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While mixing concrete may come off as being somewhat of a no brainer, find top tips and the best kept secrets on how to hand mix concrete to from the strongest and most durable concrete. Mixing concrete can’t be classed as simply adding water to a Portland cement and aggregate mix and hoping for the best. There are a number of factors to consider.

The Secret to Mixing Sephaku Cement

  • Always wear gloves, protective eyewear and old clothing - Cement is exothermic which means that there is a chemical reaction during the hydration of cement. This can also be referred to as “heat of hydration”. Although extreme heat is usually only attributed to large volumes of cement paste, always be aware of this chemical response when mixing concrete and that if splashed onto the skin, cement paste may burn.
  • Measure your mixture – A number one tip when mixing concrete and preparing the most durable concrete, always measure the amount of water used to mix with the concrete. Too much and too little water can affect the strength of the concrete where, too little water will result in a crumbly effect and too much water will cause set concrete to be weak.
  • Mix well – The best was to mix concrete is to replace a standard shovel mix with a metal rake for speedier and more effective results. Creating a similar action to that of a fork or balloon whisk (in cooking) the prongs of a metal rake agitate the mixture more effectively, combining the ingredients faster, reducing the amount of time and effort needed to create a smooth, tacky cement paste.
  • Combine (dry to wet) water and concrete slowly so that the paste gradually thickens without creating lumps and dry pocket clumps of cement paste that is difficult to combine. The goal here is to create a consistent mixture of water to concrete that’s mixed well and easy to place.
When Concrete Paste is Too Runny.
Although runny concrete will (eventually) set, the concrete will be frail and weak, vulnerable to cracks and fractures. You will know your mixture is too runny because the combined cement will have no traction or grip and will sag or slump when dug into a trough. Avoid runny and thin concrete paste by measuring the water in a bucket before mixing, instead of measuring by eye with a hose. Keep in mind that as the water is absorbed the paste can become "soupy" if not mixed properly so be sure to mix well, combining the paste well before adding more water or dry cement. You may like to set a few handfuls of dry cement aside for later, should your cement paste mixture become too runny, using this to thicken the mix up if necessary.

How to Resolve Crumbly or Dry Cement Paste.
Concrete is too dry if the mixture crumbles and clumps resulting in an inconsistent mixture. In order to resolve concrete mix that is too dry, create a well in the centre of the heap, adding one measure of water at a time. Be certain to mix well after each cup in order to review the results before adding excess water, wetting the concrete mix too much. Once the concrete mixture is thoroughly wetted, again create a well where the sides of an ideal concrete mix will remain in place without crumbling (or slosh because it’s too wet) and where the surface is slightly shiny.

We highlight the few common types of concrete cracks and other blemishes that occur most frequently and investigate its typical causes.

The main classification of concrete cracks
All concrete cracks can broadly be classified as either dormant or active. The difference can be summarised as follows:

  • Dormant cracks remain unchanged; and
  • Active cracks change over time, in direction, width or depth.
Cracks can be diagonal, longitudinal, vertical or random and the severity of a crack is also characterized by its direction, depth, width. Both dormant and active cracks create opportunities for moisture and other deleterious materials or liquids to penetrate and can lead to future damage if left unrepaired.
Types of concrete cracks and its causes
Some of the concrete cracks that are seen most often include:
  • Plastic shrinkage cracks: If it occurs, it will normally be visible shortly after placing the concrete. The shrinkage is caused by the forces and subsequent stresses that are created by the rapid removal of water from the concrete. Consequently the severity of cracking is largely influenced by the rate of drying of the concrete surface. The drying and removal of water from the surface depends on such factors as : -
    The temperature of the air and the concrete.
    • The relative humidity of the atmosphere.
    • The velocity of the wind.
    • The absorbency of the surface in contact with the fresh concrete.
  • Crazing, also known as checking or map cracks: This is a network of fine, superficial cracks that feature across the surface of the concrete. Craze cracking is usually caused by excessive floating and trowelling of the wet concrete surface or by applying driers comprising neat cement or a dry mixture of sand and cement onto the wet surface and then trowelling. This surface bear bad wear resistance and has a tendency to delaminate easy.
  • D-cracks: D-cracks run deeper than surface cracks and either originates from a concrete joint or runs parallel to it. It is caused by moisture penetration at the joints.
  • Hairline cracks: Hairline cracks are very thin, but run deep and could sometimes be a bit wider at the top than at the bottom. These cracks typically occur as the concrete settles during curing. It is also known as plastic settlement cracks.
  • Pop-outs: These are conical, slightly sunken depressions that appear on the concrete surface. It commonly appears due to pieces of aggregate close to the surface of the concrete that are highly absorptive, such as clinker ash with sometimes high free lime present. As a result, it expands and protrudes through the surface.
  • Scaling: Scaling causes minor pock-like marks on the concrete surface and exposes the aggregate underneath it.
Scaling can be caused by different issues. Firstly, it appears when concrete is not adequately finished and causes moisture to infiltrate. The moisture expands when there are temperature changes and pushes off pieces of concrete from the surface.

