Testing Concrete Aggregates ( From field to Site)

Here after is the specification for concrete aggregate testing in a hi-rise building construction.

General

Samples of the fine and coarse aggregates approved by the Engineer shall be kept on the Site and shall give a fair indication of the approved quality of the aggregate for comparison with the aggregate delivered during the course of the works.

Should a sample fail to comply with any of tests, the Engineer may at his own discretion reject the batch from which sample was taken, or order it to be washed and/or screened, or permit such to be used with variations in the proportions of the concrete mixes specified, all a the Contractor’s expense. Any batch of aggregate rejected by the Engineer shall be removed from the site forthwith and replaced entirely at the Contractor’s expense.

All sample and testing of aggregates shall be carried out in accordance with ASTM C33.

Preliminary Aggregate Tests

As soon as the source of supply of aggregate have been approved, the Contractor shall instruct  the testing agency to carry out the following tests for compliance with “Specification of Concrete Aggregates” (ASTM C33).

1. Sieve analysis
2. Tests for clay, silt and dust content
3. Tests for organic impurities
4. Tests for salt content (chloride and sulfate ions)

The results of these tests shall be submitted for approval as soon as available. Test (1) and (2) with tests for the moisture content of each aggregate shall be carried out on the sample used for each trial mix.

Testing sampling of Aggregates

The contractors shall carry out such tests on aggregate samples as are necessary for the production of the specified concrete. The minimum incremental frequency of tests on each type of concrete used for the works shall be:

1. Sieve analysis: at least once weekly
2. Moisture content: at least once weekly
3. Tests for clay, silt and dust content: at least once fortnightlu
4. Tests for organic impurities: at least once monthly
5. Tests for salt content (chloride and sulfate ions): for every 500 cubic meters of concrete placed.

If for any reason the Engineer is not satisfied with the works concrete, he may instruct the Contractor to further increase the rate of sampling. Conversely, the rate of sampling may be reduce by the Engineer when consistent high quality been well established.

Concrete Mix Design Secrets

In order to make a concrete mix design that works, you should master all the concrete theories in combination with experiences of concreting at work. Here is some important things you need to know when design concrete mixes.

A. What do you need to know before designing concrete?

1. What are the strength requirements?

- Compressive (on cube or cylinder specimen) strength

- Flexural strength

- Tensile strength

2. What is the placing method? By pump or direct pouring?

3. How far is the job site from the batching plant?

4. What is the structure for casting? Pavement, foundation, elevated slab, etc.

5. What are the projects specification?

- Maximum or minimum cement contents

- Maximum water/cement ratio

- Slump or consistency limit

- Minimum Strength requirement @28 days

- Material specifications (what is the maximum size of aggregate?)

6. Latest testing results of materials is needed in the preliminary selection of materials and design calculation

B. What are Design Precautions and Things to Remember when design concrete mixes?

1. Increasing the sand/total aggregate ratio, increases the water requirement at the same consistency.

2. Increasing the water/cement ratio decreases the strength of concrete at the same cement content.

3. Remember that adding 5 liters of water per cubic meter increases the slump by 2.5cm.

4. Remember that adding 5 liters of water per cubic meter decreases strength by approximately 4%.

5. Always follow recommended admixture dosage.

6. Always have “control” when performing trial mixes, always perform trial mixes with another mix using the same materials. This data can be useful in diagnostics if a problem occurs.

7. Always adjust batching quantities to the actual moisture condition of the aggregates.

8. Volume tolerance for 1m3 concrete is 1 ± 0.2m3.

9. Range of normal weight concrete is from 2,200 kg/m3 to 2,400 kg/m3

Lightweight Concrete Production

1. Introduction to light weight concrete

Lightweight concrete is mainly used as back-filling material. When talking about lightweight concrete, we refer to a concrete of which its specific gravity (density) is much more lower than normal concrete.

Basically normal concrete: 2.35 – 2.42 ton/m3 while lightweight concrete 0.80-1.40 t/m3.

Therefore final strength is not issue. Expected final strength would be usually lower 5 MPa at 28 dasy as such concrete contained of lot of air entrained.

The strength would be affected by type of lightweight aggregate to used.

2. Production Principle

Basically there are 2 ways to produce light weight concrete.

a. Cement/ Water/Sand/lightweight aggregate/ chemical additives (admixtures)

b. Cement/ Water/ Sand/ Chemical additives

By lightweight aggregate we can consider polystyrene balls, expanded clay for instance.  Chemical additives, we mainly means special air entrained agent such as Sika Lightcrete 1-500VP. This admixture ca entrained safely up to 30% of air entrained into the mix.

As per the use of the special air entrained, usually, the mix is batched for a designed volume the air entrained is measured in order to determine final volume of the concrete batched. Indeed in term of concrete production it is important to know about about which volume we are talking, we are referring to:

For instance: Expected air entrained 30%

Volume of fresh batch 1000 liters -> initial ar development 30% –> final hard volume ~1.3 m3

Fibre Reinforced Concrete

Conventional concrete containing discontinuous discrete fibres is called fibre-reinforced concrete. Fibers of various shape and size produced from steel, plastic,  glass, carbon and natural materials have been used.

However for any reinforcement to be effective, it must be stiffer than the concrete matrix that is reinforcing. Generally the less stiff fibres (made from plastic and natural materials) only offer benefits in improving the tensile strength of plastic and semi-hardened concrete and are therefore mainly used to reduce plastic shrinkage and plastic settlement cracking. The stiffer fibres improve both the tensile strength and the toughness of harden concrete.

The most widely used stiff fibre is steel. Low volume fractions of fibres (less than 1%) are used to reduce shrinkage cracking. Moderate volume fractions (between 1% to 2%) increase flexural strength, fracture toughness and impact resistance. High volume fractions (greater than 2%) lead to strain hardening of the composites. The shape and length of the fibres also play a role in the fibres’ effectiveness in improving the properties of the concrete. The use of fibres in concrete can have a marked effect on the workability of the concrete and this need to be taken into account  in the mix proportion of fibre-reinforced concrete.

Currently a great deal of research is being undertaken into the use of ultra-high-performance fibre-reinforced composites. One of the benefits of these materials is extremely high ductility. Fibre-reinforced concrete has been used for precast panels, airfield and highway pavements, industrial floors and in spraying concrete for slope stability and underground mining applications.