Axim Concrete Frequently Asked Questions

What is the difference between cement and concrete?
Although the terms cement and concrete often are used interchangeably, cement is actually an ingredient of concrete. Concrete is basically a mixture of aggregates, water, admixtures and cement. The aggregates are sand and gravel or crushed stone.

Chemical admixtures, like those produced by Axim are a critical part of the formulation of concrete. Admixtures can alter the chemical reaction of cement and water.

Portland cement is not a brand name, but the generic term for the type of cement used in virtually all concrete, just as stainless is a type of steel and sterling a type of silver. Cement comprises from 10 to 15 percent of the concrete mix, by volume. Through a process called hydration, the cement and water harden and bind the aggregates into a rocklike mass. This hardening process continues for years meaning that concrete gets stronger as it gets older.

So, there is no such thing as a cement sidewalk, or a cement mixer; the proper terms are concrete sidewalk and concrete mixer.

Why do you use Admixtures in concrete?
Admixtures enhance the properties of concrete or mortar in the plastic state and improve durability in the hardened state. Admixtures increase the efficiency of cementitious materials and/or improve the economy of the concrete mix.

What is an Air-Entraining Admixture?
An air-entraining agent is a liquid chemical admixture used to create bubbles that will provide adequate freeze-thaw protection in concrete by generating 5 to 7 percent of the volume with air of properly sized and properly spaced bubbles. Air-entraining admixtures are surfactants, soaps or detergents, primarily used to generate microscopic size bubbles that render hardened concrete resistant to the effects of freezing and thawing. It is also used to improve the workability/placeability of fresh concrete.

What is a Water-Reducing Admixture?
A Water-Reducing Admixture is an admixture that either increase slump of freshly mixed concrete or mortar without increasing water content or maintain slump with a reduced amount of water—the effect being due to factors other than air entrainment.

It is a product that can be used as a water reducer, plasticizer, workability agent or surface active agent

Water-Reducing Admixture can, increase strength, reduce cement and/or all mineral admixtures, facilitate placeability, improve finishability, maintain freeze-resistant concrete and increase profitability and productivity.

What are Accelerators?
Accelerators are used to reduce the set time of concrete and increase early strength gain. Although the mechanism is not fully understood, it is believed that accelerating admixture increases the reactivity and hydration of C3S and C2S. The reaction is catalytic in nature, forming CSH (calcium silica hydrate) gel and increasing rate of reaction. This results with increased reaction of Hydrates, especially at early ages.

What are Retarders?
Retarders are admixtures that cause a decrease in the rate of cement hydration and lengthen the time of setting. Retarders remove the tendency of some cement to exhibit false set and counter the accelerating effect of high temperatures. They improve the workability and the finishability in hot weather and increase the transporting distance of the ready-mixed concrete. Retarders help to reduce the possibility of early dry-shrink cracking and reduce the maximum temperature rise in mass concreting, by extending the heat-dissipation period.

What are Superplasticizers / High Range Water-Reducers?
Superplasticizers are liquid chemical admixtures used in concrete because of its ability to perform as a highly effective wetting agent. Superplasticizers increase water reduction to achieve low water/cement ratios, increase strength gain, increases slump for ease of placement and creates a denser and more durable concrete.

Why does concrete crack?
Concrete, like all other materials, will slightly change in volume when it dries out. In typical concrete this change amounts to about 500 millionths. Translated into dimensions-this is about 1/16 of an inch in 10 feet (.4 cm in 3 meters). The reason that contractors put joints in concrete pavements and floors is to allow the concrete to crack in a neat, straight line at the joint when the volume of the concrete changes due to shrinkage.

Why test concrete?
Concrete is tested to ensure that the material that was specified and bought is the same material delivered to the job site. There are a dozen different test methods for freshly mixed concrete and at least another dozen tests for hardened concrete, not including test methods unique to organizations like the Army Corps of Engineers, the Federal Highway Administration and state departments of transportation.

What are the most common tests for fresh concrete?
Slump, air content, unit weight and compressive strength tests are the most common tests.

Slump is a measure of consistency, or relative ability of the concrete to flow. If the concrete can't flow because the consistency or slump is too low, there are potential problems with proper consolidation. If the concrete won't stop flowing because the slump is too high, there are potential problems with mortar loss through the formwork, excessive formwork pressures, finishing delays and segregation.