Then there is delamination. Ineffective finishing methods and too early floating before all or most of the bleed water has evaporated can lead to excessive moisture or air in the concrete, which will in turn rise to the top and form pockets just below the surface. These pockets may create blisters, which can break open and cause scaling.
  • Spalling: Spalling is similar to scaling, but the surface depressions are larger and deeper and often linear to the length of a rebar (the reinforced steel rod used in concrete).
There are multiple issues that can lead to spalling. This includes pressure from under the surface of the concrete, inadequately constructed joints or decomposition of the rebar in the concrete.
  • Offset cracks: Offset cracks occur where the concrete on the one side of the crack is of a different height than on the other side of it. It generally arises when the surface under the concrete has not been levelled or inadequately/ inconsistently compacted.
  • Diagonal corner cracks: These cracks run diagonally from one joint to its vertical joint at the other corner of the concrete slab. It appears because of curling or warping. Curled up or warped corners are not very stable as it generally has empty spaces below it. As a result, it often cracks after curing, the moment weight is applied.

Prevention is better than cure
When analysing the causes of cracking, it becomes clear that prevention is better than cure. Be sure to avoid errors in design and detailing and apply sound construction practices right from the word go. Feel free to get in touch with Sephaku Cement for expert advice on construction of concrete fixtures.

We delve into concrete curing and the most suitable curing techniques. Curing is a process that is executed immediately after the placement and finishing of the concrete, to ensure that all the ingredients bond fully. This is done by retaining suitable moisture and temperature conditions. Adequately cured concrete will demonstrate the following characteristics:

  • Improved strength
  • Enhanced volume stability
  • Reduced likelihood of cracking
  • Resistance to freezing and thawing
  • Resistance to wear and abrasion, such as dusting, crazing and scaling
Ultimately, concrete structures that have been properly cured will be more durable in the long term.

The two curing categories
Curing techniques can generally be divided into two groups:
  • Those that add moisture
  • Those that prevent moisture loss
Each of these methods has advantages and disadvantages that need to be considered before deciding which one is most suitable.

Curing techniques that add moisture
The typical curing methods that add moisture are:
  • Sprinkling: This simply involves the continuous sprinkling of the concrete with water at intervals.
  • Wet covers: In this technique, the concrete is covered with damp, moisture-retaining materials such as straw, cotton mats or burlap, which is a coarse, hemp-like fibre. Some also use moist earth, but it can be quite messy.
  • Ponding: This is done by forming a little wall around the concrete slab normally using sand and by flooding the concrete surface with water.
Both sprinkling and use of wet covers are excellent curing techniques as it also provides cooling through evaporation, which is especially critical in hot weather. When sprinkling, it is just important to keep the intervals frequent enough - so that they concrete will not dry out in between. When using wet covers, it is important to place them as soon as the concrete hardens. Curing techniques that prevent moisture loss
The most common curing method to prevent moisture loss is to seal the surface through the use of the following types of material:
  • Plastic sheets: Waterproof plastic film is a lightweight barrier that is easy to apply, regardless of the shape and size of the concrete surface.
  • Waterproof paper: The surface needs to be wet with fine water spray before the waterproof paper is placed.
    When using waterproof paper, it is imperative to check that it is thick enough, as too thin paper may discolour the hardened concrete

An easy and inexpensive curing method
Curing compounds are membrane-forming substances that are relatively inexpensive in comparison to other curing techniques. It can easily be applied with spray equipment such as hand-operated pressure sprayers. It also allows for long periods of curing even while the concrete is in use. Here are some useful pointers:
  • In the case of heavy rains within a few hours of application, the compound must be reapplied.
  • Brushes can also be used, but only on formed surfaces; as it could mar smooth surfaces.
  • They are also available with dyes in order to see which areas are sprayed and which areas were missed. (The dyes normally degrade after a while and present no discolouration issues).

Adequate curing length
There are several factors that can influence or determine the concrete curing period:
  • Ratio of concrete mixture;
  • The level of strength required;
  • The size and shape of the concrete structure;
  • Weather conditions; and
  • Future conditions that the concrete will be exposed to.
To determine a suitable length for curing, it is therefore best to consult specialists first. For expert advice on curing techniques, feel free to get in touch with Sephaku Cement.

