Sand & Cement Jointing FAQS
Sand – Sand & Cement Mortars
Natural Sand with / without Sand Stabilisers – The use of natural sand that is simply brushed into the joints between the paving elements is probably the oldest method, and is still a widely used approach, but only suitable for concrete block paving with negligible joint widths and laid butt-jointed to the next element. The key advantages of sand filled joints are speed and cost, but the important key disadvantages are the ease of washout and ease of weed growth. These disadvantages can be partially negated, if not always completely eliminated, by the use of a so-called ‘Sand Stabiliser’, such as GftK’s vdw 870.
This is roller or spray applied to the newly laid and jointed paving, where it penetrates. As the resin dispersion dries through evaporation, the residual resin acts to bind the fine sand particles together, thus very significantly reducing the surface resistance to sand washout or weed growth. A side benefit is that the impregnation of the concrete blocks also somewhat increases their stain resistance by reducing the surface permeability. This can be a solution for this type of paving, but re-sanding and re-stabilising will likely need to be repeated every few years to maintain the protection and performance.
Sand and Cement / Polymer-modified Sand and Cement (including site-batched mixes and factory pre-batched mortars). As mentioned in the introduction to paving joint technologies, various forms of cement based paving jointing has been carried out for up to 6000 years, back to the origins of cement mortars and concretes by the Minoans, the Greeks, the Romans, and every civilisation ever since. The key advantages of sand and cement are again that these materials are cheap and widely available everywhere. However the disadvantages of using cement-based materials for paving jointing works can also be significant and these can include cement staining and the additional works due to difficulties with surface cleaning that is always necessary after using any cement jointing. Also importantly for durability, all cement mixes shrink due to hydration as they harden, and this results in cracks in the surfaces or at the bond interface, this is also known as short-term / plastic shrinkage, due to water loss from the freshly placed mortar into the substrate or the air; and / or longer-term shrinkage cracking, which is due to further chemical hydration and reactions, plus physical drying of the mortar over time.
This is the primary reason that for the last 25-35 years or so, to try and reduce / counteract these shrinkage effects, cement-based paving jointing / pointing mixes have generally also been ‘polymer-modified’ (PM), meaning that they have an additional ingredient in the mix of either a liquid ‘polymer’, such as liquid Styrene Butadiene Rubber ( SBR), which can easily be added to site batched pointing mixes. Alternatively, this in factory produced products this is done by using a freeze-dried polymer that is incorporated by industrial shear-mixers into most factory-batched, cement-based, prebagged, paving joint mortars. The polymer addition increases the mortars flow and workability etc, whilst also allowing a lower cement content to be used, as well as a more optimised water to cement ratio, which in turn improves the quality of the hardened mortar in the joints. This is because it reduces the mortar capillary volume and sizes and so reduces the rate of excess water evaporation, which therefore reduces shrinkage cracking – As well as improving the mortars flow and consistence properties for easier application. A ‘Win- Win – Win’ combination. Today the most widely used freeze-dried polymer admixtures are polycarboxylates, which have been derived from high performance concrete superplasticiser technology. In laboratory testing and under ideal site conditions, these advanced polymer-modified paving joint mortars perform extremely well and have minimal or even zero shrinkage cracking.
HOWEVER, as with all freshly mixed cement-based materials it is always necessary to pre-wet and dampen the substrates, ideally to an ‘SSD’ (Saturated Surface Dry) condition, in order to obtain a good bond and also to prevent water migration from the mortar, BUT if there is any residual water from this pre-wetting, then the cement mortar’s precisely laboratory formulated and optimised water to cement ratio, is immediately and very significantly altered. This affects the mortar mix very adversely, negating almost all of the advantages and as a result, shrinkage and cracking of the mortar will now take place to some extent, depending on the amount of residual water present in relation to the cement mortar mix. So Buyer Beware!
This potential problem will arise whenever and wherever any free water / residual moisture is present, either from pre-wetting, from earlier cleaning works, or from any recent rainfall. Overall, the problem of shrinkage cracking is by far the biggest cause of premature failure in all types and brands of cement-based paving jointing products. This can be a difficult one to diagnose though, as the very nature of this issue means that cracks are formed at the time of the pointing/grouting/jointing works on site - BUT they may effectively be invisible or seem to be of no consequence at that time. The cracks and the further problems arising from water ingress into these etc., do not arise for some time, which can be for several months, or even several years depending on the paved areas environment and exposure. Typically, paving joint cement mortar shrinkage problems will always be more visible and evident after winter, when water penetration and freeze-thaw reaction at low temperatures takes place and causes physical damage. The main reasons for this are usually that the original paving was of good quality, BUT for the paving joints, cement-based materials were selected and/or they used cement based jointing products in bad weather conditions that always leads to premature failure, so once again it is always important to remember - Buyer Beware!
These damaging effects are also greatly accelerated by the use of de-icing salts on any paved areas, especially on public roads and pavements, but typically this treatment also affects public car parks, and domestic driveways, with the salt brought in on the cars. As evidence of this issue, you will see many areas of prematurely failing cement-based joints in paved surfaces in the street and around salt treated public and commercial areas, where there are many visible patch repairs to the paving, with frequent paving re-pointing and patching being carried out, almost annually in some areas. Note: Polymer modified cement mortars can resist de-icing salt exposure very well, BUT once cracks have formed during application, then all future bets are off!
