Concrete is truly a versatile building material. Concretes in use today are formulated with very specific performance characteristics in mind and include lightweight, heavyweight, porous, fiber-reinforced, mass, high-performance and cellular concretes to name just a few. Each provides specific characteristics or properties for their intended use. These properties are achieved by intentional formulation and control of such variables as cement content and type, pozzolan type and content, aggregate type, admixtures used, the addition time and rate of those admixtures, as well as other, often subtle, differences.

By George W. Seegebrecht and Steven H. Gebler
Contributing Editors
www.concrete.Com

One widely used specialty concrete is known as “shotcrete.” The major difference between shotcrete and its close cousin, concrete, is the placement method. Concrete is discharged from a ready-mix truck, placed on the ground or in forms and then must be vibrated for compaction. By contrast, the shotcrete process, whether using wet or dry material feed, does not require forming or compaction thereby enhancing design creativity and application flexibility, often resulting in a savings of time or money

Shotcrete, was originally called “Gunite” when Carl Akeley designed a doubled chambered cement gun in 1910. His apparatus pneumatically applied a sand-cement mixture at a high velocity to the intended surface. Other trademarks were soon developed known as Guncrete, Pneucrete, Blastcrete, Blocrete, Jetcrete etc. all referring to pneumatically applied concrete. Today Gunite equates to dry-mix process shotcrete while the term “shotcrete” usually describes the wet-mix shotcrete process. At point of application, both are typically referred to as shotcrete.

Dry-mix process shotcrete, introduces and mixes the required water at the application nozzle as the dry cementitious materials (fly ash, slag, silica fume etc.) and aggregates are delivered through the “gun” The nozzleman controls mix consistency, adjusting water addition to suit the changing conditions of the work area. The dry-mix process also is well suited for sporadic application operations since the majority of the water only comes into contact with the cementitious materials as it leaves the nozzle.The wet-mix process utilizes concrete delivered to the job that is thoroughly mixed excluding of any required accelerators. The ingredients are generally delivered in ready-mix trucks as with normal concrete. Accelerators or other admixtures may still be metered into the slurry at the nozzle along with air under pressure to increase the velocity of the material and improve control of the application or “shooting” process.

The impact velocity of properly applied shotcrete instantly compacts the material, yielding an “in-place” mix that is richer in cement and higher in strength than the same mixture prior to placement. Typically, a fine aggregate dry-mix shotcrete mix delivered in a 1:3 cement to aggregate proportion upon entering the application gun results in a 1:2 cement to aggregate ratio when in place. What appears to be a waste of materials and a dust nuisance known in the trade as “rebound” and overspray, actually results in dense, high-strength shotcrete as a portion of the aggregate ricochets off the receiving surface and away from the placement location. The loss through rebound will vary depending upon the dryness of the mix, the shooting distance from the surface, wind conditions, etc. The intended thickness is generally overshot, trimmed back to the design thickness and finished to the desired surface texture and appearance.

While the dry mix process sounds quick and economical, it requires precautions to ensure application quality. The nozzleman’s workmanship and experience are critical, since the nozzleman controls the critical water-to-mix ratio going into application equipment. With the wet-mix process, the nozzleman has no control over the consistency of the mix delivered to the job site, but can control the velocity of the materials and the addition of accelerators as the mix leaves the nozzle.

Just as in concrete mix designs, the water-to-cementitious materials ratio remains the single most important parameter influencing the compressive strength, shrinkage and overall durability of the final product. Application technique is also crucial and less forgiving than ordinary ready-mix. Good “shooting” technique can mean the difference between a dense high-strength material or one that looks good on the finished surface but actually has underlying sand pockets, voids and poorly encased reinforcing steel. Poor application technique increases the probability of cracking and its negative ramifications.The shotcrete process is more versatile than conventional concrete placement. If the shooting surface is sound, clean and accessible, shotcrete can be applied in very difficult or complex shapes or sections where conventional concrete formwork would prove difficult or impossible as well as cost prohibitive. Shotcrete is especially applicable for unique shapes desired in complex shapes, swimming pools and other unique features of aquatic parks. It can also be an excellent overlay and repair material for existing structures because of its potential to achieve good bond strength and low permeability.

The nuances and differences between concrete and shotcrete are too numerous to cover in a short article. Selecting a concrete placement method, whether it be conventional concrete, wet-mix or dry-mix process shotcrete, can be a challenging task, since there are positive aspects of each for almost every application. While it is true that one approach may be more applicable, adaptable or economical than another, the final concrete placement selection for the project should be based on project design, material performance criteria and overall budget.

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.