Construction concrete is a fundamental material in civil engineering projects across Kenya, from high-rise buildings in Nairobi to county government infrastructure in Kisumu. Mastery of concrete mixing processes ensures structural integrity, durability, and cost-effectiveness in construction works. This chapter delves into the science and practical steps of concrete mixing, equipping professionals with knowledge to produce quality concrete tailored to project requirements.

9.1 Concrete Mixing

Concrete mixing involves combining cement, aggregates, water, and sometimes admixtures to form a homogeneous material that hardens to provide structural strength. In Kenya's construction sector, proper mixing techniques directly influence the performance of concrete in diverse climatic and site conditions. Understanding the terminology and manufacturing process is essential for civil engineers, site managers, and technicians to achieve consistent quality.

9.1.1 Introduction and definition of terms

Concrete mixing terminology forms the foundation for effective communication and quality control on construction sites. Clear understanding of these terms enables precise specification, testing, and troubleshooting during concrete production.

Cement

Cement is the binding material in concrete, typically Portland cement in Kenya. It reacts chemically with water (hydration) to form a hard matrix that binds aggregates together. Cement quality affects setting time, strength development, and durability.

Aggregates

Aggregates are inert granular materials such as sand, gravel, or crushed stone that occupy the bulk of concrete volume. They provide compressive strength and reduce shrinkage. Aggregates must be clean, well-graded, and free from harmful substances like clay or organic matter to ensure concrete performance.

Water

Water initiates the hydration process of cement and affects concrete workability. The water-to-cement ratio critically influences strength and durability; excess water weakens concrete, while insufficient water impedes hydration. Kenyan contractors must use clean, potable water to avoid contamination that could reduce concrete quality.

Admixtures

Admixtures are chemical additives incorporated into concrete to modify its properties, such as accelerating setting time, improving workability, or enhancing durability. Common admixtures in Kenya include plasticizers for easier placement and retarders for hot weather concreting to delay setting.

Mix Ratio

The mix ratio specifies the proportion of cement, sand, aggregates, and water in concrete by weight or volume. It determines concrete strength and workability. For example, a 1:2:4 mix (cement:sand:aggregates) is common for general construction, while structural elements may require stronger mixes like 1:1.5:3.

9.1.2 Manufacturing process of concrete

The manufacturing of concrete entails a sequence of controlled steps to ensure a uniform and durable final product. In Kenya, construction projects often face challenges such as material variability and climate effects, making adherence to process standards crucial.

9.1.2.1 Batching

Batching is measuring the quantities of cement, aggregates, water, and admixtures before mixing. It can be done by volume or weight, with weight batching preferred for accuracy. On-site concrete batching plants or manual methods are common in Kenya's urban and rural projects.

Accurate batching is essential because the strength and durability of concrete depend on the correct proportions, especially the water-cement ratio, and errors of a few shovelfuls per batch quickly add up to weak or uneven concrete. Volume batching measures materials in gauge boxes or containers and is common on small sites, while weight batching measures materials on scales and is used in batching plants and for structural concrete where accuracy matters most. Example: the contractor building classrooms for a secondary school in Siaya replaced shovel counting with timber gauge boxes sized to one bag of cement, and cube test results became much more consistent.

Equipment for Batching

Steps for Volume Batching

1. Confirm the mix: Check the specified mix ratio, water-cement ratio and any admixtures.
2. Prepare gauge boxes: Use gauge boxes sized to the cement bag, for example one box for 1 part and two boxes for 2 parts.
3. Allow for bulking of sand: Increase the sand quantity if the sand is damp, because damp sand occupies more volume.
4. Measure the aggregates: Fill the gauge boxes level with sand and coarse aggregate on the mixing platform or into the mixer skip.
5. Add the cement: Add whole bags of cement for each batch.
6. Measure the water: Measure water in a graduated container according to the water-cement ratio, adjusting for wet aggregates.
7. Measure admixtures: Add admixtures in the dose recommended by the manufacturer.
8. Record each batch: Record the materials used per batch for quality control.

Worked Example: Materials for 1 m³ of 1:2:4 Concrete

Calculate the materials needed for 1 m³ of 1:2:4 concrete with a water-cement ratio of 0.5. Take the dry volume factor as 1.54, density of cement as 1,440 kg/m³ and one bag as 50 kg.

