being that something has gone wrong and the cause and/or blame for the problem is thought to be related to cement content. The other is that an older structure is being repaired or expanded and it is desired to match the existing materials.There are ASTM standards describing how to do such determinations, but they are based on a number of assumptions that, in some cases, are not valid. This document seeks to describe what is actually measured, the source of potential errors and how big they may be, and what can be done to improve the probability of reaching an answer close to the truth. This is done by describing what mortar and concrete are made of, what can be measured, what assumptions are made, and how the whole puzzle is solved.

By Peter C. Taylor
Contributing Editor
www.concrete.com

Can we really measure cement content in hardened concrete and mortar? We can measure the raw materials in hardened concrete and mortar, but these data do not necessarily give enough information to allow us to state the cement content without some assumptions and qualifications.

WHAT IS IN CEMENT, MORTAR AND CONCRETE?

We start by describing the raw materials that go into mortar and concrete and by defining some terms. Cement is a generic term meaning “glue.” Portland cement is a gray powder that when mixed with water forms a paste that hardens and gains strength with time. This is the glue that holds mortar and concrete together. When sand or fine aggregate is added to paste the mixture is known as mortar which is suitable for thin cross sections. Grouts, plasters and stuccos are generally special mortars and contain much the same raw materials. Stone added to mortar makes concrete which can be used in structural or massive applications.Cement

The cement most often used in construction is known as portland cement. There are other types of construction cements, some used in masonry construction and other special cements used for repairs or high temperature applications. This paper addresses portland cement and its derivatives only.The predominant chemical compounds in portland cement are based upon oxides of calcium (lime), silicon (silica), aluminum (alumina) and iron. There are other compounds present in smaller quantities such as magnesia and carbon dioxide and a number of trace elements. The principal chemical compounds that combine with water (hydrate) to provide strength are calcium silicates. However, in all reported chemical analyses, the constituents of cement and concrete are reported simply as the appropriate oxides. The way in which these compounds combine is extremely complex and outside the scope of this paper. Modern portland cements, by definition, all tend to contain these compounds in a fairly tight range of values even if they come from different manufacturing facilities. Hydrated portland cement has the unusual, and desirable, property that it will continue to gain strength (albeit at a decreasing rate) when in the presence of water. This complicates chemical analysis because the system is continually changing from the time of first mixing to the time of test.

A source of further complication is when historic materials are being tested because the composition and fineness of cement made in 1920 is not the same as that made in 2000. Masonry cements are normally a blend of portland cement, crushed limestone and some polymeric additives. The manufacturers do not publicize the relative amounts of portland cement and limestone but ASTM standards do set out ranges into which the blends should fall. It is these blends that tend to cause the most complicated analyses and the broadest range of assumptions in the method.

Aggregates

The aggregates used in mortar and concrete are built from the same building blocks: lime, silica, alumina and iron oxide. Some aggregates can be physically separated from hydrated portland cement by their differing solubility in acid. Aggregates tend to fall into two very broad categories, those containing mainly silica and those containing mainly calcium and magnesia. Siliceous aggregates are generally insoluble in acid, but not always, and this is the source of one important assumption made by ASTM C 1084. Calcareous aggregates are soluble in acid, but generally do not contain soluble silica – another assumption.Supplementary Cementing Materials

Other materials coming into the market are the so-called supplementary cementing materials such as fly ash and slag. These are often waste materials that contain similar compounds as portland cement, albeit in differing proportions. By virtue of their chemistry, glassy state and fineness they will react beneficially with portland cement. They are either added to concrete to reduce costs, or to enhance properties. It is difficult to distinguish between these materials and cement in a hardened concrete. The range of their chemical compositions is large, further complicating the interpretation of chemical analysis.Unhydrated particles of fly ash and slag can be observed using microscopical techniques, and an experienced analyst can estimate the volume of residual fly ash and fly ash present. The presence of slag can also be qualitatively indicated by testing for the presence of sulfides. The extremely small size of silica fume particles (another supplementary cementing material,) and the low dosage normally added makes definitive detection of this material difficult.

Chemical Admixtures

Chemicals, generally in liquid form, are often added to cementing materials in order to modify or enhance the properties of the plastic or hardened concrete. They are generally added in very small doses and their presence does not usually interfere with cement content determination.WHAT DO WE MEASURE?

