Aug 13 2013

Check by Consumer Before Ordering the Ready Mix Concrete

The following need to be looked into by the consumer:

Reliability of the plant and transit mixers for consistent and continuous concrete supply as per requirement.
Calibrations of all measuring devices and their accuracy.
Mode of operation of plant should preferably be fully automatic and not manual.
Quality of materials proposed to be used.
Adequacy of quantity of materials proposed to be used.
Compliance of concrete specifications based on the mix parameters specified.
Adequacy of testing facilities
Time likely to be taken by transit mixers from plant to site and back.

Jul 13 2013

Test Methods of Sprayed Concrete

Determination of early strengths

To determine the very early strengths (in the range 0 to 1 N/mm²), the Proctor or Penetrating needle is used.

The following methods are well established for compressive strength testing between 2 and 10 N/mm²:

Kaindl/Meyco: Determination by the pull-out force of bolts.
HILTI (Dr. Kusterle): Determination of the impression depth (I) and pull-out force (P) of nails shot with a HILTI DX 450L shot bolt machine (load and nail size are standard).
Simplified HILTI method (Dr. G. Bracher, Sika):Determination of the impression depth (I) of nails shot with a HILTI DX 450L shot bolt machine (load and nail size are standard). Determining the required early strength by this method should be accurate to ±2 N/mm².

Jun 24 2013

Testing the Consistence by Degree of Compatibility

Principle:

The fresh concrete is placed carefully in the steel test container. Compaction must be avoided. When the container is full to overflowing, the concrete is smoothed flush with the edge without vibration. The concrete is then compacted, e.g. with a poker vibrator (max.bottle diameter 50 mm). After compaction the distance between the concrete surface and the top of the container is measured at the centre of all 4 sides. The mean figure (s) measured is used to calculate the degree of compatibility.

Container dimensions Base plate 200 x 200 mm (±2)

Height 400 mm (±2)

Concrete in container Concrete in container

before compaction after compaction

Degree of compatibility: c= h1

_________ (non dimensional)

h1 – S

Apr 21 2013

Concrete for Precast Structures

Precast concrete is used to form structures which are delivered after hardening. Long journeys in the fresh concrete state disappear, which changes the whole production sequence. Concrete used for the production of precast structures requires an industrialized production process,and a good concrete mix design with continuous optimization is essential. The following points are important through the different stages of the process:

Preparation of concrete design

When preparing the design, the concrete requirements must be defined according to the specific elements, their intended use and exposure conditions. The following parameters should normally be defined:

Strength requirements
Durability requirements
Aesthetic requirements
Maximum particle diameter
Method of placement
Placing rate
Concrete consistence
General boundary conditions (temperature etc.)
Handling of the concrete and its placing
Definition of test requirements
Consideration of the specific concrete element parameters
Curing definition
Mix design and specification
Preliminary testing
Mix design adjustment if necessary

Feb 25 2013

Concrete Vibrating Table

1. Purpose

Vibrating table is used for proper compaction of concrete while casting specimens for compressive strength determination.

2. Procedure

i. After preparing the concrete mix, put moulds on the vibration table platform.

ii. Fill concrete in the mould in layers of 50 mm deep.

iii. In placing each scoopful of concrete, move the scoop around the top edge of the mould as the concrete slides from it, in order to ensure a symmetrical distribution of the concrete within the mould.

iv. Let each layer compact by vibration.

v. After compacting concrete in 3 layers, finish the surface of the concrete level with the top of the mould.

vi. Stop the table and remove the mould from it.

Feb 24 2013

Concrete Slump Test

The slump cone is used for determining the workability of concrete where the nominal maximum size of aggregate does not exceed 38 mm.
B. Methods of sampling and analysis of concrete

1. Firstly decide the frequency of slump value to be taken during concreting.

2. Oil the interior surface of the slump cone with mould releasing oil to prevent adhesion of the concrete.

3. Place the slump cone on a leveled surface.

4. Collect the sample in a wheelbarrow after mixing the concrete properly in the transit mixture.

5. Remix the sample thoroughly in wheelbarrow with sampling scoop.

6. After remixing immediately fill the slump cone in layers approximately one – quarter of the height of the cone.

7. Each layer shall be compacted with the tamping rod by 25 strokes distributed in a uniform manner over the cross-section of the cone and for the second and subsequent layers tamping rod shall penetrate into the underlying layer.

8. After compacting the top layer the concrete shall be struck off level with the top of the slump cone, using a _______. Any mould which may have leaked out between the mould and the base plate shall be cleaned away.

9. Unscrew the slump cone from the base plate and remove it immediately from the concrete by raising it slowly and carefully in a vertical direction.

10. After the concrete subsides place the slump cone on the base plate in reverse position and place a scale on it. Measure the height between the top of the mould and the highest point of the concrete specimen being tested.

C. Reporting of slump value: The slump measured shall be reported in terms of millimeters.