W-C Ratio Problems with Cement-based Paving Joint Mortars
As outlined above, one of the key issues with any cement based paving joint mortar, be it traditional site-batched or a factory pre-batched product, is that any residual water in the joints from the paving installation and surface cleaning, or any pre- or immediate post-application rainfall, or any water run-off from adjacent surfaces, will always have seriously negative effects on the water : cement ratio of the product / design mix. These changes are negative because the increased water : cement ratio will significantly reduce the hardened performance and durability of any cement joint mortar. This is by greatly increasing the risk of both short- and long-term shrinkage cracking (by a factor of x 300% or more), plus an even more dramatic increase in the mortar capillary content (up by a factor of x 6 – 600% or more). The result in practice is reduced performance and durability of hardened cement based paving joint mortars, due to significantly reduced resistance to water ingress, de-icing salts and frost damage, plus their resulting much lower strengths and resistance to load, stress exposure and cleaning regimes.
These two graphics are well known in cement technology, and they can be used to Illustrate the dramatic changes in a cement based paving joint mortar, which will typically occur in the joints on site. This excess water can be from any residual water in the joints, or from rainfall just before, during or after installation, or as run-off from adjacent areas / structures etc.
- As can be seen in the first graph: Through the addition of any additional free water locally or along the whole joint, there can easily be 300% more shrinkage that will cause significant cracking (short and long-term), during curing and hardening of the product / design mix, than when precisely tested prebatched in a laboratory for the respective Product Data Sheets and/or any ‘Test Reports’, i.e. these are totally theoretical values and the real world is different.
- In the second graph: This graphic shows that as a result of any additional water, the resulting hardened cement based, paving joint mortar, will be anything up to 600% more permeable / porous than the laboratory test samples. Therefore, with this lower hardened density and much higher pore / capillary network in the cement matrix, the mortar will have much lower strengths and be very much weaker, with a severe loss of tensile / flexural strength. As a result, the joints will be far more susceptible to frost damage from freeze-thaw attack and de-icing salts, be more likely to stain and far less resistant to both natural erosion and modern cleaning regimes, especially jet washing and mechanical street cleaning machines.
These important differences are the reason that engineers can refer to Concrete as ‘Labcrete’ or ‘Sitecrete’ – These can be very different in performance when the environmental conditions for application are not ideal, which is never easy with the great British weather. Another thing to bear in mind is that defects and damage from this shrinkage and increased permeability may not be immediately apparent, because these damaging effects may take some time to develop and a couple of winters to show – But this damage definitely happened at the time of application.
– So once again, buyer – beware! Can you control the environment sufficiently to ensure correct application of cement based paving jointing mortars? If so, then these are a great and very cost-effective solution. However, if this is not possible then you will be faced with awfully expensive repointing or worse, and within a very few years indeed. For signs of this just look around any public or commercial paved areas where the decision on the pointing mortar was likely made largely on the initial cost, even cost per pack or kilo, instead of on the overall long term sustainable cost.
The choice of paving pointing technology and products is yours!
Figure 1. Cement Mortar Shrinkage vs Water : Cement Ratio
Figure 2. Cement Mortar Porosity (as Penetration) vs Water : Cement Ratio
Traditional stone sett paving pointing with typically 3/4 to 1 cement to sand site mixes, are traditionally made and applied by trained and experienced craftsmen by trowel in small quantities, working on their hands and knees. They are cured correctly under damp hessian and plastic sheeting, but they do not do this work when it is cold and wet, or hot and dry. Today the larger areas of paving with cement-based jointing, tend to be installed by pump or slurry and then the surfaces are mechanically cleaned to remove residual cement residue. However, once again this method is also temperamental and sensitive to environmental temperatures and humidity.
As some variation of water to cement ratio is almost inevitable with whatever method of cement pointing on any UK paving project, there will always be some risk and some inherent defects installed from the beginning. This is not to mention the issues of inadequate curing and protection of cement mortars from direct sun and winds that cause the next biggest problem of excess evaporation. This can also result in the loss of surface strength and increased permeability, as well as cracks in cement paving joint mortars – though these will not be seen at the time, they come back to cause latent defects after a relatively short period of time.
Cement based paving joint mortars can also have issues with joints that have variable dimensions which can also lead to variable shrinkage and cracking. Plus it should always be remembered that cement mortars adhere most tenaciously to roughened, permeable mineral surfaces; their adhesion to smooth impermeable surfaces such as metals and plastic is much lower and not ideal for stress transfer or accommodating any form of movement, including for thermal variations – hence the reason that elastic joint sealants are used for the joints in concrete and brickwork structures. This is also a consideration for larger areas of exposed paving.
In summary we must stress that cement based paving jointing in many different forms, has and does provide a highly effective solution in many hard-landscaping situations. In fact, it should also be stressed that cement based paving jointing is used for somewhere over 80-85% of all paving joints - more in some markets and applications so it is not all bad. However, it is important to understand the limitations of cement paving jointing, as well as the fact that it is widely available in many forms and that it is relatively cheap. BUT please consider ALL of the parameters for your paving project and make the right technical and commercial decisions based on ALL of your requirements, which should always include the desired durability and what the ease of closure and repointing would mean on your project!
For more advice or assistance with your specific paving project, please call 01257 266696 for FREE Expert Advice during normal office hours, or you can email us at any time to: sales@nccstreetscape.co.uk and we will get back to you just as soon as we can. Thank you.