Dry volume = Wet volume × 1.54

Dry volume = 1 × 1.54

Dry volume = 1.54 m³

Sum of ratio parts = 1 + 2 + 4 = 7

Cement volume = (1 ÷ 7) × 1.54

Cement volume = 0.22 m³

Mass of cement = 0.22 × 1,440

Mass of cement = 316.8 kg, which is about 6.3 bags, say 7 bags

Sand volume = (2 ÷ 7) × 1.54

Sand volume = 0.44 m³

Coarse aggregate volume = (4 ÷ 7) × 1.54

Coarse aggregate volume = 0.88 m³

Water = Water-cement ratio × Mass of cement

Water = 0.5 × 316.8

Water = 158.4 kg, which is about 158 litres

9.1.2.2 Mixing

Mixing combines all materials to produce a homogeneous blend. Mechanical mixers, such as pan or drum mixers, are preferred for consistency and efficiency. Proper mixing time, typically 2 to 4 minutes, is essential to avoid under or over-mixing, which can affect concrete strength and workability.

Mixing coats every aggregate particle with cement paste and distributes all materials evenly, so that every part of the concrete has the same strength and workability. Machine mixing is preferred for all structural and large-volume work, while hand mixing is acceptable only for small, non-structural work, and it usually requires about 10% extra cement to make up for less efficient mixing. Example: a dairy farm in Ol Kalou hired a 500-litre drum mixer for the floor slab of its new milking parlour, producing uniform concrete much faster than hand mixing for the earlier calf pens.

Equipment for Mixing

Steps for Machine Mixing

1. Prepare the mixer: Check the mixer, clean the drum and wet its inside to prevent loss of water from the first batch.
2. Add part of the water: Put about a quarter of the mixing water into the drum.
3. Add coarse aggregate: Load the coarse aggregate to scour the drum.
4. Add sand and cement: Add the sand and cement while the drum turns.
5. Add the remaining water: Add the rest of the water gradually, with any admixture.
6. Mix for the required time: Mix for at least 2 minutes after all materials are in, until the colour and consistency are uniform.
7. Discharge the concrete: Empty the drum completely into the transporting container.
8. Clean the mixer: Wash the drum with water and some aggregate at breaks and at the end of the day.

Steps for Hand Mixing

1. Prepare the platform: Clean a hard, watertight platform.
2. Spread the sand: Spread the measured sand evenly.
3. Add the cement: Spread the cement over the sand.
4. Mix dry: Turn the sand and cement at least three times until the colour is uniform.
5. Add coarse aggregate: Spread the coarse aggregate over the mix and turn it until evenly distributed.
6. Add water gradually: Form a crater in the centre, add water slowly and turn the mix inwards.
7. Mix until uniform: Continue turning until the concrete has uniform colour and consistency.

9.1.2.3 Transporting

After mixing, concrete must be transported to the placement site promptly to prevent initial setting. In urban projects like Nairobi hospitals, transit mixers ensure continuous agitation during transport. For sites without access to mixers, wheelbarrows or buckets may be used, but this increases the risk of segregation and quality loss.

Transporting concrete means moving it from the mixer to the point of placement quickly and without segregation (separation of coarse aggregate from mortar), loss of water or loss of cement paste. The method chosen depends on the quantity, distance, height and site access, and concrete should be placed as soon as possible after mixing, commonly within about 30 minutes for site-mixed concrete in hot weather and within the time stated on the delivery ticket for ready-mixed concrete. Example: during construction of a four-storey office for a bank in Nyeri, the contractor used a concrete pump to place slab concrete directly from transit mixers, avoiding the delays and segregation caused by carrying concrete up ladders in buckets.

Methods of Transporting Concrete

Precautions When Transporting Concrete

• Avoid delays: Transport concrete immediately to prevent it from stiffening before placement.
• Prevent segregation: Avoid long drops, rough routes and excessive vibration that separate stone from mortar.
• Prevent loss of water: Cover containers in hot, sunny or windy conditions.
• Use clean, watertight containers: Prevent leakage of cement paste and contamination.
• Never add water to stiffened concrete: Adding water weakens concrete and changes the water-cement ratio.
• Plan smooth routes: Provide firm runways and ramps for wheelbarrows and dumpers.

9.1.2.4 Placing

Concrete is placed into formworks or molds carefully to avoid segregation. Techniques such as pouring in layers and using vibrators ensure compaction and eliminate air pockets. For example, in the construction of county government offices in Mombasa, mechanical vibrators are used to achieve dense concrete with high strength.

