CONCRETE MIX

BASIC MIX

The physical properties of density and strength of concrete are determined, in part, by the proportions of the three key ingredients, water, cement, and aggregate. You have your choice of proportioning ingredients by volume or by weight. Proportioning by volume is less accurate, however due to the time constraints of a class time period this may be the preferred method.

A basic mixture of mortar can be made using the volume proportions of 1 water : 2 cement : 3 sand. Most of the student activities can be conducted using this basic mixture. Another "old rule of thumb" for mixing concrete is 1 cement : 2 sand : 3 gravel by volume. Mix the dry ingredients and slowly add water until the concrete is workable. This mixture may need to be modified depending on the aggregate used to provide a concrete of the right workability. The mix should not be too stiff or too sloppy. It is difficult to form good test specimens if it is too stiff. If it is too sloppy, water may separate (bleed) from the mixture.

BASIC INGREDIENTS FOR CONCRETE

MIX PROPORTION

THIS PROPORTION VARIES ACCORDING TO THE FOLLOWING CONDITIONS:

To gain complete control over the properties of a mortar/concrete mix, you must consider the following criteria:

• Cement Amount:As a thumb rule, increasing the cement increases the strength. Beyond a certain point, it also acts negatively. Since major force transfer in a concrete/mortar matrix is from sand-sand interaction, excess cement will turn the mortar very brittle since cement particles cannot transfer normal contact force - they are good at providing shear strength. Since cement is expensive, in low-strength applications like roads the amount of cement is less to optimize on the cost.

• Water Amount: Generally water content ranging from 20% - 35% (w/w cement) is considering a safe operable range. Lower water content gives low strength and less manoeuvrable mix for flat conditions like road laying. Higher water content is generally used in specific conditions like laying out pile using tremie - where the free flowing nature of mix is necessary. Although high-water also leads to lower strength, there are other workarounds (mentioned ahead).

• Sand Amount: A very high amount of sand will make your mix very brittle and weak against all kinds of forces. For M20, M25 etc, general proportion is 1:3. However, for High-strength mix (M35+) its better to go with 1:2 and somewhere about that.

• Aggregate Amount: Aggregates have two reasons to be there - economy and strength. They are cheap and provide as a good filler. Very high and very low amounts of aggregate give poor strength but a varied economic solution. A moderate amount is good enough.

• Aggregate shape: Generally, increasing crookedness of the aggregate particle gives increasing strength as it gives a higher contact area and better inter-lock capabilities.

• Air Entrainment: A high air-content in the mix leads to lower strength. That's why high-strength concrete is vibrated before laying to expel the small air-pockets. A low air content gives low manoeuvrability, so sometimes, 'air-entrainers' (chemical additives) are used to give the desired flowing nature without compromising on changing the water content and consequently the strength.

• Additives: Physical additives like 'fine-silica-powder', 'fly-ash' allow cement reduction and provide economy. fine-silica is ... very fine so it goes into the smallest of voids and provides good contact strength by reducing the air content. Fly-ash comes as a general replacement for cement. It is a by-product from thermal power stations and is very cheap. Upto 15% cement can be replaced by fine-silica and upto ~40% by fly-ash. Chemical additives like super-plasticizers do provide a effective workability increase, or even a water reduction at similar workability - thus giving a higher strength mix.

GRADE OF CEMENT MORTAR

GRADE OF CONCRETE

WILL BE CONTINUED.....................