Building experts agree that for masonry homes and offices, concrete stairs are more stable and durable than wooden stairs. Here is the lowdown on building concrete stairs.

It starts with a solid design
Concrete stairs need to meet the following requirements:

  • Comply with prescribed building codes and standards;
  • At least 90cm wide to allow sufficient space for use;
  • It must provide rails;
  • An angle and slope that facilitate comfortable use, even by older people; and
  • A solid foundation.
Considering the complexities of such a structure, it is best to commission a civil engineer to conduct the design.

Start by building the form according to the design
You will start by building your formwork according to the specifications of the design. Here are some pointers:

CUT AND LAY THE FORMWORK

You can use low-grade plywood such as 2x100mm or 2x180mm to create your formwork, but it is critical to check that it is of sufficient thickness to support the weight of the concrete you are going to pour.

  • Cut all the forms for the framework;
  • First lay the side forms to create a 2-dimensional side elevation of the stairs.


FASTEN THE FORMWORK

Ideally, the formwork of the concrete stairs should be fastened to both the foundation and the ceiling of the building. For this reason, it is recommended that the ceiling slab and stairs are poured on the same day.

  • Use screws to fasten the formwork firmly to the foundation and ceiling slab of the building;
  • Check that the steps will bond to the foundation in a seamless connection.
Utilise wooden rods to create braces to resist the outward thrust of the poured concrete. Then nail a sufficient number of rods all along the outer surface of the form.


INSTALL THE RISER FACES

The final step of the formwork is to lay the riser faces.

  • Add braces to the centre for stability;
  • Ensure that there are no gaps between the boards;
  • Use a spirit level to straighten the forms and level the treads.
Ultimately it is important that the entire framework is locked together tightly and securely.


CREATE THE CORRECT CONCRETE MIX

To design the correct concrete mix, you may contact Sephaku Technical Sales for advice on mix design/proportions needed.

One of the most critical parts of creating concrete steps that will last; is using the correct concrete mix. For this type of application, a high strength concrete mix will suffice. For an area of 5 square metres, you will need:

  • 2 x 50kg bags of Sephaku 42,5N blended cement
  • 2 x 65-litre wheelbarrows of fine sand
  • 2 x wheelbarrows of gravel
  • Water
Mix thoroughly, using enough water to create a mixture that is easily pourable.


POUR THE CONCRETE
  • Start at the bottom and work your way up
  • Pour only one step at a time
  • Spade/vibrate the mix to remove any entrapped air
  • Spread the concrete evenly and smooth the surface finish to the desired texture


FINISH THE STEPS
  • Level the steps with a screed board or wooden float
  • For a finer finish, use a hand / steel trowel



To avoid cracking, you should allow for a curing period of at least two weeks. Click here for best practices on curing techniques (link to our blog article about curing techniques). As soon as the concrete has set, the form boards can be removed with a hammer and crowbar. Just be sure to exercise patience to avoid damaging the stairs’ edges in the process. The supporting formwork at the bottom of the stairs must stay in place until sufficient strength has been achieved, which will most likely be in around 28 days.

To build structures that will last, you need the right type of cement. We take a look at the different types of cement and its applications.

The three main strength classes of cement All cement is mainly classified into three strength classes.

  • 32,5
  • 42,5
  • 52,5
This figure is typically accompanied by either an “N” or ”R”, which denotes the period of time it takes for the cement to reach the suitable strength.
  • The N implies that it will have normal strength gain; and
  • The R implies early/rapid strength gain.
This is important to know, as should you for example build a structure that requires rapid setting and hardening, you would use an “R” type of cement that will in other words reach strength at an earlier age.

Cement standards
Questionable materials will produce questionable results. For the South African built environment, it is recommended to use cement that complies with the requirements of South African National Standards (SANS) 50197-1. All the Sephaku Cement products fully conform to the requirements of the strength classes as specified by SANS 50197-1, as well as the National Regulatory Of Compulsory Specifications (NRCS) requirements.

General purpose cement that gains strength at an early stage
The Sephaku 32 is a 32,5N general purpose blended cement that reaches strength at a normal age and that is suitable for general use in:
  • Concrete for home or building structures
  • Mortar, plaster and screed applications.


Sephaku 32 applications
The Sephaku 32 can be blended to achieve low, medium or high strength concrete for a myriad of applications:
  • High strength concrete: At the correct proportions columns, ground slabs and light -duty industrial floors;
  • Medium strength concrete: paths, driveways and light-duty floors;
  • Low strength concrete: foundations, domestic flooring, steps, paths and fencing;
  • General purpose mortar: bricklaying and plastering; and
  • Dry, lean mixes: brick- and block manufacturing.

