A B C D ... of Concrete for Civil Engineers

In this article, we will discover twenty-six points to focus on for concrete work.

A – Aim for Target

Engineers specify concrete by characteristic compressive strength. Concrete compressive strength follows a normal distribution curve, and characteristic strength is linked to a value below which only five per cent of samples will fall. Mix design is performed to achieve target strength, which is higher than characteristic strength.

B – Batching of Ingredients by Weight

The concrete mix design process shall start ahead of the planned first pour on site. The ingredients shall be mixed by weight as per the approved mix design.

C – Cement Content

Minimum cement content and type of cement shall be based on exposure class (carbonation class, corrosion induced by chlorides from seawater class, corrosion induced by chloride other than from seawater class, sulphate attack class) and life of the structure.

D – Durability Tests

As the saying goes, “An idle mind is the Devil’s playground,” the voids in hardened concrete give a path for the ingress of Carbon Dioxide (CO2) and other gases.

• Water Permeability Test DIN 1048 Part 5
• Rapid Chloride Penetration Test ( ASTM C1202)
• Wenner 4 Probe Resistivity Test
• Carbonation Chamber Test
• Bulk Diffusion Test

E –Encast (Embedded) Items

The design engineer shall coordinate with disciplines (Mechanical, Electrical, Instrument and Telecom, Piping) for requirements of encast items in concrete. The drawing shall show the location, level and layout of all Insert plates/angles, and anchor bolts. The construction engineer shall coordinate and track the availability of inserts before the planned pour. Check the requirement of the template to maintain the position and level of pre-fixed anchor bolts, For Example, concrete pour for Gas Turbine Generator anchor bolts, steel members (Angle/Channel) are supplied by the vendor for final assembly and used to position anchor bolts.

F - Formwork

Formwork shall be designed by a competent and experienced Temporary Works Designer. Soffit formwork should be designed for the self-weight of the wet concrete plus construction loading from operatives, equipment, and plants. Vertical formwork should be designed to resist concrete pressures. Fresh concrete acts like a liquid and high pressures can be produced in deep sections. This pressure acts in all directions and can tend to lift formwork as well as to ‘burst’ it. The formwork and the associated falsework shall be braced for lateral stability.

G – Grading of Aggregates

Proper grading of aggregates (Fine and Coarse) ensures a dense and workable concrete mix. Well-graded aggregates reduce the voids in the concrete mix, leading to higher strength and durability.

H – Heat of Hydration

The chemical change of cement paste is an exothermic reaction, releasing heat. This heat of hydration becomes a critical design aspect in mass concrete (large concrete pour, e.g., Nuclear containment raft, tower raft, dam). The temperature difference between the core and the surface is crucial.

I - Insulated Curing

This practice is used in cold weather to ensure the proper hydration and strength development of concrete. It addresses maintaining temperature and preventing freezing.

J - Joints

All three types of joints, contraction, expansion, and construction joints, shall be considered in pre-pour planning. Joints are provided to minimize cracks in fresh and hardened concrete.

K - Keeping Surface Moist

To support curing, the concrete mix with supplementary cementitious materials like GGBS (Ground Granulated Blast Slag), Fly Ash, and Micro Silica requires continuous longer-duration curing to accomplish desired properties..

L - Lightweight Concrete

Long-span composite floors use lightweight concrete that optimizes steel requirements in secondary beams ( supporting metal deck floor) and primary lattice girders or cellular/castellated girders. Engineers are working on the development of Floatable, Lightweight Cement Composite (FLCC) for application in offshore structures

M – Maintaining Clear Cover

Ensuring the correct cover to reinforce is essential for durability and protection against corrosion.

N – No to Water addition

Do not add water to the mix delivered by truck to the site. A small dosage of admixture may be added based on pre-alignment during mix design

O - Optimizing Admixture

Admixtures are catalysts for enhancing both the fresh and hardened properties of concrete. For high-strength concrete with a cube strength exceeding 60 MPa, High-Range Water-Reducing Admixtures (HRWR) are added to lower the water-cement ratio to below 0.3, thereby improving strength and durability. In applications requiring pumping concrete at elevated heights, Viscosity Modifying Admixtures (VMAs) are used to prevent segregation and ensure consistent flow. Pre-construction concrete performance shall be conducted using a test pour or simulating the actual conditions.

P - Placement of concrete

Place concrete using tools and techniques to avoid segregation of the fresh concrete mix.

Q - Quality Assurance and Quality Control (QA / QC)

Physical, mechanical and chemical parameters of the ingredients shall be tested at planned frequency and planned intervals. Test results of both fresh and hardened concrete shall be maintained. Maintaining a record of moving mean compressive strength and standard deviations helps to qualify/accept cube failure results below characteristic cube/cylindrical strength.

R – Reinforcement

Proper placement and securing of reinforcement are crucial for the structural integrity of concrete elements. Reinforcement shall stored and covered to avoid corrosion. if reinforced corrosion is the prime problem, option of using GFRP rebar

S - Supplementary Cementitious Materials (SCMs)

SCMs ( GGBS, Flyash, Microsolica) play a significant role in improving the durability of hardened concrete. They help to reduce embodied carbon if locally available.

T - Temperature

Both Hot Weather and Cold Weather concreting require special precautions to complete the hydration of fresh concrete. Temperature Forecast shall be considered before planning pour of concrete

U - Use Cement not older than three months

Use cement not older than 3 Months (12 weeks). Water used shall be tested as per BS EN 1008:2002 for quality, meeting limits on pH, Dissolved Chemicals ( Chlorides, Sulphates, organic and inorganic solids)

V - Vibration

Fresh concrete shall be vibrated/compacted using an appropriate type of vibrator (Poker, External, Vibrating Beams) to remove entrapped air and ensure a dense and homogeneous mix

W - Workability of Fresh Concrete

It defines how easily concrete can be placed considering reinforcement congestion, height of pour, and access for compaction. Normal concrete workability is tracked using the slump test before the pour. For self-compacting concrete following tests are performed:

• Slump-flow test (SF1,SF2, SF3 Class)
• J-Ring test
• L-Box test
• Orimet test
• GTM screen stability

The slump-flow test measures the diameter of the spread and the time taken to reach a spread of 500 mm. The J-ring test measures the passing ability of fresh concrete (clear gap of 10 mm, concrete shall not exhibit signs of segregation).

X - X-ray Inspection

X-ray inspection can be used to detect internal defects in concrete structures, such as voids, cracks, and honeycombing. It is a non-destructive testing method that provides valuable information about the internal condition of concrete.

Y - Young’s Modulus of Elasticity (MOE)

Tall buildings' lateral sway and acceleration felt by users at the topmost floor are linked to Flexural Rigidity (product of E and Moment of Inertia I). Young’s modulus of elasticity of concrete is tested at 90 days or 180 days to meet desired values.

Z - Zoom into the Future of Concrete

3-D printing of concrete is an emerging technology that offers new possibilities for construction, including complex shapes, reduced waste, and faster construction times.

 References

1. Myth and Misinterpretation of "Overdesign"
2. Durability Requirements for Concrete
3. 3-D Printing Concrete
4. BRMCA Ready Mixed Practical Guide
5. A Novel, Multifunctional, Floatable, Lightweight Cement Composite: Development and Properties
6. CRMCA ( Colorado Ready Mixed Concrete Association)
7. High-Strength, High-Performance Iconic Supertall Towers
8. Successful High-Strength Concrete in Tall Buildings in North America
9. A Tall Building Engineer’s Perspective on Specifying Modulus of Elasticity (MOE)
10. The Successful Use of High-Strength Concrete in United States High-Rise Buildings