INTRODUCTION

Concrete is a composite material composed of coarse granular material (aggregate or filler) embedded in a hard matrix of material (the cement or binder) that fils the space between the aggregate particles and glues them together.

Concrete is a composite building material made from the combination of aggregate and cement binder. The most common form of concrete is portland cement concrete, which consists of mineral aggregate (generally gravel and sand), portland cement and water. It is commonly concerned that concrete dries after mixing and placement. In fact, concrete does not solidify because water evaporates, but rather cement hydrates and hardens into a stone-like material. When used in the generic sense, this is the material referred to be the term concrete. Concrete is used to make sidewalks, building foundations, freeways, overpasses, parking structures, bases for gates/fences/poles, and cement in brick or block walls. An old name for concrete is liquid stone.

The most important requirement for concrete buildings is long - lasting durability. With proper materials and techniques, it can withstand many acids, fertilizers, water, fire, and abrasion. Concrete can be finished to produce surfaces ranging from glass-smooth to coarsely textured, and it can be colored with pigments or painted. Major signs of concrete deterioration are cracking, spalling, deflection, stains, erosion and corrosion.

Deterioration in concrete can be caused by environmental factors, inferior materials, poor workmanship, inherent structural design defects and inadequate maintenance.

Chemical analysis of hardened concrete can provide a wealth of information about the mix constituents and possible causes of deterioration. This analysis is usually required 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 some methods describing how to do such determinations:

• chemical composition of crumbled sample of concrete, especially regarding two components: soluble silica in HCl and /or CaO,

• the content of insoluble parts in HCl,

• sieve analysis of concrete,

• differential thermal analysis,

• microscopy analysis,

• density of mortar.

In Poland analysis of composition of hardened concrete describes instruction ITB nr. 277. The laboratory method is based on determination: apparent density of concrete, ascertainment of presence of slag and/or ash, the content of insoluble parts in HCl and the content of component affiliated to binder during hardening.

During laboratory practice it is necessary to determinate only content of aggregate and cement in hardened concrete.

CHOICE OF PLACE AND THE WAY TO TAKING SPECIMEN

The largest difficulty in chemical analysis of concrete is heterogeneity of this material. This is why the way and the place of taking specimen is very important for precision results. The mass of sample depends on grain's size of aggregate. The minimum mass of concrete's sample (the sum of individual pieces) should be 3 kg if the aggregate's size is up to 40mm. If aggregate size is bigger the sample's mass - up to 5 kg.

Choosing place of taking samples one should consider: external appearance of concrete, local changes of colour, presence of efflorescence and others in whole building.

Realization of practice

DETERMINATION THE CONTENT OF AGGREGATE AND CEMENT IN HARDENED CONCRETE

It is necessary to determine apparent density of concrete, amount of insoluble residue in HCl and amount of affiliated components during hydration, hydrolysis and carbonization and make proper calculation using these data.

1. Determination of apparent density

Apparent density - the weight (dried to constant mass in temp. 105 - 110ºC) per unit volume of a material including voids inherent in the material.

2. Determination of aggregate content

The amount of insoluble residue in HCl is assumed to be aggregate.

Sample of concrete (1 kg) one should crumble and sieved through sieve 1 mm. According to proper standard reduce mass of sample to 50 - 100 g and dry to constant mass in temp. 105ºC. After drying and precise mixing one should weigh 5 - 10 g of sample and powder in mortar until the grain's size will be less than 0,2 mm. Sample (1 - 2 g) is taken for further investigation: determination of insoluble parts in HCl.

The weighed sample should be placed into 250 cm³ beaker, add 100 cm³ water solution of HCl 1:3 (1,19). Mix all with glass rod, leave for 15 min. and decant the liquid over the precipitate. Then add to the precipitate 50 cm³ HCl and keep on water bath (temp. 90 - 100ºC). Content of the beaker wash twice hot water and decant. Add to the beaker 50cm³ 5% Na ₂CO₃ and keep it on water bath for 15 min. and wash twice hot water and decant. Place 50 cm³ of water into the beaker with precipitate, then acidify with HCl at the presence of methyl orange and filter. The precipitate wash 6 times with hot water until chlorides are completely washed. In the end make the test for chloride ions: place some drops of the solution on watch glass, add a drop of Ag NO₃ - if you don't get precipitate it means that there are no chloride ions. Put the paper filter with content into the porcelain crucible (first weight the crucible) and burn the paper filter, then ignition of the precipitate in the temp.1000ºC. After the cooling in the exsiccator weight the crucible with the precipitate (m₁). Calculate the content of insoluble components according to the formula:

[%]

m₁ - the mass of the crucible with the precipitate [g]

m₂ - the mass of the crucible [g]

m₃ - the mass of the sample [g].

ρ - apparent density [kg/m³]

There is assumed that the content of insoluble parts equals the content of aggregates.

If aggregates consists components soluble in HCl it will be necessary to change formula using a correction:

[%]

R_k - content in aggregates of component soluble in HCl [%].

The value of R_k have to be estimate on the base of chemical analysis of aggregate's sample. This analysis should be made like the analysis of hardened concrete using average sample of aggregates.

Determination affiliated components in the hardened concrete

The content of the affiliated components (S) to the concrete during binding and hardening determine on the base of differential thermal analysis (DTA).

Value of amount of affiliated components is equal ignition loss in weight observed in a sample heated to 1000ºC.

Results of investigation should be represented in the report by formula.

REPORT

Subject data

Name of the sample:

sample's weight:

concrete's composition
	%	kg/m^3
apparent density
aggregate's content
affiliated components
cement's content

conclusions:

cement

+

water

cement paste

+

fine aggregates

mortar

+

coarse aggregates

concrete

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