Placing is depositing concrete in its final position in the formwork, as close as possible to where it will remain, in layers of suitable thickness and without disturbing the reinforcement. Before placing, the formwork and reinforcement must be checked and approved, and placing should continue without long interruptions so that cold joints do not form between hardened and fresh concrete. Example: the site engineer on a water storage tank for a county water company in Machakos inspected the formwork, cover blocks and reinforcement before approving placing, and the walls were cast continuously to avoid leaking joints.

Steps for Placing Concrete

1. Inspect formwork and reinforcement: Check dimensions, alignment, strength, cover and fixing of reinforcement.
2. Clean the formwork: Remove debris, sawdust, tie wire and standing water.
3. Apply release agent: Coat the formwork with release agent where required.
4. Wet absorbent surfaces: Dampen timber formwork and existing concrete surfaces.
5. Place close to final position: Deposit concrete near where it is required instead of moving it sideways with vibrators.
6. Limit the free fall: Avoid dropping concrete more than about 1.5 m; use chutes or tremie pipes for greater heights.
7. Place in layers: Place in even layers of about 300 mm to 450 mm, compacting each layer before the next.
8. Work continuously: Keep placing until the element or planned construction joint is complete.

Good Practices During Placing

• Avoid displacing reinforcement: Keep bars and cover blocks in position while placing.
• Do not place in heavy rain: Rain washes out cement and weakens the surface.
• Protect from heat: In hot weather, place early in the day and keep materials cool.
• Form proper construction joints: Locate joints where specified and prepare them before continuing.
• Watch the formwork: Check for leaks, bulging or movement during placing.

9.1.2.5 Compacting

Compacting is the process of removing entrapped air from freshly placed concrete so that it becomes dense, fills the formwork completely and surrounds the reinforcement fully. Freshly placed concrete can contain 5% to 20% entrapped air, and every 1% of air left in it reduces its strength by about 5% to 6%, as well as leaving voids, honeycombing and weak bond with the reinforcement. Compaction is done mainly with vibrators, which make the concrete flow and release air bubbles, or by hand methods such as rodding and tamping for small, non-structural work. Example: on a footbridge for a county roads office in Bungoma, honeycombing found in an earlier beam led the contractor to train workers on correct poker vibrator spacing, and the next beams were dense and smooth.

Equipment for Compacting

Steps for Compacting with a Poker Vibrator

1. Check the vibrator: Confirm the vibrator works and the head size suits the section and reinforcement spacing.
2. Insert vertically: Insert the poker vertically and let it sink quickly under its own weight.
3. Penetrate the previous layer: Allow the poker to enter about 100 mm to 150 mm into the layer below to knit the layers together.
4. Vibrate for the right time: Hold it for about 10 to 15 seconds, until air bubbles stop rising and the surface looks shiny.
5. Withdraw slowly: Pull the poker out slowly so that the hole closes behind it.
6. Space insertions correctly: Insert at regular intervals, usually about 300 mm to 600 mm, so that zones of influence overlap.
7. Avoid contact with formwork and bars: Keep the poker away from formwork faces and avoid vibrating the reinforcement directly.
8. Complete before setting: Finish compaction of each layer before the concrete begins to set.

Signs of Adequate and Poor Compaction

• Adequate compaction: Air bubbles stop rising, the surface becomes level and shiny with a thin layer of paste, and the sound of the vibrator becomes steady.
• Under-compaction: Leaves honeycombing, voids, poor bond with reinforcement and low strength.
• Over-vibration: Causes segregation and bleeding, bringing excess water and fine material to the surface.
• Uneven compaction: Results from irregular insertion spacing, giving variable quality in the same element.
• Formwork problems: Excessive vibration near formwork can cause leaks of grout and surface blemishes.

9.1.2.6 Curing

Curing maintains adequate moisture and temperature for hydration after placement. Proper curing in Kenya's hot climate is vital to prevent premature drying and cracking. Methods include water spraying, wet coverings, or curing compounds applied for at least 7 days to achieve desired strength.

Curing starts as soon as the concrete surface is hard enough not to be damaged, usually after the final finishing, and continues for at least 7 days for ordinary Portland cement and longer, often 10 to 14 days, for blended cements and in hot, dry or windy conditions. Concrete that is not cured can lose a large part of its potential strength and develop surface cracks and dusting. Example: a hotel in Garissa ponded water on its new roof slab and covered the beams with wet hessian for 14 days because of the area's high temperatures and drying winds.