It is not sufficient to just measure the chemical composition of the hardened material to determine cement content because all the constituents of hardened concrete contain the same chemical elements. This section describes what other means can be used to complement the chemical analysis.Chemistry

The basic procedure is to take a representative sample of the mortar or concrete, crush it to a fine powder, dissolve it in acid and then use standard chemical analytical techniques to measure the relative proportions of calcium, silica, alumina, magnesia. The amount of insoluble residue is also determined and assumed to be aggregate. A portion of the sample is also heated to 1000°C and the loss in mass measured at certain temperatures. These losses represent different phases of the material (including water and carbon dioxide) breaking down into gas and leaving the sample. The original unit weight of the sample is also a useful parameter that is regularly determined. The reliability of these analyses is strongly influenced by the sampling techniques used. The size and number of pieces of mortar or concrete taken from the structure have to be sufficient to represent the concrete being tested. The ASTM methods specify minimum sample sizes, but it is not uncommon to receive much smaller samples from the field. These analyses are often the ones that cause problems.Petrography

Petrographic (microscopical) analysis of the sample is invaluable in addressing a number of questions:

• What type of cement has been used?
• Does the sample contain fly ash, slag, ground limestone or other mineral admixtures, and if so, approximately how much?
• What is the aggregate type and is it possibly soluble in acid?
• What is the water – cement ratio?
• What is the extent of hydration?
• What is the condition of the sample?
• Are there deposits or contaminants?
• Has leaching removed constituents?

Not all of these questions can always be answered, and often the answers are given as ranges of values, all of which have to be built into the final interpretation. Microscopical point-count methods can be useful in determining the presence and amount of fly ash, but this approach requires refinement.ASTM C 1084 – STANDARD TEST METHOD FOR PORTLAND-CEMENT CONTENT OF HARDENED HYDRAULIC CONCRETE

The broad approach in C1084 is to use analytical chemical means to measure soluble calcium oxide, silica and insoluble residue. Allowing for the aggregate type and composition, the amount of soluble oxide is attributed to the cement, and used to calculate the total cement content. Similarly the insoluble material is attributed to the aggregate and used to calculate the aggregate content.The method makes the following assumptions (and qualifies itself accordingly):

• There are no supplementary cementing materials.
• Soluble calcium oxide and silica contents of cement are assumed as fixed values unless given from another source.
• Soluble silica and calcium in aggregate is assumed to be negligible (where appropriate.)

If any of these assumptions are not correct the results of the analysis are likely to be inaccurate. Many aggregates contain soluble calcium and / or soluble silica, while supplementary cementing materials are soluble. The ASTM method recommends that the type of aggregate be assessed but does not require a petrographic examination. This means that even strict compliance with the method is no guarantee of finding out what went into a given concrete sample.ASTM C 1324 – STANDARD TEST METHOD FOR EXAMINATION AND ANALYSIS OF HARDENED MASONRY MORTAR

Tests on mortar are complicated by the larger range of cementing binders used, and by the frequent addition of ground limestone or hydrated lime into the mix. The basic chemical analysis of the sample is similar to that conducted on concrete. The method also requires that a petrographic examination be carried out in order to ascertain what components have been used in the mortar, i.e. masonry cement, masonry lime, and type of aggregate. Estimates are also made of the air content, water – cement ratio and degree of hydration. All of these are used as inputs into the matrix when solving the chemical calculations. This method is somewhat empirical in that estimated values are compared with results from calculations based on assumptions and measured data. The assumptions are modified based on the observations in order to bring the two sets of information into agreement. This type of iterative practice is at the heart of engineering calculations, but is unsettling to pure scientists. What it does mean is that any set of reported results are open to some variation, the extent of which is difficult to assess, and may be large. Again, the method is not a black box that takes a limited set of inputs and returns a neat, absolute, result.WHAT IS NEEDED?

It is important that a sufficient number of concrete samples are extracted so that at least 1 kg (2.2 lb) is available for chemical analysis, with sufficient remaining for petrographic analysis. Two 4 x 8 inch cores are a minimum when concrete is being assessed.For mortar, ASTM C 1324 requires a minimum sample of 10 g. Samples extracted from at least two zones are desirable – one set from the concrete in question and another from similar concrete that is considered acceptable. It is then possible to report on differences between the concretes with some confidence, even if the absolute answers are difficult to extract.

Ideally, the concrete batch materials (cement, supplementary cementing materials and aggregates) should be provided, in which case the amount of each material in the mix can be solved as a set of simultaneous equations.

All information about the aggregate source, mill test certificates for the cements, the use of supplementary cementing materials and the age of the concrete will assist the determination. The more data and material that are provided, the narrower will be the range of error reported at the end of the analysis.