General purpose cement that has normal strength gain
The Sephaku 42 blue is a highly versatile 42,5N general purpose blended cement that is suitable for a comprehensive repertoire of concrete, mortar and screed application. It can be blended to achieve all the required strengths of concrete for the same applications as the Sephaku 32 mentioned above.
Sephaku 42 blue benefits
  • Suited for a comprehensive range of building and civil work;
  • Used to manufacture concrete products;
  • Appropriate for any domestic concrete use; and
  • It uses less water to achieve a mix that is cohesive and durable.

Early strength cement for fast demoulding and striping of formwork
The Sephaku 42 green is a 42,5R cement that develops strength at an early stage, which makes it most suitable for the following applications:
  • Any type of building and civil work;
  • Instances where fast demoulding, striking or striping of formwork is advantageous;
  • Ready-mix; and
  • Site-batched concrete, mortar and screeds.
This cement also has a significantly reduced water demand which in turn leads to concrete that flows and pumps effortlessly and creates an improved surface finish with enhanced durability.

High performance cement that requires day one strength
The Sephaku 52 is a 52,5N high performance cement that is suitable for civil engineering and structural concrete applications that require day one strength and cold-weather concreting. This includes:
  • High-strength concrete, precasting and ready-mix;
  • Water-tight, structural and pre-stressed concretes; and
  • Precision grouts, repairs and formulated cement products.
It is also ideal for use with cement extenders that enhance the concrete properties such as silica fume and fly ash or slagment.
For a comprehensive guideline on the types of cement and its applications, as well as the mix proportions for the various concrete strengths, click here to download the Sephaku Cement product brochure now.

Concrete is made up of four elements: cement, sand, stone (aggregate) and water. The cement: sand: stone ratio is critical as it determines the compressive strength of the concrete, which in turn establishes the capacity of the structure you build to withstand loads. We offer a guideline to solid concrete mixing proportions for optimal compressive strength.

Choose the right cement for the right application

Point of departure would be to select the right cement for the right application and to check that it complies with the relevant specifications to ensure a lasting end product. Click here for more information on the types of cement and its applications.

Input equals output

Apart from choosing the right type of cement for the right application, it is also critical to take an uncompromising approach to the quality of the cement you choose, to ensure structures that will last. The range of cement supplied by Sephaku Cement does not only meet, but also exceeds the relevant standards of the built environment in South Africa:

  • Manufactured according to the requirements of the National Regulator for Compulsory Standards;
  • Sold under a Letter of Authority (LOA);
  • Carry the SABS mark certification ; and
  • In accordance with the South African National Standards (SANS) 50197-1.

Consult manufacturers’ guidelines for ideal mixing proportions
As soon as you’ve selected the cement that is most suited to your application, you will need to consult the manufacturers’ guidelines to determine the ideal concrete mixing proportions. The mixing proportions are also subject to the strength of concrete you need, which can generally be classified in the following five groups:
  • High strength (25 - 30 MPa)
  • Medium strength (20 – 25 MPa)
  • Low strength (10 – 15 MPa)
  • Mortar (5 MPa)
  • Dry, lean mixes (7 – 14 MPa)

Examples of concrete mixing ratios
For the purposes of this illustration, we will take a look at the Sephaku 32, a 32,5N general purpose blended cement that reaches compressive strength at a normal rate and that is suitable for an extensive range of building applications, including general domestic or building concrete, mortar, plaster and screed.
The guidelines allow for minimal wastage of between 5-10% and are based on a 65-litre builder’s wheelbarrow. Please note that one 65-litre wheelbarrow load of cement is equal to two 50kg bags in volume.
  • Dry, lean mixes for brick and block manufacturing: One wheelbarrow of cement (= 2 x 50kg pockets) and eight wheelbarrows of sand.
  • General purpose mortar for bricklaying and plastering: One wheelbarrow of cement and six wheelbarrows of sand.
  • Low strength concrete for foundations, domestic flooring, steps, paths and fencing: One wheelbarrow of cement, four wheelbarrows of sand and four wheelbarrows of stone.
  • Medium strength concrete for paths, driveways and light-duty floors: One wheelbarrow of cement, three wheelbarrows of sand and three wheelbarrows of stone.
  • High strength concrete for columns, slabs and heavy-duty industrial flooring: One wheelbarrow of cement, two wheelbarrows of sand and two wheelbarrows of stone.

These concrete mixing proportions will vary, subject to the type of cement that is required. For a detailed guideline on concrete mixing proportions of 42, 5 and 52, 5 types of cement, click here to download the Sephaku Cement product brochure now.