Air content measures the total air content in a sample of fresh concrete, but does not indicate what the final in-place air content will be, because a certain amount of air is lost in transportation, consolidating, placement and finishing. Three field tests are widely specified: the pressure meter and volumetric method are ASTM standards and the Chace Indicator is an AASHTO procedure.

Compressive strength is tested by pouring cylinders of fresh concrete and measuring the force needed to break the concrete cylinders at proscribed intervals as they harden. According to Building Code Requirements for Reinforced Concrete (ACI 318), as long as no single test is more than 500 psi below the design strength and the average of three consecutive tests equals or exceeds the design strength then the concrete is acceptable. If the strength tests don't meet these criteria, steps must be taken to raise the average.

How do you control the strength of concrete?
The easiest way to add strength is to add cement. The factor that most predominantly influences concrete strength is the ratio of water to cement in the cement paste that binds the aggregates together. The higher this ratio is, the weaker the concrete will be and vice versa. Every desirable physical property that you can measure will be adversely affected by adding more water. You can reduce the water/cement ratio by adding water-reducing admixtures or superplasticizers.

What is alkali-silica reactivity (ASR)?
Alkali-silica reactivity is an expansive reaction between reactive forms of silica in aggregates and potassium and sodium alkalis, mostly from cement, but also from aggregates, pozzolans, admixtures and mixing water. External sources of alkali from soil, deicers and industrial processes can also contribute to reactivity. The reaction forms an alkali-silica gel that swells as it draws water from the surrounding cement paste, thereby inducing pressure, expansion and cracking of the aggregate and surrounding paste. This often results in map-pattern cracks, sometimes referred to as alligator pattern cracking. ASR can be avoided through 1) proper aggregate selection, 2) use of blended cements, 3) use of proper pozzolanic materials 4) contaminant-free mixing water and 5) adding Lithium-based admixtures.

How much does concrete weigh?
Fresh concrete weighs about 3900 pounds per cubic yard using normal weight aggregates. It weighs about 3500 to 3700 pounds per cubic yard when cured.

What is Portland cement?
Portland cement is hydraulic cement produced by pulverizing clinker consisting essentially of hydraulic calcium silicates, usually containing one or more of the forms of calcium sulfate as an interground addition. Portland cement was first patented in 1824 and is named after the natural limestone quarried on the Isle of Portland in the English Channel.

I	Normal
IA	Normal, air-entraining
II	Moderate sulfate resistance
IIA	Moderate sulfate resistance, air-entraining
III	High early strength
IIIA	High early strength, air-entraining
IV	Low heat of hydration
V	High sulfate resistance

What is Fly Ash?
Fly Ash is a pozzolan. A pozzolan is a siliceous and/ or siliceous and aluminous material which in itself is not cementitious but will, in the presence of moisture and temperature > 40°F (> 5C), combine with calcium hydroxide to form cementitious compounds. The major use of fly ash is as a partial replacement for cement in concrete. The amount of cement replaced depends on the fly ash type, the fly ash quality, the application and the atmospheric conditions.

What is Blast Furnace Slag?
It's non-metallic element and consists essentially of silicates and aluminosilicates of calcium, and of other bases. It's developed in a molten condition simultaneously with iron in a blast furnace. Blast Furnace Slag is cementitious and pozzolanic. It improves concrete performance.

What is Silica Fume?
Silica fume is a by-product resulting from the reduction of high purity quartz with coal in an electric arc furnace in the manufacture of ferro silicon and silicon metal. Silica fume, which has a high content of amorphous (uncrystallized) silicon dioxide and consists of very fine spherical particles, is collected by filtering gases escaping from the electric arc furnaces. Silica fume helps to improve the compressive strength and to reduce the permeability of concrete by two methods:

What is Self Consolidating Concrete?
Self Consolidating Concrete (SCC) is concrete that can flow around reinforcement and consolidate within formwork under its own weight without additional effort, while retaining its homogeneity. SCC is different from traditional concrete and it has high fluidity, stability and deformability. It has an improved surface finish and greater durability.

SCC was developed in the late 1980s by Prof. Hajime Okamura at the University of Tokyo in Japan. AXIM has been a leader in North America in the development and mix design of SCC.