Methods of Curing Concrete

• Ponding: Low bunds of mortar or sand hold a layer of water on flat slabs.
• Water spraying: Concrete surfaces are sprayed with water several times a day.
• Wet coverings: Hessian, jute sacks, sand or sawdust are kept wet on the concrete surface.
• Polythene sheeting: Plastic sheets are laid over damp concrete to prevent evaporation.
• Curing compounds: Liquid membranes are sprayed on the surface to seal in moisture.
• Leaving formwork in place: Formwork kept on longer retains moisture in columns and beams.

Steps for Curing Concrete

1. Start promptly: Begin curing as soon as the surface can be wetted without damage.
2. Choose the method: Select a method suited to the element, weather and water availability.
3. Keep surfaces continuously moist: Do not allow the concrete to dry out between wettings.
4. Protect from sun and wind: Shade and shelter fresh concrete in hot, windy conditions.
5. Protect from damage: Keep people, materials and loads off fresh concrete.
6. Continue for the specified period: Maintain curing for at least 7 days or as specified.
7. Record curing: Note the start and end dates in the site diary.

9.1.2.7 finishing

Finishing is the process of levelling, smoothing, texturing or otherwise treating the exposed surface of freshly placed and compacted concrete to give it the required level, appearance and performance. The type of finish depends on the use of the surface: a rough or broomed finish provides grip on ramps and pavements, a wood-float finish gives an even surface for screeds and tiles, and a steel-trowel finish produces a dense, smooth surface for floors. Timing is critical, because finishing too early brings water and fine material to the surface, while finishing too late makes the concrete difficult to work. Example: the warehouse floor of an agricultural cooperative in Eldoret was power floated and steel trowelled to produce a hard, dense surface that resisted wear from loaded trolleys.

Tools and Equipment for Finishing

Types of Concrete Finishes

• Screeded (tamped) finish: The concrete is struck off to level with a screed board, leaving a rough surface for further layers.
• Wood-float finish: A uniform, slightly gritty surface suitable for receiving screeds, tiles or carpets.
• Steel-trowel finish: A smooth, dense, hard-wearing surface for floors exposed to foot traffic.
• Power-floated finish: A very flat, dense surface produced by machines for warehouses and industrial floors.
• Broom (brushed) finish: A ridged, non-slip surface for ramps, pavements, driveways and parking areas.
• Exposed aggregate finish: The surface paste is washed or brushed away to expose the decorative aggregate.

Steps for Finishing a Concrete Slab

1. Screed the surface: Strike off the compacted concrete to level using a screed board moved in a sawing action along guide rails.
2. Bull float the surface: Smooth the surface with a bull float to remove ridges and fill small hollows.
3. Wait for bleed water to evaporate: Allow surface water to disappear before further finishing.
4. Float the surface: Use a wood or power float to produce an even surface.
5. Edge and joint: Round the edges and form control joints with edging and jointing tools.
6. Trowel the surface: For smooth floors, steel trowel the surface, increasing pressure with each pass as the concrete stiffens.
7. Apply texture if required: Broom the surface for a non-slip finish where specified.
8. Begin curing: Start curing immediately after finishing to protect the surface.

Common Finishing Defects and Their Causes

• Dusting: A powdery surface caused by finishing while bleed water is present or by poor curing.
• Crazing: Fine surface cracks caused by rapid drying or overworking the surface.
• Scaling: Flaking of the surface due to over-trowelling or weak surface paste.
• Blistering: Bubbles under a trowelled surface caused by trowelling too early and trapping air or water.
• Uneven surface: Hollows and ridges caused by poor screeding or inadequate guide rails.

Chapter Summary

This chapter provides a comprehensive overview of concrete mixing, beginning with the key terms and the detailed manufacturing process that includes batching, mixing, transporting, placing, compacting, curing, and finishing. It then explores the main components of concrete such as cement, aggregates, water, and additives, while distinguishing between different types of concrete including ready-mix, site-mixed, and precast varieties. The principles of mixing, whether by volume or weight, are discussed alongside the importance of mix design and proportioning, with special emphasis on the water-cement ratio and adjustments for specific work requirements. Various types of concrete mixes are examined, covering both standard and special mixes tailored for unique applications. The chapter also reviews the tools and equipment used in mixing concrete, comparing hand mixing with machine mixing and highlighting essential safety procedures. Finally, it addresses the critical properties of concrete, including mechanical aspects like compressive strength and workability, as well as durability factors such as chemical resistance, density, fire resistance, and sound insulation.